Solution processed small molecule bulk heterojunction organic photovoltaics based on a conjugated donor-acceptor porphyrin

Article abstract of DOI:10.1039/c2jm34429g

Due to the π-conjugation of the whole molecule along with the push-pull property of a newly designed conjugated donor-acceptor porphyrin, the solution processed solar cells using the blend of the porphyrin small molecule with PC₇₁BM exhibit a p

Full text of DOI:10.1039/c2jm34429g

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Solution processed small molecule bulk heterojunction organic photovoltaics
based on a conjugated donor–acceptor porphyrin†
*
Yuying Huang, Lisheng Li, Xiaobin Peng, Junbiao Peng and Yong Cao
Received 7th July 2012, Accepted 6th September 2012
DOI: 10.1039/c2jm34429g
plays a very important role in the significant progress.11 The D–A
Due to the p-conjugation of the whole molecule along with the push–
pull property of a newly designed conjugated donor–acceptor
porphyrin, the solution processed solar cells using the blend of the
porphyrin small molecule with PC71BM exhibit a promising
performance with a power conversion efficiency of up to 4.02%.
molecules were considered as efficient donor materials for OSCs
because of several key advantages:14 (1) the internal charge transfer
(ICT) is facilitated by the push–pull property of a D–A structure; (2)
the more efficient ICT leads to more desirable double-bond charac-
teristics of the p-electrons and therefore a smaller band gap; and (3)
the highest occupied molecular orbital (HOMO) and lowest unoc-
cupied molecular orbital (LUMO) energy levels and the band gap of
a D–A molecule can be easily tuned by introducing various electron-
donors or acceptors into the molecule.15
Most natural photosynthetic systems utilize chlorophylls to absorb
light and carry out photochemical charge separation to store energy,
and this remarkable process provides all of our food and most of our
energy resources.1 Porphyrins were studied earlier as photoactive
materials for organic photovoltaics (OPVs) partly because the
porphyrin structure is a synthetically tractable analogue of chloro-
phylls.2 Furthermore, porphyrins have extensively p-conjugated
systems, are amenable to fast electron transfer to acceptors, and
absorb light with high molar absorption coefficients, and their
properties can be tuned easily via synthetic modifications of the
periphery or by metal insertion into the cavity.3 These physical and
chemical properties make porphyrins suitable for OPV studies, and
significant efforts have been devoted to porphyrin-based organic
solar cells (OSCs) since porphyrins may have a significant potential to
provide major efficiency breakthroughs.4–7 Indeed porphyrins have
been successfully employed as organic dyes in dye-sensitized solar
cells (DSSCs).5 However, the studies on porphyrin bulk heterojunc-
tion (BHJ) OPVs, which show many advantages over DSSCs, have
been less successful, especially the reports on the BHJ OPVs based on
solution-processed porphyrin small molecules as the donors are very
limited.8,9
It was reported that the short exciton diffusion length or the low
charge carrier mobilities of porphyrins are likely the root of the low
efficiencies of porphyrin-based BHJ OSCs.16–18 After a close investi-
gation of the reports on porphyrin BHJ OSCs, we found that a
conjugated molecule and a D–A structure were not considered
simultaneously for the design of porphyrin-related BHJ OPV mate-
rials,19–22 and the strategy of conjugated D–A molecules, which is
being applied in OSCs very successfully, is almost unemployed in
porphyrin OSCs though this approach has been applied to porphyrin
two-photon absorption studies.23 Perhaps unsurprisingly, enhanced
performance can be anticipated in OSCs of the conjugated D–A
porphyrin derivatives.19,20,23–27
In this context, the very typical acceptor 2,1,3-benzothiadiazole
(BT) end-capped with 3-hexylthienyl (TBT) was linked by ethyny-
lenes to a porphyrin core to form 5,15-bis(7-(4-hexyl-thiophen-2-yl)-
2,1,3-benzothiadiazole-4-yl-ethynyl)-10,20-bis(3,5-di(dodecyloxy)-
phenyl)-porphyrin zinc (DHTBTEZP, Scheme 1). The ethynylenes
can make the whole molecule planar so that the whole molecule
is p-conjugated. The p-conjugation of the whole molecule along
with the push–pull property of the porphyrin core and BT
moieties can facilitate the ICT.19 The solution processed OSC
using the blend of DHTBTEZP with phenyl C71-butyric acid
methyl ester (PC71BM) exhibited a promising performance with
a PCE of up to 4.02%. For comparison, two TBTs were linked to
a porphyrin core directly (without ethynylene linkages) to form
5,15-bis(7-(4-hexyl-thiophen-2-yl)-2,1,3-benzothiadiazole-4-yl)-
10,20-bis(3,5-di(dodecyloxy)-phenyl)-porphyrin zinc (DHTBTZP,
Scheme 1), and the performance of the OSC of DHTBTZP with
PC71BM was also investigated.
