1. JOURNAL OF APPLIED PHYSICS 110, 044509 (2011) In-Situ growth of cadmium telluride nanocrystals in poly(3-hexylthiophene) matrix for photovoltaic application Mohd Taukeer Khan,1,2,3 Amarjeet Kaur,3 S. K. Dhawan,1,a) and Suresh Chand2,b) 1 Polymeric and Soft Materials Section, National Physical Laboratory, (CSIR), Dr. K. S. Krishnan Road, New Delhi-110 012, India 2 Organic and Hybrid Solar Cell Group, National Physical Laboratory, (CSIR), Dr. K. S. Krishnan Road, New Delhi-110 012, India 3 Department of Physics and Astrophysics, University of Delhi, Delhi-110 007, India (Received 30 April 2011; accepted 17 July 2011; published online 29 August 2011) In the present study, nanocrystals of cadmium telluride (CdTe) have been directly synthesized in poly(3-hexylthiophene) (P3HT) matrix without use of any surfactant. In situ synthesis of nanoparticles in polymer matrix improves the polymer-nanoparticles interface, which facilitates efﬁcient electronic interaction between them. Spectral results suggest that CdTe nanocrystals are bound with P3HT via dipole-dipole interaction and form a charge transfer complex. Structural and morphological studies reveal that CdTe works as transport media along/between the polymer chains, which facilitate percolation pathways for charge transport. Therefore, enhancement in current density has been observed for the bulk heterojunction (BHJ) device of P3HT-CdTe nanocomposites blended with PCBM. An open circuit voltage (VOC) of 0.80 V was obtained from the BHJ device due to the increase in the energy level offset between the donor and acceptor. This new photovoltaic element could provide a new nanoscale criterion for the investigation of photoinduced energy/charge transport in organic-inorganic interfaces. V 2011 American Institute of C Physics. [doi:10.1063/1.3626464] I. INTRODUCTION phase in organic phase, thus ensuring a large, distributed sur- face area for charge separation. Moreover, nanocrystals are Organic-inorganic hybrid nanocomposites are potential uniformly distributed to the entire device thickness and thus systems for organic photovoltaic devices1–4 because it contains a built in percolation pathway for transport of includes the desirable characteristics of organic and inor- charge carriers to the respective electrodes. ganic components within a single composite. They have In surfactant-assisted synthesis, nanoparticles growth is advantage of tunability of photophysical properties of the controlled by electrostatic interactions of the surfactant func- quantum dots (QDs) and retained the polymer properties like tional group and steric hindrance of surfactant side alkyl solution processing, fabrication of devices on large and ﬂexi- chains. P3HT provides a combination of both effects as it ble substrates.5–9 Due to different electron afﬁnities of QDs contains an electron donating sulfur functionality, a potential and polymer, a built in potential is generated at polymer- anchorage for the nucleation, and growth of nanoparticles QDs interfaces which assist the charge transfer between along with steric hindrance due to long hexyl side polymer and QDs. Up to now, the organic-inorganic hybrid chains.13,14 This new photovoltaic element could provide a thin ﬁlm solar cells have shown power conversion efﬁciency new nanoscale criterion for the investigation of photoinduced of $3% (Ref. 10). In conventional synthesis of QDs (CdTe, energy/charge transport in organic-inorganic interfaces. CdSe), they were capped with organic aliphatic ligands, such Direct synthesis of CdS nanorods13 and CdSe nanopar- as tri-n-octylphosphine oxide (TOPO) or oleic acid. It has ticles14 in P3HT matrix, and also PbS nanorods in poly(2- been shown that when the QDs are capped with organic methoxy-5-(2-ethyl-hexyloxy)-p-phenylene vinylene) (MEH- ligands, they hinder the efﬁcient electron transfer from the PPV),15 have been reported previously. As CdTe have opti- photoexcited polymer to the nanoparticles.11 To remove the mal bandgap for solar cells and absorb higher amount of so- organic ligands, polymer-nanoparticles were treated with lar radiation compared to the CdSe, CdS, PbS nanocrystals, pyridine. However, pyridine is an immiscible solvent for the therefore replacement of theses nanocrystals with CdTe polymer and ﬂocculation of the P3HT chains in an excess of would enable these hybrid devices to further enhancement in pyridine may lead to the large-scale phase separation result- power conversion efﬁciency. ing in poor photovoltaic device performance.12 To overcome The present investigation reports the synthesis of CdTe the effects of the capping ligands on charge transport we nanocrystals in P3HT matrix with three different combina- have directly synthesized uncapped nanoparticles inside the tions of P3HT and CdTe (P3HT1.5, P3HT11 and P3HT12). polymer matrix. The in situ growth of the nanocrystals in These P3HT-CdTe nanocomposites can be dissolved in all polymer templates controls the dispersion of the inorganic common solvents for the polymer, from which thin ﬁlms can be readily cast. Structural and morphological study revealed a) Electronic mail: email@example.com. that CdTe nanoparticles have been successfully synthesized b) Electronic mail: firstname.lastname@example.org. in P3HT matrix. Optical measurements of nanocomposites 0021-8979/2011/110(4)/044509/7/$30.00 110, 044509-1 V 2011 American Institute of Physics CDownloaded 03 Sep 2011 to 188.8.131.52. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
2. 044509-2 Khan et al. J. Appl. Phys. 110, 044509 (2011) ﬁlms show that photoinduced charge separation occurs at the Growth of CdTe NCs has been completed when the solution P3HT-CdTe interfaces, indicating this is a promising color turned to black. On the completion of the reaction, approach for the fabrication of efﬁcient organic-inorganic unreacted cadmium acetate and precursor of tellurium have hybrid photovoltaic devices. Photovoltaic performance of been removed by treating nanocomposites with hexane. The P3HT:PCBM as well as P3HT-CdTe:PCBM have been reaction mixture was separated by centrifugation and dried investigated in device conﬁguration viz. indium tin oxide in vacuum at 80 C. Similarly, other compositions of P3HT (ITO)/poly(3,4-ethylendioxythiophene)-poly(styrene sulfonate) containing different mole ratios of Cd-acetate were synthe- (PEDOT:PSS)/P3HT:PCBM/Al and ITO/PEDOT:PSS/ sized and are designated as P3HT1.5 for 0.1 mmol and P3HT-CdTe:PCBM/Al, respectively. These devices are des- P3HT12 for 0.4 mmol having Te precursor in the ratios of ignated as device A and device B, respectively. Based on 0.2 mmol for P3HT1.5 and 0.8 mmol for P3HT12. The syn- these investigations very interesting and important results theses of different P3HT-CdTe compositions have been also have been found wherein the JSC and VOC of device B have carried out at 220 C using the same procedure discussed increased. Improvement in JSCis due to enhancement of solar above. absorption and the formation of charge transfer complex (CTC) which reduces the defect states and barrier height at B. Characterization the polymer-nanoparticles interfacial boundaries. The enhance- For optical and scanning electron microscope (SEM) ment in VOC is explained by the increase in the energy level studies, P3HT and P3HT-CdTe nanocomposites were dis- offset between the LUMO of the acceptor and the HOMO of solved in chlorobenzene and thin ﬁlms of these solutions the donor. The fundamental facet revealed in situ synthesis of were deposited on glass substrates by spin casting at 1500 P3HT-CdTe nanocomposites via surfactant-free method, rpm for 120 s, and annealed at 120 C for 30 min. Absorption which, to our best knowledge, is designed for the ﬁrst time spectra were recorded by Shimadzu UV-1601 spectropho- and its optical, electrical study shows that a P3HT-CdTe tometer. Photoluminescence measurement was carried out at nanocomposite is highly plausible for solar cell application. room temperature. The samples were excited with the wave- length of 510 nm optical beam and the photoluminescence II. EXPERIMENTAL METHODS (PL) signal was detected with the Perkin Elmer LF 55 having Xenon source spectrophotometer (in the wavelength region A. Synthesis of 530–850 nm). Fourier transform infrared spectroscopy P3HT has been synthesized by chemical oxidative cou- (FTIR) spectra were recorded on Nicolet 5700 in transmis- pling method as described previously.16 In situ growth of sion mode, wavenumber range 400–4000 cmÀ1 with a reso- CdTe nanoparticles in P3HT matrix was carried out as sche- lution of 4 cmÀ1 performing 32 scans. Samples for high- matically illustrated in Fig. 1. In a typical synthesis of resolution transmission electron microscopy (HRTEM) were P3HT11, 50 mg of P3HT has been dissolved in 10 ml of prepared by putting a drop of nanocomposites solutions on chlorobenzene to which 0.2 mmol of cadmium acetate dihy- carbon grids and images were taken using a Tecnai G2 F30 drade in chlorobenzene has been added. The reaction mix- S-Twin instrument operated at an accelerating voltage of ture has been heated for 2 hrs at 160 C. Tellurium precursor 300 kV, having a point resolution of 0.2 nm and a lattice has been made by treating 0.4 mmol of tellurium powder resolution of 0.14 nm. (Acros Organics) in trioctylphosphine (TOP) (Sigma Aldrich, USA), at 160 C for 2 hrs under argon ﬂow. The tel- C. Solar cells fabrication and testing lurium precursor has been then injected in to the P3HT-Cd For the fabrication of device A and device B, ITO (sheet solution and the resultant bright orange reaction mixture was resistance $18 X/cm2) substrates have been carefully cleaned allowed to react for 2 hrs at 160 C under argon atmosphere. in ultrasonic baths of acetone and isopropyl alcohol and dried at 120 C for 60 min in vacuum. Prior to use, substrate have been treated with oxygen plasma. PEDOT: PSS (Sigma Aldrich, USA) layers were spin- coated at 2000 rpm for 2 min. onto the ITO substrate and cured at 120 C for 60 min in vacuum. P3HT:PCBM and P3HT11:PCBM both have been taken in the ratio of 1:0.8 with a concentration of 1 wt. % in chlorobenzene. The solu- tion containing P3HT plus PCBM was designated as solution A and other containing P3HT-CdTe nanocomposite plus PCBM were designated as solution B. The chlorobenzene so- lution A and B have been spin casted at 1500 rpm for 2 min on the top of PEDOT:PSS layer in an inert atmosphere, fol- FIG. 1. (Color online) Proposed mechanism for in situ growth of the CdTe lowed by annealing at 130 C for 30 min. Finally, Aluminum nanoparticles in the P3HT matrix. (a) P3HT was synthesized by chemical (Al) contacts 150 nm has been applied via evaporation oxidative polymerization route. (b) Schematic of Cd2þ ions were assumed to be coupled with the unpaired S along the P3HT planar chain network. (c) through a shadow mask at 2 Â 10À6 Torr. The device active Schematic diagram of P3HT capped CdTe nanoparticles after reaction of area has been taken $0.1 cm2 for all the devices discussed in TOPTe with Cd2þ ions coupled P3HT. this work. The J-V characteristics of device A and deviceDownloaded 03 Sep 2011 to 184.108.40.206. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
3. 044509-3 Khan et al. J. Appl. Phys. 110, 044509 (2011) B have been performed in the dark and under halogen lamp has been suppressed. Hence, further device investigation has illumination with irradiance of 80 mWcmÀ2, using a Keithley been carried out in P3HT11. However, this interaction 2400 Source-Measure unit, interfaced with a computer. between polymer and nanoparticles indicates that nanocom- posites have potential for the charge transfer at polymer- III. RESULTS AND DISCUSSION nanoparticles interfaces, which results in the PL quenching and the improvement of short circuit current density. The A. Morphological study mechanism of this interaction has revealed that the sulfur High resolution transmission electron micrograph atom of P3HT can interact with the CdTe nanoparticles by (HRTEM) images and electron diffraction (insets) patterns dipole-dipole interaction and CdTe nanocrystals have been of the synthesized P3HT-CdTe nanocomposites P3HT1.5, deposited uniformly and compactly on or in-between the P3HT11, and P3HT12 at 160 C are shown in Fig. 2. Trans- P3HT chains to form nanoparticles as suggested in Fig. 1(c). mission electron micrograph images reveal that ratio of The selected area electron diffraction patterns of P3HT1.5, P3HT and cadmium acetate plays a signiﬁcant role in con- P3HT11 and P3HT12 are shown in the inset of Figs. 2(b), trolling the size and shape of the nanocomposites. A differ- 2(d) and 2(f), respectively, which conﬁrmed the high crystal- ence in contrast at different places indicates that the CdTe linity of the CdTe. nanocrystals are capped by P3HT. It is evident from the Figs HRTEM images of the synthesized P3HT-CdTe nano- 2(a) and 2(b) that at low CdTe concentration the P3HT ma- composites P3HT1.5, P3HT11, and P3HT12 at 220 C are trix shows more binding with CdTe nanocrystals and forma- shown in Fig. 3. In the present case nanorod formation of tion of even nanorods structure of P3HT-CdTe as seen by P3HT-CdTe is absent due decrease in the bonding between enlarged image [Fig. 2(b)]. However as the CdTe concentra- P3HT and CdTe and nanocrystals shows better crystallinity. tion increase Figs. 2(c) and 2(e) the binding between CdTe Moreover, the particle size in the present case is large com- and P3HT reduces and saw the precipitation of CdTe nano- pared with that of above, due to at higher temperature aggre- crystals appear rather than percolated network. The optimum gation of particle take place. Therefore further studies have percolation and interaction between P3HT and CdTe take place in P3HT11 as shown in Figs. 2(c) and 2(d), where the nanorods formation as well as individual CdTe precipitation FIG. 2. HRTEM images and electron diffraction (ED; insets) patterns of (a)–(b) P3HT1.5, (c)–(d) P3HT11 and (e)–(f) P3HT12 nanocomposites syn- FIG. 3. HRTEM images and electron diffraction (ED; insets) patterns of thesized at 160 C. Bar scale 20 nm for panels (a), (c), and (e) and 5 nm for (a)–(b) P3HT1.5, (c)–(d) P3HT11 and (e)–(f) P3HT12 nanocomposites syn- (b) (d) and (f). thesized at 220 C.Downloaded 03 Sep 2011 to 220.127.116.11. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
