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JOURNAL OF APPLIED PHYSICS 110, 044509 (2011)     In-Situ growth of cadmium telluride nanocrystals in poly(3-hexylthiophen...
044509-2       Khan et al.                                                                                   J. Appl. Phys...
044509-3       Khan et al.                                                                                           J. Ap...
044509-4      Khan et al.                                                                                 J. Appl. Phys. 1...
044509-5       Khan et al.                                                                                    J. Appl. Phy...
044509-6     Khan et al.                                                                                   J. Appl. Phys. ...
044509-7       Khan et al.                                                                                              J....
JOURNAL OF APPLIED PHYSICS 109, 114509 (2011)    Hole transport mechanism in organic/inorganic hybrid system based    on i...
114509-2     Khan et al.                                                                                    J. Appl. Phys....
114509-3     Khan et al.                                                                               J. Appl. Phys. 109,...
114509-4     Khan et al.                                                                                    J. Appl. Phys....
114509-5     Khan et al.                                                                                     J. Appl. Phys...
APPLIED PHYSICS LETTERS 97, 163113 ͑2010͒    Memory effect in cadmium telluride quantum dots doped ferroelectric    liquid...
Charge Transport in organic semiconductors
Charge Transport in organic semiconductors
Charge Transport in organic semiconductors
Charge Transport in organic semiconductors
Charge Transport in organic semiconductors
Charge Transport in organic semiconductors
Charge Transport in organic semiconductors
Charge Transport in organic semiconductors
Charge Transport in organic semiconductors
Charge Transport in organic semiconductors
Charge Transport in organic semiconductors
Charge Transport in organic semiconductors
Charge Transport in organic semiconductors
Charge Transport in organic semiconductors
Charge Transport in organic semiconductors
Charge Transport in organic semiconductors
Charge Transport in organic semiconductors
Charge Transport in organic semiconductors
Charge Transport in organic semiconductors
Charge Transport in organic semiconductors
Charge Transport in organic semiconductors
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Charge Transport in organic semiconductors

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Charge Transport in organic semiconductors

