Crystalline donor
–acceptor conjugated polymers for
bulk heterojunction photovoltaics
Jian-Ming Jiang,aMao-Chuan Yuan,aK. Dinakaran,bA. Hariharanb and Kung-Hwa Wei*a
Molecular engineering of conjugated polymers for tuning their energy bands is an important process in the quest for highly efficient bulk heterojunction (BHJ) polymer photovoltaic devices. One effective approach is to construct a conjugated polymer by conjugating two chemical units possessing different electron donating (donor) and accepting (acceptor) capabilities. Conjugated copolymers featuring donor– acceptor (D/A) subunits are promising materials for solar cell applications because of their tunable energy bands and solubility that can be tailored to the performances of the photovoltaic devices. Under proper processing conditions, the conjugated polymers with rigid and planar D/A segments can undergo self-assembly to form crystalline structures that improve charge carrier mobility and provide better resistance to the permeation of water and oxygen compared to amorphous polymers. Conjugated polymers with D/A structure have been investigated thoroughly over the last few years. In this highlight, we present an overview of recent developments in BHJ organic photovoltaics employing D/A crystalline copolymers as active layer materials for photon-to-electron conversion, with particular emphasis on crystalline D/A polymers featuring newly developed acceptor structures, including thieno [3,4-c]pyrrole-4,6-dione, diketo-pyrrole-pyrrole, bithiazole, thiazolothiazole and thieno[3,2-b]thiophene moieties, and conclude with future perspectives.
Introduction
Organic solar cells provide a path to inexpensive renewable energy, with signicant improvements in processing and manufacturing scalability relative to traditional silicon cells.1–5
In recent years theeld of polymer photovoltaics has experi-enced tremendous advances in (i) the understanding of the underlying photo-physical processes;6 (ii) the development of new materials with tailored energy levels and solubility;7and (iii) controlling the morphology of the active layer of the devices.8To date, bulk heterojunctions (BHJs), where the active layer comprises a blend of electron-donating conjugated poly-mers and electron-accepting fullerene derivatives, have been the most prevalent active layer structures in polymer solar cells (PSCs) exhibiting high power conversion efficiencies.9–16
The power conversion efficiency (PCE) of a solar cell device is determined by the product of its short-circuit current density (Jsc), open-circuit voltage (Voc), and ll factor (FF). In a device
having a BHJ-structured active layer, the open-circuit voltage is typically linearly proportional to the difference in energy between the highest occupied molecular orbital (HOMO) of the polymer and the lowest unoccupied molecular orbital (LUMO)
of the fullerene; therefore, the value of Voc can be increased
either by elevating the LUMO energy level of the fullerene or decreasing the HOMO energy level of the polymer. Low-band gap polymers that are designed to harvest a broader solar spectrum, however, tend to have high-lying HOMOs and low-lying LUMOs; the resulting small difference between the LUMO energy level of the fullerene and the HOMO energy level of the polymer frequently leads to a low value of Voc. Therefore,
opti-mizing the band gap and the energy levels of the polymer is necessary in the quest for BHJ PSCs exhibiting high values of Voc
and Jscsimultaneously.17–20
The HOMO–LUMO band gap (Eg) of the photoactive polymer
can be tuned and modied by varying the molecular weight, bond length alternation, torsion angles, aromatic resonance energy, substituents, and intermolecular interactions. The synthesis of conjugated polymers comprising alternating elec-tron donor (D) and acceptor (A) units has proven to be a particularly efficient method for the production of organic semiconductors for BHJ solar cell applications because of the facile tunability of the HOMO and LUMO energy levels and their solubility. For instance, alternating donor and acceptor units that allow an internal charge transfer process along the conju-gated chain increase the effective resonance length of the p-electrons, leading to smaller bandgaps as a result of facilitated p-electron delocalization through planarization. Through careful design and selection of the donor and acceptor aNational Chiao Tung University, Materials Science and Engineering, Hsinchu,
Taiwan. E-mail: khwei@mail.nctu.edu.tw
bAnna University, MIT Campus, Department of Chemistry, Chennai, India Cite this:J. Mater. Chem. A, 2013, 1,
4415 Received 3rd November 2012 Accepted 20th December 2012 DOI: 10.1039/c2ta00965j www.rsc.org/MaterialsA
Materials Chemistry A
HIGHLIGHT
Published on 20 December 2012. Downloaded by National Chiao Tung University on 28/04/2014 02:07:18.
