A comprehensive experimental study is reported on the optical and electrical characteristics of 2-((5-(4- (diphenylamino)phenyl)thiophen-2-yl)methylene)malononitrile (DPTMM) when used as molecular donor in an organic solar cell (OSC) device structure.

1. RSC Advances
PAPER
Cite this: RSC Adv., 2014, 4, 5236
Exploiting the potential of 2-((5-(4-(diphenylamino)phenyl)thiophen-2-yl)methylene)malononitrile as
an efficient donor molecule in vacuum-processed
bulk-heterojunction organic solar cells†
Jin Woo Choi,a Chang-Hyun Kim,a Jonathan Pison,a Akinola Oyedele,a
Denis Tondelier,a Antoine Leli` ge,b Eva Kirchner,b Philippe Blanchard,b Jean Roncalib
e
and Bernard Geffroy*ac
A comprehensive experimental study is reported on the optical and electrical characteristics of 2-((5-(4(diphenylamino)phenyl)thiophen-2-yl)methylene)malononitrile (DPTMM) when used as molecular donor
in an organic solar cell (OSC) device structure. A major property of this new donor-type material is an
unusually deep highest-occupied molecular orbital (HOMO) level that leads to a high open-circuit
voltage (Voc). A reasonably high hole-mobility was also observed in a hole-injection diode configuration.
These are both promising factors for high-performance OSCs. In order to fully explore the potential of
DPTMM in bulk-heterojunction-based OSCs, a step-wise experimental strategy was applied to optimize
film composition and cell architecture. By co-evaporating the DPTMM with C60 to promote exciton
dissociation by maximizing the heterojunction area power conversion efficiency (PCE) of 3.0% was
Received 25th October 2013
Accepted 9th December 2013
achieved. Finally, inserting a buffer layer and a spatial gradient of the donor/acceptor ratio was found to
provide better conduction paths for charge carriers. The maximum obtained PCE was 4.0%, which
DOI: 10.1039/c3ra47059h
compares favorably with the state-of-the-art of high-performance OSCs. All optimized devices show
www.rsc.org/advances
quite unusual high Voc values up to 1 V.
1. Introduction
Organic solar cells (OSCs) are considered to be a promising
future energy technology – one which benets from the possibility of tailoring materials and enabling cost-effective production of mechanically exible solar modules. Over the last two
decades, the synthesis of new functional materials1–5 and the
design of novel device architectures have been the foremost
factors that have led to dramatic improvements in the power
conversion efficiency (PCE) of OSCs.6–9 However, their absolute
level of performance is still too low to respond to existing
commercial demand. It is well-established that three key
parameters dene the power conversion efficiency of OSCs: the
open-circuit voltage (Voc), the short circuit current ( Jsc) and the
ll factor (FF).10–12 Therefore, it is crucial to understand
the critical factors that correlate best with changes in these
a
LPICM, Ecole Polytechnique, CNRS UMR-7647, 91128 Palaiseau, France. E-mail:
bernard.geffroy@polytechnique.edu; Tel: +33-1-69-33-43-82
b
LUNAM, University of Angers, CNRS UMR-6200, MOLTECH-Anjou, Linear Conjugated
Systems Group, 2 Bd Lavoisier, 49045 Angers, France
c
CEA Saclay, DSM/IRAMIS/SPCSI/LCSI, 91191 Gif Sur Yvette, France
† Electronic supplementary
10.1039/c3ra47059h
information
5236 | RSC Adv., 2014, 4, 5236–5242
(ESI)
available.
See
DOI:
parameters and concomitant strategic improvement based on
relevant device physics is of central importance.
