Carbazole linked phenylquinoline-based fullerene derivatives as acceptors for bulk heterojunction polymer solar cells: Effect of interfacial contacts on device performance

Article abstract of DOI:10.1039/c4ta00013g

To understand the effect of interfacial contact between the hole transporting layer (HTL) and fullerene derivatives in the active layer of bulk heterojunction polymer solar cells (BHJ PSCs), carbazole (Cz) linked phenylquinoline (PhQ)-based fullerene derivatives, PhQHCz-C₆₁BM and PhQEOCz-C₆₁BM, have been successfully synthesized. They are used as acceptors with a poly(3-hexylthiophene) (P3HT) donor in the active layer, and PEDOT:PSS and MoO₃ were used as the HTL. Both the derivatives are highly soluble in common organic solvents and possess high thermal stability. BHJ PSCs are fabricated with configurations of ITO/PEDOT:PSS/P3HT:PhQHCz-C ₆₁BM/LiF/Al, ITO/PEDOT:PSS/P3HT:PhQEOCz-C₆₁BM/LiF/Al, and ITO/MoO₃/P3HT:PhQHCz-C₆₁BM/LiF/Al, ITO/MoO ₃/P3HT:PhQEOCz-C₆₁BM/LiF/Al, and the device characteristics were measured under AM1.5G (100 mW cm^-2). Both derivatives exhibited much lower power conversion efficiencies (PCE) of ～0.1% when PEDOT:PSS was employed as the HTL. In contrast, the PCE increases to ～2.2% upon replacing PEDOT:PSS with MoO₃ as the HTL. This is due to the fact that protonation of the pyridyl nitrogen of the acceptor in the active layer by the -SO₃H group of PEDOT:PSS in the HTL, establishes a charge injection barrier at the interfacial contact and leads to restricted charge collection at the electrodes. This was indirectly confirmed by protonation of pyridyl nitrogen in PhQHCz-C₆₁BM by the -SO ₃H group in p-toluenesulphonic acid.

Full text of DOI:10.1039/c4ta00013g

Journal of
Materials Chemistry A
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PAPER
Carbazole linked phenylquinoline-based fullerene
derivatives as acceptors for bulk heterojunction
polymer solar cells: effect of interfacial contacts on
device performance†
Cite this: J. Mater. Chem. A, 2014, 2,
6916
Pachagounder Sakthivel,‡§a Kakaraparthi Kranthiraja,‡a Chinnusamy Saravanan,‡a
Kumarasamy Gunasekar,a Hong Il Kim,b Won Suk Shin,b Ji-Eun Jeong,c
c
a
*
Han Young Woo and Sung-Ho Jin
To understand the effect of interfacial contact between the hole transporting layer (HTL) and fullerene
derivatives in the active layer of bulk heterojunction polymer solar cells (BHJ PSCs), carbazole (Cz) linked
phenylquinoline (PhQ)-based fullerene derivatives, PhQHCz-C61BM and PhQEOCz-C61BM, have been
successfully synthesized. They are used as acceptors with a poly(3-hexylthiophene) (P3HT) donor in the
active layer, and PEDOT:PSS and MoO3 were used as the HTL. Both the derivatives are highly soluble in
common organic solvents and possess high thermal stability. BHJ PSCs are fabricated with
configurations of ITO/PEDOT:PSS/P3HT:PhQHCz-C61BM/LiF/Al, ITO/PEDOT:PSS/P3HT:PhQEOCz-
C61BM/LiF/Al, and ITO/MoO3/P3HT:PhQHCz-C61BM/LiF/Al, ITO/MoO3/P3HT:PhQEOCz-C61BM/LiF/Al,
and the device characteristics were measured under AM1.5G (100 mW cmꢀ2). Both derivatives exhibited
much lower power conversion efficiencies (PCE) of ꢁ0.1% when PEDOT:PSS was employed as the HTL.