4. 044509-4 Khan et al. J. Appl. Phys. 110, 044509 (2011) been carried out on P3HT-CdTe hybrid system synthesized at 160 C. The surface morphology of the of the pristine P3HT and P3HT-CdTe nanocomposite ﬁlms have been examined by SEM images. Figure 4(a) shows the nearly ﬂat surface mor- phology of pristine P3HT ﬁlm. The SEM images with differ- ent P3HT and CdTe compositions (P3HT1.5, P3HT11, P3HT12) have been shown in Fig. 4(b)–(d). At low concen- tration of CdTe (P3HT1.5), the nanocrystals aggregate to form sludge like structure due to binding between P3HT and CdTe as shown in Fig. 4(b). However, with increase of the CdTe concentration [Fig. 4(c)], the binding between CdTe and P3HT reduces leading to the formation of multifoliated leaf like structures. On the contrary, further increase in CdTe concentration multifoliated leaf like structures reduces lead- ing to precipitation of CdTe nanocrystals (as evident from FIG. 5. (Color online) FTIR spectra of P3HT and P3HT-CdTe the difference in brightness) shown in Fig. 4(d). nanocomposites. B. Spectral study stretching decreases. Also a shift by 25 cmÀ1 (from 1110 to 1135 cmÀ1), to the higher energy region of C-S characteristic The success of formation of P3HT-CdTe nanocompo- band has been observed in P3HT-CdTe, indicating the sites have been conﬁrmed by the Fourier transform infra-red enhancement of the C-S bond energy. Moreover, the charac- (FT-IR) spectra as shown in Fig. 5. Strong absorption bands teristic band of thiophene ring shows a redshift from 822 to of P3HT at 2953, 2920 and 2854 cmÀ1 have been assigned 816 cmÀ1 with the increase of concentration of CdTe in to the asymmetric C–H stretching vibrations in –CH3, –CH2, polymer matrix. These ﬁndings suggest additional intermo- and the symmetric C–H stretching vibration in –CH2 respec- lecular interaction between polymer and nanocrystals which tively. They are ascribed to the alkyl-side chains. The bands arises due to strong dipole-dipole interaction between the at 1456, 1377 cmÀ1 are due to the thiophene ring stretching Cd2þ ions and S atoms as shown in Fig. 1(b). and methyl deformation respectively. The C-C vibrations The normalized UV-vis spectra of the P3HT and P3HT- appear at 1260 cmÀ1. The characteristic C-S band stretching CdTe nanocomposites ﬁlms are shown in Fig. 6(a). The max- has been observed at 1111 cmÀ1 while absorption band at imum absorption of pristine P3HT ﬁlms has been observed 822 cmÀ1 and 725 cmÀ1 have been assigned to the aromatic at 510 nm which corresponds to the p-p* transition of the C-H out-of plane stretching and methyl rocking respectively. conjugated chain in the P3HT (Ref. 17 and 18). For the In nanocomposites of P3HT-CdTe, the intensity of peaks P3HT-CdTe composite ﬁlms, the absorption spectrum has corresponding to C-S bond and aromatic C-H out-of plane been broader compared to that of pristine P3HT. The broad- ness in absorption spectra indicates the presence of CdTe nanocrystals in polymer matrix.14 Maximum absorption for P3HT1.5 and P3HT11 have been red shifted to 515 nm and 518 nm, respectively. Redshift in P3HT-CdTe nanocompo- sites suggest the formation of charge transfer states in P3HT- CdTe nanocomposites resulting in partial electron transfer from P3HT to CdTe (Ref. 19). On further increase of the amount of CdTe in P3HT (P3HT12) there is a blueshift in absorption spectra compared to that of P3HT1.5 and P3HT11, which is observed at 514 nm. This means at higher concentration of CdTe in P3HT, there is smaller shift in absorption spectra. The smaller shift in absorption at high concentration of CdTe in P3HT compared to that of low con- centration of CdTe (P3HT1.5, P3HT11) is due to weak inter- action between polymer-nanocrystals. The photoluminescence quenching can be used as a powerful tool for evaluation of charge transfer from the excited polymer to the nanoparticles.20,21 Once the photo- generated excitons are dissociated, the probability for recom- bination should be signiﬁcantly reduced. In Fig. 6(b), we compared PL spectra of pristine P3HT ﬁlms with that of dif- ferent nanocomposites P3HT-CdTe ﬁlms. The P3HT and FIG. 4. SEM micrograph of spin casted thin ﬁlms of panels (a) P3HT, (b) P3HT-CdTe nanocomposites exhibited emission maximum P3HT1.5, (c) P3HT11 and (d) P3HT12 annealed at 120 C for 30 mins. around 660 nm. PL intensity of the nanocomposite ﬁlm hasDownloaded 03 Sep 2011 to 18.104.22.168. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
5. 044509-5 Khan et al. J. Appl. Phys. 110, 044509 (2011) FIG. 6. (Color online) (a) Normalized absorption spectra of P3HT and P3HT- CdTe nanocomposites ﬁlms synthesized at different ratio of P3HT and CdTe pre- cipitated with hexane. (b) Photolumines- cence spectra of P3HT, P3HT-CdTe nanocomposites, P3HT-PCBM and P3HT- CdTe-PCBM ﬁlms after excitation by radi- ation of 510 nm wavelengths. been signiﬁcantly reduced as compared with the value of the P3HT11. The PL quenching upon addition of PCBM in P3HT ﬁlm and the reductions of PL intensity have been P3HT and P3HT11 further conﬁrm the electron transfer increased with the increase of the CdTe concentration in from P3HT to CdTe or PCBM and CdTe to PCBM. The QY polymer. Reduced PL intensity of the composites relative to of P3HT decreases from initially 26% to 11% on incorpora- the pristine P3HT indicates that charge transfer, thereby tion of CdTe nanocrystals in to the P3HT matrix. The P3HT, exciton dissociation at interface of CdTe and P3HT occurred P3HT1.5, P3HT11, P3HT12 shows the QY of 26%, 20%, (Fig. 7) (Ref. 22). This PL quenching experiment provides 17%, 14%, respectively. Reduction in QY of polymer/nano- us with good evidence that the nanoparticles will be able to crystal composites compared to that of pristine P3HT, imply- transfer their excited state hole to the polymer. Charge trans- ing that a large amount of singlet excitons are not able to fer take place in conjugated polymer-semiconductor nano- radiate onto ground state and they dissociate at the polymer/ crystals composites at the interface where the P3HT with a nanocrystals interface as suggested in Fig. 7. higher electron afﬁnity (À3.37 eV) transferred electron onto CdTe with relatively lower electron afﬁnity (À3.71). In con- C. P3HT-CdTe:PCBM solar cells version, the polymer absorb the solar photons (charge gener- ation), the electron is transferred to the CdTe nanocrystals Figure 8(a) shows the J-V characteristics of device A and the hole potentially can transfer to the polymer (charge and B under AM 1.5 illuminations with an intensity of 80 separation). This is a well known effect of the ultrafast elec- mWcmÀ2. The performance of device A showed a short-cir- tron transfer from the donor to acceptor, and it is expected to cuit photocurrent (JSC) of 2.25 mAcmÀ2, an open-circuit increase the exciton dissociation efﬁciency in photovoltaic voltage (VOC) of 0.58 V, a ﬁll factor (FF) of 0.44, and a devices.23,24 Moreover, the PL spectra of P3HT-PCBM and power conversion efﬁciency (PCE) of 0.72%. However, in P3HT11-PCBM are also shown in Fig. 6(b). On incorpora- situ growth of CdTe nanocrystals in P3HT matrix (device tion of PCBM in P3HT and P3HT11 (P3HT-CdTe), the PL B), the PCE value increased up to 0.79% thereby improving spectra has been quenched relative to the P3HT and the Jsc to 3.88 mAcmÀ2, VOC of 0.80 V, while FF diminish- ing to 0.32. The increase in the value of JSC of device B can be understood in terms of host (P3HT) and guest (CdTe) charge transfer type interaction. In fact, there are various possibil- ities by which CdTe can interact with host P3HT. It can ei- ther go structurally into P3HT main chain or forms donor acceptor charge transfer complex (CTCs) or form molecular aggregates. However, the enhancement in JSC in P3HT11 nanocomposites indicates that CTCs formation between the host and guest may be the dominant mechanism of interac- tion between the two. This suggested mechanism is indeed supported by the PL quenching in P3HT-CdTe nanocompo- sites, decrease in QY and energy levels of different materials used shown in Fig. 7. On incident of light, both P3HT and CdTe absorb light and generate excitons. Here, electron afﬁnities of P3HT, CdTe and PCBM are 3.37 eV, 3.71 eV and 4.2, respectively, hence it is energetically favorable for FIG. 7. (Color online) Schematic illustration of the energy diagram of con- electron transfer from P3HT to CdTe or PCBM and CdTe to ﬁguration of device B. The P3HT, CdTe and PCBM have HOMO levels at PCBM or hole injection from CdTe to P3HT as indicated by 5.27, 5.48 and 6.0 eV while LUMO levels at 3.37, 3.71 and 4.2 eV, respec- tively for facilitating the charge transfer at the P3HT-CdTe nanocomposites arrows in Fig. 7 (Ref. 25). and PCBM interface. The arrows indicate the expected charge transfer in Moreover, enhancement in JSC may also results in process P3HT-CdTe-PCBM. improvement in light absorption in P3HT-CdTe compositesDownloaded 03 Sep 2011 to 22.214.171.124. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
6. 044509-6 Khan et al. J. Appl. Phys. 110, 044509 (2011) FIG. 8. (Color online) (a) J-V curves obtained from device A and device B under AM 1.5 illuminations at irradia- tion intensity of 80 mW/cm2 (b) J-V characteristics of pristine and P3HT- CdTe nanocomposites ﬁlms in hole only device conﬁguration viz. ITO/ PEDOT:PSS/P3HT or P3HT-CdTe/Au at room temperature in dark. compared to that of pristine P3HT. As in composite device and change of I-V shape of device B from device A. The P3HT and CdTe both absorb light compared to that of addition of one hole-blocking layer at cathode and another P3HT: PCBM device where PCBM contribution is very electron-blocking layer at anode can prevent the polymer small, hence light harvesting is more in hybrid system so and nanocrystal from shorting the two electrodes under illu- that number of exciton generated upon incident of light is mination. This and other similar approaches aimed at increases and as a results current density increases. increasing FF in hybrid PV cells are currently under investi- Figure 8(b) shows the J-V characteristics of P3HT and gation within our laboratory. P3HT-CdTe (P3HT11) nanocomposites thin ﬁlms in hole only device conﬁguration viz. ITO/PEDOT:PSS/P3HT/Au IV. CONCLUSIONS and ITO/PEDOT:PSS/P3HT-CdTe/Au. The nature of J-V characteristic of composites thin ﬁlm is different from that of In conclusion, the successful synthesis of CdTe nanopar- pristine P3HT. In case of composites ﬁlm the hole current ticles in a P3HT matrix without need of any surfactant has has been observed to be more than that in pristine P3HT. been demonstrated. BHJ solar cells device of P3HT-CdTe: The enhancement in hole current in P3HT-CdTe composites PCBM have been fabricated and these results were compared compared to that of pristine P3HT can be understood in with those of the P3HT:PCBM. Incorporation of CdTe in terms of host (P3HT) and guest (CdTe) charge transfer type P3HT:PCBM device increases the JSC and VOC. Increase in interaction. In the composites ﬁlm the CdTe nanocrystals are JSC is due to CTCs formation between the host (P3HT) and bound with P3HT via dipole-dipole interaction and form a guest (CdTe QDs) duly supported by UV-vis absorption and CTC. The charge carriers which had to jump from one chain PL quenching studies and decrease of QY. Improvement in to another to transport through P3HT are now assisted by the VOC is due to the increase in the energy level offset between CdTe nanocrystals. The calculated value of activation energy the LUMO of the acceptor and the HOMO of the donor. Fur- of localized states has been found to be 52 meV for P3HT ther improvement can be achieved by controlling over the and 11 meV for P3HT-CdTe (Ref. 26). As activation energy morphology of the photoactive layer, improving the contacts in P3HT-CdTe is lower compared to the pristine P3HT, the between photoactive layer and cathode and reducing the cur- CdTe nanocrystals support transportation of holes which rent leakage by introducing the electron and hole blocking improves their mobility and results into enhancement in the layers before respective electrodes. hole current. The enhancement in VOC in device B can be understood ACKNOWLEDGMENTS in terms of lower HOMO level of CdTe compared to P3HT The authors would like to thank Director NPL for his (Fig. 7). VOC is correlated with the energy difference keen interest in work. We sincerely thank Dr. Ritu Srivas- between the HOMO of the donor polymer and the LUMO of tava, Mr. Neeraj Chaudhary and Dr. Kuldeep Singh all from the acceptor.27,28 Clearly, a lower HOMO energy level pro- NPL, for their cooperation and useful discussions. Our spe- vides a higher open circuit voltage (Voc). The measured dif- cial thanks to Mr. K.N. Sood for recording the SEM micro- ference (0.21 eV) of the HOMO energy levels between graph and to Dr. Renu Pasricha for recording HRTEM P3HT and CdTe almost completely translated into the images. The authors are also thankful to Dr. Avadesh Prasad observed difference in Voc ($0.22 V). from Department of Physics and Astrophysics, University of The cells suffered from low ﬁll factors which may be Delhi for his useful suggestions. One of us (M.T.K.) is thank- caused by low shunting and a high series resistance.29–31 The ful to CSIR, New Delhi, India, for the award of Senior presence of polymer or nanocrystal pathways that connect Research Fellowship. the anode to the cathode is a source of current leakage or electrical shorts, depending on the conductivity of the path- 1 Y. Y. Lin, T. H. Chu, S. S. Li, C. H. Chuang, C. H. Chang, W. F. Su, C. P. way.32 The incorporation of CdTe nanocrystals into a Chang, M. W. Chu, and C. W. Chen, J. Am. Chem. Soc. 131, 3644 (2009). 2 P3HT–PCBM matrix results enhancement in photoconduc- H. C. Leventis, S. P. King, A. Sudlow, M. S. Hill, K. C. Molloy, and S. A. Haque, Nano Lett. 10, 1253 (2010). tivity of the active layer.33 Thus increased photoconductivity 3 W. U. Huynh, J. J. Dittmer, and A. P. Alivisatos, Science 295, 2425 of the active layer is responsible for the decreasing ﬁll factor (2002).Downloaded 03 Sep 2011 to 126.96.36.199. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
7. 044509-7 Khan et al. J. Appl. Phys. 110, 044509 (2011) 4 20 P. K. Sudeep and T. Emrick, ACS Nano, 3, 2870 (2009). J. Xu, J. Wang, M. Mitchell, P. Mukherjee, M. Jeffries-EL, J. W. Petrich, 5 H. Xin, O. G. Reid, G. Ren, F. S. Kim, D. S. Ginger, and S. A. Jenekhe, and Z. Lin, J. Am. Chem. Soc. 129, 12828 (2007). 21 ACS Nano, 4, 1861 (2010). J. Yu D. H. Hu, and P. F. Barbara, Science 289, 1327 (2000). 6 22 K. M. Coakley and M. D. McGehee, Chem. Mater. 16, 4533 (2004). D. S. Ginger and N. C. Greenham, Phys. Rev. B 59, 10622 (1999). 7 23 B. G. Kim, M. S. Kim, and J. Kim, ACS Nano, 4, 2160 (2010). G. D. Scholes and D. S. Larsen, Phys. Rev. B 61, 13670 (2000). 8 24 S. Zhang, P. W. Cyr, S. A. McDonald, G. Kostantatos, and E. H. Sargent, N. C. Greenham, X. Peng, and A. P. Alivisatos, Phys. Rev. B 54, 17628 Appl. Phys. Lett. 87, 233101 (2005). (1996). 9 25 C. Groves, O. G. Reid, and D. S. Ginger, Acc. Chem. Res. 5, 612 (2010). J. N. Freitas, I. R. Grova, L. C. Akcelrud, E. Arici, N. S. Sariciftci, and 10 S. Dayal, N. Kopidakis, D. C. Olson, D. S. Ginley, and G. Rumbles, Nano A. F. Nogueira, J. Mater. Chem. 20, 4845 (2010). 26 Lett. 10, 239 (2010). M. T. Khan, A. Kaur, S.K. Dhawan, and S. Chand, J. Appl. Phys. 109, 11 W. U. Huynh, J. J. Dittmer, W. C. Libby, G. L. Whiting, and A. P. Alivisa- 114509 (2011). 27 tos, Adv. Funct. Mater. 13, 73 (2003). H. Zhou, L. Yang, S. Stoneking, and W. You, ACS Applied Material and 12 D. Cui, J. Xu, T. Zhu, G. Paradee, S. Ashok, and M. Gerhold, Appl. Phys. Interface 2, 1377 (2010). 28 Lett. 88, 183111 (2006). Z. T. Liu, M. F. Lo, H. B. Wang, T. W. Ng, A. L. V. Roy, C. S. Lee, and 13 H. C. Liao, S. Y. Chen, and D. M. Liu, Macromolecules 42, 6558 (2009). S. T. Lee, Appl. Phys. Lett. 95, 093307 (2009). 14 29 S. Dayal, N. Kopidakis, D. C. Olson, D. S. Ginley, and G. Rumbles, C. Ulzhofer, S. Hermann, N. P. Harder, P. P. Altermatt, and R. Brendel, ¨ J. Am. Chem. Soc. 131, 17726 (2009). Phys. Status Solidi (RRL) 6, 251 (2008). 15 30 A. Stavrinadis, R. Beal, J. M. Smith, H. E. Assender, and A. A. R. Watt, M. S. Kim, B. G. Kim, and J. Kim, ACS Applied Material and Interface 6, Adv. Mater. 20, 3105 (2008). 1264 (2009). 16 31 M. T. Khan, M. Bajpai, A. Kaur, S. K. Dhawan, and S. Chand, Synth. Met. D. Gupta, M. Bag, and K. S. Narayan, Appl. Phys. Lett. 92, 093301 160, 1530 (2010). (2008). 17 32 B. K. Kuila and A. K. Nandi, J. Phys. Chem. B 110, 1621 (2006). W. U. Huynh, J. J. Dittmer, N. Teclemariam, D. J. Milliron, and A. P. 18 R. D. McCullough, Adv. Mater. 2, 93 (1998). Alivisatos, Phys. Rev. B 67, 115326 (2003). 19 33 J. Xu, J. Hu, X. Liu, X. Qiu, and Z. Wei, Macromol. Rapid Comm. 30, H.Y. Chen, M. K. F. Lo, G. Yang, H. G. Monbouquette, and Y. Yang, Nat. 1419 (2009). Nanotech. 3, 543 (2008).Downloaded 03 Sep 2011 to 188.8.131.52. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