Employing bulk-heterojunction (BHJ) architecture in an OSC
is greatly advantageous in comparison to planar bilayer heterojunction devices. A BHJ organic layer is formed by deposition of
mixed solution or co-evaporation of donor and acceptor materials, whereas a planar device is based on two independently and
sequentially deposited organic lms. It has been generally
proven that the overall characteristics of BHJ cells showed better
device performance than planar ones, especially in terms of Jsc
and FF. These improvements are mainly explained by the
increased interfacial area at which a donor (p-type) and an
acceptor (n-type) materials form an intimate contact and where
the charge transfer between two materials takes place.13–16
Accordingly, such a structure is thought to enhance the dissociation of excitons at interfaces. However, random structural
phases in a BHJ layer may disrupt the transport of generated
charge carriers, because there is possibility of clusters that are
not well electrically connected to the other clusters. Therefore,
the control of the morphology of BHJ layer represents a key issue
whereas high-mobility organic semiconductors which can overcome deciencies in carrier transport in BHJ are desirable.
There are intensive research efforts to nd organic semiconductors that can meet the material requirements for BHJ
This journal is © The Royal Society of Chemistry 2014

2. Paper
OSCs. Although low band gap processable conjugated polymers
remain a major class of donor materials for organic photovoltaics,2,3 the past few years have witnessed the growing importance of small conjugated molecules.1 Due to their perfectly
dened chemical structures, the reproducibility of their
synthesis and their easier purication, molecular donors allow
a more straightforward analysis of structure–property relationships than polydisperse polymers. In addition small conjugated
molecules allow the preparation of BHJ solar cells either by
solution5 or vacuum process.4 Small p-conjugated molecules
have represented a major class of active materials in OSCs
during the early developments of the eld.17,18 For example,
phthalocyanine derivatives have been considered as promising
materials due to their high molar absorptivity, high hole
mobility, and long exciton lifetime.19,20 However, photovoltaic
devices with phthalocyanine derivatives generally exhibited
relatively low Voc. Beyond any consideration of the potential
impact of the lm microstructure on Voc,21 the most critical
factor determining Voc is the highest-occupied molecular orbital
(HOMO) of the donor material itself relative to the lowestunoccupied molecular orbital (LUMO) of the paired acceptor
material.22 In this context, the search for molecular donor
materials showing an increased ionization potential is a key
strategy for nding alternative phthalocyanine-based materials
that might present a higher Voc in an OSC device.
An interesting approach consists of the design of molecular
donors in which an internal charge transfer (ICT) is created by
the introduction of electron acceptor (A) groups in the structure
of the donor.23 This ICT concomitantly leads to the extension of
the absorption spectrum towards longer wavelengths and
particularly to the decrease of the HOMO level which results in
an increase of Voc of the corresponding OSC and also in a better
stability of the molecule against oxidation. Thus conjugated
molecules such as D–A, D–A–D or A–D–A systems, combining
electron-donor (D) and electron-acceptor (A) groups, have led to
molecular donors exhibiting interesting photovoltaic performance.24,25 In addition, the use of a buffer layer with the photoconversion layer has recently gained increasing attention in
OSC research.26,27 An organic buffer layer is usually inserted
between the charge-injection electrode and the active layer for
the purpose of enhanced device operation. The buffer layer is
introduced to improve charge injection/collection by providing
an intermediate transport level. Morphological tuning of the
active layer can be also expected in the case where the active
layer is deposited on top of a buffer layer for which the surface
energy is different from that of the bare electrode. The overall
improvement in PCEs observed in this type of system has been
found to be linked to more ideal diode current–voltage curves,
lacking the so-called s-shape related to a leakage current
contribution.28
We herein report on the demonstration of high-performance,
evaporation-based BHJ OSCs using a small donor molecule based
on a D–A structure, namely 2-((5-(4-(diphenylamino)phenyl)thiophen-2-yl)methylene)malononitrile (DPTMM) which has been
recently used in planar bilayer heterojunction OSCs.29,30 Through
multiple series of experiments, and by employing a co-evaporated
BHJ system design and the tailored lm composition with a
This journal is © The Royal Society of Chemistry 2014
RSC Advances
proper buffer layer at the hole-collection side, the PCE of a
DPTMM/C60 cell was raised to 4.0%. In particular, an outstanding
value of Voc near 1.0 V suggests that DPTMM can be considered as
a promising candidate for an efficient molecular donor in highperformance OSCs. Also, the simplicity of the molecular structure
of DPTMM associated to its ease of preparation represents
important parameters which must be taken into account for the
future development of OSCs.
