Tailorable PC71BM Isomers: Using the Most Prevalent Electron Acceptor to Obtain High‐Performance Polymer Solar Cells

Despite being widely used as electron acceptor in polymer solar cells, commercially available PC₇₁BM (phenyl‐C₇₁‐butyric acid methyl ester) usually has a “random” composition of mixed regioisomers or stereoisomers. Here PC₇₁BM has been isolated into three typical isomers, α‐, β₁‐ and β₂‐PC₇₁BM, to establish the isomer‐dependent photovoltaic performance on changing the ternary composition of α‐, β₁‐ and β₂‐PC₇₁BM. Mixing the isomers in a ratio of α/β₁/β₂=8:1:1 resulted in the best power conversion efficiency (PCE) of 7.67 % for the polymer solar cells with PTB7:PC₇₁BM as photoactive layer (PTB7=poly[[4,8‐bis[(2‐ethylhexyl)oxy]benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl][3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl]thieno[3,4‐b]thiophenediyl]]). The three typical PC₇₁BM isomers, even though sharing similar LUMO energy levels and light absorption, render starkly different photovoltaic performances with average‐performing PCE of 1.28–7.44 % due to diverse self‐aggregation of individual or mixed PC₇₁BM isomers in the otherwise same polymer solar cells.     

Low‐bandgap quinoxaline‐based D–A‐type copolymers: Synthesis,characterization, and photovoltaic properties

Three classes of quinoxaline (Qx)‐based donor–acceptor (D–A)‐type copolymers, poly[thiophene‐2,5‐diyl‐alt‐2,3‐bis(4‐(octyloxy)phenyl‐quinoxaline‐5,8‐diyl] P(T‐Qx), poly{4,8‐bis(2‐ethylhexyloxy)benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl‐alt‐2,3‐bis(4‐(octyloxy)phenyl‐quinoxaline‐5,8‐diy} P(BDT‐Qx), and poly{4,8‐bis(2‐ethylhexyloxy)benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl‐alt‐(5′,8′‐di‐2‐thienyl‐2,3‐bis(4‐octyloxyl)phenyl)‐quinoxaline‐5,5‐diyl} P(BDT‐DTQx), were synthesized via a Stille coupling reaction. The Qx unit was functionalized at the 2‐ and 3‐positions with 4‐(octyloxy)phenyl to provide good solubility and to reduce the steric hindrance. The absorption spectra of the Qx‐containing copolymers could be tuned by incorporating three different electron‐donating moieties. Among these, P(T‐Qx) acted as an electron donor and yielded a high‐performance solar cell by assuming a rigid planar structure, confirmed by differential scanning calorimetry, UV–vis spectrophotometer, and density functional theory study. In contrast, the P(BDT‐Qx)‐based solar cell displayed a lower power conversion efficiency (PCE) with a large torsional angle (34.7°) between the BDT and Qx units. The BDT unit in the P(BDT‐DTQx) backbone acted as a linker and interfered with the formation of charge complexes or quinoidal electronic conformations in a polymer chain. The PCEs of the polymer solar cells based on these copolymers, in combination with [6,6]‐phenyl C70 butyric acid methyl ester (PC₇₁BM), were 3.3% [P(T‐Qx)], 1.9% [P(BDT‐Qx)], and 2.3% [P(BDT‐DTQx)], respectively, under AM 1.5G illumination (100 mW cm^?2). © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Synthesis and characterization of cyclopentadithiophene‐based low bandgap copolymers containing electron‐deficient benzoselenadiazole derivatives for photovoltaic devices