8. JOURNAL OF APPLIED PHYSICS 109, 114509 (2011) Hole transport mechanism in organic/inorganic hybrid system based on in-situ grown cadmium telluride nanocrystals in poly(3-hexylthiophene) Mohd Taukeer Khan,1,2 Amarjeet Kaur,2 S. K. Dhawan,1,a) and Suresh Chand1,b) 1 Polymeric and Soft Materials Section, National Physical Laboratory, (CSIR), Dr. K. S. Krishnan Road, New Delhi-110 012, India 2 Department of Physics Astrophysics, University of Delhi, Delhi-110 007, India (Received 29 March 2011; accepted 21 April 2011; published online 9 June 2011) The present manuscript demonstrates the hole transport mechanism in an organic/inorganic hybrid system based on in-situ grown cadmium telluride (CdTe) nanocrystals in a poly(3-hexylthiophene) (P3HT) matrix. The increase of hole current in the hybrid system is correlated with the formation of a host-guest (P3HT-CdTe) charge transfer complex duly supported by photoluminescence quenching. The hole transport mechanism in P3HT is governed by a space charge limited current with temperature, carrier density, and ﬁeld dependent mobility. Incorporation of CdTe nanocrystals in a polymer matrix results in enhancement in the value of trap density Hb from 2.8 Â 1018 to 5.0 Â 1018 cmÀ3 and reduction in activation energies from 52 meV to 11 meV. At high trap density, trap potential wells start overlapping; this results in decrease of activation energies. V 2011 American Institute of Physics. [doi:10.1063/1.3594647] C I. INTRODUCTION i.e., P3HT incorporated with in-situ grown CdTe nanocrys- tals, could provide a new nanoscale criterion for the investi- Conjugated polymer poly(3-hexylthiophene) (P3HT) is gation of photoinduced energy/charge transport in organic/ well known for its application in solar cells as an electron inorganic interfaces. The improved performance of hybrid donor material in combination with n-type inorganic nano- solar cells is expected because of the improved charge trans- crystals, which results in better electron transportation and fer at the organic/inorganic interfaces10 and enhanced solar improved efﬁciency.1–6 In comparison to the all organic radiation absorption. There are practically no discussions electron donor:acceptor devices, this hybrid approach of the available on P3HT incorporated with CdTe nanocrystals photo-active medium is supposed to be the better option as related to the study on its charge transport. they possess the desirable characteristics of both organic The objective of this paper is to study the effect of (absorption) and inorganic (transport) systems. Up to now, in-situ incorporation of CdTe nanocrystals on the hole trans- the organic-inorganic hybrid thin ﬁlm solar cells have exhib- port in P3HT conjugated system. For this, we have studied ited a power conversion efﬁciency of up to g $ 3%,7 which current density versus voltage (J–V) characteristics of pure is relatively small compared to the P3HT:PCBM system P3HT as well as P3HT-CdTe nanocomposite thin ﬁlms in a (g $ 6%).8 The lower g in hybrid system is because of an hole-only device conﬁguration as a function of temperature. inadequate charge transfer between polymer-nanocrystals Based on these investigations, we have found very interest- and the poor nanoscale morphology of the composites ﬁlm. ing and important results wherein the incorporation of CdTe In a conventional route, nanocrystals are synthesized through nanocrystals in P3HT matrix results in the enhancement in the use of surface ligands. The incorporation of such nano- the hole current and change the transport mechanism from crystals with the polymer would form an insulating interface, the mobility model to the trap model. which hinders the electron transfer from the photo-excited polymer to the nanoparticles.9 The in-situ growth of the II. THEORY nanocrystals in polymer templates controls the dispersion of the inorganic phase in organic thus providing a large distrib- A. Exponential distribution of trap states uted surface area for charge separation. Moreover, nanopar- Due to the low mobility of charge carriers in organic ticles are uniformly dispersed in the entire device thickness semiconductors, the injected carrier forms a space charge. and thus contain a built in percolation pathway for transport This space charge creates a ﬁeld that opposes the applied of charge carriers to the respective electrodes. bias and thus decreases the voltage drop across junction; as a The main elements used in hybrid solar cells are P3HT result, space charge limited currents (SCLCs) have been pro- (donor) and CdSe nanocrystals (acceptor). As CdTe absorb a posed as the dominant conduction mechanism in organic higher amount of solar radiation compared to the cadmium semiconductors.11,12 Ohmic conduction can be described by Selenide (CdSe), replacement of CdSe with CdTe therefore V would enable these hybrid devices to be further enhanced in J ¼ qnl ; (1) power conversion efﬁciency. This new photovoltaic element, d where q is the elementary charge, d is the thickness of the a) Electronic mail: email@example.com. ﬁlm, l is the carrier mobility, and n is the carrier density. b) Electronic mail: firstname.lastname@example.org. Pure SCLC with no traps is given by Child’s law13 0021-8979/2011/109(11)/114509/5/$30.00 109, 114509-1 V 2011 American Institute of Physics CDownloaded 10 Jun 2011 to 184.108.40.206. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
9. 114509-2 Khan et al. J. Appl. Phys. 109, 114509 (2011) 9 V2 with j ¼ er e0 l 3 ; (2) Ej À Ei þ jEi À EF j þ Ej À EF 8 d sij ¼ 2arij þ : (8) 2kB T where ere0 is the dielectric permittivity. SCLC theory with an exponential trap distribution pro- All conductive pathways between sites with Gij G are poses that the space charge that limits conduction is stored in electrical insulators, while conductive pathways between the traps. The exponential distribution of traps in energy and sites with Gij ! G are electrical conductors. At some critical space is described as14 conductance in between, therefore, a threshold conductance GC exists where for the ﬁrst time, electrical current can per- Hb E colate from one edge to the other. NðEÞ ¼ exp À ; (3) Et Et A bond is deﬁned as a link between two sites that have a conductance Gij ! G. The average number of bonds B is where N(E) is the distribution function of hole trap density at equal to the density of bonds; (Nb) divided by the density of an energy level E above the valence bandedge, Hb is the total sites that form bonds, (Ns) in the material. Critical bond trap density at the edge of valence band. Et is the characteris- number BC is the average number of bonds per site for which tic trap energy that is often expressed in terms of the charac- threshold percolation occurs. The onset of percolation is teristic temperature of trap distribution TC as Et ¼ kBTC and determined by calculating the critical average number of l ¼ Et/kBT ¼ TC/T. where kB is the Boltzmann constant. The bonds per site17 parameter l determines the distribution of traps in the forbid- den gap. Nb In case of exponential distribution of traps, assuming BðG ¼ GC Þ ¼ BC ¼ : (9) NS that the trapped hole carrier density (pt) free hole carrier density (p) and using continuity equation and boundary con- Vissenberg and Matters16,19,20 set the critical bond number dition for current density (J) and applied voltage (V) as to Bc 5 2.8. The total density of bonds is given by ððð J ¼ qlpðxÞFðxÞ; (4) Nb ¼ 4p 2 rij gðEi ÞgðEj Þhðsc À sij ÞdEi dEj drij : (10) ð V ¼ FðxÞdx: (5) The density of sites Ns ð The expression for J is given by Ns ¼ gðEÞhðsc kB T À jE À EF jÞdE: (11) 1Àl 2l þ 1 lþ1 l er e0 l V lþ1 J ¼ q lNv ; (6) lþ1 l þ 1 Hb d 2lþ1 At a low carrier concentration, exponential density of states in amorphous organic semiconductors is given by:17,20 where F(x) is the electric ﬁeld inside the ﬁlm and Nv is the 8 ! effective density of states. From Eq. (6), the slope of the cur- N0 exp E rent-voltage characteristics on a log-log plot is l þ 1. There- gðEÞ ¼ kB T0 kB T0 ; À1 E 0; (12) : fore from the slopes on the log–log plots of current density 0; E 0 versus voltage, one can extract the trap energy width Et. where No is the total density of states (molecular density) per B. Unified mobility model unit volume and To is a characteristic temperature that deter- mines the width of the exponential distribution. This model is based on percolation in a variable range Combining Eqs. (8) to (12), the expression for Bc hopping (VRH) system with an exponential distribution of localized states.15–17 Percolation is the term used for move- 3 ! T0 Emax ment of charge carriers through a random network of BC ¼ pN0 exp ; (13) 2aT kB T0 obstacles. Consider a square lattice, where each site is ran- domly occupied or empty. Occupied sites are assumed to be where Emax ¼ EF þ sC kB T is the maximum energy that par- electrical conductors while the empty sites represent insula- ticipates in bond formation. According to the percolation tors, and the electrical current can ﬂow between nearest theory, the conductivity of the system can be expressed as neighbor conductor sites. Percolation paths are the most opti- mal paths for current and transport of charge carriers, which r ¼ r0 exp½ÀsC ; (14) are governed by the hopping of charge carriers between these conducting sites. The system can be described as a random where r0 is the prefactor and sc is the critical exponent of the resistor network,18 a system made up of individual discon- critical conductance when percolation ﬁrst occurs (when nected clusters of conducting sites the average size of which B 5 Bc). Using Eqs. (13) and (14), we get is dependent on a reference conductance G. The conductance 2 3T0 =T between sites is given by sin p T 4 T0 T0 Â Ã r ¼ r0 4 p5 : (15) T BC ð2aÞ3 G ¼ G0 exp Àsij (7)Downloaded 10 Jun 2011 to 220.127.116.11. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
10. 114509-3 Khan et al. J. Appl. Phys. 109, 114509 (2011) The conductivity can be converted into mobility by dividing the highest occupied molecular-orbital (HOMO) of P3HT by e. p, where e is the electronic charge and p is the carrier and CdTe as well as far below the lowest unoccupied molec- density:21,22 ular-orbital (LUMO) energy level (Fig. 2(a)); as a result, in 3 the Au: ITO device, the transport is dominated by holes. For 2 sin p T T0 =T 4 the preparation of devices A and B, ITO (sheet resistance, r0 T0 T0 T0 lðT; p; FÞ ¼ 4 3 5 pð T ÞÀ1 : (16) $18 X/cm2) substrates have been carefully cleaned in ultra- q T BC ð2aÞ sonic baths of acetone and isopropyl alcohol and dried at 120 C for 60 min in vacuum. PEDOT:PSS layers have been The average charge carrier density as a function of the spin-coated at 2000 rpm for 2 min onto the ITO substrate applied bias voltage V is given by13 and cured at 120 C for 60 min in vacuum. P3HT and P3HT- ! CdTe both have been taken with a concentration of 20 mg/ e0 er V ml in chlorobenzene. Thin ﬁlms of P3HT and P3HT-CdTe pðVÞ ¼ 0:75 : (17) qd 2 have been spin casted in inert atmosphere, followed by annealing at 120 C for 30 min. Finally, gold (Au) contact, 200 nm, has been applied via evaporation through a shadow III. EXPERIMENTAL mask at 2 Â 10À6 Torr. The device active areas have been taken $0.1 cm2 for all the devices discussed in this work. CdTe nanocrystals ($ 5 nm) have been grown in-situ in J–V characteristics of the devices have been measured with a P3HT matrix. In a typical synthesis of P3HT11, 50 mg Keithley 2400 Source-Measure unit, interfaced with a P3HT has been dissolved in 10 ml chlorobenzene. Cadmium computer. acetate dihydrate, 0.2 mmol (0.1 mmol for P3HT1.5, 0.4 mmol for P3HT12, and 0.6 mmol for P3HT13), was added in IV. RESULTS AND DISCUSSION P3HT-chlorobenzene solution. The reaction mixture was heated for 2 hr at 160 C. A tellurium precursor was made by Figure 1(a) shows the transmission electron micrograph treating 0.4 mmol (0.2 mmol for P3HT1.5 or 0.8 mmol for (TEM) of P3HT-CdTe nanocomposite. Nanocrystals of P3HT12, and 1.2 mmol for P3HT13) tellurium powder CdTe have been found in between the polymers chain. The (Acros organics) in trioctylphosphine (TOP) (Sigma Aldrich, inset of Fig. 1(a) shows the high resolution TEM (HRTEM) USA), at 160 C for 2 hr under argon ﬂow. The tellurium and electron diffraction pattern of CdTe nanocrystals in precursor so obtained has then injected into the P3HT-Cd so- P3HT matrix. In Fig. 1(b), we compared photoluminescence lution, and the resultant bright orange reaction mixture was (PL) spectra of pristine P3HT with that of different nano- allowed to react for 2 hr at 160 C under argon atmosphere. composites of P3HT-CdTe. The PL intensity of the nano- Growth of CdTe NCs has been completed when the solution composites has been signiﬁcantly reduced as compared to color turned black. After completion of the reaction, the the value of pristine P3HT, and the reduction of PL intensity unreacted Cd-acetate and Te precursor were been removed has been increased with the increase of the CdTe concentra- by treating nanocomposites with hexane. The reaction mix- tion in polymer. The reason for the photoluminescence ture was separated by centrifugation and dried in vacuum at quenching in composites may be due to the p–p interaction 80 C. of P3HT with CdTe,23 forming additional decaying paths of The effect of CdTe nanocrystals on the hole conduction the excited electrons through the CdTe (see Fig. 2(a)). in P3HT has been carried out at different temperatures in Reduced PL intensity of the composites relative to the refer- hole only device conﬁguration. For this purpose, the hole ence P3HT, indicates the host-guest charge transfer complex conduction mechanisms of P3HT as well as P3HT-CdTe formation and, thereby, the occurrence of exciton dissocia- nanocomposites thin ﬁlms have been investigated in device tion at interface between P3HT and CdTe-NCs.24 This PL conﬁguration viz. indium tin oxide (ITO)/poly(3,4-ethylen- quenching experiment provides good evidence that the nano- dioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS)/ particles will be able to transfer their excited state hole to the P3HT/Au and ITO/PEDOT:PSS/P3HT-CdTe/Au, respec- polymer. tively. These devices are designated as device A and device Figure 2 shows the J–V characteristics of device A B, respectively. Work functions of Au and ITO are close to measured in the temperature range of 290-150 K. On FIG. 1. (Color online) (a) TEM images of above syn- thesized CdTe nanocrystals in P3HT matrix (Bar scale 10 nm). Inset shows electron diffraction patterns of P3HT-CdTe nanocomposites. (b) Photoluminescence spectra of P3HT and different P3HT-CdTe nanocompo- sites ﬁlms after excitation by radiation of 375 nm wavelengths.Downloaded 10 Jun 2011 to 18.104.22.168. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