2.
Experimental
2.1. Materials
Fig. 1 shows the chemical structures of all materials used in this
study. DPTMM (Fig. 1a) was used as the donor material in all
experiments. The synthesis of this new donor material has been
reported in a recent paper.29 The HOMO and LUMO levels of
DPTMM were found to be 5.96 eV and 3.79 eV, respectively.
2.2. Solar cell fabrication and measurements
All solar cell samples were fabricated on a pre-cleaned,
patterned indium-tin-oxide (ITO) coated glass substrate
purchased from Xinyan Technologies and with a sheet resistance of 20 U ,À1. Before depositing organic layers, the
patterned ITO glasses were cleaned by ultrasonication in
isopropyl alcohol (IPA) and treated by UV–ozone with a
NOVASCAN UV-exposure system. Subsequently, the substrates
were spin-coated with poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS) (Baytron 4083). The thickness
of this layer was approximately 40 nm aer drying at 108 C for
2 minutes. The samples were then transferred into the thermal
evaporator. Small molecules were thermally evaporated at a
pressure below 10À7 Torr. For the BHJ active layer, the material composition ratio (DPTMM and C60) was obtained by two
quartz sensors. One quartz sensor measured C60 evaporation
rate while another sensor detected the deposition rate of both
DPTMM and C60, from both of which could be estimated the
lm composition (see ESI†). The devices were nished by
Chemical structures of the investigated compounds. (a)
DPTMM, (b) C60, (c) a-NPB and (d) PEDOT:PSS.
Fig. 1
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3. RSC Advances
depositing 1.2 nm of lithium uoride (LiF) and a 100 nm thick
aluminum layer as a top electrode through a shadow mask,
dening a device area of 0.28 cm2. The structures of the devices
used in this study are summarized in Table 1. The current–
voltage (I–V) measurements of the fabricated OSCs were performed under the illumination of a simulated AM 1.5G solar
light (100 mW cmÀ2) connected to a computer-controlled
Keithley 2635 source measurement unit (SMU) inside a
nitrogen-lled glove box.
2.3. Hole only device fabrication and measurements
Hole only devices were fabricated using the same indium-tinoxide (ITO) coated glass substrates following the same cleaning
procedure as for solar cell fabrication. Firstly, PEDOT:PSS
(Baytron AI4083) was spin-coated on the ITO anode, followed by
thermal annealing at 108 C for two minutes. The thickness of
the PEDOT:PSS layer was approximately 40 nm. Subsequently,
120 nm of DPTMM was deposited by thermal evaporation at a
pressure of 10À7 Torr. Twenty nanometers of Au was then
evaporated on top of the DPTMM layer, with a nal capping
layer of 30 nm of Al to avoid mechanical deterioration of the Au
contact during the electrical characterization. The active layer of
2.9 Â 10À2 cm2 was dened by shadow-masking of the top
contact. The current–voltage (I–V) characteristics were acquired
with a Keithley 4200 semiconductor characterization system.
2.4. Optical characterization
Organic layers for absorption spectra measurement were
deposited on glass substrates and the lms for ellipsometric
measurements were deposited on Si wafers. The absorption
spectra were measured using a Jenway 6800 spectrophotometer.
The refractive and extinction indexes n and k were obtained
from measurement performed by MM16 Muller-ellipsometer
from Horriba Jobin-Yvon. Spectroscopic ellipsometry is a useful
method to study optical characteristics in OSCs.31–34 This technique is based on the relative change of polarization in incident
and reected light. The refractive and extinction indexes n, k of
C60, DPTMM, and DPTMM:C60 (60 : 40) blend layers were
obtained by tting the results from Muller ellipsometric
measurements. In order to t the optical data, a parametric
model based on four oscillators was used. This model was
derived from the simple Forouhi-model, and was used in order
to give a Lorentzian-shape to the expression of n and k.35–37
Paper
3.