Solution processed small molecules for BHJ OSCs are attractive
due to several advantages superior to their polymer counterparts,
such as a defined molecular structure, a definite molecular weight,
and high purity without batch to batch variations, showing great
prospect in low-cost and large-scale OSC commercialization appli-
cations.10,11 The power conversion efficiencies (PCEs) of solution
processable small molecule based OSCs have steadily improved over
the past decade from about 0.03%12 to over 6.7%,13 and the approach
of using the conjugated donor–acceptor (D–A) molecular structure
Institute of Polymer Optoelectronic Materials and Devices, State Key
Laboratory of Luminescent Materials and Devices, South China
University of Technology, 381 Wushan Road, Guangzhou 510640, China.
E-mail: chxbpeng@scut.edu.cn
DHTBTEZP and DHTBTZP, which were synthesized by Sono-
gashira coupling reaction and Suzuki coupling reaction, respectively
(see ESI†), show good solubility in common organic solvents such as
chloroform, THF, and toluene, and can be readily processed to form
† Electronic supplementary information (ESI) available: Experimental
section and detailed results. See DOI: 10.1039/c2jm34429g
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J. Mater. Chem., 2012, 22, 21841–21844 | 21841

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p-conjugation between the BT and porphyrin units in DHTBTEZP
to enable the p-electrons to be delocalized within the whole molecule,
which was demonstrated by the ab initio calculations reported below.
The absorption spectrum of a DHTBTEZP thin film versus that
in solution showed a red-shift of 35 nm at the Q band region,
which was attributed to the strong intermolecular interaction in
the condensed solid state. From the onset of the absorption
spectrum of the film (957 nm), the optical energy band gap of
DHTBTEZP was estimated to be 1.3 eV, which belongs to typi-
cally low band gaps. Electrochemical properties of DHTBTEZP
were investigated by cyclic voltammetry (CV) in solutions of
Bu4NPF6 (0.1 M) in acetonitrile at a scan rate of 50 mV sꢁ1 at
room temperature under the protection of argon. The onset
oxidation potential (Eox) was 0.8 eV vs. Fe/Fe2+ (ESI†). The
HOMO energy level of DHTBTEZP was estimated to be ꢁ5.2 eV
according to the empirical formula EHOMO ¼ ꢁe(Eox + 4.4) (eV),
and the LUMO energy level was calculated to be ꢁ3.9 eV from the
corresponding HOMO energy level and the optical energy gap.
The BHJ OSCs were fabricated by spin-coating a DHTBTEZP or
DHTBTZP/PC71BM (1 : 3 w/w) solution in 1,2-dichlorobenzene/
Scheme 1 The chemical structures of DHTBTEZP and DHTBTZP.
33–35
chlorobenzene (DCB/CB) with 1 vol% pyridine as an additive
onto a PEDOT/PSS layer in a conventional device configuration of
ITO/PEDOT:PSS (40 nm)/DHTBTEZP or DHTBTZP:PC71BM
(1 : 3, w/w, 60 ꢂ 2 nm)/PFN/Al (80 nm) (ITO: indium tin oxide;
PEDOT:PSS: poly(styrene sulfonate)-doped poly(ethylene-dioxy-
thiophene); PFN: poly[(9,9-bis(30-(N,N-dimethylamino)propyl)-2,7-
fluorene)-alt-2,7-(9,9-dioctylfluorene)). A detailed device fabrication
process is described in the Experimental section (see ESI†). The
devices based on DHTBTEZP exhibited promising photovoltaic
properties with an average PCE (from eight devices) of 3.86% under
simulated AM 1.5G illumination (100 mW cmꢁ2). Fig. 2a shows the
current density–voltage (J–V) curve of a device with a short-circuit
current density (Jsc) of 9.46 mA cmꢁ2, an open-circuit voltage (Voc) of
0.85 V, and a fill factor (FF) of 50%, resulting in a PCE of 4.02%.
Although a PCE of up to 5.2% was reported for an OSC based on
tetrabenzoporphyrin as the donor and bis(dimethylphenylsi-
lylmethyl)[60]fullerene (SIMEF) as the acceptor, the PCE was only
2.0% when PCBM was used in place of the SIMEF acceptor through
thermally managing the precursor.9 The PCE reported here is the
highest PCE for BHJ OSCs based on the solution-processed
porphyrin small molecules as the donors and PCBM as the acceptors
reported so far.22,28 Compared with the parameters of non-func-
tionalized or non-conjugated porphyrin OSCs, all these values of Voc,
Jsc and FF are improved for the DHTBTEZP-based OSCs.