In contrast, the PCE increases to ꢁ2.2% upon replacing PEDOT:PSS with MoO3 as the HTL. This is due to
the fact that protonation of the pyridyl nitrogen of the acceptor in the active layer by the –SO3H group
of PEDOT:PSS in the HTL, establishes a charge injection barrier at the interfacial contact and leads to
restricted charge collection at the electrodes. This was indirectly confirmed by protonation of pyridyl
nitrogen in PhQHCz-C61BM by the –SO3H group in p-toluenesulphonic acid.
Received 2nd January 2014
Accepted 28th January 2014
DOI: 10.1039/c4ta00013g
www.rsc.org/MaterialsA
materials, have attracted great attention since last decade. In
particular, the BHJ formed by the electron donor poly(3-hex-
1. Introduction
ylthiophene) (P3HT) and the electron acceptor [6,6]-phenyl-C61
-
Organic photovoltaics (OPVs) have attracted much attention in
solar cell research due to their broad range of advantages, such
as low cost, exibility, light weight, and large area devices
through roll-to-roll printing and environmental stability, over
traditional solar cell technologies.1–3 In OPVs, bulk hetero-
junction (BHJ) approaches,4 formed by the binary blends of
electron donor (low band gap conjugated polymers or small
molecules) and electron acceptor (fullerene derivatives)
butyric acid methyl ester (PC61BM), has become a benchmark
structure in OPVs with a maximum power conversion efficiency
(PCE) of around 5%.5,6
To further improve the PCE of BHJ organic solar cells (OSCs),
the following two factors are mainly considered by the scientic
community: (1) design of new donor and acceptor materials for
BHJ with suitable energy levels and nano scale morphology; (2)
development of suitable hole and electron transport materials
for interfacial layers to minimize the energy barrier at interfa-
cial contacts for charge-extraction. Accordingly, huge numbers
of conjugated polymers and small molecules have been
synthesized and utilized as electron donors with PCBM as an
electron acceptor in BHJ OSCs and a maximum PCE of ꢁ10%
has been achieved so far.7–9 When compared to the develop-
ments in low band gap conjugated polymers and small mole-
cule based donor materials, the research on acceptor materials
is very scarce. To date, two kinds of acceptor materials have
generally been utilized in BHJ OSCs: (1) fullerene10 and (2) non
fullerene11,12 based materials. In particular, fullerene based
aDepartment of Chemistry Education, Graduate Department of Chemical Materials,
Institute for Plastic Information and Energy Materials, Pusan National University,
Busan, 609-735, Republic of Korea. E-mail: shjin@pusan.ac.kr; Fax: +82 51 581
2348; Tel: +82 51 510 2727
bEnergy Materials Research Center, Korea Research Institute of Chemical Technology,
141 Gajeong-ro, Yuseong-gu, Daejeon 305-600, Republic of Korea
cDepartment of Cogno-Mechatronics Engineering (WCU), Pusan National University,
Miryang, 627-706, Republic of Korea
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ta00013g
‡ These authors contributed equally to this work.
§ Present Address: School of Advanced Sciences, Organic Chemistry Division, VIT
University, Vellore-632 014, Tamil Nadu, India.
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materials show very good acceptor properties compared with their purication except for tetrahydrofuran and toluene, which were
non-fullerene counterparts due to their high electron affinity, dried and puried by distillation over sodium and under N2.
electron mobility, tuneable solubility in organic solvents, tuneable Other solvents were used without additional purication.