Results and discussion
3.1. Optical properties of DPTMM
Fig. 2 shows the absorption spectra of the DPTMM and C60
compounds, as well as a thin lm of co-deposited DPTMM:C60
(1 : 1). The DPTMM compound has a broad absorption spectrum from 300 nm to 700 nm, with an absorption peak centered
at 520 nm. The extracted values for n and k of the C60 layer
(Table 2) are close to those found in the literature.33 Ellipsometric measurements of the DPTMM:C60 blend are well tted
by using a combination of both ellipsometric models obtained
for C60 and DPTMM separately. Note that the measured k
spectra are in good agreement with the absorption spectra of
Fig. 2. Equally important as lm absorption, the propagation of
light in a multilayer structure is inuenced by interference
effects. More specically, the electromagnetic eld distribution
strongly depends on the thickness and optical constants of each
layer.38,39 The spatial distribution of absorption and exciton
generation characteristics were modeled by using a constant
value of n and k at 520 nm (where DPTMM shows the highest
absorption). The selected values of n and k are shown in Table 2.
Fig. 3a displays the two corresponding intensity proles (the
squared moduli of the electromagnetic eld amplitude |E(x)|2)
as a function of position in the device for both a bi-layer device
and BHJ structure. It is observed that the BHJ structure has its
maximum light intensity in the co-deposited layer, whereas a
bilayer structure has its maximum near the PEDOT:PSS region.
Fig. 3b shows the calculated exciton generation rate G(x), taking
into account the extinction coefficient of each layer. Total
Fig. 2 Absorption spectra of compounds DPTMM, C60, and
DPTMM:C60 in solid film.
Table 1
Device X
A
B
C
D
E
F
G
Summary of the structures of the solar cells used is this study
Structure (nm) ITO/PEDOT:PSS
(40 nm)/X/C60 (30 nm)/LiF (1.2 nm)/Al (100 nm)
DPTMM (10 nm)/DPTMM:C60 60 : 40 (30 nm)
DPTMM (15 nm)/DPTMM:C60 60 : 40 (30 nm)
DPTMM (20 nm)/DPTMM:C60 60 : 40 (30 nm)
a-NPB (5 nm)/DPTMM (15 nm)/DPTMM:C60 60 : 40 (30 nm)
a-NPB (5 nm)/DPTMM (15 nm)/DPTMM:C60 55 : 45 (30 nm)
a-NPB (5 nm)/DPTMM (15 nm)/DPTMM:C60 50 : 50 (30 nm)
a-NPB (5)/DPTMM (15 nm)/DPTMM:C60 70 : 30.
DPTMM:C60 50 : 50 (30 nm)
5238 | RSC Adv., 2014, 4, 5236–5242
Table 2 Selected values of n and k at 520 nm used for optical
simulation
Material
Glass
n
k
ITO
PEDOT:
PSS
DPTMM
DPTMM:
C60 (60 : 40)
C60
LiF
Al
1.51
0
2.00
0.01
1.52
0.03
1.81
0.83
1.92
0.39
2.26
0.16
1.41
0
0.71
5.37
This journal is © The Royal Society of Chemistry 2014

4. Paper
RSC Advances
Fig. 3 (a) Distribution of electromagnetic field intensity for bi-layer device (ITO/PEDOT:PSS (40 nm)/DPTMM (30 nm)/C60 (40 nm)/LiF (1.2 nm)/Al
(100 nm)) and BHJ device (ITO/PEDOT:PSS (40 nm)/DPTMM (10 nm)/DPTMM:C60 (60–40%) (30 nm)/C60 (30 nm)/LiF (1.2 nm)/Al (100 nm)) at
520 nm incident wavelength. (b) Profiles of the exciton generation rate versus the x-position in the solar cell structures.