smooth and pinhole-free films upon spin-coating. The 3,5-bis(dode-
cyloxy)phenyl groups at meso-positions 10 and 20 of the zinc
porphyrin ring of DHTBTEZP and DHTBTZP play an important
role in solubilizing. The thermal properties of DHTBTEZP were
determined by thermogravimetric analysis (TGA), which exhibited
an onset decomposition temperature at 416 ꢀC (see ESI†), indicating
that this porphyrin has good thermal stability.
The UV-vis absorption spectra of DHTBTEZP and DHTBTZP in
CH2Cl2 and thin-films are shown in Fig. 1. In solution, DHTBTZP
exhibited a Soret band at 423 nm and very weak Q bands at 550 and
590 nm, which are quite similar to those of non-functionalized
porphyrins, indicating that the ICT in DHTBTZP is very weak. In
contrast to the absorption spectrum of DHTBTZP, DHTBTEZP
exhibits two broad peaks centered at 511 and 801 nm in CH2Cl2 and
the intensity of absorbance of the peak at 801 nm dramatically
increased. The peaks at 511 and 801 nm were assigned to the
porphyrin characterized Soret band and Q band, respectively. The
dramatical red-shifts of ca. 90 nm at the Soret band and ca. 250 nm at
Q bands along with the significant increase of the absorbance at the Q
bands were contributed by the efficient ICT within DHTBTEZP. The
difference was attributed to the co-planarity and therefore the
Fig. 2 (a) Current density–voltage (J–V) curve of DHTBTEZP or
DHTBTZP/PC71BM based solar cell devices under AM1.5G illumina-
tion (100 mW cmꢁ2) and (b) external quantum efficiency curve of the
DHTBTEZP/PC71BM devices.
Fig. 1 UV-vis absorption spectra of DHTBTEZP and DHTBTZP in
CH2Cl2 solution and thin films.
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In contrast, the device based on DHTBTZP only exhibited a PCE
of 0.71% with a Jsc of 2.81 mA cmꢁ2, a Voc of 0.88 V, and a FF of
28.7%. Although the Voc is similar to those of the DHTBTEZP-based
OSCs, the Jsc and FF are much lower probably due to the lack of
efficient ICT in DHTBTZP as shown by the absorption spectra.
The external quantum efficiency (EQE) of a device based on
DHTBTEZP is also shown in Fig. 2b. Similar to the absorption curve
(see ESI, Fig. S4†), the DHTBTEZP/PC71BM device exhibited a
broad and high response ranging from 380 nm to 950 nm, suggesting
that most of the light energy absorbed by the DHTBTEZP/PC71BM
blend film is to some extent converted into electricity. Therefore,
much more photons can be converted into electrons leading to a high
photovoltaic efficiency. The highest EQE value is 59% at 500 nm, and
the EQE value at 870 nm is 12%. The Jsc calculated from the EQE
curve is 9.3 mA cmꢁ2, which matches the measured value of 9.46 mA
cmꢁ2. We also tried to measure the EQE of the device based on
DHTBTZP, but could not obtain due to the very low Jsc.
Fig. 4 The optimum geometry and electron-state-density distribution of
DHTBTZP: (a) geometry, (b) LUMO and (c) HOMO.