energy levels and superior packing arrangement in the solid state by PC61BM was purchased from SES Research Co., Ltd. P3HT was
introducing various functional groups on the fullerene core.10,13 obtained from Rieke Specialty Polymers Co., Ltd. Synthetic
Thus, the development of new fullerene based acceptors toward details and characterization data are described in the ESI.†
increasing the PCE in combination with suitable polymer donors is
still highly desirable. On the other hand, many types of hole14,15 and
2.2 Measurements
electron16,17 transporting materials are employed between the active
1H and 13C NMR spectra were recorded on a Varian Mercury
layer and the indium tin oxide (ITO) substrate and also between the
Plus 300 and 600 MHz spectrometer in CDCl3 using tetrame-
active layer and the metal electrode as hole and electron trans-
thylsilane as an internal standard. The UV-visible absorption
porting layers, respectively, to improve the PCE. Even though
spectra were recorded with a JASCO V-570 spectrophotometer at
research on interlayersin OPVs has not yet been fullycompleted, the
room temperature. The absorption spectra were measured
number of papers has recently increased.18–21 In particular, poly(3,4-
using 10ꢀ5 mol Lꢀ1 chloroform solution. Thermal analyses were
ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a
carried out on a Mettler Toledo thermogravimetric analyzer/
well-known conducting polymer that has been used as a benchmark
simultaneous differential thermal analyzer (TGA/SDTA) 851e
hole transporting layer (HTL) in BHJ OSCs due to its energy level
instrument and a differential scanning calorimetry (DSC) 822e
matching a wide range of low band gap donor polymers/small
analyzer under N2 at a heating rate of 10 C minꢀ1. The cyclic
ꢂ
molecules.22–24 However, the highly acidic and hygroscopic nature of
voltammetry (CV) studies were carried out with a CHI 600C
PEDOT:PSS causes a reduction in device performance and long-
potentiostat (CH Instruments) at a scan rate of 100 mV sꢀ1 in a
term stability.25,26 Very recently, Olson et al. demonstrated that the
0.1 M solution of tetrabutylammonium tetrauoroborate in o-
–SO3H group of PEDOT:PSS in the HTL protonates the basic pyridyl
dechlorobenzene (o-DCB). A platinum wire and Ag/AgNO3 were
nitrogen in the donor material of the active layer, resulting in poor
used as the counter and reference electrodes, respectively. All of
deviceperformance.27 To overcome these challenges, wide band gap
the electrochemical studies were carried out at room tempera-
30
metal oxides such as MoO3,28 V2O5,29 and WO3 were used as
ture. The lowest unoccupied molecular orbital (LUMO) energy
substitutes for PEDOT:PSS. In contrast, the highest reported PCE
levels of the new acceptors were calculated using the equation
value (10.6%) of PSCs consists of PEDOT:PSS as the HTL.31 Thus,
there is still room to mandate the structure–property relationship
when using PEDOT:PSS as the HTL.
onset
LUMO ¼ (E
ꢀ Ef1e/r2rocene + 4.8) eV.34 Where, E1/2 ¼ 0.35 V was
red
calculated from the oxidation and reduction potential of ferrocene.
According to the Ikkala et al. report, each PCBM molecule can
form non-covalent bonds with up to six 4-vinylpyridine monomer
units.32 As a continuation of this work, Mezzenga et al. reported
that rod–coil poly(3-hexylthiophene)-block-poly(4-vinylpyridine)
(P3HT-P4VP) block copolymers blend with PCBM via supramo-
lecular weak interactions, in which the micro-phase segregated
P3HT-rod domains act as electron-donating and the homoge-
neous P4VP-block-PCBM blends act as electron-accepting species,
and they are applied to BHJ PSCs using PEDOT:PSS as the HTL.
In this case, the PCE is decreased due to the hole collection
barrier and/or interfacial dipoles nearer the device anode.33
Therefore, in order to develop new acceptor materials and
explore further structure–property relationships within the
active layer and with the adjacent layer, in this work, we have
introduced basic pyridyl nitrogen on the carbazole (Cz) linked
phenylquinoline (PhQ)-based fullerene derivatives, PhQHCz-
2.3 Charge carrier mobility studies
The apparent charge carrier mobility was evaluated from the
current density–voltage (J–V) characteristics of single charge
carrier devices and the results were subsequently t using the
space-charge-limited-current (SCLC) method. In order to
measure the SCLC of only one type of charge carrier in a blend,
the other one must be suppressed by a large injection barrier,
resulting in an electron or a hole-only device. The hole mobility
was measured using the following device conguration: ITO/
PEDOT:PSS/P3HT:acceptor/MoO3/Al and ITO/MoO3/P3HT:ac-
ceptor/MoO3/Al. The Mott–Gurney law, which includes the eld-
dependent mobility, is described by the following equation:35
!