11. 114509-4 Khan et al. J. Appl. Phys. 109, 114509 (2011) FIG. 2. (Color online) (a) Schematic illustration of the energy diagram of conﬁguration of device B. The P3HT, CdTe have HOMO levels at 5.27, 5.48 while LUMO levels at 3.37, 3.71, respectively for facilitating the charge transfer at the P3HT-CdTe. (b) Experi- mental (symbols) and calculated (solid lines) J–V characteristic of P3HT thin ﬁlms at different temperatures in hole only device conﬁguration viz. ITO/ PEDOT:PSS/P3HT/Au. lowering the temperature, the current has been found to showed a good agreement with same value of parameters at decrease. In organic semiconductors, charge transport is gov- different temperatures. Solid curves in Fig. (3) represent the erned by hopping of a carrier from site to site of an empty plot of Eq. (6) at respective temperatures. The values of pa- density of states. The thermal energy helps to cross the ener- rameters used in the calculations are; Hb ¼ 5.0 Â 1018 cmÀ3, getic barrier between two adjacent sites. This implies that Nv ¼ 6.0 Â 1018 cmÀ3, l ¼ 6.0 Â 10À5 cm2 VÀ1 sÀ1, d ¼ 110 the charge transport in organic semiconductor is thermally nm, and Tc ¼ 400 K. For the characteristics measured at 250 activated. Therefore on lowering the temperature, the current K, 220 K, 195 K, 175 K, and 150 K, the agreement was has been found to decrease. obtained for l ¼ 7.8 Â 10À5, 1.16 Â 10À4, 2.4 Â 10À4, At low applied bias, the J–V characteristics showed ohmic 3.55 Â 10À4, 7.5 Â 10À4 cm2 VÀ1 sÀ1, respectively. behavior [Eq. (1)] as injected carriers are negligible compared The enhancement in current density in P3HT-CdTe thin to that of the applied bias.25 At moderate ﬁeld, the injected ﬁlm can be understood in terms of host (P3HT) and guest carrier density becomes so high that the ﬁeld due to the car- (CdTe) charge transfer type interaction. In fact, there are var- riers dominates the applied bias. At this point, the J–V charac- ious possibilities by which CdTe can interact with host teristics may switch to pure SCLC and follow Eq. (2). On P3HT. It can either go structurally into P3HT main chain or further enhancement of ﬁeld, the quasi-Fermi level intersects form donor/acceptor charge transfer complexes (CTCs) or an exponential trap distribution, and characteristics will begin form molecular aggregates. However, the enhancement in J to follow Eq. (6). The hole mobility up to this ﬁeld is constant in device B indicates that CTCs formation between the host and also independent of the hole density. The ﬁt of the J–V characteristics of the P3HT device using the Eq. (6) is poor at high applied bias where current density deviates strongly as expected from Eq. (6). This discrepancy has been analyzed by uniﬁed mobility model given by Eq. (16). This model accounts the inﬂuence of temperature, carrier density, and applied ﬁeld on the carrier mobility.17 The solid curves in Fig. (2) have been obtained by combining Eqs. (6) and (16) using a com- puter program. The value of different parameters for solid curves are; d ¼ 110 nm, er ¼ 3, e0 ¼ 8.85 Â 10À14 F/cm, Hb ¼ 2.8 Â 1018 cmÀ3, Nv ¼ 1 Â 1019 cmÀ3, TC ¼ 400 K, ˚ T0 ¼ 325 K, r0 ¼ 4 Â 104 S/m, aÀ1 ¼ 1.12 A, and Bc ¼ 2.8. Figure 3 shows the J-V characteristics of device B meas- ured at different temperatures. Interestingly the nature of P3HT-CdTe composite thin ﬁlm is different than that of pris- tine P3HT. In case of composite ﬁlm, the hole current has been observed to be more than that in pristine P3HT at all temperatures. The inset of Fig. 3 shows the comparison of J–V characteristics of devices A and B at 150 K. The compo- sites exhibited an S-shaped characteristic, and the rate of reduction of current with temperature is low compared to that in pristine P3HT. We tried to ﬁt the experimental data with FIG. 3. (Color online) Experimental (symbols) and calculated (solid lines) mobility model. The data did not show agreement with the J-V characteristics of device B at different temperature in hole only device mobility model for single set of parameter values. On the conﬁguration viz. ITO/PEDOT:PSS/P3HT-CdTe/Au. The inset shows the other hand, the comparison of experimental data with Eq. (6) comparison of J–V characteristics of device A and B at 150 K.Downloaded 10 Jun 2011 to 22.214.171.124. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
12. 114509-5 Khan et al. J. Appl. Phys. 109, 114509 (2011) and guest may be the dominant mechanism of interaction overlapping of traps potential wells; this results in a decrease between the two, duly supported by photoluminescence in activation energies. Our results constitute important pro- quenching [Fig. 1(b)]. In the composites ﬁlm, the CdTe gress for the use of inorganic nanocrystals in polymer for the nanocrystals are bound with P3HT via dipole-dipole interac- large area solution processed hybrid photovoltaic cells. tion and form a CTC. The charge carriers, which had to jump from one chain to another to transport through P3HT, ACKNOWLEDGMENTS are now assisted by the CdTe nanocrystals. The calculated value of activation energy of localized states has been found Authors would like to thank Director NPL for his keen to be 52 meV for P3HT and 11 meV for P3HT-CdTe. As interest in their work. We sincerely thank Dr. Ritu Srivas- activation energy in P3HT-CdTe is lower compared to the tava, Dr. Rajeev Singh, Dr. Pankaj Kumar, and Dr. Kuldeep pristine P3HT, the CdTe nanocrystals support transportation Singh, all from NPL, for their cooperation and useful discus- of holes; this improves their mobility and results into sions. M.T.K. is thankful to CSIR, India, for the award of enhancement in the current. Senior Research Fellowship. The change of mobility from ﬁeld dependent in P3HT to 1 M. Reyes-Reyes, K. Kim, and D. L. Carroll, App. Phys. Lett. 87, 083506 ﬁeld independent in the P3HT-CdTe thin ﬁlm can be (2005). 2 explained on the basis of increase of trap density (Hb) and S. D. Oosterhout, M. M. Wienk, S. S. van Bavel, R. Thiedmann, L. J. A. reduction in activation energy. Usually, an electric ﬁeld Koster, J. Gilot, J. Loos, V. Schmidt, and R. A. J. Janssen, Nat. Mater. 8, 818 (2009). raises the mobility because it lowers the activation barriers. 3 W. Ma, C. Yang, X. Gong, K. Lee, and A. J. Heeger, Adv. Funct. Mater. In organic semiconductors, most of the charge carriers are 15, 1617 (2005). 4 trapped in localized states. An applied ﬁeld gives rise to the T. Stoferle, U. Scherf, and R. F. Mahrt, Nano Lett. 9, 453 (2009). 5 accumulation of charge in the region of the semiconducting W. U. Huynh, J. J. Dittmer, and A. P. Alivisatos, Science 295, 2425 (2002). layer. As these charges are accumulated, (i) spatial overlap 6 Y. Zhou, Y. Li, H. Zhong, J. Hou, Y. Ding, C. Yang, and Y. Li, Nanotech- between the trap potential increases that lower the activation nology 17, 4041 (2006). barriers26 and (ii) only a fraction of total charge carriers are 7 S. Dayal, N. Kopidakis, D. C. Olson, D. S. Ginley, and G. Rumbles, Nano required to ﬁll all the traps, the remaining carriers will on av- Lett. 10, 239 (2010). 8 K. Kim, J. Liu, M. A. G. Namboothiry, and D. L. Carroll, Appl. Phys. erage require less activation energy to hop away to a neigh- Lett. 90, 163511 (2007). boring site. This results in a higher mobility with increasing 9 W. U. Huynh, J. J. Dittmer, W. C. Libby, G. L. Whiting, and A. P. Alivisa- ﬁeld. Incorporation of CdTe nanocrystals in the P3HT matrix 10 tos, Adv. Funct. Mater. 13, 73 (2003). simultaneously enhances the value of trap density from L. J. A. Koster, W. J. Van Strien, W. J. E. Beek, and P. W. M. Blom, Adv. Funct. Mater. 17, 1297 (2007). 2.8 Â 1018 to 5.0 Â 1018 cmÀ3 and produces extrinsic charge 11 K. C. Kao and W. Hwang, Electrical Transport in Solids (Pergamon, carriers.27 At high trap density, the trap potential wells over- Oxford, 1981). 12 lap; this results in decreasing activation energies (from 52 M. A. Lampert and P. Mark, Current Injection in Solids (Academic, New York, 1970). meV to 11 meV). Furthermore, increase in the charge carrier 13 P. W. M. Blom and M. C. J. M. Vissenberg, Mater. Sci. Eng. 27, 53 density on incorporation of CdTe nanocrystals in P3HT ma- (2000). 14 trix results in only partial ﬁlling of carriers even in deeper M. T. Khan, M. Bajpai, A. Kaur, S.K. Dhawan, and Suresh Chand, Synth. intrinsic states; this leads to an upward shift of the Fermi Met. 160 1530 (2010). 15 V. Ambegaokar, B. I. Halperin, and J. S. Langer, Phys. Rev. B 4, 2612 level to the effective transport level and concomitant (1971). increase of the jump rate. This implies that even at low ﬁeld 16 M. Sahimi, Applications of Percolation Theory (Taylor and Francis, Lon- larger numbers of free charge carriers are available for trans- don, 1994). 17 port and hence the mobility in P3HT-CdTe ﬁlms is inde- F. Torricelli and L. Colalongo, IEEE Electron. Device Lett. 30, 1048 (2009). pendent of applied ﬁeld. 18 H. Scher and E. W. Montroll, Phys. Rev. B 12, 2455 (1975). 19 G. E. Pike and C. H. Seager, Phys. Rev. B 10, 1421 (1974). 20 V. CONCLUSIONS M. C. J. M. Vissenberg and M. Matters, Phys. Rev. B 57 12964 (1998). 21 C. Tanase, E. J. Meijer, P. W. M. Blom, and D. M. de Leeuw, Phys. Rev. In conclusion, on in-situ incorporation of CdTe-NCs Lett. 91, 216601 (2003). 22 into P3HT matrix, a pronounced change in the transport F. Torricelli, D. Zappa, and L. Colalongo, Appl. Phys. Lett. 96, 113304 mechanism and enhancement in hole current of the hybrid (2010). 23 M. T. Khan, R. Bhargav, A. Kaur, S. K. Dhawan, and S. Chand, Thin Solid system have been observed. Hole transport in pristine P3HT Films 519, 1007 (2010). has been observed to follow the uniﬁed mobility model, 24 D. S. Ginger and N. C. Greenham, Phys. Rev. B 59, 10622 (1999). 25 whereas the hybrid ﬁlm exhibited agreement with the trap A. J. Campbell, D. D. C. Bradley, and D. G. Lidzey, J. Appl. Phys. 82, 6326 (1997). conduction mechanism, and mobility is ﬁeld independent. 26 P. N. Murgatroyd, J. Phys. D 3, 151 (1970). This change of the conduction mechanism is an important 27 V. I. Arkhipov, E. V. Emelianova, P. Heremans, and H. Bassler, Phys. ¨ ﬁnding and has been attributed to the enhancement in the Rev. B 72, 235202 (2005).Downloaded 10 Jun 2011 to 126.96.36.199. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
14. 163113-2 Kumar et al. Appl. Phys. Lett. 97, 163113 ͑2010͒ ! $ (8* (* ! %! ! ! ! ! !! ! ! ε+ ! ε+ %! $! $! ! ! !7 ε+ !! ! ε+ # $ % ! ! ! ! ! # $ % ,-./0.123 (56* ! ! ! ! ! %7 !7 ,-./0.123 (56* %! ! 7 898;1 7 $! ! 7 898;1 ! ! % ! ! # ! $ ! % ! ! ! # ! $ % ! ! ,-./0.123 (56* ,-./0.123 (56* (2* ! ()* !7 ! %7 ! 7 89: %! ! ! $! #! ! !7 $! ε+ ε+ ε+ %7 ! ! 7 89: ! $! ! # $ % ! ! ! ! ! FIG. 2. ͑Color online͒ Polarizing optical micrographs of ͑a͒ bright state, ͑b͒ ,-./0.123 (56* ! dark state of pure and ͑c͒ bright state, ͑d͒ dark state of ϳ4 wt % CdTe-QDs # $ % ! # $ % ! ! ! ! ! ! ! ! ! ! doped LAHS19 material. ,-./0.123 (56* ,-./0.123 (56* ͑pure and CdTe-QDs doped͒ were ﬁlled into these cells by FIG. 3. ͑Color online͒ Behavior of dielectric permittivity ͑Ј͒ at different dc capillary action above their respective isotropic tempera- biases of CdTe-QDs doped ͑a͒ LAHS19, ͑b͒ LAHS18, and ͑c͒ FLC 6304 in ϳ4 m thick cells, and ͑d͒ KCFLC 7S in ϳ10 m thick cell at room tures. The homogeneous alignment of FLC cells has been temperature. achieved by using rubbed polyimide technique. The phase sequences of FLC materials used are as follows: helicoidal structure͒ and results in the lower and static value 7 °C 59.5 °C 60 °C ء ء of Ј. On the removal of bias, FLC molecules attain their cryt . ↔ SmC ↔ SmA ↔ iso . ͑LAHS19͒ helicoidal structure again due to restoring forces and Ј 1 °C 58 °C 64 °C reaches its original value. If somehow, FLC molecules retain cryt . ↔ SmC ↔ ءSmA ↔ ءiso . ͑LAHS18͒ their switched state ͑unwound state͒ even after removal of bias then it means the material exhibits memory. Figure 3͑a͒ −14 °C 60.5 °C 64 °C shows the occurrence of dielectric memory in CdTe-QDs cryt . ↔ SmC ↔ ءSmA ↔ ءiso . ͑FLC 6304͒ doped LAHS19 material. It can be seen clearly from ﬁgure that the switched state has been retained for CdTe-QDs ? 60.8 °C 86.8 °C 100.8 °C doped LAHS19 even after removal of bias whereas it re- cryt . ↔ SmC ↔ ءSmA ↔ ءN ↔ iso . ͑KCFLC 7S͒ stored its original state instantaneously in case of pure The dielectric measurements of pure and CdTe-QDs doped LAHS19 ͓inset of Fig. 3͑a͔͒. Similarly, Figs. 3͑b͒–3͑d͒ depict FLC materials were performed using an impedance analyzer the occurrence of dielectric memory in CdTe-QDs doped ͑Wayne Kerr, 6540A, U.K. ͒ in the frequency range 20 Hz–1 LAHS18, FLC6304, and KCFLC 7S materials, respectively. MHz with measuring voltage of 0.5 V. The optical micro- It is evident from Figs. 3͑b͒–3͑d͒ that all FLC materials show graphs were taken using polarizing optical microscope ͑Carl dielectric memory by doping CdTe-QDs whereas pure coun- Zeiss, Germany͒. terparts do not show the same as can be seen from the insets Figure 2 shows the optical micrographs of bright and ͓of Figs. 3͑b͒ and 3͑d͔͒. The memory effect in pure FLC dark states of pure and ϳ4 wt % CdTe-QDs doped FLC 6304 has been studied and found that it does not show ͑LAHS19͒. It can be seen clearly from Fig. 2, that the doping memory effect by dielectric measurements.1 It has been dem- of CdTe-QDs does not perturb the alignment of LAHS19 onstrated earlier that surface stabilization ͑SS͒ leads to the material remarkably. However, the presence of CdTe-QDs in bistability and memory effect in FLCs.19 To exclude the pos- the material can be clearly seen in the form of light scattering sibility of SS, we used either DHFLC material ͑which have centers in the dark state of CdTe-QDs doped LAHS19 ͓Fig. ultrashort pitch͒ or FLC material keeping cell thickness 2͑d͔͒. It is found that the dispersion of spherical particles greater than pitch value. This means that by doping CdTe- such as cadmium sulphide QDs into the LCs typically intro- QDs in the conventional FLCs, one can get a memory state duce random surfaces that can disrupt the uniform LC even in the thicker non-SS FLC geometries ͓Fig. 3͑d͔͒. alignment.17 It has also been observed that the presence of It has been observed that the dielectric memory persisted large colloidal particles in LCs is usually associated with a over time scales exceeding minutes. The value of Ј CdTe- defect in the texture.18 The memory effect in CdTe-QDs QDs doped LAHS19 material does not reach its original doped various FLC material has been observed using dielec- ͑0 V͒ value instantaneously but takes some remarkable time. tric measurements. Figure 3 shows the behavior of dielectric The value of Ј after 4 min was found almost 50% of its permittivity ͑Ј͒ with frequency at different dc biases of original value indicating that almost half of the FLC mol- CdTe-QDs doped FLCs. The behavior of memory in pure ecules were still in switched state even after 4 min of re- FLC materials has been shown in insets of ﬁgures. All FLC moval of bias. The retention of memory state for other FLCs/ materials used exhibit SmC ءphase at room temperature. In CdTe-QDs composites has also been observed. It has been SmC ءphase, the FLC molecules are arranged in a helicoidal found that the retention of memory state follows the same manner and show higher value of Ј due to the contribution trend in case of FLCs/CdTe-QDs as CdTe-QDs doped of Goldstone mode ͑GM͒ which occurs due to phase ﬂuctua- LAHS19. However, the retention time is found to be depen- tion of FLC director. The application of sufﬁcient bias across dent on the concentration of CdTe-QDs and the FLC mate- the cell suppresses the GM contribution ͑due to unwinding of rials used.Downloaded 22 Oct 2010 to 188.8.131.52. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions