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molecular units, we can tune the HOMO and LUMO energy levels—and, therefore, the bandgap—of a synthesized conju-gated polymer, because its HOMO and LUMO energy levels are largely localized on the donor and acceptor moieties, respec-tively.15 For instance, a weakly electron-donating unit
conju-gated to a strongly electron-withdrawing unit is necessary to simultaneously decrease the HOMO energy level and the bandgap of a D/A polymer and therefore to increase Vocand Jsc
of the device simultaneously.
Signicant progress has been made in D/A copolymers such that photovoltaic devices containing these polymers are capable of exhibiting high PCEs. On the other hand, the hole mobility of a crystalline conjugated polymer is usually higher than that of an amorphous conjugated polymer and therefore is more comparable with the electron mobility in the active layer, where the electron mobility is dominated by fullerene. Recent studies of crystalline polymer-based solar cells have revealed that the crystallinity of the polymer can signicantly impact the device efficiency: crystalline polymers facilitate charge carrier trans-port, which enhances the FF of the device.21–23In this highlight,
we will review the performances of BHJ solar cells that were based on various crystalline D/A polymers featuring newly developed acceptor structures, including thieno[3,4-c]pyrrole-4,6-dione, diketo-pyrrole-pyrrole, bithiazole, and thiazolothia-zole moieties, and conclude with future perspectives.
Thieno[3,4-c]pyrrole-4,6-dione (TPD)-based
D/A crystalline polymers
The thieno[3,4-c]pyrrole-4,6-dione (TPD) unit is attractive because its compact planar structure benets electron delocal-ization.24,25Moreover, it promotes intra- and chain inter-actions along and between coplanar polymer chains, while its strongly electron withdrawing effect can lead to lower HOMO and LUMO energy levels—a desirable property for increasing the stability and values of Vocin BHJ solar cells. Furthermore,
TPD can impart crystallinity in D/A copolymers when it conju-gates with a planar donor unit and is readily prepared in only a few steps from commercially available compounds.
In 2010, a crystalline D/A copolymer with benzodithiophene (BDT) and TPD units as the donor and the acceptor, respec-tively,PBDTTPD, was synthesized through Pd2(dba)3/P(o-tolyl)3
-mediated Stille coupling.26The photovoltaic properties of the BHJ devices having the device structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly-styrenesulfonate (PEDOT:PSS)/PBDTTPD:PC71BM (w/w, 1 : 2)/LiF/Al exhibited a
value of Jscof 9.8 mA cm2, a value of Vocof 0.85 V, and an FF of
0.66, resulting in a PCE of 5.5%. That study revealed, for therst time, that TPD and related derivatives could be employed to efficiently tailor the optical and electronic properties of gated polymers. A slight chemical modication of the conju-gated backbone can promote both high molecular weight and processability, while allowing tuning of the electronic proper-ties. Hence, a series of high-molecular-weight PBDTTPD that possess either linear or branched alkyl chains (P1–P3) have been investigated for their use in devices having the structure ITO/PEDOT:PSS/polymer:PC61BM (w/w, 1 : 1.5)/LiF/Al.27 The
active layers were spin-coated from chlorobenzene (CB); in some cases, a small amount of 1,8-diiodooctane (DIO), an additive with a high boiling point, was added to optimize the morphology. The devices incorporating the alkyl-substituted TPD-basedPBDTTPD, P1–P3, exhibited power conversion effi-ciency ranging from 4.0 to 6.8%, depending on the structure of the solubilizing alkyl groups. The same type of copolymers that were synthesized with substituted thiophene spacers (alkyl chains facing the BDT unit) between the BDT and TPD units,P4, exhibit decent morphologies and provide PCEs of up to 3.9%.28