We have synthesized two cyclopentadithiophene (CDT)‐based low bandgap copolymers, poly[(4,4‐bis(2‐ethyl‐hexyl)‐4H‐cyclopenta[2,1‐b:3,4‐b′]dithiophene‐2,6‐diyl)‐alt‐(benzo[c][1,2,5]selenadiazole‐4,7‐diyl)] (PCBSe) and poly[(4,4‐bis(2‐ethyl‐hexyl)‐4H‐cyclopenta[2,1‐b:3,4‐b′]dithiophene‐2,6‐diyl)‐alt‐(4,7‐dithiophen‐2‐yl‐benzo[c][1,2,5]selenadiazole‐5,5′‐diyl)] (PCT2BSe), for use in photovoltaic applications. Through the internal charge transfer interaction between the electron‐donating CDT unit and the electron‐accepting benzoselenadiazole, we realized exceedingly low bandgap polymers with bandgaps of 1.37–1.46 eV. The UV–vis absorption maxima of PCT2BSe were subjected to larger hypsochromic shifts than those of PCBSe, because of the distorted electron donor–acceptor (D–A) structures of the PCT2BSe backbone. These results were supported by the calculations of the D–A complex using the ab initio Hartree‐Fock method with a split‐valence 6‐31G* basis set. However, PCT2BSe exhibited a better molar absorption coefficient in the visible region, which can lead to more efficient absorption of sunlight. As a result, PCT2BSe blended with [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) exhibited a better photovoltaic performance than PCBSe because of the larger spectral overlap integral with respect to the solar spectrum. Furthermore, when the polymers were blended with PC₇₁BM, PCT2BSe showed the best performance, with an open circuit voltage of 0.55 V, a short‐circuit current of 6.63 mA/cm², and a power conversion efficiency of 1.34% under air mass 1.5 global illumination conditions. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1423–1432, 2010     

Particle plasmon‐induced charge trapping at heterointerfaces in PCDTBT:PC70BM blends

We investigate the influence of particle plasmons on exciton and charge generation and recombination processes in the blend of poly (9‐(1‐octylnonyl)‐9H‐carbazole‐benzothiadiazole‐4,7‐diyl‐2,5‐thiophenediyl) (PCDTBT) and [6,6]‐phenyl‐C₇₀butyric acid methyl ester (PC₇₀BM). The particle plasmons are generated from gold nanoparticles, which are embedded into PCDTBT:PC₇₀BM blend. For the blend with gold nanoparticles, we observe enhance light harvesting. Despite the enhanced light collection, we find that the quasi‐steady‐state charge generation has not been influenced by the particle plasmons. However, the generation and recombination of long‐lived (sub‐millisecond) polaron paris have been significantly enhanced: from untrapped state in the pristine blend to the trapped state in the gold nanoparticle‐embedded blend. This result implies that the plasmon‐influenced polarons are trapped at the broadband geminate polaron pair (GPP) state. This state acts as an intermediate state, which either leads to the formation of charge transfer excitons (CTXs) or free charge carriers. In our case, the particle plasmon‐influenced polarons are trapped in the GPP state, which leads to the formation of CTXs. For this reason, we do not observe the enhanced charge generation in PCDTBT:PC₇₀BM blend with particle plasmon resonance. Finally, we revealed that the long‐lived polarons mainly resulted from the localization by particle plasmons. The macroscopic modification in the blend film made negligible contributions to this influence. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 940–947     

Synthesis and photovoltaic properties of thieno[3,4‐b]pyrazine or dithieno[3′,2′:3,4;2″,3″:5,6]benzo[1,2‐d]imidazole‐containing conjugated polymers

Novel conjugated polymers composed of benzo[1,2‐b:4,5‐b′]dithiophene and thieno[3,4‐b]pyrazine or dithieno[3′,2′:3,4;2″,3″:5,6]benzo[1,2‐d]imidazole units are synthesized by Stille polycondensation. The resulting polymers display a longer wavelength absorption and well‐defined redox activities. The effective intramolecular charge‐transfer and energy levels of all polymers are elucidated by computational calculations. Bulk‐heterojunction solar cells based on these polymers as p‐type semiconductors and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) as an n‐type semiconductor are fabricated, and their photovoltaic performances are for the first time evaluated. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 1067–1075     

Morphological study of an intrinsically stretchable photovoltaic bulk heterojunction