15. 163113-3 Kumar et al. Appl. Phys. Lett. 97, 163113 ͑2010͒ The occurrence of memory effect can be understood by found to be dependent on the concentration of CdTe-QDs taking the interaction of FLC molecules with CdTe-QDs into and the FLC material used. account. The operation of a memory device in a junctionlike CdS nanocomposites/conducting polymer poly͓2-methoxy-5- The authors sincerely thank Professor R. C. Budhani, the ͑2-ethylhexyloxy͒-1,4-phenylene-vinylene͔ heterostructure Director, National Physical Laboratory, for continuous en- embedded in polyvinyl alcohol ͑PVA͒ matrix has been dem- couragement and interest in this work. We sincerely thank onstrated by studying C-V characteristics, and attributed this Dr. Poonam Silotia of Delhi University for useful discus- memory behavior to the trapping, storage, and emission of sions. The authors ͑A.K. and J.P.͒ are thankful to University holes in the CdS nanoneedles embedded in PVA matrix.20 Grant Commission ͑UGC͒ and Council of Scientiﬁc and In- The phenomena such as QD memory21 and electrical dustrial Research ͑CSIR͒, New Delhi for providing ﬁnancial bistability22 in CdSe-QDs based devices due to their charge assistance. trapping and storage characteristics have also been demon- 1 strated. The CdTe-QDs used for this study have been grown J. Prakash, A. Choudhary, A. Kumar, D. S. Mehta, and A. M. Biradar, in situ in the matrix of conducting polymer P3HT, i.e., the Appl. Phys. Lett. 93, 112904 ͑2008͒. 2 A. Kumar, J. Prakash, D. S. Mehta, W. Haase, and A. M. Biradar, Appl. CdTe-QDs are capped with P3HT. The applications of dc Phys. Lett. 95, 023117 ͑2009͒. bias across the CdTe-QDs doped FLCs sample cells induce 3 L. A. Holt, B. J. Richard, E. D. Stephen, B. Andrew, and S. Gordon, J. charge transfer from FLC molecules to CdTe-QDs. The Appl. Phys. 103, 063712 ͑2008͒. 4 P3HT is a conducting polymer and hence it provided a low H. Qi and T. Hegmann, Appl. Mater. Inter 1, 1731 ͑2009͒. 5 resistive path to the charges in reaching toward CdTe-QDs. T. Joshi, A. Kumar, J. Prakash, and A. M. Biradar, Appl. Phys. Lett. 96, 253109 ͑2010͒. The charge stored on CdTe-QDs can retain for several min- 6 W.-K. Lee, J.-H. Choi, H.-J. Na, J.-H. Lim, J.-M. Han, J.-Y. Hwang, and utes and hence it does not remove instantaneously after re- D.-S. Seo, Opt. Lett. 34, 3653 ͑2009͒. moval of bias. The FLC molecules nearby these CdTe-QDs 7 X. Tong and Y. Zhao, J. Am. Chem. Soc. 129, 6372 ͑2007͒. 8 retain their switched state in the effect of charge stored on S. W. Lee, C. Mao, C. E. Flynn, and A. M. Belcher, Science 296, 892 the CdTe-QDs which has been resulted in the form of pro- ͑2002͒. 9 nounced dielectric memory. It is found that the charge stored A. Balandin, K. L. Wang, N. Kouklin, and S. Bandyopadhyay, Appl. Phys. Lett. 76, 137 ͑2000͒. on the CdTe-QDs has been released in a slower fashion and 10 M. Tamborra, M. Striccoli, R. Comparelli, M. Curri, A. Petrella, and A. taken upto 5 min duration to discharge completely in case of Agestiano, Nanotechnology 15, S240 ͑2004͒. ϳ4 wt % CdTe-QDs doped LAHS19 material. Moreover, 11 A. G. Nassiopoulou, A. Olzierski, E. Tsoi, I. Berbezier, and A. Karmous, the charge retention time on CdTe-QDs is found to depend J. Nanosci. Nanotechnol. 7, 316 ͑2007͒. 12 on the FLC material used. The maximum charge retention J. H. Lee, J. S. Choi, S. Hong, I. Hwang, Y.-I. Kim, S. J. Ahn, S.-O. Kang, time of ϳ10 min has been observed in case of ϳ4 wt % and B. H. Park, Jpn. J. Appl. Phys. 46, L1246 ͑2007͒. 13 N. Nedev, D. Nesheva, E. Manolov, R. Bruggemann, S. Meier, and Z. CdTe-QDs doped LAHS18 material which has slightly dif- Levi, Proceedings of the 26th International Conference on Microelectron- ferent composition than that of LAHS19 whereas in case of ics, 11–14 May, pp. 117–120 ͑2008͒. KCFLC 7S/CdTe-QDs composite, Ј reached its original 14 T. Lundstrom, W. Schoenfeld, H. Lee, and P. M. Petroff, Science 286, value ͑0 V state͒ in comparatively shorter duration 2312 ͑1999͒. 15 ͑ϳ1 min͒. However, to understand the exact nature of inter- J. J. Finley, M. Skalitz, M. Arzberger, A. Zrenner, G. Bohm, and G. Abstreiter, Appl. Phys. Lett. 73, 2618 ͑1998͒. actions between FLC molecules and QDs in achieving im- 16 D. Nesheva, N. Nedev, E. Manolov, I. Bineva, and H. Hofmeister, J. Phys. proved electro-optical properties of FLCs are still a challeng- Chem. Solids 68, 725 ͑2007͒. ing task to be investigated. 17 R. Basu and G. S. Iannacchione, Phys. Rev. E 80, 010701͑R͒ ͑2009͒. 18 A pronounced memory effect in FLCs/CdTe-QDs com- L. S. Hirst, J. Kirchhoff, R. Inman, and S. Ghosh, Proc. SPIE 7618, posites has been observed. The observed memory effect has 76180F ͑2010͒. 19 been attributed to conducting polymer mediated charge N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett. 36, 899 ͑1980͒. 20 S. P. Mondal, V. S. Reddy, S. Das, A. Dhar, and S. K. Ray, Nanotechnol- transfer from FLC molecules to CdTe-QDs. It is found that ogy 19, 215306 ͑2008͒. the stored charges on CdTe-QDs do not remove instanta- 21 M. D. Fischbein and M. Drndic, Appl. Phys. Lett. 86, 193106 ͑2005͒. neously after removal of bias but they take several minutes 22 K. Mohanta, S. K. Majee, S. K. Batabyal, and A. J. Pal, J. Phys. Chem. B to discharge completely. The retention of memory state is 110, 18231 ͑2006͒.Downloaded 22 Oct 2010 to 184.108.40.206. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions
16. This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright
18. Authors personal copy1008 M.T. Khan et al. / Thin Solid Films 519 (2010) 1007–1011(CdS QDs) duly supported by UV–vis absorption and PL quenching designated as solution B. For preparation of device A and device B, thestudies. We have also studied the effect of post thermal annealing on ITO-coated glass substrate was ﬁrst cleaned with detergent, ultra-device performance and found improved efﬁciency of devices after sonicated in acetone, trichloroethylene and isopropyl alcohol, andthermal treatment at 150 °C for 30 min due to improved nanoscale subsequently dried in an vacuum oven. Highly conducting PEDOT:PSSmorphology, increased crystallinity and improved contact to the (Aldrich USA) was spin casted at 4000 rpm for 2 min. The substrateelectron-collecting electrode. was dried for 10 min at 150 °C in vacuum and then moved into a glove box for spin casting the photoactive layer. The chlorobenzene solution2. Experimental section A and B was then spin casted at 1500 rpm for 2 min on the top of the PEDOT:PSS layer. Subsequently 120 nm Al ﬁlm was deposited on top2.1. Materials of the active layer. Thermal annealing was carried out by directly placing the completed device at 150 °C in a vacuum oven. The 3-Hexylthiophene, 1-dodecanethiol and ferric chloride (FeCl3) performance of the devices has been studied by their J–V character-were purchased from Sigma Aldrich and used as received. Sodium istics in the dark and under halogen lamp illumination with irradiancesulphide fused ﬂakes (Na2S), was purchased from Fisher Scientiﬁc, of 80 mWcm− 2, using a Keithley 2400 Source-Measure unit, inter-and cadmium nitrate (Cd(NO3)2.4H2O) was purchased from Thomas faced with a computer.Baker. HPLC grade chloroform (CHCl3), hexane and methanol werepurchased from MERCK. All solvents were freshly distilled prior to 2.5. Structural and optical properties analysisuse. The TEM sample of CdS was prepared by drying a droplet of the CdS2.2. Synthesis of poly(3-hexylthiophene) nanoparticles dispersed in deionised water on a carbon grid while the TEM sample of P3HT/CdS was prepared by drying a droplet of the above The polymerization of P3HT was done by the oxidative coupling hybrid solution on an uncoated grid. High resolution transmissionmethod [20–22]. This method is based on the use of a Lewis acid such electron microscopy (HRTEM) images were obtained with an apparatusas ferric chloride (FeCl3) to initiate the polymerization. The reaction equipped with a HAADF detector (image size: 2014 × 2014 pixels; scanmixture was kept stirred under a nitrogen environment at − 40 °C for times: 5–20 s; camera length: 200 mm). Absorption spectra were taken24 h. After polymerization, the mixture color turned into green andsubsequently washed with methanol and deionized water to removeundesired impurities like unprocessed oxidant, monomer andoligomers. This polymer contains residual iron salts and chlorine asan impurity. To get a pristine polymer, the chloroform (CHCl3)solution of the polymer was subsequently treated with an aqueoussolution of ethylene diamine tetra-acetic acid (EDTA), ammoniasolution at 62 °C. After this treatment, polymer was dried in vacuumat 60 °C for 2 h and kept in vacuum.2.3. Synthesis of cadmium sulphide (CdS) quantum dots CdS nanoparticles were synthesized according to a previousliterature [23,24].Two hexane solutions of Aerosol OT (AOT) (0.2 M,50 ml) were prepared. An aqueous solution of cadmium nitrate Cd(NO3)2.4H2O (0.4 M) was added to one hexane solution, while anaqueous solution of Na2S (0.4 M) was added to the other solution inorder to achieve a [H2O]/[AOT] ratio of 6 for both solutions. Thesolutions were stirred for 3 h. The micellar solution containing Cd(NO3)2 was then added slowly to the micelle solution containing Na2Sat room temperature under a nitrogen atmosphere. CdS nanoparticleswere obtained after the solution was stirred for 3 h. 1-Decanethiol(DT) molecules (4.3 mmol) were added to a hexane solution of CdSnanoparticles (1.5 M). This solution was stirred for 5 h, and methanolwas subsequently added in order to remove the AOT molecules. Afterthe methanol phase was removed, the hexane phase was evaporated.The residual solution was then dropped into a large volume ofmethanol, and the resultant yellow precipitate was ﬁltered off using a0.2-μm membrane ﬁlter, yielding puriﬁed DT-protected CdSnanoparticles.2.4. Fabrication and measurement of device In our experiments, the ratio of P3HT:PCBM was taken 1:0.8 andP3HT:CdS:PCBM was taken 1:1:0.8 with a concentration of 1 wt.% inchlorobenzene. Two solutions of P3HT were prepared in chloroben-zene and in one of them CdS was added and sonicated for 4 h in orderto well disperse CdS in P3HT. PCBM solution in chlorobenzene wasadded in above solutions and the mixed solution was ultrasonicatedfor 2 h. The solution containing P3HT plus PCBM was designated assolution A and other containing P3HT plus CdS and PCBM was Fig. 1. XRD spectra of CdS QDs, P3HT and P3HT/CdS nanocomposite ﬁlms.
19. Authors personal copy M.T. Khan et al. / Thin Solid Films 519 (2010) 1007–1011 1009on Shimadzu UV-1601 spectrophotometer. Photoluminescence (PL) 3.2. Structural characterizationspectra were recorded on a Perkin Elmer LF 55 with Xenon sourcespectrophotometer. XRD of the polymer ﬁlm was taken in RINT 3.2.1. XRD analysis2000 Rigaku make X-ray diffractometer (40 KV, 30 mA, λ = 1.54059 A, Fig. 1 shows XRD patterns for pure P3HT, CdS/P3HT nanocompo-Cu-Kα1). The HRTEM apparatus is equipped with a HAADF detector site and CdS powder. In the CdS XRD spectra, three broad peaks at(image size: 2014 × 2014 pixels; scan times: 5–20 s; camera length: 2θ = 27°, 44° and 52° belong to the (111), (220) and (311) planes200 mm). An energy-dispersive X-ray spectrometer attached to the respectively of cubic CdS. The XRD peaks are broadened due to theTecnai G2 F30 was used to perform elemental analyses at spots selected small size of QDs. The average crystallite size determined from thein the HAADF-STEM images. Image processing was performed using Debye–Scherrer formula d = 0.9λ/β cosθ is estimated to be aboutDigital Micrograph software (Gatan). 2.33 nm, where λ is the wavelength of the X-rays used, β is the full width at half maximum and θ is the angle of reﬂection. The strong ﬁrst order reﬂection, (100), of P3HT observed at 2 angle 5.45°,3. Result and discussion corresponds to interlayer spacing 16.4 Å. The second order reﬂection (200) of P3HT, observed at 2 angle 10.86°, corresponds to interlayer3.1. Regio-regularity and molecular weight of P3HT spacing 8.402 Å. In comparison, XRD data of CdS/P3HT shows that the 2θ values match the (1 0 0), (1 0 0), (1 1 1), (2 2 0) and (3 1 1) planes. A comparative study of the photophysical properties of the P3HT The appearances of few additional peaks in composites are attributedpolymer synthesized for the present investigation with those reported to the presence of QDs in the P3HT matrix.earlier for P3HT synthesized by various routes  shows the qualityof P3HT. The reported band for aromatic C–H out of plain vibration isat 820 cm− 1, which is the characteristics of 2,5-disubstituted-3- 3.2.2. HRTEM imageshexylthiophene for rr-P3HT whereas the corresponding band for rdm- HRTEM images of CdS QDs and P3HT/CdS nanocomposite areP3HT occurs at 827 cm− 1 . The observed band for aromatic C–H shown in Fig. 2(a–c) and (d–f), respectively. It is seen from Fig. 2(a)out of plain vibration at 822 cm− 1 [Fig. 1(a)] of pristine P3HT of the that the size of the QDs ranges from 5 to 6 nm and their shape ispresent work, conﬁrms the synthesis of P3HT having rr-P3HT like spherical. Also it is seen from Fig. 2(b) on a higher resolution thatchain conformation. Regio-regular P3HT has solid-state absorptions there exist (1 1 1), (2 2 0) and (3 1 1) planes of cubic CdS havingranging from λmax = 520–530 nm and for regio-random P3HT to 480– interplanar spacing 3.36, 2.06 and 1.76 Å, respectively. This formation500 nm [25,26]. The absorption spectrum of our P3HT ﬁlm has an of different planes is explicitly conﬁrmed by the diffraction patternabsorption peak at 526 nm with an edge at 606 nm which conﬁrms its shown in Fig. 2(c). Further Fig. 2(d) and (e) shows that QDs are evenlyregio-regularity. 1H NMR study also conﬁrms the regio-regularity of distributed within the P3HT and form uniform hybrid nanocompositeP3HT (δ = 6.977). Molecular weights of P3HT of the present work are: ﬁlms. Different planes of CdS QDs in the P3HT matrix are shown by theMw = 39,278 and Mn = 27,298 with a PD index of 1.44. diffraction pattern in Fig. 2(e).Fig. 2. High resolution TEM images of (a) CdS nanoparticles in the range of 5–6 nm, (b) lattice resolution of cubic CdS QDs, (c) diffraction image of CdS QDs, (d–e) CdS nanoparticlesdispersed in poly(3-hexylthiophene) matrix and (f) diffraction image of CdS QDs in the P3HT matrix.
20. Authors personal copy1010 M.T. Khan et al. / Thin Solid Films 519 (2010) 1007–10113.3. Optical study 3.3.2. Photoluminescence spectra Photoluminescence quenching in a bulk heterojunction is a useful3.3.1. UV–vis absorption spectra indication of the degree of success of exciton dissociation and UV–vis absorption spectra of P3HT and P3HT/CdS nanocomposite efﬁciency of charge transfer between the donor–acceptor compositesolution in chloroform and of thin ﬁlms are shown in Fig. 3(a). Regio- materials [30,31]. P3HT has a photoluminescence property, [32,33]regular P3HT have solid-state absorptions ranging from λmax = 520– and the photoluminescence spectra of P3HT and P3HT/CdS solution in530 nm and solution absorption ranging 442–456 nm. The absorption CHCl3 at excitation wavelength 448 nm, are presented in Fig. 3(b).spectrum of our P3HT solution has an absorption peak at 448 nm and Signiﬁcant PL quenching was observed for the nanocompositethin ﬁlms show absorption at 526 nm which conﬁrms its regio- solution. The PL intensity of the composite solution is signiﬁcantlyregularity. Strong absorption band at 448 nm for P3HT is attributed to reduced compared with the value of the P3HT in Fig. 3(b). Thisthe excitation of electrons in the π-conjugated system. The P3HT/CdS indicates that charge transfer, thereby exciton dissociation at interfacenanocomposite shows that absorption band at 438 nm is 10 nm blue between CdS and P3HT, has taken place. Higher exciton dissociationshifted relative to the pristine P3HT solution. The blue shift in efﬁciency accounts for higher device performance. For an excitationabsorption of the P3HT/CdS nanocomposite can be assigned to the wavelength of 448 nm the solution show an emission at 674 andquantum conﬁnement effect from the CdS nanoparticles [27–29]. 668 nm for P3HT and P3HT/CdS respectively. The reason for theMaximum absorption intensity in the nanocomposite is lower due to photoluminescence quenching of P3HT/CdS may be due to the π–πscattering caused by the nanoparticles in the P3HT matrix. As shown interaction of P3HT with CdS [34,35], forming additional decayingin Fig. 1(a) CdS quantum dots show a broad absorption from 290 to paths of the excited electrons through the CdS. The small blue shift700 nm with a maximum absorption peak at 292 nm and an edge at (4 nm) in the nanocomposite emission spectra indicates that the440 nm. The absorption spectrum of the P3HT ﬁlm has an absorption ground state energy level is more stable in the nanocomposite thanpeak at 526 nm with an edge at 606 nm, and exhibited a red shift that of pristine P3HT. This may be possible through the resonancecompared with that of a comparable solution, indicating that it is stability of π clouds of P3HT and CdS through π–π interaction.easier for the P3HT to form a planar orientation in the solid ﬁlm. Theabsorption spectrum of the P3HT/CdS thin ﬁlm also exhibits a 15 nmblue shift relative to P3HT, indicating that the CdS nanoparticles in theﬁlm also have a quantum conﬁnement effect.Fig. 3. (a) UV–visible absorption spectra of P3HT and P3HT/CdS QD nanocomposite Fig. 4. J–V curves obtained from device A and device B under AM 1.5 illumination at anﬁlms and solution in the chloroform (b) photoluminescence spectra of P3HT and P3HT/ irradiation intensity of 80 mW/cm2, (a) devices without thermal annealing andCdS QD nanocomposite solution in chloroform at the excitation wavelength 448 nm. (b) devices with postproduction heat treatment at 150 °C for 30 min.