At about the same time, D/A copolymers with bithiophene as the donor and TPD as the acceptor,PBTTPD, were synthesized through Pd2(dba)3/P(o-tolyl)3-mediated Stille polymerization.29
PBTTPD exhibited intensive (100), (200), and (300) X-ray diffraction peaks at 3.4, 6.8, and 10.2, respectively, revealing a
highly ordered structure having a d-spacing of 26 ˚A, consistent with the interchain separation dened by its alkyl side chains; a broad feature at 24.6, corresponding to a short distance of 3.6 ˚A, revealedp-stacking between the polymeric backbones. A solar cell device prepared using the polymerPBTTPD/PC61BM
(w/w, 1 : 1.5) and CHCl3as the solvent achieved a PCE of 4.7%
with a high value of Vocof 0.95 V, a value of Jscof 8.0 mA cm2,
and an FF of 0.62. Later, aPBTTPD/PC71BM solar cell device was
prepared with CHCl3 solvent and diiodohexane (DIH), an
additive, which effectively induced higher polymer crystallinity and removed the grain boundary of the large PC71BM-rich
grains, resulting in a more-uniform lm morphology on the mesoscale.30As a result, Jscincreased to 13.1 from 9.1 mA cm2,
and the PCE increased to 7.3% from 5%, as compared to the case that the processing of thePBTTPD/PC71BM active layer was
carried out without the additive. Another polymer,PDTTTPD, comprising 2,5-di(thien-2-yl)thieno[3,2-b]thiophene (DTT) and TPD units, exhibits a relatively high crystallinity, owing to the orderly packing of the backbone because of the incorporation of the rigid fused-thiophene units, and displays excellent thermal stability;31a device incorporatingPDTTTPD and PC
71BM (w/w,
1 : 1) exhibited a PCE of 5.1% (Table 1).
Moreover, a series of new D/A alternating polymers based on C-, Si-, and N-bridged dithiophene moieties as the donor units and TPD as the acceptor units have also been synthesized. The change from a C to a Si to a N atom in the bridged dithiophene induced a redshi in the resulting thin lm absorption spectra. Electrochemical measurements revealed that these polymers possess lower HOMO energy levels relative to the previously reported analogs. The photovoltaic properties of these polymers were investigated in devices having the conguration ITO/ PEDOT:PSS/polymer:PC71BM (w/w, 1 : 2)/Ca/Al, processed with
2 vol% of 1-chloronaphthalene (CN). The highest achievable PCE forPCTTPD consisting of cyclopentadithiophene and TPD was 3.74%.32 Using a similar architecture, Watson et al.
obtained a PCE of 3.15% for a PSC based onPCTTPD with a linear alkyl chain and PC71BM (w/w, 1 : 2).33By changing the
bridging atom from C to Si in the cyclopentadithiophene unit, silole-containing polymers with low-lying HOMO energy levels have been developed to enhance the values of Voc.34–37In
addi-tion, silole-containing polymers that had been applied in photovoltaic devices have also proved to be effective at
Table 1 Thieno[3,4-c]pyrrole-4,6-dione based D/A crystalline polymers
Polymer structure Reference, published year, composition Hole mobility, Voc, Jsc, FF, PCE
PBDTTPD
26, 2010,PBDTTPD/PC71BM, (weight ratio, 1 : 2), DCB N/A, 0.85 V, 9.8 mA cm2, 0.66, 5.5%
27, 2010,P1/PC61BM,P2/PC61BM,P3/PC61BM,
(weight ratio, 1 : 1.5), CB + 2% DIO
N/A, 0.85 V, 11.5 mA cm2, 0.68, 6.8%, (P3-based)
P4
28, 2011,P4/PC61BM, (weight ratio, 1 : 2), CF N/A, 0.89 V, 7.6 mA cm2, 0.57, 3.9%
PBTTPD 29, 2010,PBTTPD/PC61BM, (weight ratio, 1 : 1.5), CF 1.0 104cm2V1s1, 0.95 V, 8.0 mA cm2, 0.62, 4.7% 30, 2011,PBTTPD/PC71BM, (weight ratio, 1 : 1.5), CF + 0.5% DIH N/A, 0.92 V, 13.1 mA cm2, 0.61, 7.3% PDTTTPD 31, 2011,PDTTTPD/PC71BM,
(weight ratio, 1 : 1), CF + 1% DIO
