Paper
Journal of Materials Chemistry A
materials show very good acceptor properties compared with their purication except for tetrahydrofuran and toluene, which were
non-fullerene counterparts due to their high electron affinity, dried and puried by distillation over sodium and under N2.
electron mobility, tuneable solubility in organic solvents, tuneable Other solvents were used without additional purication.
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 tetrauoroborate 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 conguration: 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 congurations 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
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. A, 2014, 2, 6916–6921 | 6917