Thin‐film polymer solar cell consisting of [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC₇₁BM) and poly[[4,8‐bis[(2‐ethylhexyl)oxy]benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl][3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl]thieno[3,4‐b]thiophenediyl]] (PTB7) demonstrates elastic stretchability with the aid of a high boiling point additive, 1,8‐diiodooctane (DIO). The usage of DIO not only helps to form uniformly distributed nanocrystalline grains, but may also create free volumes between the nano‐grains that allow for relative sliding between the nano‐grains. The relative sliding can accommodate large external deformation. Large dichroic ratios of the optical absorption of both PC₇₁BM and PTB7 were observed under large‐strain deformation, indicating reorientation of the nanocrystalline PC₇₁BM and PTB7 polymer chains along stretching direction. The dichroic ratio decreases to nearly 1.0 as the blend was relaxed to 0% strain. Therefore, the nanometer‐size grain blending morphology provides an approach to impart stretchability to organic semiconductors that are otherwise un‐stretchable. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 814–820     

One donor–two acceptor (D–A1)‐(D–A2) random terpolymers containing perylene diimide,naphthalene diimide,and carbazole units

A series of one donor–two acceptor (D–A₁)‐(D–A₂) random terpolymers containing a 2,7‐carbazole donor and varying compositions of perylene diimide (PDI) and naphthalene diimide (NDI) acceptors was synthesized via Suzuki coupling polymerization. The optical properties of the terpolymers are weighted sums of the constituent parent copolymers and all show strong absorption over the 400 to 700 nm range with optical bandgaps ranging from 1.77 to 1.87 eV, depending on acceptor composition. The copolymers were tested as acceptor materials in bulk heterojunction all‐polymer solar cells using poly[(4,8‐bis‐(2‐ethylhexyloxy)‐benzo[1,2‐b;4,5‐b′]dithiophene)‐2,6‐diyl‐alt‐(4‐(2‐ethylhexanoyl)‐thieno[3,4‐b]thiophene)‐2,6‐diyl] (PBDTTT‐C) as the donor material. In contrast to the optoelectronic properties, the measured device parameters are not composition dependent, and rather depend solely on the presence of the NDI unit, where the devices containing any amount of NDI perform half as well as those using the parent polymer containing only carbazole and PDI. Overall this is the first example of a one donor–two acceptor random terpolymer system containing perylene diimide (PDI) and naphthalene diimide (NDI) acceptor units, and demonstrates a facile method of tuning polymer optoelectronic properties while minimizing the need for complicated synthetic and purification steps. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3337–3345     

Control of vertical distribution of thiophene‐based copolymers containing 4,7‐Dithien‐2‐yl‐benzo[C][1,2,5]thiadiazole and 3,6‐Dithien‐2‐yl‐pyrrolo[3,4‐C]pyrrole‐1,4(2H,5H)‐dione as Side Groups for Photovoltaics

Four new D—A type copolymers with 2D‐conjugated side‐chain identified PfToBT, PbToBT, PfTDPP and PbTDPP, containing two acceptors 4,7‐dithien‐2‐yl‐benzo[c][1,2,5]thiadiazole (DTBT), and diketopyrrolopyrrole (DPP) linked by thiophene donors, are obtained using Pd‐catalyzed Stille‐coupling reaction. These polymers show a broad visible‐near‐infrared absorption band (E_g = 1.79–1.66 eV) and possess a relatively low‐lying HOMO level at ?5.34 to ?5.12 eV. All the polymer:PC₇₀BM blend films showed edge‐on structure and have similar d_π‐spacing values. According to the structure of conjugated side‐chain, the vertical distributions of polymer chains and PC₇₀BM within the BHJ (bulk heterojunction) were different. When DPP used as an acceptor, conjugated side chains of the polymer coexisted with PC₇₀BM in same position. The BHJ film prepared from PfToBT, PbToBT had a discontinuous network between polymer and PC₇₀BM, whereas films from PfTDPP and PbTDPP formed continuous and evenly distributed network between them. This optimized vertical morphology promotes hole transport along respective pathways of polymers and fullerenes in the vertical direction, leading to high J_SC. PbTDPP shows PCE up to 2.9% (Jsc of 9.4 mA/cm², Voc of 0.68 V, and FF of 0.44). © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2746–2759     

Isoindigo‐3,4‐Difluorothiophene Polymer Acceptors Yield “All‐Polymer” Bulk‐Heterojunction Solar Cells with over 7 % Efficiency

Poly(isoindigo‐alt‐3,4‐difluorothiophene) (PIID[2F]T) analogues used as “polymer acceptors” in bulk‐heterojunction (BHJ) solar cells achieve >7 % efficiency when used in conjunction with the polymer donor PBFTAZ (model system; copolymer of benzo[1,2‐b:4,5‐b′]dithiophene and 5,6‐difluorobenzotriazole). Considering that most efficient polymer‐acceptor alternatives to fullerenes (e.g. PC₆₁BM or its C₇₁ derivative) are based on perylenediimide or naphthalenediimide motifs thus far, branched alkyl‐substituted PIID[2F]T polymers are particularly promising non‐fullerene candidates for “all‐polymer” BHJ solar cells.     