generated exciton was calculated by integration of G(x)
throughout the entire active zone of 190 nm x 260 nm in
both structures. It is found that the total density of generated
excitons is $20% higher in the bilayer structure than that in the
BHJ structure. However, it should be noted that from the total
‘generated’ excitons, only ‘dissociated’ excitons create electron–
hole pairs that nally contribute to the energy conversion. We
can compare the numbers of effective dissociable excitons by
assuming an average exciton diffusion length of 15 nm.40–42
Integration zone was therefore conned to the 15 nm distance
zone from the DPTMM/C60 interface for the bilayer structure
and to the mixed layer plus 15 nm margins for the BHJ structure
(Fig. 3b). The exciton generation by PEDOT:PSS layer was
neglected because generated excitons dissociate ineffectively at
a PEDOT:PSS/DPTMM interface. By using this method it is
found that the maximum dissociable exciton in the BHJ structure is $65% higher than in the bilayer structure.
a situation, an energetic asymmetry leads to a formation of nonzero internal eld, so that the measured J–V characteristics
should be corrected by the corresponding built-in potential
(Vbi).45 Fig. 4b shows that, by assuming Vbi of 0.3 V, the current
injected from the PEDOT:PSS follows a quadratic dependence
on V–Vbi at high voltage, indicating space-charge-limited
conduction (SCLC).46,47 We can t the experimental data in this
regime to the Mott–Gurney law including the voltage correction
term as
9 ðV À Vbi Þ2
J ¼ m3s
8
L3
(1)
where m is the charge-carrier mobility, 3s is the semiconductor
permittivity, and L is the semiconductor thickness. By assuming
a dielectric constant of 3, the extracted hole mobility m of
DPTMM in Fig. 4b is 1.0 Â 10À5 cm2 VÀ1 sÀ1, which favorably
compares to the reported SCLC mobility values of various
organic semiconductors in a diode conguration.48–50
3.2. Mobility measurement on DPTMM
To estimate the hole-transport properties of the DPTMM
molecular lms, we fabricated and analyzed a hole-injection
diode. Fig. 4a shows the current density–voltage ( J–V) characteristics of such a device, containing a 120 nm DPTMM layer. It
is observed that the current is higher when the PEDOT:PSS side
is more positively biased than the Au electrode, which is mainly
attributed to the difference in the injection barriers.43,44 In such
3.3. Optimization and analysis of DPTMM-based solar cells
The performance characteristics of all the photovoltaic devices
in this study are summarized in Table 3. The device performance of these OSCs exhibited good reproducibility, for every
set of experiments several cells were tested (see ESI†).
Fig. 5 shows the illuminated J–V characteristics of the BHJ
solar cells made with DPTMM and C60. Devices A, B and C differ
(a) Current–voltage characteristics of a DPTMM hole-only diode. The inset is the device structure. (b) The current–voltage curve for the
injection from PEDOT:PSS corrected by the built-in potential (Vbi). The mobility extraction is done by fitting the measured data to the Mott–
Gurney law in the high-voltage region.
Fig. 4
This journal is © The Royal Society of Chemistry 2014
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5. RSC Advances
Table 3
Paper
Summary of the device parameters of the fabricated solar
cells
Device
Voc (V)
Jsc (mA cmÀ2)
FF (%)
PCE (%)
A
B
C
D
E
F
G
0.94
0.95
0.88
1.02
1.00
1.00
0.99
5.58
6.29
4.00
5.64
5.46
5.85
6.29
55
49
56
55
60
65
64
3.0
2.9
2.0
3.2
3.3
3.8
4.0
Measured current–voltage characteristics of bulk hetero
junction solar cells with different thicknesses of DPTMM layer under
1sun AM 1.5, (ITO/PEDOT:PSS (40 nm)/DPTMM (10–20 nm)/
DPTMM:C60 (60–40%) (30 nm)/C60 (30 nm)/LiF (1.2 nm)/Al (100 nm)).