2.2 104cm2V1s1, 0.85 V, 9.0 mA cm2, 0.67, 5.1% PCTTPD 32, 2011,PCTTPD/PC71BM, (weight ratio, 1 : 2), DCB + 2% CN 1.5 103cm2V1s1, 0.80 V, 10.0 mA cm2, 0.47, 3.7% PDTSTPD 38, 2011,PDTSTPD/PC71BM,
(weight ratio, 1 : 2), CB + 3% DIO
1.0 104cm2V1s1, 0.88 V,
12.2 mA cm2, 0.68, 7.3%
improving hole mobility and rendering a higher crystallinity; the packing of polymer chains is enhanced due to the fact that the larger silicon atom results in a longer silicon–carbon bond that modies the geometry of the fused dithiophene unit.37For
instance, a low-band gap polymer, PDTSTPD, prepared by conjugating TPD moieties with electron-rich dithieno[3,2-b:20,30,d]silole (DTS) units, exhibits excellent thermal stability, a broad spectral absorption, a low-lying HOMO energy level and crystalline characteristics, due to the highly planar structures of DTS and TPD units. Manipulating the compositions and modulating the morphologies of the blends allowed optimiza-tion of devices based on thesePDTSTPD/PC71BM blends; the
best-performing device, prepared using CB incorporating 3% DIO, reached a PCE of 7.3%, with a value of Vocof 0.88 V, a value
of Jscof 12.2 mA cm2, and an FF of 0.68.38
Diketo-pyrrole-pyrrole (DPP)-based D/A
crystalline polymers
Diketo-pyrrole-pyrrole (DPP)-based conjugated polymers have been exploited for use in BHJ solar cells. DPP has great potential as a building block for the preparation of low-band gap poly-mers, because of its planar conjugated bicyclic structure and strong electron-withdrawing ability resulting from its two amide groups. ThePDTPDPP39copolymer that incorporates dithieno
[3,2-b:20,30-d]pyrrole (DTP) and DPP units as donor and acceptor, respectively, possessed a low band gap of 1.13 eV and exhibited a low-angle X-ray diffraction peak around 6 that
could be attributed to a lamellar structure in a thinlm state. A photovoltaic device prepared using PDTPDPP/PC71BM (1 : 2,
w/w) and a mixed solvent system of CHCl3/DCB (v/v, 4 : 1)
achieved a low PCE of 2.7%, mainly because of its low value of Voc(0.37 V), even though its value of Jscwas quite high (14.9 mA
cm2) (Table 2).
In another case, a family of semi-random P3HTTDPP copolymers that consist of 3-hexyl thiophene (3HT), thiophene (T) and various contents (5–15%) of the DPP acceptor units was prepared.40 These polymers combine several attractive properties: broad absorption proles, high absorption coeffi-cients, high hole mobilities, and semicrystalline structures, which are induced by a good packing of the backbone units that combine 3-hexyl thiophene and planar DPP units along the in-plane direction and also strongp–p interactions in the out-of-plane directions of polymer backbones. In the active layer of BHJ solar cells that were fabricated with PC61BM,
these polymers exhibited effective lm formation, with opti-mized polymer-to-fullerene ratios that varied based on the content of DPP in the polymer backbone; an efficiency of nearly 5.0% was observed for the device incorporating P3HTTDPP(10%) at a polymer-to-fullerene ratio of 1 : 1.3. A
Table 2 Diketo-pyrrole-pyrrole based D/A crystalline polymers
Polymer structure Reference, published year, composition Hole mobility, Voc, Jsc, FF, PCE
PDTPDPP 39, 2010,PDTPDPP/PC61BM, (weight ratio, 1 : 2), CF/DCB (4 : 1) 5.0 102cm2V1s10.38 V, 14.9 mA cm2, 0.48, 2.7% P3HTTDPP(10%):m ¼ 0.8; n ¼ 0.10; o ¼ 0.10 40, 2011,P3HTTDPP(10%)/PC61BM, (weight ratio, 1 : 1.3), DCB 2.3 104cm2V1s1, 0.57 V, 13.9 mA cm2, 0.63, 4.9% PTDPP 41, 2012,PTDPP/PC70BM,
(weight ratio, 1 : 1), DCB/CF (1 : 4) N/A, 0.63 V, 14.8 mA cm
2, 0.60, 5.6% PFTDPP 42, 2012,PFTDPP/PC61BM, (weight ratio, 1 : 3), CF + 5% CN 7.0 104cm2V1s1, 0.65 V, 14.8 mA cm2, 0.64, 6.5%