21. Authors personal copy M.T. Khan et al. / Thin Solid Films 519 (2010) 1007–1011 1011Table 1 interface result in a better charge collection at the electrodes withPerformance of the P3HT/PCBM solar cell with and without CdS QD doping and thermal reduced series resistance and higher ﬁll factor.annealing. Devices Voc (Volts) Jsc (mA/cm2) FF (%) Efﬁciency (%) 4. Conclusions Device A 0.45 2.57 30.0 0.45 Device B 0.45 4.65 32.0 0.87 Device A annealed 0.58 2.26 45.0 0.74 In conclusion, we have constructed and studied a bulk heterojunc- Device B annealed 0.58 2.98 43.99 0.95 tion photovoltaic device that contains P3HT:CdS:PCBM and the results are compared with those of the P3HT:PCBM. Incorporation of CdS in P3HT:PCBM device increases the device performance due to CTC3.4. J–V characteristics formation between the host (P3HT) and guest (CdS QDs) duly supported by UV–vis absorption and PL quenching studies. Postpro- Fig. 4 shows the J–V characteristics of device A and B under AM 1.5 duction thermal annealing decreases the series resistance and improvesillumination with an intensity of 80 mWcm− 2. The performance of the contact between the active layer and Al, resulting in enhanceddevice A showed a short-circuit photocurrent (Jsc) of 2.57 mAcm− 2, device efﬁciency. Further improvement can be anticipated if betteran open-circuit voltage (VOC) of 0.45 V, a ﬁll factor (FF) of 0.30, and a control over the morphology of the photoactive blend can be gained.power conversion efﬁciency (PCE) of 0.45%. When we incorporate CdSQDs in the P3HT matrix (device B), the PCE value increased up to Acknowledgements0.87% by improving the Jsc of 4.65 mAcm− 2, VOC of 0.45 V, and FF of0.32. The performance of devices A and B thermal annealing at 150 °C The authors MTK is thankful to CSIR, New Delhi, India, for the awardfor 30 min are shown in Fig. 4(b). After thermal treatment device A of Senior Research Fellowship. We sincerely thank, Dr. Ritu Srivastavadelivers an VOC of 0.58 V, a JSC of 2.26 mA/cm2, a FF of 0.45 and a device and Dr. Renu Paschricha for their cooperation and useful discussion.efﬁciency of 0.74%. Device B after thermal annealing gives an VOC of0.58 V, a JSC of 2.98 mA/cm2 and a FF of 0.44, resulting in an estimateddevice efﬁciency of 0.95 %. These data are summarized in Table 1. References The modulation of device parameters i.e. increase in the value of  R.J. Kline, M.D. Mcgehee, M.F. Toney, Nat. Mater. 5 (2006) 222.VOC, JSC and FF in device B can be understood in terms of host (P3HT)  D.H. Kim, Y. Jang, Y.D. Park, K. Cho, J. Phys. Chem. B 110 (2006) 15763.and guest (CdS QDs) charge transfer type interaction. In fact there are  M. Mas-Torrent, D. den Boer, M. Durkut, P. Hadley, A.P.H.J. Schenning,various possibilities by which doped CdS can interact with host P3HT. Nanotechnology 15 (2004) S265.  B.C. Thompson, J.M.J. Fr_chet, Angew. Chem. Int. Ed. 47 (2008) 58.It can either go structurally into the P3HT main chain or form donor  W. Ma, C. Yang, X. Gong, K. Lee, A.J. Heeger, Adv. Funct. Mater. 15 (2005) 1617.acceptor charge transfer complex (CTC) or form molecular aggregates.  M. Reyes-Reyes, K. Kim, D.L. Carroll, Appl. Phys. Lett. 87 (2005) 083506.However, the enhancement in JSC in P3HT on CdS dispersion indicates  P. Schilinsky, U. Asawapirom, U. Scherf, M. Biele, C.J. Brabec, Chem. Mater. 17 (2005) 2175.that CTC formation between the host and guest may be the dominant  G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Nat. Mater. 4mechanism of interaction between the two. This suggested mecha- (2005) 864.nism is indeed supported by the UV–vis absorption and PL emission  T. Stoferle, U. Scherf, R.F. Mahrt, Nano Lett. 9 (2009) 453.  W.U. Huynh, J.J. Dittmer, A.P. Alivisatos, Science 295 (2002) 2425.studies in pure P3HT and CdS dispersed P3HT as shown in Fig. 3(a)  S.D. Oosterhout, M.M. Wienk, S.S. van Bavel, R. Thiedmann, L.J.A. Koster, J. Gilot, J. Loos,and (b), respectively. Fig. 3(a) shows a blue shift in the UV–vis V. Schmidt, R.A.J. Janssen, Nat. Mater. 8 (2009) 818.absorption peak from 523 nm to 512 nm on incorporation of CdS QDs  Y. Zhou, Y. Li, H. Zhong, J. Hou, Y. Ding, C. Yang, Y. Li, Nanotechnology 17 (2006) 4041.in the P3HT matrix which may be attributed to the CTC/quantum  W.J.E. Beek, M.M. Wienk, R.A.J. Janssen, Adv. Mater. 16 (2004) 1009.conﬁnement effect from the CdS nanoparticles . Also small blue  K. Kumari, S. Chand, P. Kumar, S.N. Sharma, V.D. Vankar, V. Kumar, Appl. Phys. Lett.shift (4 nm) in the nanocomposite PL spectra indicates that during the 92 (2008) 263504.  S. Bhattacharya, S. Malik, A.K. Nandi, A. Ghosh, J. Chem. Phys. 125 (2006) 174717.CTC formation ground state energy level is more stable in the  J.Y. Kim, K. Lee, N.E. Coates, D. Moses, T.Q. Nguyen, M. Dante, A.J. Heeger, Sciencenanocomposite than that of pristine P3HT. This may be possible 317 (2007) 222.through the resonance stability of the π clouds of P3HT and CdS  L. Wang, Y.S. Liu, X. Jiang, D.H. Qin, Y. Cao, J. Phys. Chem. C 111 (2007) 9538.through π–π interaction  as a result of CTC formation.  C.R. Bullen, P. Mulvaney, Nano Lett. 4 (2004) 2303.  W.W. Yu, X.G. Peng, Angew. Chem. Int. Ed. 41 (2002) 2368. Similarly PL quenching seen in Fig 3(b) on CdS dispersion in P3HT  R. Singh, J. Kumar, R.K. Singh, A. Kaur, K.N. Sood, R.C. Rastogi, Polymer 46 (2005)is a direct evidence of CTC formation between the host and guest since 9126.PL quenching is an indication of the degree of success of exciton  S. Amou, O. Haba, K. Shirato, T. Hayakawa, M. Ueda, T. Takeuchi, M. Asai, J. Polym. Sci. A: Polym. Chem. 37 (1999) 1943.dissociation and efﬁciency of charge transfer between the donor–  M.R. Andersson, D. Selse, M. Berggren, H. Järvinen, T. Hjertberg, Inganass,acceptor composite materials. The PL quenching in P3HT/CdS has O. Wennerstoerm, J.E. Oesterholm, Macromolecules 27 (1994) 650.been attributed to the π–π* interaction of P3HT with CdS, forming  T. Nakanishi, B. Ohtani, K. Uosaki, J. Phys. Chem. B 102 (1998) 1571.  T. Tsuruoka, K. Akamatsu, H. Nawafune, Langmuir 20 (2004) 25.additional decaying paths of the excited electrons through CdS. To be  R.D. McCullough, Adv. Mater. 10 (1998) 93.more precise during CTC formation CdS QDs may diffuse into the  T.A. Chen, X. Wu, R.D. Rieke, J. Am. Chem. Soc. 117 (1995) 233.amorphous-crystalline boundaries of the P3HT polymer and intro-  R.R. Prabhu, M.A. Khadar, J. Phys. 65 (2005) 801.  P. Sonar, K.P. Sreenivasan, T. Madddanimath, K. Vijayamohanan, Mater. Res. Bull.duces the conducting path thus reducing the defect states and barrier 41 (2006) 198.height at these interfacial boundaries.  R. Maity, U.N. Maiti, M.K. Mitra, K.K. Chattopadhyay, Physica E 33 (2006) 104. The increase in PCE (and the improved FF) after thermal treatment  J. Yu, D.H. Hu, P.F. Barbara, Science 289 (2000) 1327.  G. Yu, A.J. Heeger, J. Appl. Phys. 78 (1995) 4510.implies a signiﬁcant decrease in the series resistance , thermally  B.K. Kuila, A.K. Nandi, Macromolecules 37 (2004) 8577.induced morphology modiﬁcation, thermally induced crystallization  B.K. Kuila, A.K. Nandi, J. Phys. Chem. B 110 (2006) 1621.and improved transport across the interface between the bulk  J. Xu, J. Wang, M. Mitchell, P. Mukherjee, M. Jeffries-EL, J.W. Petrich, Z. Lin, J. Am.heterojunction material and aluminum (Al) electrode . The Chem. Soc. 129 (2007) 12828.  H.C. Liao, S.Y. Chen, D.M. Liu, Macromolecules 42 (2009) 6558.improved nanoscale morphology results in a more efﬁcient charge  H.C. Raus, Solar Cell Array Design Handbook, Van Nostrand Reinhold, New York,generation. The higher crystallinity and improved transport across the 1980, p. 56.
23. M.T. Khan et al. / Synthetic Metals 160 (2010) 1530–1534 1531 Fig. 1. UV–vis absorption spectra of all polymers (a) thin solid ﬁlm on glass substrate and (b) solution in chloroform.tetraﬂuoroborate (TBATFB) were purchased from MERCK, Ger- of 3-octylthiophene (3OT, 0.05 M) and 3-hexylthiophene (3HT,many. The sample for XRD and UV absorption were prepared 0.05 M) were added drop wise in ferric chloride (0.4 M) suspen-by spin-coating of polymer solution onto glass substrate at sion in chloroform. The reaction mixture was kept stirring under2000 rpm for 60 s and annealed at 393 K for 30 min. The J–V nitrogen environment at 233 K for 24 h. After polymerization,characteristics of the copolymer device having conﬁguration to remove undesired impurity like unreacted oxidant, monomerITO/PEDOT:PSS/P3OT-HT/Au was studied for determining temper- and oligomers, polymer was subsequently washed with methanolature and ﬁeld dependent hole mobility. A thin ﬁlm of PEDOT:PSS and deionised water and dried in vacuum at 350 K for 3 h. FeCl3was spin coated onto pre-cleaned and plasma treated ITO (sheet entrapped in the polymer matrix was removed by successiveresistance ∼ 18 / ) coated glass substrates at 2000 rpm and cured treatment with aqueous ammonia and disodium salt of ethyleneat 393 K for 30 min in vacuum. Subsequently, homogeneous solu- diamine tetra-acetic acid (EDTA)  at 335 K and dried in vac-tion was made by dissolving P3OT-HT in binary solvent solution uum at 350 K. Above procedures were repeated several times toof chloroform and chlorobenzene in 1:1 ratio with material con- minimize the FeCl3 impurity present in the polymer matrix. Thintent of 30 mg/ml. Copolymer P3OT-HT thin ﬁlms were spin casted ﬁlms of synthesized polymers were prepared by solution casting ofin an inert atmosphere on the PEDOT:PSS ﬁlms at 2000 rpm, chloroform solution on ﬂat glass substrate at room temperature.and cured at 393 K for 15 min. On top of these ﬁlms, Au elec-trodes ∼250 nm were deposited by vacuum thermal evaporation 3. Results and discussionat 2 × 10−6 Torr. These hole only devices were sealed using UVirradiated epoxy resin to inhibit undesired atmospheric oxida- 3.1. UV–vis absorption spectroscopytion. Absorption spectra were recorded by Shimadzu UV-1601spectrophotometer. Thermogravemetric analysis and differential UV–vis spectra of all polymer thin ﬁlms are shown in Fig. 1(a). Inscanning calorimetry measurement was performed on a METTLER conjugated polymers, the extent of conjugation directly affects theTOLEDO, TGA/SDTA 851e and DSC822e respectively with heating observed energy of the –* transition, which appears as the max-rate of 10 ◦ C/min under nitrogen atmosphere. XRD of polymer imum absorption . The wavelength of maximum absorptionﬁlms were taken in RINT 2000 Rigaku powder X-ray diffractome- ( max ) in solid ﬁlm of the P3OT, P3OT-HT and P3HT are observedter (40 kV, 30 mA, = 1.54059 A, Cu-K␣1 ). The J–V characteristics of at 511 nm, 512 nm and 518 nm, respectively. The blue shift ofP3OT-HT in the temperature range of 290–110 K was performed in the absorption of the P3OT-HT ﬁlm than that of P3HT has beena liquid nitrogen cryostat, under a reduced pressure of 10−6 Torr attributed to steric hindrance of octyl side chain attached to copoly-coupled with Keithley 2400 source meter unit interfaced with PC. mer matrix which may be difﬁcult to rotate compared to hexyl side chain to form the more advantageous arrangement. Polymer ﬁlm2.2. Synthesis of polymer P3HT, P3OT and copolymer P3OT-HT also shows an absorption shoulder at 600 nm, 595 nm and 598 nm for P3OT, P3OT-HT and P3HT respectively are assigned to the 1 Bu The polymerization and copolymerization of 3-alkylthiophenes vibronic sidebands  which conﬁrm the interchain absorption inwas done by oxidative coupling method [16,17]. This method is polymer [20,21]. Fig. 1(b) shows the absorption spectra of all poly-based on the use of a Lewis acid such as ferric chloride (FeCl3 ) to mers in chloroform solution. The maximum absorption of P3OT,initiate the polymerization. For the polymerization of 3HT and 3OT, P3OT-HT and P3HT in chloroform appeared at 442 nm, 439 nm andthe monomer to oxidant ratio was taken 1:4. Equal molar ratio 443 nm respectively which has been attributed to HOMO -LUMO * transition . The absorption spectra of polymers showed blue shift in solution compared with that of the solid ﬁlms. The blue shift in solution is attributed to coil like structure in solution whereas solid ﬁlms have rod like structure. Coil like structure have short effective conjugation length compared to rod like structure with higher conjugation length, this results in increase of – stacking and blue shift in solution phase. 3.2. Thermal studiesScheme 1. Scheme of copolymer copoly(3-octylthiophene-3-hexylthiophene) In order to investigate the thermal stability of polymers, ther-P3OT-HT. mogravimetric analysis (TGA) experiments were performed under
24. 1532 M.T. Khan et al. / Synthetic Metals 160 (2010) 1530–1534Fig. 2. (a) TGA of P3OT, P3HT and copolymer P3OT-HT with scan rate 10 ◦ C/min under nitrogen atmosphere (b) DSC scan of polymers with heating rate 10 ◦ C/min undernitrogen atmosphere.nitrogen gas. As shown in Fig. 2(a), the onset point of weight loss 8.40 Å, and 9.34 Å respectively. Observed dP3OT-HT values (18.786 Åfor P3OT, P3OT-HT and P3HT are observed at 427 ◦ C, 434 ◦ C and and 9.34 Å) in the copolymer P3OT-HT are smaller than the P3OT440 ◦ C respectively, possibly caused by decomposition of the end homopolymer and larger than the P3HT homopolymer, suggestinggroups; indicating that the polymer have good thermal stability. partial inter-digitation between the side chains and/or the occur-From above results it is concluded that long alkyl side group poly(3- rence of tilting of the octyl chains in P3OT-HT. XRD study shows thatalkylthiophenes) decompose earlier than short alky side group the interlayer spacing increases with elongation of alkyl side chain.poly(3-alkylthiophenes). Differential scanning calorimetry (DSC) This shows that the stacks of planer thiophene main chain werescan of the polymers are shown in Fig. 2(b). In the DSC of the copoly- uniformly spaced by alkyl side chain. Copolymer P3OT-HT showsmer P3OT-HT, two melting transitions with endothermic peaks at two strong peaks at 2Â angle 16.860◦ , 14.04◦ which corresponds to164 ◦ C and 228 ◦ C were observed. The observed two melting tran- different two d020 values of 5.254 Å and 6.303 Å respectively. Thesitions are characteristic of its copolymer architecture composed 6.303 Å spacing is due to the interlayer stacking distance betweenof P3OT and P3HT, which have melting transitions at 186 ◦ C and P3OT in a layered packing structure (dP3OT ), whereas the 5.254 Å215 ◦ C, respectively. spacing comes from the interlayer stacking distance between P3HT (dP3HT ). These peaks conﬁrm the formation of copolymer3.3. XRD studies P3OT-HT. Fig. 3 shows X-ray diffraction (XRD) pattern of solution cast ﬁlms 3.4. Hole transport in thin ﬁlm of P3OT-HTof the above polymers, precured at 120 ◦ C. The strong ﬁrst orderreﬂection, (1 0 0), of P3OT, P3HT and P3OT-HT are at 2Â angle 4.24◦ , Fig. 4 shows the J–V characteristics of P3OT-HT thin ﬁlm in5.08◦ , and 4.7◦ , corresponding to interlayer spacing 20.83 Å, 17.38 Å, hole only conﬁguration as mentioned above at different temper-and 18.786 Å respectively . Observed intensity of copolymer is atures in the range 290–110 K. These experimental results weredecreased compared to homopolymer P3HT and P3OT due to regio- analysed based on the theory of space charge limited conductionrandom structure (HT:HH 65:35) of copolymer P3HTOT which was (SCLC) [24–27] with traps distributed exponentially in energy andcalculated by 1 H NMR study in CDCl3 . Regio-random structure of space. For a case when the traps are distributed exponentially incopolymer is attributed to the random repeating of hexyl and octyl the energy space within the forbidden gap, distribution functiongroup attached to the polymer matrix. The second order reﬂection for the hole trap density as a function of energy level E above the(2 0 0) of P3OT, P3HT, and P3OT-HT are observed at 2Â angle 8.62◦ , valence band, and at a distance x from the injecting contact can be10.52◦ , and 9.48◦ , corresponding to interlayer spacing 10.25 Å, Fig. 4. Experimental (symbols) and calculated [solid line using Eqs. (2) and (3)] J–V characteristics of hole only device of copolymer P3OT-HT for the temperature range ◦ Fig. 3. XRD spectra of solution cast polymer ﬁlms, annealed at 120 C. 290–110 K.