values derives from many factors, including the measurement
methods, the porphyrin structures, as well as the material purities and
the aggregation properties. Hence, a detailed hole mobility study of
the DHTBTEZP or DHTBTZP/PC71BM blend film was conducted
by using the electric-field dependent space-charge-limited-current
(SCLC) method, which has been widely used to estimate the charge
transporting ability of the OSC active layers.32 As shown in Fig. 5, the
blend film based on DHTBTEZP exhibited a moderate hole mobility
of 7.4 ꢃ 10ꢁ6 cm2 Vꢁ1 sꢁ1, which is much higher than that based on
As shown in Fig. 3 and 4, the optimum geometries and electron-
state-density distributions of the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO) of
DHTBTEZP and DHTBTZP were investigated by performing
density functional theory (DFT) calculations at the B3LYP/(6-
31G(D)+LANL2DZ) level using the Gaussian 09 program suite.29 Ab
initio calculations indicated that DHTBTEZP is planar and the
p-electrons are delocalized in the whole molecule both at LUMO and
HOMO. However, the BT units are almost perpendicular to the
porphyrin moiety and the p-electrons are almost confined to the
porphyrin moiety at HOMO and to BT units at LUMO in
DHTBTZP. It is also worthy to note that the thiophene units are
coplanar with the BTs and there are some p-electron distributions at
LUMOs in both DHTBTEZP and DHTBTZP. All these results
suggested that the co-planarity of the units in DHTBTEZP enable the
whole molecule to be conjugated and the p-electrons to be delocalized
within the whole molecule. The highly delocalized p-electrons along
with the push–pull property of the D–A structure are beneficial for the
efficient ICT as revealed by its absorption spectrum. In contrast, due
to the perpendicular conformation, the p-electrons could not be
delocalized efficiently in DHTBTZP and thus led to poor ICT even if
DHTBTZP was a D–A molecule. A more efficient ICT should lead to
a higher charge mobility,13,14,36,37 which is very important in OPVs.
Literature values for hole mobilities in porphyrins vary tremen-
DHTBTZP with the hole mobility of 2.5 ꢃ 10ꢁ9 cm2 Vꢁ1 sꢁ1
,
probably due to the more efficient p-electron conjugation and
therefore the delocalized p-electrons in DHTBTEZP as revealed by
ab initio calculations and the more efficient ICT shown by the
absorption spectra. Owing to the high charge-carrier mobility, an
efficient charge transport in the OSCs is anticipated that contributes
to the high PCEs of the DHTBTEZP-based OSCs.
Several factors contribute to the excellent photovoltaic properties
of DHTBTEZP. Firstly, the whole molecule became planar after the
introduction of ethynylene linkages and the planarity of the whole
molecule is the premise for the efficient p-conjugation and highly
delocalized p-electrons. Secondly, ICT was facilitated by the push–
pull property of the D–A structure and the p-conjugation of the
whole molecule, which contributed to not only the high hole mobility
and high FF but also to a small band gap to absorb more light energy
to generate the high Jsc. Finally, both DHTBTZP and DHTBTEZP-
based OSC devices exhibited a relatively higher Voc (0.85 V)
compared to most of the previously reported porphyrin-based OSCs,
which was also attributed to the D–A structures.
dously from 10ꢁ16 to 10ꢁ1 cm2 Vꢁ1
s
ꢁ1, and most of the reported
values are below 10ꢁ8 cm2 Vꢁ1 sꢁ1 17,25,26,30,31 The discrepancy in the
.
Fig. 5 Variations of the current density with the Vapplied–Vbi–Vs value
for the film based on DHTBTEZP or DHTBTZP/PC71BM at a 1 : 3
weight ratio.
Fig. 3 The optimum geometry and electron-state-density distribution of
DHTBTEZP: (a) geometry, (b) LUMO and (c) HOMO.
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In conclusion, a new conjugated D–A porphyrin small molecule
has been designed and synthesized. The typical acceptor BT end-
capped with 3-hexylthienyl was linked by an ethynylene bridge to a
porphyrin core. Due to the p-conjugation of the whole molecule and
the push–pull property of the porphyrin core and BTs after the
introduction of ethynylene and the BT acceptor, the porphyrin
showed significant red-shifts both at the Soret band and Q bands and
enhanced absorbance at the Q bands of the absorption spectrum. The
BHJ OSCs fabricated by solution spin-coating with DHTBTEZP as
the donor and PC71BM as the acceptor showed a Jsc of 9.46 mA
cmꢁ2, a Voc of 0.85 V, and a FF of 50%, resulting in a PCE of 4.02%.
This is the highest PCE for BHJ OSCs based on the solution-
processable porphyrin small molecules as the donors and PCBM as
the acceptor reported so far. This study shows that the performance
of porphyrin-based OSCs can be significantly improved upon suit-
able molecular design, indicating that porphyrins are promising for
the application in BHJ OSCs.
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Acknowledgements
This work was financially supported by the grants from the National
Natural Science Foundation of China (51073060, 50990065, and
51010003) and International Science and Technology Cooperation
Program of China (2010DFA52150).
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