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
9
ðV ꢀ VbiÞ
V ꢀ Vbi
J ¼ 3r30 m0
exp 0:89b
8
L3
L
C
61BM and PhQEOCz-C61BM, and applied them as new accep-
where 30 is the permittivity of free space (8.85 ꢃ 10ꢀ12 F mꢀ1), 3r
is the dielectric constant (assumed to be 3, which is a typical
value for conjugated polymers), L is the thickness of the active
layer, m0 is the zero-eld mobility, b is the eld activation factor
and Vbi is the built-in voltage due to the difference in work
function of the two electrodes.
tors for BHJ polymer solar cells (PSCs) with a P3HT donor. We
expected the pyridyl and fullerene units to be located in the
same molecule, which led it to self-assemble in a proper way
that might prevent the protonation of the pyridyl nitrogen by
the materials in the adjacent layer.
2. Experimental
2.4 Fabrication of BHJ PSCs
2.1 Materials and characterization
The BHJ PSCs were fabricated with congurations of ITO/
All chemicals and reagents were purchased from Aldrich PEDOT:PSS, MoO3/P3HT:PhQHCz-C61BM or PhQEOCz-C61BM/
Chemical Co., Ltd and TCI and used without further LiF/Al. The glass substrate was coated with a transparent ITO
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electrode (110 nm thick, 10–15 U squareꢀ1) and ultrasonically
cleaned with detergent, distilled water, acetone, and isopropyl
alcohol. It was then subjected to UV/ozone treatment for 20 min
and used as an anode. A thin, 40 nm thick layer of PEDOT:PSS
(Clevios PH1000) was spin-coated onto the ITO glass substrate.
PEDOT:PSS spin-coated lms were annealed at 150 ꢂC for 30
min. The devices with MoO3 (9 nm) as the HTL were thermally
evaporated onto the ITO glass substrate. P3HT:PhQHCz-C61BM
and P3HT:PhQEOCz-C61BM were dissolved in chlorobenzene
(CB) or o-DCB with appropriate weight ratios and stirred for 24
h. The blend solutions were then ltered with a 0.45 mm PTFE
syringe lter, spin-coated on top of the ITO and dried at room
temperature for 30–40 min. LiF (0.5 nm) and the Al cathode (120
nm) were deposited on the top of the active layer under a
vacuum of less than 5.0 ꢃ 10ꢀ6 Torr, to yield an active area of 9
mm2 per pixel. The lm thickness was measured with an a-Step
IQ surface proler (KLA Tencor, San Jose, CA). The performance
of the BHJ PSC devices was measured using a calibrated AM1.5G
solar simulator (Oriel® Sol3A™ Class AAA solar simulator,
models 94043A) with a light intensity of 100 mW cmꢀ2 adjusted
using a standard PV reference cell (2 cm ꢃ 2 cm mono crystal-
line silicon solar cell, calibrated at NREL, Colorado, USA) and a
computer-controlled Keithley 236 source measure unit. The
incident photon to current conversion efficiency (IPCE) spec-
trum was measured using Oriel® IQE-200™. All fabrication
steps and characterizations were carried out in an ambient
Scheme 1 Synthesis of PhQHCz-C61BM and PhQEOCz-C61BM.
of the 2-methoxyethoxyethyl group, which may reduce the
crystallinity of the fullerene derivative. PhQHCz-C61BM and
PhQEOCz-C61BM have glass transition temperatures (Tg) of 216
and 212 ꢂC, respectively, without any crystallization between 25
ꢂ
and 350 C in their DSC thermograms, indicating their amor-
phous nature.