polymer with a similar chemical structure, PTDPP, which consists of thiophene and DPP units, is another promising low-band gap material for OPVs41that exhibits good
crystal-linity, because not only the DPP unit but also the thiophene unit has a good planar structure as described before. The morphologies of PTDPP/PC71BM blends cast from mixed
solvents exhibit features with multiple length scales, comprisingbrils of PTDPP that form during the early stages of solvent evaporation. Because of this polymer’s deeper
HOMO energy level, the value of Voc of its optimized device
was 0.63 V, leading to a higher PCE of 5.6%, with a value of Jsc of 14.8 mA cm2 and an FF of 0.6. Another polymer,
PFTDPP, featuring an alternating furan/thiophene (FT) back-bone, was chosen as a model system because of the signi-cant contribution of the furan moiety to the overall polymer solubility.42 A solar cell incorporating PFTDPP/PC
71BM (w/w,
1 : 2) yielded a PCE of 6.5%, with a value of Voc of 0.65 V, a
value of Jscof 14.8 mA cm2, and an FF of 0.64.
Table 3 Bithiazole based D/A crystalline polymers
Polymer structure Reference, published year, composition Hole mobility, Voc, Jsc, FF, PCE
PBDTBTz 51, 2010,PBDTBTz/PC71BM, (weight ratio, 1 : 1), DCB 6.8 104cm2V1s1, 0.86 V, 7.8 mA cm2, 0.57, 3.8% PDTSBTBTz 52, 2011,PDTSBTBTz/PC71BM, (weight ratio, 1 : 1), DCB 6.8 104cm2V1s1, 0.72 V, 8.7 mA cm2, 0.61, 3.8% PBDTBTBTz 52, 2011,PBDTBTBTz/PC71BM, (weight ratio, 1 : 1), DCB 3.1 104cm2V1s1, 0.82 V, 9.0 mA cm2, 0.60, 4.5% PTTBTz 53, 2012,PTTBTz/ICBA,
(weight ratio, 1 : 1), DCB + 3% DIO
6.4 103cm2V1s1, 1.03 V, 8.6 mA cm2, 0.61, 5.4% PDTSTTz-4 56, 2011,PDTSTTz-4/PC71BM, (weight ratio, 1 : 1.3), DCB 7.8 102cm2V1s1, 0.73 V, 11.3 mA cm2, 0.72, 5.9% PCTTTz 57, 2011,PCTTTz/PC71BM, (weight ratio, 1 : 3), CB 1.0 103cm2V1s1, 0.67 V, 11.1 mA cm2, 0.54, 4.0%
Bithiazole (BTz)-based D/A crystalline
polymers
Bithiazole (BTz) is an electron-decient molecule containing two electron withdrawing (C]N) groups and a relatively simple coplanar structure.43–47Incorporation of BTz units can lead to
copolymers exhibiting low HOMO energy levels, a desirable feature for increasing the values of Voc of PSCs.48–50 A D/A
copolymer, PBDTBTz, containing BDT donor units and BTz acceptor units, has been prepared through Pd-catalyzed Stille coupling.51 PBDTBTz exhibits a relatively low HOMO energy level of5.15 eV. The PCE of an optimized PSC incorporating PBDTBTz/PC71BM (w/w, 1 : 1) was 3.8%, with a value of Jscof
7.8 mA cm2, a value of Vocof 0.86 V, and an FF of 0.57. In
addition, the two D/A copolymers PDTSBTBTz and
PBDTBTBTz—containing dithienosilole (DTS) and benzodi-thiophene (BDT) donor units, respectively, BTz acceptor units, and bithiophene (BT) bridges between them—have also been synthesized;52they undergo strong inter-chain interactions and
feature low HOMO energy levels and high hole mobilities. The PCE of an optimized PSC incorporatingPBDTBTBTz/PC71BM
(1 : 1, w/w) reached 4.5%, with a value of Jscof 9.0 mA cm2, a
value of Vocof 0.82 V, and an FF of 0.60. The synthesis of a D/A
crystalline copolymer, PTTBTz, containing thienothiophene donor units and BTz acceptor units, was also reported.53Films ofPTTBTz exhibit decent crystallinity because of the relatively good packing of the polymer chains resulting from the structure similarity between BTz and thiophene-based units. PTTBTz exhibits a band gap of 1.89 eV and a hole mobility of 6.45 103 cm2 V1 s1. The photovoltaic performance of the polymer
was improved signicantly by using DIO as the additive and indene-C60 bisadduct (ICBA) as the acceptor. The PCE of the
optimized PSC incorporating PTTBTz/ICBA (w/w, 1 : 1) was 5.4%, with a high value of Vocof 1.03 V, a value of Jscof 8.6 mA
cm2, and an FF of 0.61.