New selenophene‐based low‐band gap conjugated polymers for organic photovoltaics

Low‐band gap selenophene‐based polymers were synthesized. Their optoelectronic and photovoltaic properties and space‐charge limited currents were compared with those of the related thiophene‐based polymers. The band gaps of the Se‐based derivatives were approximately 0.05–0.12 eV lower than those of their thiophene counterparts. Organic photovoltaic (OPV) devices based on the blends of these polymers and 1‐(3‐methoxycarbonyl)propyl‐1‐phenyl‐[6,6]‐C₇₁ (PC₇₁BM) were fabricated, and the maximum power conversion efficiency of the OPV device based on PSPSBT and PC₇₁BM was 3.1%—with a short‐circuit current density (J_sc) of 9.3 mA cm^?2, an open‐circuit voltage (V_oc) of 0.79 V, and a fill factor of 0.42—under AM 1.5 G illumination (100 mW cm^?2). © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4550–4557     

Synthesis and Photovoltaic Properties of Cyclopentadithiophene‐Based Low‐Bandgap Copolymers That Contain Electron‐Withdrawing Thiazole Derivatives

We have synthesized four types of cyclopentadithiophene (CDT)‐based low‐bandgap copolymers, poly[{4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b:3,4‐b′]dithiophene‐2,6‐diyl}‐alt‐(2,2′‐bithiazole‐5,5′‐diyl)] ( PehCDT‐BT ), poly[(4,4‐dioctyl‐4H‐cyclopenta[2,1‐b:3,4‐b′]dithiophene‐2,6‐diyl)‐alt‐(2,2′‐bithiazole‐5,5′‐diyl)] ( PocCDT‐BT ), poly[{4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b:3,4‐b′]dithiophene‐2,6‐diyl}‐alt‐{2,5‐di(thiophen‐2‐yl)thiazolo[5,4‐d]thiazole‐5,5′‐diyl}] ( PehCDT‐TZ ), and poly[(4,4‐dioctyl‐4H‐cyclopenta[2,1‐b:3,4‐b′]dithiophene‐2,6‐diyl)‐alt‐{2,5‐di(thiophen‐2‐yl)thiazolo[5,4‐d]thiazole‐5,5′‐diyl}] ( PocCDT‐TZ ), for use in photovoltaic applications. The intramolecular charge‐transfer interaction between the electron‐sufficient CDT unit and electron‐deficient bithiazole (BT) or thiazolothiazole (TZ) units in the polymeric backbone induced a low bandgap and broad absorption that covered 300 nm to 700–800 nm. The optical bandgap was measured to be around 1.9 eV for PehCDT‐BT and PocCDT‐BT , and around 1.8 eV for PehCDT‐TZ and PocCDT‐TZ . Gel permeation chromatography showed that number‐average molecular weights ranged from 8000 to 14 000 g mol^?1. Field‐effect mobility measurements showed hole mobility of 10^?6–10^?4 cm² V^?1 s^?1 for the copolymers. The film morphology of the bulk heterojunction mixtures with [6,6]phenyl‐C₆₁‐butyric acid methyl ester (PCBM) was also examined by atomic force microscopy before and after heat treatment. When the polymers were blended with PCBM, PehCDT‐TZ exhibited the best performance with an open circuit voltage of 0.69 V, short‐circuit current of 7.14 mA cm^?2, and power conversion efficiency of 2.23 % under air mass (AM) 1.5 global (1.5 G) illumination conditions (100 mW cm^?2).     