The UV-visible absorption spectra of PhQHCz-C61BM and
PhQEOCz-C61BM in chloroform solution are shown in Fig. 1a.
The absorption spectra of both derivatives are very similar to
each other with a peak maximum around 320 nm. This indi-
cates that there is no electronic effect on the nature of alkyl
chains in the Cz unit. The optical band gaps were calculated
from their absorption edges and found to be 1.71 and 1.73 eV
for PhQHCz-C61BM and PhQEOCz-C61BM, respectively. These
photophysical properties indicate that these materials are
potentially good candidates as electron acceptors for BHJ PSCs.
The electrochemical properties of these derivatives were inves-
tigated by cyclic voltammetry (CV) to determine the highest
occupied molecular orbital (HOMO) and LUMO energy levels.
Fig. 1b shows the cyclic voltammograms of PhQHCz-C61BM and
PhQEOCz-C61BM, and the data are listed in Table 1.
environment without
a
protective atmosphere. While
measuring the J–V curves for the OPV devices, a black mask was
used and only the effective area of the cell was exposed to light
irradiation. The device data reported here were conrmed over
more than 5 iterations under the same conditions.
3. Results and discussion
The synthesis of PhQHCz-C61BM and PhQEOCz-C61BM is out-
lined in Scheme 1. The alkyl and alkoxy groups were introduced
into Cz in the presence of sodium hydride to obtain compounds
1 and 2, subsequent Friedel–Cras acylation yields compounds
3 and 4, respectively. Friedlander condensation of 3 and 4 with
2-aminobenzophenone followed by Friedel–Cras acylation
with methyl 5-chloro-5-oxopentanoate yields 7 and 8, respec-
tively. The keto-ester of 7 and 8 reacted with p-toluenesulfonyl
hydrazide to give 9 and 10, respectively. Finally, the desired
fullerene derivatives, PhQHCz-C61BM and PhQEOCz-C61BM,
were synthesized via the standard 1,3-dipolar cycloaddition
reaction of 9 and 10 with C60. The structure of the fullerene
derivatives was conrmed by 1H, 13C NMR, FTIR and mass
spectral studies (Fig. S1–S7, ESI†) Both PhQHCz-C61BM and
PhQEOCz-C61BM are highly soluble in common organic
solvents such as methylene chloride, tetrahydrofuran (THF),
chloroform, CB, o-DCB, and toluene.
As shown in Fig. 1b, both derivatives exhibit well dened
quasi reversible reduction peaks indicating their n-type semi-
conductor nature. The LUMO energy levels of PhQHCz-C61BM
and PhQEOCz-C61BM were calculated from their onset reduc-
tion potentials and found to be ꢀ3.57 and ꢀ3.62 eV for
The thermal properties of PhQHCz-C61BM and PhQEOCz-
C
61BM were measured by TGA and DSC analysis (Fig. S8 and S9,
ESI†). The TGA data reveal that the 5% weight loss temperatures
of PhQHCz-C61BM and PhQEOCz-C61BM are at 478 and 423 ꢂC,
respectively, indicating their high thermal stability; the former
Fig. 1 (a) UV-visible absorption spectra in CF solution and (b) cyclic
voltammograms in o-DCB at a scanning rate of 100 mV sꢀ1, of
has a higher thermal stability than the latter due to the presence PhQHCz-C61BM and PhQEOCz-C61BM.