Relative to BTz, thiazolothiazole (TTz) has a more-rigid and more-coplanar fused ring.54Thus, within the structural motif of
PDTSBTz, replacement of the BTz unit with TTz affords a more-planar copolymer,PDTSTTz.55Through the 4-position alkyl side
chain engineering on a DTS-alt-TTz copolymer, the polymer PDTSTTz-4 was obtained exhibiting a low band gap of 1.76 eV and a high hole mobility.56A PSC incorporating PDTSTTz-4/ PC71BM (1 : 1.3, w/w) exhibited a high PCE of 5.9%, with a value
of Vocof 0.73 V, a value of Jscof 11.3 mA cm2, and an FF of 0.72.
Subsequently, a narrow-band gap copolymer PCTTTz, with asymmetrical alkyl substitution on the cyclopenta[2,1-b:3,4-b0 ]-dithiophene (CT) building blocks and additional hexyl side chains on the thienyl subunits of the TTz constituents, was prepared.57PCTTTz exhibits good solubility and semicrystalline
characteristics because the addition of the extra substituents improved the solubility, and also encouraged the stacking of the polymer. An organic solar cell constructed with PCTTTz/ PC71BM as the active layer afforded a photovoltaic PCE of 4.0%
(Table 3).
Other acceptor based D/A crystalline
polymers
Benzothiadiazole (BT) is one of the strongest electron-with-drawing moieties used widely in PSCs, due to the combination
Table 4 Other acceptor based D/A crystalline polymers
Polymer structure Reference, published year, composition Hole mobility, Voc, Jsc, FF, PCE
PAFDTBT
59, 2011,PAFDTBT/PC71BM,
(weight ratio, 1 : 3), DCB + 0.5% DIO
1.0 102cm2V1s1, 0.89 V,
9.9 mA cm2, 0.70, 6.2%
PTTTBO
63, 2011,PTTTBO/PC61BM,
(weight ratio, 1 : 1), DCB + 0.5% DIO
1.2 103cm2V1s1, 0.85 V, 11.6 mA cm2, 0.54, 5.3%
PBFTT
64, 2011,PBFT/PC71BM,
(weight ratio, 1 : 1.5), DCB + 3% DIO
4.1 104cm2V1s1, 0.74 V,
14.1 mA cm2, 0.69, 7.2%
PTATT
65, 2011,PTAT/PC61BM,
(weight ratio, 1 : 1), CF + 2% DIO
1.7 104cm2V1s1, 0.66 V, 15.0 mA cm2, 0.58, 5.6%
of its electron accepting properties and its ability to adopt a quinoid structure, resulting in low-band gap coplanar poly-mers.58Recently, the 9-alkylidene-9H-uorene (AF)-containing main chain D/A-type polymer PAFDTBT was applied in BHJ PSCs.59A PCE of 6.2%—with a value of V
ocof 0.89 V, a value of Jsc
of 9.9 mA cm2, and an FF of 0.70—was achieved in a PSC based onPAFDTBT/PC71BM, indicating thatPAFDTBT is a promising
candidate for high-efficiency solar cells. Replacing the sulphur atom with an oxygen atom forms benzooxadiazole (BO) and leads to a decrease in both the HOMO and LUMO energy levels of the corresponding polymers relative to that of polymers with BT as the acceptor unit for conjugating with the same donors;60,61this molecular structure can result in not only air-stable polymers but also high values of Vocwhen blended with
fullerenes.62By using BO and thiophene derivatives, a series of crystalline low-band gap conjugated polymers were realized,63 with excellent crystallinity due to the incorporation of the symmetric and planar BO structure, thermal stability and low-lying HOMO energy levels. A device incorporatingPTTTBO and PC61BM (blend weight ratio, 1 : 1), with DIO (0.5 vol%) as an
additive, exhibited a high value of Vocof 0.85 V and a PCE of