25. M.T. Khan et al. / Synthetic Metals 160 (2010) 1530–1534 1533Fig. 5. (a) Experimental (symbols) and calculated [solid line using Eq. (4) with activation energy = 0.21 eV and 0 = 3.6 × 10−5 cm2 /Vs] Arrhenius plot of the zero ﬁeldmobility versus temperature T. (b) The coefﬁcient (which described the ﬁeld dependence of the mobility) as a function of temperature T. The solid line is according to Eq.(5), using T0 = 500 K and ˇ = 6.9 × 10−5 eV/V1/2 cm1/2 .written as: Temperature dependent high ﬁeld J–V characteristics is under- Hb −E stood in terms of the coefﬁcient ␥(T). From the J–V curve (Fig. 4) andh(E, x) = exp S(x) (1) Eqs. (2) and (3) we calculate the value of (T) at each temperature. kB TC Et Fig. 5(b) shows the variation of (T) as a function of temperature.where Hb is the density of traps at the edge of valence band, Et is The experimental results show that there is a linear dependencecharacteristic trap energy and kB is the Boltzmann constant. Et is according to equation :also often expressed in terms of the characteristics temperature TCof trap distribution Et = kB TC . 1 1 Density of trapped holes at position x is given by pt (x) = (T ) = ˇ − (5) h(E, x)f (E)dE where f(E) is the Fermi-Dirac distribution function. kB T kB T0Assuming that the trapped hole carrier density (pt ) » free hole car-rier density (p) and using continuity equation J = q p(x)F(x) and with ˇ = 6.9 × 10−5 eV/V1/2 cm1/2 , T0 = 500 K.boundary condition V = F(x)dx the J–V characteristic is governed Expressions (3)–(5) describe the Arrhenius dependence of theby [27,28]: mobility which arises if moving charges must hope over a coulomb l+1 l barrier of height in energy. In such a case, an electric ﬁeld depen- 2l + 1 l εε0 V l+1J = q1−l Nv (2) dence arises √because the barrier height is lowered electric ﬁeld by l+1 l + 1 Hb d2l+1 an amount ˇ E.where J is the current density, V is the applied voltage, q the is ele- The set of J–V characteristics as a function of temperaturementary charge, d is the thickness of the material ﬁlm, is the ﬁeld can be fully described by Eqs. (2)–(5) using the parame-independent hole carrier mobility, F(x) is the electric ﬁeld inside the ters Hb = 3.8 × 1018 cm−3 , Nv = 3 × 1019 cm−3 , ε = 3, ε0 = 8.85 × 10−14ﬁlm, Nv is the effective density of states, ε is the dielectric constant F/cm, TC = 560 K, Et = 46 meV, d = 150 nm, 0 = 3.6 × 10−5 cm2 /Vs,of material, ε0 is permittivity of the free space and l = Et /kB T = TC /T. T0 = 500 K = 21 eV and ˇ = 6.9 × 10−5 eV/V1/2 cm1/2 .The parameter l determines the distribution of traps in the forbid- A microscopic interpretation of this ubiquitous mobility is thatden gap. the charge transport in disordered organic conductors is thought When the experimental data in Fig. 4 has been analysed in terms to proceed by means of hopping in a Gaussian site-energy distri-of Eq. (2), it has been found that the theory ﬁts up to intermediate bution. This density of states (DOS) reﬂects the energetic disorderﬁelds and at high ﬁelds (corresponding to ≥6 V), the current grad- of hopping site due to ﬂuctuation in conjugation lengths, struc-ually deviates from the above proposed theory and becomes larger tural disorder [31,32]. Copolymerizing of P3OT and P3HT couldthan as expected from Eq. (2). This discrepancy has been analyzed create regio-random structural due to random repetition of hexylin terms of ﬁeld dependent mobility [29–31]: and octyl unit and energetic disorder due to different energy levels √ P3HT and P3OT. Due to these structural and energetic disorders in p (E, T ) = (0, T )exp( (T ) E) (3) copolymer, the hole mobility is strongly dependent on temperaturewith (0,T) the hole mobility at zero ﬁeld and (T) the ﬁeld acti- and electric ﬁeld. The introduction of the hexyl group into the P3OTvation factor, which reﬂects the lowering of the hopping barriers matrix can also lead to structural defects and hence increase of trapin the direction of the applied electric ﬁeld. The applied ﬁeld gives density, so that a fraction of the charges moving inside the P3OT-HTrise to increase of the charge carrier density. In order to describe ﬁlms are trapped thereby reducing the mobility. Thus copolymer-the hole conduction in P3OT-HT at high ﬁelds, we combine the SCLC ization is expected to diminish the mobility and increase its electric(Eq. (2)) with the ﬁeld dependent mobility (Eq. (3)). Temperature ﬁeld dependence for hole.dependence of zero ﬁeld mobility is shown in Fig. 5(a) in an Arrhe- It is thus explicitly established from above that hole transportnius plot. They decrease with decrease of temperature. We observe in P3OT-HT copolymer thin ﬁlms shows ﬁeld and temperaturea thermally activated behaviour of zero ﬁeld mobility according to dependent mobility at higher ﬁelds with hole transport ﬁttingthe equation: parameters as ˇ = 6.9 × 10−5 eV/V1/2 cm1/2 , 0 = 3.6 × 10−5 cm2 /Vs, T0 = 500 K and = 21 eV, respectively. It is important to mention (0, T ) = 0 exp − (4) here that the hole mobility in P3OT-HT copolymer estimated as kB T above lies between that of P3HT and P3OT and hence it can ﬁndwhere is the thermal activation energy and kB is the Boltzmann application in polymer solar cells with a new donor:acceptor com-constant. bination.
26. 1534 M.T. Khan et al. / Synthetic Metals 160 (2010) 1530–15344. Conclusions  E. Kymakis, I. Alexandrou, G.A.J. Amaratunga, J. Appl. Phys. 93 (2003) 1764.  G. Kalita, S. Adhikari, H.R. Aryal, R. Afre, T. Soga, M. Sharon, W. Koichi, M. Umeno, In conclusion, we demonstrate the synthesis of a copolymer J. Phys. D: Appl. Phys. 42 (2009) 115104.of 3-alkylthiophenes and establish the mechanism of hole trans-  B.J. Landi, S.L. Castro, H.J. Ruf, C.M. Evans, S.G. Bailey, R.P. Raffaelle, Solar Energyport in it. It has been suggested that hole transport in P3OT-HT Mater. Solar Cells 87 (2005) 733.  E. Kymakis, P. Servati, P. Tzanetakis, E. Koudoumas, N. Kornilios, I. Rompogian-copolymer in governed by SCLC with traps distributed in energy nakis, Y. Franghiadakis, G.A.J. Amaratunga, Nanotechnology 18 (2007) 435702.and space and hole mobility being strongly dependent on temper-  V.D. Mihailetchi, H. Xie, B.D. Boer, L.J.A. Koster, P.W.M. Blom, Adv. Funct. Mater.ature and electric ﬁeld. The estimated value of zero ﬁeld mobility 16 (2006) 699.  Ph. Leclere, A. Calderone, D. Marsitzky, V. Francke, Y. Geerts, K. Mullen, J.L.( 0 ) ∼ 3.6 × 10−5 cm2 /Vs, is quite good and shows the promise of Bredas, R. Lazzaroni, Adv. Mater. 12 (2000) 1042.using P3OT-HT as a new donor material in the development of poly-  A. Zen, M. Saphiannikova, D. Neher, U. Asawapirom, U. Scherf, Chem. Mater. 17mer solar cells. Efforts are in progress to fabricate solar cells using (2005) 4.  R. Singh, J. Kumar, R.K. Singh, A. Kaur, K.N. Sood, R.C. Rastogi, Polymer 46 (2005)P3OT-HT as donor with PCBM and even CNTs as acceptor materials. 9126.  S. Amou, O. Haba, K. Shirato, T. hayakawa, M. Ueda, K. Takeuchi, M. Asai, J.Acknowledgments Polym. Sci. A: Polym. Chem. 37 (1999) 1943.  M.R. Andersson, D. Selse, M. Berggren, H. Järvinen, T. Hjertberg, Inganass, O. Wennerstoerm, J.E. Oesterholm, Macromolecules 27 (1994) 650. The authors MTK and MB are thankful to CSIR, New Delhi, India,  K. Inoue, R. Ulbricht, P.C. Madakasira, W.M. Sampson, S. Lee, J. Gutierrez, J.for the award of Junior Research Fellowship. We sincerely thank Dr. Ferraris, A.A. Zakhidov, Synth. Met. 154 (2005) 41.M.N. Kamalasannan, Dr. Ritu Srivastava and Dr. Rajeev K. Singh for  P.J. Brown, D.S. Thomas, A. Kohler, J.S. Wilson, J.S. Kim, C.M. Ramsdale, H. Sir- ringhaus, R.H. Friend, Phys. Rev. B 67 (2003) 064203.their cooperation and useful discussion.  Y. Kim, S.A. Choulis, J. Nelson, D.D.C. Bradley, S. Cook, J.R. Durrant, J. Mater. Sci. 40 (2005) 1371.References  T.-A. Chen, X. Wu, R.D. Rieke, J. Am. Chem. Soc. 117 (1995) 233.  C.Y. Yang, C. Soci, D. Moses, A.J. Heeger, Synth. Met. 155 (2005) 639.  A.K. Kapoor, S.C. Jain, J. Pootmans, V. Kumar, R. Mertens, J. Appl. Phys. 92 (2002) Y. Kim, S. Cook, S.M. Tuladhar, S.A. Choulis, J. Nelson, J.R. Durrant, D.D.C. Bradley, 3835. M. Giles, I. McCulloch, C. Ha, M. Ree, Nat. Mater. 5 (2006) 197.  N.F. Mott, R.W. Gurney, Electronic Processes in Ionic Crystals, Oxford University M. Reyes-Reyes, K. Kim, D.L. Carroll, Appl. Phys. Lett. 87 (2005) 083506. Press, London, 1940. W.U. Huynh, J.J. Dittmer, A.P. Alivisatos, Science 295 (2002) 2425.  P. Kumar, S.C. Jain, A. Misra, M.N. Kamalasanan, V. Kumar, J. Appl. Phys. 100 Y. Kang, N.-G. Park, D. Kimb, Appl. Phys. Lett. 86 (2005) 113101. (2006) 114506. I. Khatri, S. Adhikari, H.R. Aryal, T. Soga, T. Jimbo, M. Umeno, Appl. Phys. Lett.  K.C. Kao, W. Hwang, Electrical Transport in Solids, Pergamon, Oxford, 1981. 94 (2009) 093509.  M.A. Lampert, P. Mark, Current Injection in Solids, Academic, New York, Z. Liu, Q. Liu, Y. Huang, Y. Ma, S. Yin, X. Zhang, W. Sun, Y. Chen, Adv. Mater. 20 1970. (2008) 3924.  P.W.M. Blom, M.J.M. de Jong, M.G. van Munster, Phys. Rev. B 55 (1997) R656. K. Kim, J. Liu, M.A.G. Namboothiry, D.L. Carroll, Appl. Phys. Lett. 90 (2007)  L. Bozano, S.A. Carter, J.C. Scott, G.G. Malliaras, P.J. Brock, Appl. Phys. Lett. 74 163511. (1999) 1132. B.J. Landi, R.P. Raffaelle, S.L. Castro, S.G. Bailey, Prog. Photovolt: Res. Appl. 13  D.H. Dunlap, Phys. Rev. B 52 (1995) 939. (2005) 165.  W.D. Gill, J. Appl. Phys. 43 (1972) 5033.
27. OFFPRINT Polymeric-nanoparticles–induced vertical alignment in ferroelectric liquid crystalsA. Kumar, J. Prakash, P. Goel, T. Khan, S. K. Dhawan, P. Silotia and A. M. Biradar EPL, 88 (2009) 26003 Please visit the new website www.epljournal.org
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29. October 2009EPL, 88 (2009) 26003 www.epljournal.orgdoi: 10.1209/0295-5075/88/26003Polymeric-nanoparticles–induced vertical alignmentin ferroelectric liquid crystalsA. Kumar1 , J. Prakash1(a) , P. Goel1 , T. Khan2 , S. K. Dhawan2 , P. Silotia3 and A. M. Biradar1(b)1 Liquid Crystal Group, National Physical Laboratory - Dr. K. S. Krishnan Road, New Delhi-110012, India2 Conducting Polymer Group, National Physical Laboratory - Dr. K. S. Krishnan Road, New Delhi-110012, India3 Department of Physics and Astrophysics, University of Delhi - Delhi-110007, India received 13 July 2009; accepted in ﬁnal form 5 October 2009 published online 5 November 2009 PACS 61.30.-v – Liquid crystals PACS 77.84.Nh – Liquids, emulsions and suspensions; liquid crystals PACS 42.79.Kr – Display devices, liquid-crystal devices Abstract – Here, we report the polymeric (copolymer of benzene and pentacene) nanoparticles (PNPs) induced vertical alignment in ferroelectric liquid crystals (FLCs). The nanoparticles used in this study have been synthesized via chemical route method. The PNPs have been doped in FLC mixture. It has been observed that pentacene molecules (presented in PNPs used) prefer an upright orientation on the indium-tin-oxide–coated surfaces, which in turn provide assistance to align FLC molecules vertically (or homeotropically). It has also been observed that the addition of PNPs into the FLC materials improves the electro-optical response. However, the transition temperatures of the PNPs-doped FLC materials have been lowered. These ﬁndings will provide a fascinating tool to align FLC materials devoid of any surface treatment. Moreover, these studies would be helpful in the realization of low threshold and faster liquid crystal display devices. Copyright c EPLA, 2009 The impact of nanotechnology on liquid crystal (LC) researchers around the world. It has been reported thatmedia has resulted in fascinating improvements in the the surfaces of the substrates such as glass, oxides, andoverall performance of LCs-based display and non-display metals exhibits the ability to align LC molecules verticallyapplications. It has long been appreciated that the minute provided some cleaning processes on the substrates .addition of nanoparticles (NPs) to LC materials has However, the vertical alignment in these cases showed aimproved many special characteristics in the form of poor reproducibility and uniformity. The surface treat-frequency modulation response , non-volatile memory ment of silanes and large alkyl side chain alcohols aseﬀect , faster electro-optic response , low driving volt- homeotropic coupling agents has been a most widelyage , enhanced photoluminescence  and reduced resid- used technique for homeotropic alignment of LCs .ual dc . For electro-optical devices based on LCs, the The oblique evaporation of silicon mono-oxide ﬁlms onvertical (also called as homeotropic) alignment of LCs the substrate surface has been used to align LCs verti-has been a challenging topic of research from both the cally . Hwang et al. have studied the vertical align-fundamental and technological point of view for a long ment of LCs on an amorphous silicon oxide (a-SiOx ) thinperiod of time. The vertically aligned LCs has been exten- ﬁlm and found this as the consequence of anisotropicsively used for liquid crystal displays (LCDs) such as interactions (such as LC-LC interaction, Debye interac-large-area LCD televisions, information display devices tion, and London dispersion) between LCs and a-SiOxand digital displays in medical devices due to its unprece- thin ﬁlms . The vertical alignment of LCs using ﬂuo-dented contrast ratio and wide viewing angle charac- rinated diamond like carbon thin ﬁlms is also an exampleteristics . The various techniques have been investi- of such techniques . Cheng et al. have explored thegated to accomplish the vertical alignment of LCs by the possibility of favorable vertical alignment of LCs having large negative dielectric anisotropy (∆ε) with a minute (a) Also at Instrument Design Development Center, IIT Delhi - addition of the material possessing a longitudinal dipoleNew Delhi-110016, India. and therefore a positive ∆ε in the LCs . The verti- (b) E-mail: email@example.com cal alignment of LCs by incorporating NPs has emerged 26003-p1
30. A. Kumar et al.as a fascinating ﬁeld of research as it provides a bettertool to align LCs without the use of any alignment (a) (b)techniques. It has been shown that the NPs of polyhe-dral oligomeric silsesquioxane (POSS) can induce verticalalignment in nematic LCs having positive anisotropy .The electro-optical properties of POSS NPs-induced verti-cally aligned LC cells are very similar to the conventionalhomeotropic cells with alignment layers. It has also beendiscussed that the aligning capability induced by POSS isdue to adsorption of POSS on the inner surfaces of the (c) (d)substrates and highly inﬂuenced by doping concentrationof POSS . Few studies have been reported the uprightorientation of pentacene molecules (C22 H14 ), a promisingcandidate for organic electronics to fabricate organic thin-ﬁlm transistors , photodiodes  and high-frequencyrectiﬁers , on the weakly interacting surfaces of thesubstrates such as glass, ITO etc. This property (havingupright orientation) of pentacene has attracted our atten- Fig. 1: (Color online) Optical micrographs of (a) Felix 20,tion to use it for achieving homeotropic alignment in ferro- (b) Felix 17/100, (c) CS 1016, and (d) LAHS 19 at roomelectric liquid crystals (FLCs) by a minute addition of NPs temperature.having pentacene. In this letter, we report how the polymeric (copolymerof benzene and pentacene) nanoparticles (PNPs) help toinduce vertical alignment in FLCs. We have also studied The cell gaps were maintained using 3.5 µm thick Mylarthe eﬀect of these PNPs on the intrinsic parameters like spacers. The optical micrographs of the FLC materialstransition temperature, spontaneous polarization, thresh- have been recorded using a polarizing optical microscopeold voltage, response time etc. of the FLC mixtures. (Ax-40, Carl Zeiss, Germany) ﬁtted with a CCD camera. The PNPs have been synthesized by the chemical The dielectric measurements have been carried out byroute method and the typical size of the particles using impedance analyzer 6540 A (Wayne Kerr, UK). Theprepared was between 80 and 120 nm. The proposed probe amplitude during the dielectric measurements wasmolecular structure of synthesized PNPs is shown below: kept 0.5 V. The material constants such as spontaneous polarization, rotational viscosity, and reponse time have been determined by using an automatic liquid crystal tester (ALCT, Instec, USA). The probe amplitude and AC frequency during the determination of material constants were 8 V and 10 Hz, respectively. The phase sequences of the used FLC mixtures are as follows: −8 ◦ C 15−18 ◦ C 75 ◦ C 92−103 ◦ C n cryst ←→ SmC∗ ←→ SmA ←→ N ←→ Iso, (Felix 20) −28 ◦ C 73 ◦ C 77 ◦ C 84−87 ◦ C cryst ←→ SmC∗ ←→ SmA ←→ N ←→ Iso, The weight average molecular mass of the PNPs (Felix 17/100)used is 1086–1314 depending on the n value which −21 ◦ C 56 ◦ C 67 ◦ C 73 ◦ Cremains 7–10. The PNPs show solubility in many organic cryst ←→ SmC∗ ←→ SmA ←→ N ←→ Iso, (CS 1016)solvents like chlorobenzene, N-methyl-1-pyrollidine andchloroform etc. A few wt% of PNPs were dissolved in 3−7 ◦ C 60 ◦ C 62.5 ◦ C cryst ←→ SmC∗ ←→ SmA ←→ Iso. (LAHS 19)the FLC mixtures through the chloroform solvent andsonicated in ultrasonic bath for 15 min. The sonicated The optical micrographs of diﬀerent FLC materialsmixture was baked at 65 ◦ C for 3 h ensuring the complete doped with PNPs have been shown in ﬁg. 1. One canevaporation of chloroform. The mixture of FLCs and clearly see from the ﬁgure that ﬁrst three FLC materialsPNPs has been introduced into the LC sample cells by have attained a vertical alignment. However, in the casemeans of capillary action at temperatures just above the of LAHS 19, the vertical alignment has not been favoredisotropic temperatures of the respective FLC material. much since the traces of planar (multidomain) alignmentThe LC sample cells were prepared using indium tin oxide can be clearly seen in the optical micrograph (ﬁg. 1(d)).(ITO)-coated glass substrates cleaned well with acetone. The LAHS 19 is a special kind of FLCs named as deformed 26003-p2