Synthesis and characterization of naphtho[2,1‐b:3,4‐b′]dithiophene‐based polymers with extended π‐conjugation systems for use in bulk heterojunction polymer solar cells

Two novel polymeric semiconductor materials based on naphtho[2,1‐b:3,4‐b']dithiophene (NDT), PNDT‐TTT and PNDT‐TET , were designed and synthesized. These synthesized polymers were tested in bulk heterojunction solar cells as blends with the acceptor [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC₇₁BM). PNDT‐TTT contained tri‐thiophene units, and PNDT‐TET contained bi‐thiophene units coupled by ethylenic linkages. Comparison to the properties of PNDT‐T , which contained single thiophene units, these polymers exhibit red‐shifted absorption spectra as a result of the enhanced conjugation lengths. These effects resulted in high short circuit currents (J_SC) in the organic solar cells. The PNDT‐TET ‐ and PNDT‐TTT ‐based devices exhibited considerably better photovoltaic performances, with power conversion efficiencies of 3.5 and 3.3%, respectively, compared to the PNDT‐T ‐based device (1.3%). © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4742–4751     

Donor–acceptor photovoltaic polymers based on 1,4‐dithienyl‐2,5‐dialkoxybenzene with intramolecular noncovalent interactions

Donor–acceptor (D–A) conjugated polymers bearing non‐covalent configurationally locked backbones have a high potential to be good photovoltaic materials. Since 1,4‐dithienyl‐2,5‐dialkoxybenzene ( TBT ) is a typical moiety possessing intramolecular S…O interactions and thus a restricted planar configuration, it was used in this work as an electron‐donating unit to combine with the following electron‐accepting units: 3‐fluorothieno[3,4‐b]thiophene ( TFT ), thieno‐[3,4‐c]pyrrole‐4,6‐dione ( TPD ), and diketopyrrolopyrrole ( DPP ) for the construction of such D–A conjugated polymers. Therefore, the so‐designed three polymers, PTBTTFT , PTBTTPD , and PTBTDPP , were synthesized and investigated on their basic optoelectronic properties in detail. Moreover, using [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) as acceptor material, polymer solar cells (PSCs) were fabricated for studying photovoltaic performances of these polymers. It was found that the optimized PTBTTPD cell gave the best performance with a power conversion efficiency (PCE) of 4.49%, while that of PTBTTFT displayed the poorest one (PCE = 1.96%). The good photovoltaic behaviors of PTBTTPD come from its lowest‐lying energy level of the highest occupied molecular orbital (HOMO) among the three polymers, and good hole mobility and favorable morphology for its PC₇₁BM‐blended film. Although PTBTDPP displayed the widest absorption spectrum, the largest hole mobility, and regular chain packing structure when blended with PC₇₁BM, its unmatched HOMO energy level and disfavored blend film morphology finally limited its solar cell performance to a moderate level (PCE: 3.91%). © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 689–698     

Synthesis of three new 1‐(2,6‐diisopropylphenyl)‐2,5‐di(2‐thienyl) pyrrole‐based donor polymers and their bulk heterojunction solar cell applications

A series of three new 1‐(2,6‐diisopropylphenyl)‐2,5‐di(2‐thienyl)pyrrole‐based polymers such as poly[1‐(2,6‐diisopropylphenyl)‐2,5‐di(2‐thienyl)pyrrole] ( PTPT ), poly[1,4‐(2,5‐bis(octyloxy)phenylene)‐alt‐5,5'‐(1‐(2,6‐diisopropylphenyl)‐2,5‐di(2‐thienyl)pyrrole)] ( PPTPT ), and poly[2,5‐(3‐octylthiophene)‐alt‐5,5'‐(1‐(2,6‐diisopropylphenyl)‐2,5‐di(2‐thienyl)pyrrole)] ( PTTPT ) were synthesized and characterized. The new polymers were readily soluble in common organic solvents and the thermogravimetric analysis showed that the three polymers are thermally stable with the 5% degradation temperature >379 °C. The absorption maxima of the polymers were 478, 483, and 485 nm in thin film and the optical band gaps calculated from the onset wavelength of the optical absorption were 2.15, 2.20, and 2.13 eV, respectively. Each of the polymers was investigated as an electron donor blending with PC₇₀BM as an electron acceptor in bulk heterojunction (BHJ) solar cells. BHJ solar cells were fabricated in ITO/PEDOT:PSS/polymer:PC₇₀BM/TiO_x/Al configurations. The BHJ solar cell with PPTPT :PC₇₀BM (1:5 wt %) showed the power conversion efficiency (PCE) of 1.35% (J_sc = 7.41 mA/cm², V_oc = 0.56 V, FF = 33%), measured using AM 1.5G solar simulator at 100 mW/cm² light illumination. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