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Table 1 Electronic energy levels of PC61BM, PhQHCz-C61BM, and
PhQEOCz-C61BM
onset
Acceptor
PC61BM
E1red (V) E2red (V)
E
red
(V) LUMO (eV) HOMO (eV)
ꢀ0.98 ꢀ1.41 ꢀ0.82
ꢀ3.63
ꢀ3.57
ꢀ3.61
ꢀ5.36
ꢀ5.28
ꢀ5.34
PhQHCz-C61BM ꢀ1.01 ꢀ1.43 ꢀ0.88
PhQEOCz-C61BM ꢀ1.00 ꢀ1.42 ꢀ0.84
PhQHCz-C61BM and PhQEOCz-C61BM, respectively, which are
both higher than the LUMO value of PCBM (ꢀ3.63 eV). Gener-
ally, the fullerene derivative with higher lying LUMO values
improve the Voc of the resulting OPV devices.36 The HOMO
energy levels were calculated from their optical band gap and
LUMO energy levels and the results are summarized in Table 1.
These electrochemical results indicate that PhQHCz-C61BM and
PhQEOCz-C61BM are suitable acceptor materials for BHJ PSCs
with a P3HT donor.
In order to analyze the effect of interfacial contact on the
photovoltaic properties, PhQHCz-C61BM and PhQEOCz-C61BM
were individually blended with P3HT and used as an active layer
with a transparent ITO/PEDOT:PSS or ITO/MoO3 as the front
contact and the reective LiF/Al as the back contact of BHJ PSCs.
The photovoltaic properties were measured under AM1.5G
illumination at 100 mW cmꢀ2. Several processing parameters
such as the P3HT to acceptor weight ratio, active layer thickness
(150 nm), and processing solvent were carefully optimized. The
J–V plots of PhQHCz-C61BM and/or PhQEOCz-C61BM based
devices with PEDOT:PSS as the HTL are shown in Fig. 2 and the
data are listed in Table 2. As can be seen in Table 2, both
PhQHCz-C61BM and PhQEOCz-C61BM show much lower Voc, Jsc,
and FF values yielding a very poor PCE of 0.12 and 0.13%,
respectively. However, devices based on PC61BM acceptors and
P3HT donors with similar processing conditions have a PCE of
2.53%.
This efficiency difference is quite usual and believed to be a
result of restricted charge extraction at the interfacial contact
between the HTL and active layer due to a charge injection/
extraction barrier; this may be caused by the protonation of the
pyridyl nitrogen by a –SO3H group in the PEDOT:PSS. This was
further supported by using MoO3 as the HTL instead of
PEDOT:PSS, with similar device conguration and optimization
conditions, because both had almost similar work functions
(PEDOT:PSS: 5.2 eV and MoO3: 5.3 eV).14 Fig. 2b shows the J–V
plots of the optimized devices and the data are presented in
Table 2. Surprisingly, both derivatives show drastically
improved device performance compared to their PEDOT:PSS
counterparts, as shown in Table 2. Particularly, the PCE of
Fig. 2 J–V plots of devices based on PhQHCz-C61BM and PhQEOCz-
C61BM with (a) PEDOT:PSS and (b) MoO3 as the HTLs.
PhQHCz-C61BM based devices increases from 0.12% to 2.27%
when replacing the HTL PDOT:PSS by MoO3. This conrms the
existence of the charge injection/extraction barrier at interfacial
contacts when PEDOT:PSS was used as the HTL.
The protonation reaction was further conrmed indirectly by
UV-vis absorption spectra of PhQHCz-C61BM in o-DCB upon
addition of p-toluenesulphonic acid (PTSA) as shown in Fig. 3;
one new band appeared at 444 nm, accordingly, the pink col-
oured solution turned yellow, which turned back to its original
colour by the addition of dilute base solution and conrms the
protonation of pyridyl nitrogen by the –SO3H group. To gain
insight about this effect, the hole mobility of PhQHCz-C61BM
and PhQEOCz-C61BM based devices was measured using
PEDOT:PSS and MoO3 as HTLs by the SCLC method and the
resulting data are summarized in Table 3. As can be seen in
Table 3, both PhQHCz-C61BM and PhQEOCz-C61BM show
higher hole mobility values with MoO3 as the HTL compared
with PEDOT:PSS. In addition, we measured the dark current of
the devices based on PhQHC61BM using MoO3 and PEDOT:PSS
as the hole transporting layer (Fig. S9, ESI†). The J–V curves
clearly show that the Voc of the devices based on PEDOT:PSS is
signicantly decreased compared to MoO3, due to the injection
barriers generated at the contacts leading to induced charge
recombination at the HTL/active layer interface.