Fig. 5
in the thickness of the pure DPTMM layer (10, 15 and 20 nm
respectively) in contact with the BHJ layer. Note that a pure
DPTMM layer was also inserted, and this provides an energy
level gradient favoring hole extraction from the mixed layer.16 It
has, however, been reported that a pure hole transporting layer
can hinder electron transport toward the PEDOT:PSS side.51 A
comparison of devices A, B and C provides an insight into the
effect of DPTMM thickness on photovoltaic characteristics.
When increasing the DPTMM thickness from 10 to 20 nm, a
reasonable outcome would be an increase in the series resistance. In general, the series resistance (Rs) is estimated by the
slope of a J–V curve at the Voc point. However, all three devices in
Fig. 5 have a serious kink effect52 which does not allow the
determination of reliable Rs value at the Voc point. Therefore, we
alternatively dened a quasi-series resistance Rs at 0.7 V for all
three curves, where a reliable linear regression toward the Voc
point is possible. The extracted values are 134, 147, and 248 U
cm2 for devices A, B and C, respectively. It is expected that an
increasing Rs would be linked to a deterioration of other cell
parameters. However, the observed monotonous increase in Rs
does not lead to any simple tendency in Jsc and FF as shown in
Table 1. The performance enhancements are observed in an
increase in Jsc from device A to B and in an increase in FF from
device B to C. It is inferred that the increased DPTMM thickness
from device A to B provides greater light absorption volume,
which leads to a higher Jsc, compensating the negative effect of
5240 | RSC Adv., 2014, 4, 5236–5242
Rs. The increase in FF from device B to C can be explained by
improved charge balance property which is further investigated
hereinaer.
It is thought that the serious kink shape observed in devices
A to C (Fig. 5) is due to the energy level mismatch at the hole
extraction/collection side, mainly due to the deep HOMO level
of DPTMM (À5.96 eV) comparatively to the work function of the
PEDOT:PSS material (À5.2 eV). Another set of experiments was
carried out by adding a buffer layer of (4,40 -bis[N-(1-naphtyl)-Nphenylamino]biphenyl) (a-NPB) to provide an intermediate
HOMO level (À5.4 eV) between PEDOT:PSS and DPTMM.53 A
thickness of ve nanometers was selected from various reports
concerning hole extraction layers.54–56 The effect of inserting a
5 nm a-NPB layer is best seen by comparing device D with device
B (Fig. 5 and 6), both of which have the same DPTMM thickness
and the same BHJ composition. By comparing J–V curves and
the extracted parameters in Table 2, it is observed that the
buffer layer effectively removed the kink and a concomitant
overall performance improvement was achieved. In spite of a
slight reduction in Jsc due to an added bulk resistive component, a promising PCE value of 3.2% was obtained in device D.
It is well established that the donor/acceptor volume ratio in
a BHJ layer has a signicant inuence on photovoltaic characteristics, as this ratio determines hole/electron charge balance
properties.10–12 Keeping the a-NPB buffer layer geometry, we
changed the bulk composition of the DPTMM:C60 layer by
controlling the deposition rate of each material in device D, E,
and F. As shown in Table 1, it is found that reducing the relative
amount of the donor DPTMM leads to a further performance
improvement, mainly through improved Jsc and FF, which gives
a signicant PCE of 3.8%. When decreasing the DPTMM ratio
from 60 to 50%, Jsc may be decreasing due to poorer exciton
generation. However, a better hole–electron balance in the
devices leads to a lesser amount of accumulated space charge in
the electrode. This counter effect provides better charge transport and carrier collection at each electrode, nally leading to a
higher Jsc.57–59
Fig. 6 J–V characteristics of device with a-NPB hole extraction layer
(circle) ITO/PEDOT:PSS (40 nm)/a-NPB (5 nm)/DPTMM (10 nm)/
DPTMM:C60 (60 : 40) (30 nm)/C60 (30 nm)/LiF (1.2 nm)/Al (100 nm)
and gradient doped BHJ device (triangle) ITO/PEDOT:PSS/a-NPB
(5 nm)/DPTMM (15 nm)/DPTMM (70%):C60 (30%).DPTMM (50%):C60
(50%) (30 nm)/C60 (30 nm)/LiF (1.2 nm)/Al (100 nm).