31. Polymeric-nanoparticles–induced vertical alignment in ferroelectric liquid crystals 0.20 30 0C 6.5 (a) 0V (b) 32 0C 1V 34 0C 6.0 2V 36 0C 4V 0.15 38 0C 5.5 6V 8V 40 0C 5.0 42 0C tan δ 10 V 0.10 44 0C ε 4.5 12 V 0 V again 46 0C 4.0 48 0C 50 0C 3.5 0.05 52 0C 3.0 55 0C 102 103 104 105 106 103 104 105 106 Frequency (Hz) Frequency (Hz)Fig. 2: (Color online) Frequency dependence of (a) dielectric permittivity (ε′ ) for diﬀerent bias voltages at room temperature,(b) dielectric loss factor (tan δ) measured in the temperature range 30 ◦ C–55 ◦ C in the homeotropically aligned cell of PNPs-doped Felix 17/100.helix ferroelectric liquid crystal (DHFLC) having ultrashort pitch (∼ 0.3–0.7 µm) and high value of spontaneous (a) (b)polarization (∼ 90 nC/cm2 ). So, it is diﬃcult to explainwhy PNPs induced vertical alignment is very poor inthe case of DHFLC. It might be due to denser packing(because of ultra short pitch) of the molecules of DHFLCwhich results in the poor dispersion of PNPs in the mate-rial and hence a moderate vertical alignment. The verti-cal alignment in FLC materials has also been conﬁrmedby dielectric relaxation spectroscopy. Figure 2 shows the (c)behavior of dielectric permittivity (ε′ ) with frequency atdiﬀerent applied voltages at room temperature (ﬁg. 2(a))and of dielectric loss factor (tan δ) with frequency at diﬀer-ent temperatures (ﬁg. 2(b)) for FLC material Felix 17/100.It is clear from ﬁg. 2(a) that the value of ε′ does notsuppress even at higher values of applied voltages suggest-ing the alignment to be homeotropic. In the case of thevertical alignment of the liquid crystals, the long molecu- Fig. 3: (Color online) Optical micrographs of PNPs-doped Felixlar axis is perpendicular to the substrates surface prevent- 17/100 in the SmC∗ phase at (a) 0 V, (b) 20 V, and (c) 25 Ving the Goldstone mode to take place and leaving the (which changes the orientation from homeotropic to planar) atonly possibility of molecular rotation around their short room temperature (30 ◦ C).molecular axis which gives the lower values of ε′ . Also,on the application of bias, the ε′ value does not suppress correspond to the molecular relaxations due to the motionremarkably for the vertically aligned LC cells. On the other of molecules around their short axis.hand, in the case of homogeneous alignment of LCs, the It is worth mentioning here that the vertically alignedlong molecular axis is parallel to the substrates surface cells can be transformed into homogeneous conﬁgurationgiving the larger values of ε′ due to the Goldstone mode by applying a high electric ﬁeld across the cell. It iswhich, on the application of bias, get suppressed to give noticeable that the transformation from homeotropiclower values of ε′ . We have also observed the behavior to homogeneous conﬁguration has been favored in LCof ε′ with frequency at diﬀerent biases for FLC mixtures materials because of the negative dielectric anisotropyFelix 20 and found a similar behavior of ε′ with bias as (∆ε). In materials having such transformation, the dipolethat of Felix 17/100. But the DHFLC material LAHS 19 moment along the short molecular axis (µS ) dominates.shows the presence of Goldstone mode due to poor verti- On the application of suﬃcient electric ﬁeld acrosscal alignment. Figure 2(b) shows the behavior of tan δ for the cell, the LC molecules attain orientation parallelthe vertically aligned Felix 17/100 which shows beauti- to the substrate surface due to a stronger coupling offul relaxation peaks in sub-kHz frequency regimes that µS with the electric ﬁeld. Figure 3 shows the optical 26003-p3
32. A. Kumar et al. 4.4 4.5 (a) (b) 4.4 4.2 4.3 ε′⊥ 4.2 ε′⊥ 4.0 ε′ 4.1 ε′ 3.8 ε ε 4.0 3.6 3.9 3.8 3.4 3.7 3.2 30 35 40 45 0 50 55 30 35 40 45 0 50 55 Tempearture ( C) Temperature ( C)Fig. 4: (Color online) A comparison of the dielectric permittivity (ε′ : planar state; ε′ : homeotropic state) as a function of ⊥temperature at frequency (a) 10 kHz and (b) 100 kHz. This ﬁgure estimates the dielectric anisotropy.micrograph of the transformed (on the application of ﬂat or standing up, on the gold substrates. It has beenelectric ﬁeld) Felix 17/100 material under crossed polar- observed that the deposition of pentacene molecules onizers which exhibits multidomain nature due to random SiO2 results in well-ordered monolayers as well as thickermolecular arrangement as the substrate surfaces were ﬁlms with the upright orientation of the molecules .untreated. The application of such transformation (from Recently, Zhang et al. have investigated the dynamichomeotropic to homogeneous) can be used to calculate growth process and morphology of pentacene on diﬀerentthe dielectric anisotropy of the FLC material using only substrates . They showed experimentally that thesingle sample cell. We have determined the parallel and pentacene molecule is perpendicular to the silicon waferperpendicular components of ε′ corresponding to vertical surface with a slight tilt angle. However, they couldand homogeneous alignment at various frequencies (10 not found any indication of perpendicular orientation ofand 100 kHz) for PNPs-doped FLC Felix 17/100. The the pentacene molecules in the case of polymer-treatedcomparison of parallel and perpendicular components ITO-coated substrates. The roughness of the underlyingε′ (ε′ and ε′ ) has been shown in the ﬁg. 4. ⊥ substrate also plays an important role in the growth of The explanation why the PNPs have induced the organic ﬁlm on it. The surfaces having the roughness onvertical alignment in the FLCs lies in the nature of a molecular scale do not provide well-deﬁned adsorptionthe interactions of pentacene molecules with diﬀerent sites for large organic molecules such as pentacene andsubstrate surfaces. The growth morphology of pentacene hence an upright molecular geometry can then be favoredat various surfaces has been investigated extensively by for the ﬁrst monolayer . The precise determinationvarious groups around the world [19–21]. The molecular of the growth morphology of pentacene on diﬀerentstructure of thin pentacene ﬁlms grown on a Cu (110) substrate surfaces is still a very challenging problemsurface has been investigated by using various experimen- to be investigated as it depends on a lot of factorstal techniques such as He atom scattering, low-energy involved in the deposition process, deposition parameters,electron diﬀraction, thermal desorption spectroscopy, nature of substrates etc. We have used bare ITO-coatedX-ray photoelectron spectroscopy, and X-ray absorption glass substrates cleaned with acetone to assemble thespectroscopy  which reveal that the orientation of LC sample cells and ﬁlled them with PNPs-dopedpentacene molecules on the Cu (110) depends on the ﬁlm FLC materials above their isotropic temperatures. Wethickness of the pentacene. In the monolayer regime, the proposed that during the ﬁlling of PNPs-doped FLCmolecules form ordered structure with planar adsorption materials, the PNPs near the ITO surfaces may form ageometry, whereas for ﬁlm thicknesses greater than 2 nm, thin-ﬁlm phase with stand-up orientation of the moleculessubsequent growth proceeds with the stand-up orienta- because the pentacene molecules grown on substratestion of pentacene molecules. Zheng et al. found that the with low-surface energy tend to align vertically , whichmolecular orientations of pentacene thin-ﬁlm structures works as the template for the FLC molecules resultingare strongly inﬂuenced by the metal substrates . in the uniform vertical alignment of FLC materials.They proposed that the pentacene molecules stand up on However, this phenomenon of vertical alignment of FLCsthe surface and form thin-ﬁlm phase structure on silver by addition of PNPs is rather unclear and needs furtherwhereas grow in domains with molecules, either lying investigations. 26003-p4
33. Polymeric-nanoparticles–induced vertical alignment in ferroelectric liquid crystals 24 3.0 (a) 35 0C 20 2.5 40 0C 16 45 0C PS (nC/cm2) 2.0 50 0C tan δ 12 1.5 55 0C 8 1.0 57 0C 4 58 0C 0.5 60 0C 0 0.0 -1 0 1 2 3 4 5 6 7 8 9 10 102 103 104 105 106 Applied Voltage (V) Frequency (Hz) 400 (b) Fig. 6: (Color online) Frequency dependence of the dielec- tric loss factor (tan δ) measured in the temperature range 300 30 ◦ C–60 ◦ C in the planar state of the cell of PNPs-doped Felix η (mPa*s) 17/100. PNPs doped Felix 17/100 Pure Felix 17/100 200 and PNPs-doped Felix 17/100. The eﬀect of PNPs on the material constants of Felix 17/100 has been shown 100 in ﬁg. 5. It is clear from ﬁgs. 5(a) and (b) that the spontaneous polarization and rotational viscosity of the PNPs-doped Felix 17/100 material have been lowered. 0 It is noticeable that the combined eﬀect of reduction in 0 1 2 3 4 5 6 7 8 9 10 these parameters has resulted in the faster response of the PNPs-doped Felix 17/100 material as can be seen from Applied Voltage (v) ﬁg. 5(c). Also, the addition of PNPs is more advantageous as the value of threshold voltage of PNPs-doped Felix 80 (c) PNPs doped Felix 17/100 17/100 material has been lowered remarkably as reﬂected Pure Felix 17/100 from ﬁg. 5. The addition of PNPs into FLCs has aﬀected the τR(x10-4)sec 60 order of FLC molecules and hence produces a smaller order which has been reﬂected in the reduction of the SmC∗ -SmA transition temperature (TC ) of PNPs-doped 40 FLC materials. It has been reported earlier that the addition of POSS NPs into the nematic LC perturbed the order parameter (S ) and produced a smaller S in POSS- 20 doped nematic LC [14,26]. Figure 6 shows the behavior of tan δ with frequency at diﬀerent temperatures for Felix 17/100 FLC material in homogeneous conﬁguration in 0 order to show the reduction of the SmC∗ -SmA transition 0 1 2 3 4 5 6 7 8 9 temperature (TC ) of PNPs-doped FLC materials. One can Applied Voltage (V) clearly see that TC has been shifted from 73 ◦ C to 58 ◦ C. The reduction in TC has also occurred in the case of otherFig. 5: (Color online) Behavior of (a) spontaneous polarization FLC mixtures under investigations except Felix 20 for(Ps ), (b) rotational viscosity (η) and (c) response time (τR ) which the reduction in TC is very low (∼ 1%).with applied voltage for PNPs-doped Felix 17/100 in planarconﬁguration at room temperature. The PNPs-induced vertical alignment of FLC molecules has been demonstrated. It has been proposed that PNPs induce homeotropic alignment of FLC mixtures which is a The inﬂuence of the PNPs on the physical parameters consequence of the surface interaction between the ITOof the FLCs has also been taken into account. For this, and PNPs in such a fashion that the PNPs stand upwe have determined various physical parameters in pure on the ITO surface with tilted orientation. The upright 26003-p5
34. A. Kumar et al.orientation of pentacene molecules provides assistance to  Nie X., Lu R., Xianyu H., Wu T. X. and Wu S.-S.,FLC molecules in attaining a uniform vertical alignment. J. Appl. Phys., 101 (2007) 103110.We have also observed that the addition of PNPs into the  Cognard J., Mol. Cryst. Liq. Cryst. Suppl. Ser., 1FLC mixtures improves the physical parameters of the (1982) 1.FLCs. The response time as well as threshold voltage has  Janning J. L., Appl. Phys. Lett., 21 (1972) 173.  Hiroshima K., Jpn. J. Appl. Phys., 21 (1982) L761.been lowered in PNPs-doped FLC materials. The PNPs-  Hwang B. H., Ahn H. J., Rho S. J., Chae S. S. andinduced vertical alignment of FLCs will certainly provide Baik H. K., Langmuir, 25 (2009) 8306.a promising tool to align FLCs devoid of the use of any  Ahn H. J., Rho S. J., Kim K. C., Kim J. B., Hwangsurface treatment techniques as well as to achieve low B. H., Park C. J. and Baik H. K., Jpn. J. Appl. Phys.,threshold and faster liquid crystal display devices. 44 (2005) 4092.  Cheng C., Anderson J. E. and Bos P. J., Jpn. J. Appl. ∗∗∗ Phys. Part 2, 44 (2005) L1126.  Jeng S.-C., Kuo C.-W., Wang H.-L. and Liao C.-C., The authors sincerely thank Dr. Vikram Kumar, Appl. Phys. Lett., 91 (2007) 061112.Director of the National Physical Laboratory, for contin-  Huang S.-J., Jeng S.-C., Yang C.-Y., Kuo C.-W.uous encouragement and interest in this work. We and Liao C.-C., J. Phys. D.: Appl. Phys., 42 (2009)sincerely thank Dr. S. S. Bawa, Dr. I. Coondoo, 025102.Mr. A. Choudhary, Mr. G. Singh, Ms. A. Malik  Dimitrakopoulos C. D. and Malenfant P. R. L., Adv.and Mr. T. Joshi for fruitful discussions. We are also Matter, 14 (2002) 99.thankful to the Surface Science Group, NPL for providing  Lee J., Hwang D. K., Park C. H., Kim S. S. and Imultrasonication facility. The authors (AK and JP) are S., Thin Solid Films, 12 (2004) 451.thankful to UGC, New Delhi and CSIR, New Delhi for  Steudel S., Myny K., Arhipov V., Deibel C., Vusser S., Genoe J. and Heremans P., Nat. Mater., 4 (2005)ﬁnancial assistance. 597.  Bette M. G., Kanjilal A. and Mariani C., J. Phys.REFERENCES Chem. A, 111 (2007) 12454.  Yang S.Y., Shin K. and Park C. E., Adv. Funct. Mater.,  Shiraishi Y., Toshima N., Maeda K., Yushikawa H., 15 (2005) 1806. Xu J. and Kobayashi S., Appl. Phys. Lett., 81 (2002)  Zhang F., Park B.-N., Seo S., Evans P. G. and 2845. Himpsel F. J., J. Chem. Phys., 126 (2007) 154702.  Prakash J., Choudhary A., Kumar A., Mehta D. S.  Sohnchen S., Lukas S. and Witte G., J. Chem. Phys., and Biradar A. M., Appl. Phys. Lett., 93 (2008) 112904. 121 (2004) 525.  Prakash J., Choudhary A., Mehta D. S. and  Zheng Y., Qi D., Chandrasekhar N., Gao X., Biradar A. M., Phys. Rev. E, 80 (2009) 012701. Troadec C. and Wee A. T. S., Langmuir, 23 (2007)  Lee W., Wang C.-Y. and Shih Y.-C., Appl. Phys. Lett., 8336. 85 (2004) 513.  Schwieger T., Liu X., Olligs D. and Knupfer M.,  Kumar A., Prakash J., Mehta D. S., Hasse W. and J. Appl. Phys., 96 (2004) 5596. Biradar A. M., Appl. Phys. Lett., 95 (2009) 023117.  Zhang F., Xu Z., Liu X., Zhao S., Lu L., Wang Y.  Baik L.-S., Jeon S. Y., Lee S. H., Park K. A., Jeong and Xu X., Superlattices Microstruct., 45 (2009) 612. S. H., An K. H. and Lee Y. H., Appl. Phys. Lett., 87  Chen H.-Y., Lee W. and Clark N. A., Appl. Phys. (2005) 263110. Lett., 90 (2007) 033510. 26003-p6