  1. 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: skdhawan@mail.nplindia.ernet.in. that CdTe nanoparticles have been successfully synthesized b) Electronic mail: schand@mail.nplindia.ernet.in. 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 14.139.60.97. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
  2. 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 14.139.60.97. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
  3. 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 14.139.60.97. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
  4. 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 14.139.60.97. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
  5. 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 14.139.60.97. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
  6. 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 14.139.60.97. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
  7. 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 14.139.60.97. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
  8. 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: skdhawan@mail.nplindia.ernet. film, l is the carrier mobility, and n is the carrier density. b) Electronic mail: schand@mail.nplindia.ernet.in. 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 59.144.72.1. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
  9. 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 Š; (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  à r ¼ r0 4 p5 : (15) T BC ð2aÞ3 G ¼ G0 exp Àsij (7)Downloaded 10 Jun 2011 to 59.144.72.1. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
  10. 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 59.144.72.1. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
  11. 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 59.144.72.1. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
  12. 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 59.144.72.1. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
  13. 13. APPLIED PHYSICS LETTERS 97, 163113 ͑2010͒ Memory effect in cadmium telluride quantum dots doped ferroelectric liquid crystals A. Kumar,1 J. Prakash,2 Mohd Taukeer Khan,3 S. K. Dhawan,3 and A. M. Biradar1,a͒ 1 Liquid Crystal Group, National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi 110012, India 2 Instrument Design Development Centre, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India 3 Conducting Polymer Group, National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi 110012, India ͑Received 17 August 2010; accepted 5 September 2010; published online 21 October 2010͒ A pronounced memory effect has been observed in cadmium telluride quantum dots ͑CdTe-QDs͒ doped ferroelectric liquid crystals ͑FLCs͒ by using dielectric and electro-optical methods. The memory effect has been attributed to the charge storage on the CdTe-QDs upon the application of dc bias across the sample cell. The FLC molecules remain in the switched state in vicinity of the charge stored on QDs even after removal of bias. It has been observed that the memory effect depends on doping concentrations of CdTe-QDs and the FLC material used. © 2010 American Institute of Physics. ͓doi:10.1063/1.3495780͔ Ferroelectric liquid crystals ͑FLCs͒ have been employed tifunctional switchable devices. However, the effect of QDs in various promising applications, such as flat panel displays, on the properties of LC materials is rarely reported. The spatial light modulators, optical antennas, etc., due to their understanding of the interaction between QDs and LCs is a important characteristic features such as good optical con- challenging area of research for its utilization to achieve con- trast, fast response, low threshold voltage, and memory ef- trolled self-assembly of QDs and improved electro-optical fect. The metal nanoparticles ͑NPs͒ have been doped into characteristics of LCs. liquid crystals ͑LCs͒ to observe various interesting phenom- In this paper, we observed the effect of cadmium tellu- ena, such as nonvolatile memory effect,1 enhanced ride ͑CdTe͒-QDs on the electro-optical properties of FLCs. It photoluminescence,2 enhanced electrical conductivity,3 and has been observed that the doping of CdTe-QDs in various induced LC alignments.4 The doping of NPs has improved FLC materials favors pronounced memory effect. The ob- the electro-optical properties such as good contrast and low served memory state does not come back to its original ͑un- power operation of LC based display devices.5,6 The semi- switched͒ state instantaneously but it retains in that state for conducting quantum dots ͑QDs͒ have attracted a great deal a remarkable time. We also observed that retention of of interest in the scientific community for their promising memory state depends on the doping concentration of QDs applications such as next generation photonic devices, QD and FLC materials. displays, and biomedical imaging.7–10 Apart from them, vari- The CdTe-QDs have been synthesized in the form of ous studies have been carried out by researchers for utilizing P3HT ͑Poly-3͑hexylthiophene͒͒-CdTe nanocomposites. The the QDs in the realization of nonvolatile memory devices. P3HT polymer is used for capping to prevent the agglomera- Nassiopoulou et al.11 have observed a large shift in the tion of QDs. The high resolution transmission electron mi- croscopy ͑HRTEM͒ images and electron diffraction pattern capacitance-voltage ͑C-V͒ hysteresis of the metal-oxide- of the synthesized P3HT-CdTe nanocomposites have been semiconductor ͑MOS͒ structures containing germanium shown in Fig. 1. Highly resolved HRTEM image has also QDs. The memory effect has been observed in a MOS struc- been given in the inset of Fig. 1͑a͒. The typical size of CdTe- ture containing zirconium ͑Zr͒ nanocrystals embedded in QDs varies between 2–7 nm in diameter. The sample cells ZrO2 dielectric layer by Lee et al.12 The high frequency C-V for the present study were prepared using indium tin oxide characteristics of the structures with silicon NPs have shown coated glass plates. The desired ͑squared͒ electrode area was hysteresis indicating the charging of NPs with electrons/ 0.45ϫ 0.45 cm2. The thickness of the cell was maintained holes due to charge carrier tunneling from the substrate by using ϳ4 ␮m thick Mylar spacers. The FLC materials through the thin oxide at positive/negative biases.13 Charge retention properties of QDs have been studied14,15 and found that the charge carriers stored on QDs could persist over time $ ! scales exceeding seconds or even hours.16 On the other hand, the anisotropic ordering of LCs can impart order onto the nanosized guest particles and hence researchers have utilized this property to organize QDs in the form of their self-assembly.17 Hirst et al.18 studied the mechanism to organize QDs by using anisotropic LC me- dium and explored the possibility for the fabrication of mul- FIG. 1. ͑Color online͒ ͑a͒ HRTEM image with scale bar: 20 nm ͑the inset a͒ Author to whom correspondence should be addressed. Electronic mail: shows highly magnified image with scale bar: 5 nm ͑b͒ electron diffraction abiradar@mail.nplindia.ernet.in. pattern of P3HT/CdTe-QDs nanocomposites. 0003-6951/2010/97͑16͒/163113/3/$30.00 97, 163113-1 © 2010 American Institute of PhysicsDownloaded 22 Oct 2010 to 59.144.72.1. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

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