This journal is © The Royal Society of Chemistry 2014

6. Paper
Current density–voltage response plots for gradiently doped
BHJ device. ITO/PEDOT:PSS (40 nm)/a-NPB (5 nm)/DPTMM (15 nm)/
DPTMM (70 nm):C60 (30%).DPTMM (50%):C60 (50%) (30 nm)/C60 (30
nm)/LiF (1.2 nm)/Al (100 nm). Total amount of DPTMM and C60
maintain the ratio of 60:40. Voc ¼ 0.99 V, Jsc ¼ 6.29 mA cmÀ2,
FF ¼ 64%, and PCE ¼ 4.0%.
Fig. 7
As a last development strategy, a BHJ layer with a spatial
gradient in donor/acceptor ratio (detailed in ESI†) was fabricated. This strategy was based on previous reports on a similar
technique, which is known to promote charge generation and
improve transport properties while providing a gradually
varying electronic structure from p- to n-side.60 In addition, a
relatively low Jsc in device F compared to that in device B implies
that there is a need for additional energetic adjustment to
achieve both the optimum exciton harvest and charge balance
properties. Therefore, an average donor : acceptor ratio of
60 : 40 was employed, with a gradient in composition from
DPTMM (70%):C60 (30%) at the anode side to DPTMM
(50%):C60 (50%) at the cathode side. Fig. 7 shows the J–V
characteristics of the gradient BHJ device G in the dark and
under illumination. This device shows an impressive PCE of
4.0% and a Jsc of 6.29 mA cmÀ2. This high performance is a
result of the extensive investigation of geometrical and material
factors in order to improve exciton dissociation, charge balance
and collection properties. The PCE of 4.0% proves that the
DPTMM:C60 system has a high potential for evaporation-based
BJH solar cells when an optimized device structure is employed.
It is worthy to note that in all optimized devices Voc up to 1 V
have been achieved. Such high Voc value is quite unusual for
OSCs devices.
4. Conclusion
In summary, a novel material (DPTMM) was studied in order to
investigate its potential as a donor material in organic solar
cells (OSCs). In this study bulk-heterojunction (BHJ) devices
were fabricated by vacuum co-evaporation process. This study
put emphasis on combining thin-lm characterization and
photovoltaic device optimization schemes. By means of optical
modeling, the BHJ architecture of DPTMM and C60 was shown
to promote exciton dissociation, which in turn led to a signicant improvement in device performance. The use of an a-NPB
buffer layer eliminated the contact-related kink shape near Voc
This journal is © The Royal Society of Chemistry 2014
RSC Advances
point by providing an energetic bridge between PEDOT:PSS and
DPTMM. The charge-balance property was controlled through
various donor/acceptor compositions and a fully optimized
device was fabricated with a composition gradient that favors
both the exciton separation and charge transport balance in the
complex BHJ cell geometry. The maximum obtained PCE was
4.0%, which compares favorably to the state-of-the-art of highperformance OSCs. For all devices, unusual high values of Voc
up to 1 V have been achieved. The overall PCE improvement to
the fully optimized BHJ device clearly shows that there is
signicant room for device design and process optimization to
fully exploring the potential of a given material. This result
demonstrates that simple and easily to produce small conjugated molecules such as PTMM is a promising strategy for highperformance small molecule-based organic solar cells.
Acknowledgements
The work of C. H. Kim was supported by the Vice Presidency for
External Relations (DRE) in Ecole Polytechnique through a PhD
fellowship. The authors would like to thank Dr H. Derbal-Habak
for ellipsometric measurements. The authors gratefully thank
the Renewable Energy Science Technology (REST) Master
program from ParisTech for enabling supervised student access
to the research lab. The Minist`re de la Recherche is acknowle
edged for the PhD grant to A. Leli`ge.
e
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a
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