5.3%. Another attractive example of a strongly electron-with-drawing unit is thieno[3,2-b]thiophene (TT). The incorporation of these linearly symmetrical and coplanar units of fused rings into a conjugated polymer facilitates the adoption of a low-energy backbone conformation, leading to strong inter-chain interactions and ordered packing. Yu et al. successfully devel-oped an excellent semiconducting polymer, PBFTT, incorpo-rating such units.64 An optimized BHJ solar cell based on a blend lm of PBFTT and PC71BM (1 : 1.5, w/w) exhibited an
efficiency of 7.2%, with a value of Vocof 0.74 V, a value of Jscof
14.1 mA cm2, and an FF of 0.69. The copolymerPTATT, con-taining an extendedp-conjugated system, in which the tetra-thienoanthracene (TA) unit is conjugated with T acceptor, has been reported.65 The TA moiety features two-dimensional
extended p-conjugated structures that provide enhanced co-facialp–p stacking and facilitates charge carrier transport in the polymer. As a result, in thePTATT/PC61BM blendlms, the
optimized device exhibited a PCE of 5.6%, with a value of Vocof
0.66 V, a high value of Jscof 15 mA cm2and an FF of 0.58
(Table 4).
Conclusions
Great improvement in the device efficiency of donor–acceptor (D/A) polymer:fullerene bulk heterojunction solar cells can be achieved by tuning the polymers’ energy bands and solubility, through molecular engineering of both electron donor and acceptor segments. We have shown in this highlight that by adopting fused electron-accepting heterocyclic rings featuring coplanar congurations to conjugate with the appropriate donor units, the energy bands of the D/A conjugated polymers such as the HOMO energy level and the bandgap of a D/A copolymer can be decreased, which in turn increase the values of both Vocand Jscof the device simultaneously. Additionally,
the fused heterocyclic rings can promote strong intra- and inter-chain interactions and impart crystalline characteristics in the
synthesized polymers for facilitating charge transport. High power conversion efficiency BHJ crystalline polymer solar cells have been obtained through the incorporation of fused heterocyclic rings such as thieno[3,4-c]pyrrole-4,6-dione, diketo-pyrrole-pyrrole, bithiazole, thiazolothiazole and thieno[3,2-b] thiophene in the polymer backbones. The future perspectives on the structural design of conjugated polymers for photovol-taics are (i) to rationally construct a new acceptor on the basis of the fused heterocyclic structure inspired by dyes and pigments; (ii) to substitute a specic functional group or atom, such as electron-withdrawing ester, ketone, imine, nitrile, imide or uorine, on an acceptor for lowering the HOMO energy level of a polymer; (iii) to attach proper alkyl chains to the polymer backbone for enhancing the molecular weight, solubility, and packing of the conjugated polymer; and (iv) to develop a cross-linkable conjugated polymer for a greater thermal stability.
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![Table 1 Thieno[3,4-c]pyrrole-4,6-dione based D/A crystalline polymers](https://thumb-ap.123doks.com/thumbv2/9libinfo/7534901.120288/3.892.61.826.86.1101/table-thieno-pyrrole-dione-based-d-crystalline-polymers.webp)