Among the acceptors, PhQHCz-C61BM based devices show
superior photovoltaic properties compared with PhQEOCz-
C
61BM. This can be explained on the basis of external quantum
efficiency (EQE) and morphological studies. The EQE curves of
PhQHCz-C61BM and PhQEOCz-C61BM based devices are shown
in Fig. 3a. They clearly show that PhQHCz-C61BM based devices
cover a broad wavelength from 400 to 650 nm with a maximum
EQE of 43.6%, which is higher than that of the PhQEOCz-C61BM
based device. This is consistent with the Jsc value obtained from
the J–V measurements. The morphology of P3HT:PhQHCz-
C
61BM and P3HT:PhQEOCz-C61BM blend lms was studied by
Table 2 Photovoltaic properties of PhQHCz-C61BM and PhQHCz-C61BM
P3HT/acceptor ratio
HTL
Voc (V)
Jsc (mA cmꢀ2
)
FF (%)
PCE (%)
PhQHCz-C61BM (1 : 0.8)
PhQEOCz-C61BM (1 : 0.8)
PhQHCz-C61BM (1 : 1)
PhQEOCz-C61BM (1 : 1)
PEDOT:PSS
PEDOT:PSS
MoO3
0.26
0.28
0.62
0.57
2.50
2.27
6.40
6.17
19
20
58
60
0.12
0.13
2.27
2.09
MoO3
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active layer. We demonstrated that there is protonation of the
pyridyl nitrogen of the acceptor in the active layer by the –SO3H
group of PEDOT:PSS in the HTL, which restricts charge collec-
tion at the interface and leads to a drastically lower PCE of
ꢁ0.12%, compared to the PCE of ꢁ2.2% obtained when using
MoO3 as the HTL. This is due to the fact that the protonation
reaction creates a barrier at the interface between the active
layer and the HTL interface, leading to enhanced charge
recombination at the interface. In addition, among the two new
acceptors, PhQHCz-C61BM shows quite high PCE values
compared with PhQEOCz-C61BM, due to the good phase sepa-
ration of the former with P3HT, compared to the latter. Finally,
we conclude that pyridyl nitrogen in either the acceptor or
donor derivatives is not suitable for BHJ OSCs when using
PEDOT:PSS as the HTL.
Fig. 3 Visible colour change of PhQHCz-C61BM upon addition of
PTSA in o-DCB.
Table 3 Hole mobility data for P3HT:acceptor blends
Hole mobility (m)
Acknowledgements
P3HT/acceptor device type
HTL
(cm2 Vꢀ1 sꢀ1
)
This work was supported by a grant fund from the National
Research Foundation of Korea (NRF) of the Ministry of Science,
ICT & Future Planning (MSIP) of Korea (NRF-2011-0028320) and
the Pioneer Research Center Program through the National
Research Foundation of Korea funded by the Ministry of
PhQHCz-C61BM
PhQEOCz-C61BM
PhQHCz-C61BM
PhQEOCz-C61BM
PEDOT:PSS
PEDOT:PSS
MoO3
3.00 ꢃ 10ꢀ6
5.79 ꢃ 10ꢀ6
2.05 ꢃ 10ꢀ4
2.51 ꢃ 10ꢀ4
MoO3
Science, ICT
& Future Planning (MSIP) of Korea (NRF-
2013M3C1A3065522).
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6920 | J. Mater. Chem. A, 2014, 2, 6916–6921
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