An Oligothiophene–Fullerene Molecule with a Balanced Donor–Acceptor Backbone for High‐Performance Single‐Component Organic Solar Cells

A new balanced donor–acceptor molecule, namely, benzodithiophene (BDT)‐rhodanine‐[6,6]‐phenyl‐C₇₁ butyric acid methyl ester (Rh‐PC₇₁BM) comprising two covalently linked blocks, a p‐type oligothiophene‐containing BDT‐based moiety and an n‐type PC₇₁BM unit was designed and synthesized. The single‐component organic solar cell (SCOSC) fabricated from Rh‐PC₇₁BM molecules showed a power conversion efficiency (PCE) of 3.22 % with an open‐circuit voltage (V_oc) of 0.98 V. These results rank are among the highest values for SCOSCs based on a monomolecular material. In particular, the one‐molecule Rh‐PC₇₁BM device exhibits excellent thermal stability compared to reference Rh‐OH:PC₇₁BM device. The success of our monomolecular strategy can provide a new way to develop high‐performance SCOSCs.     

Synthesis and photovoltaic properties of 2,6‐bis(2‐thienyl) benzobisazole and 4,8‐bis(thienyl)‐benzo[1,2‐B:4,5‐B′]dithiophene copolymers

In an effort to design efficient low‐cost polymers for use in organic photovoltaic cells the easily prepared donor–acceptor–donor triad of a either cis‐benzobisoxazole, trans‐benzobisoxazole or trans‐benzobisthiazole flanked by two thiophene rings was combined with the electron‐rich 4,8‐bis(5‐(2‐ethylhexyl)‐thien‐2‐yl)‐benzo[1,2‐b:4,5‐b′]dithiophene. The electrochemical, optical, morphological, charge transport, and photovoltaic properties of the resulting terpolymers were investigated. Although the polymers differed in the arrangement and/or nature of the chalcogens, they all had similar highest occupied molecular orbital energy levels (?5.2 to ?5.3 eV) and optical band gaps (2.1–2.2 eV). However, the lowest unoccupied molecular orbital energy levels ranged from ?3.1 to ?3.5 eV. When the polymers were used as electron donors in bulk heterojunction photovoltaic devices with PC₇₁BM ([6,6]‐phenyl C₇₁‐butyric acid methyl ester) as the acceptor, the trans‐benzobisoxazole polymer had the best performance with a power conversion efficiency of 2.8%. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 316–324     

Synthesis and Photovoltaic Properties of Thieno[3,4‐c]pyrrole‐4,6‐dione‐based donor–acceptor Copolymers

Three donor–acceptor (D–A) 1,3‐di(thien‐2‐yl)thieno [3,4‐c]pyrrole‐4,6‐dione‐based copolymers, poly{9,9‐dioctylfluorene‐2,7‐diyl‐alt‐1,3‐bis(4‐hexylthien‐2‐yl)‐5‐octylthieno[3,4‐c]pyrrole‐4,6‐dione}, poly{N‐(1‐octylnonyl)carbazole‐2,7‐diyl‐alt‐1,3‐bis(4‐hexylthien‐2‐yl)‐5‐octylthieno[3,4‐c]pyrrole‐4,6‐dione}, and poly {4,8‐bis(2‐ethylhexyloxyl) benzo[1,2‐b:3,4‐b′]dithiophene‐alt‐1,3‐bis(4‐hexylthien‐2‐yl)‐5‐octylthieno[3,4‐c] pyrrole‐4,6‐dione} were synthesized by Suzuki or Stille coupling reaction. By changing the donor segment, the bandgaps and energy levels of these copolymers could be finely tuned. Cyclic voltammetric study shows that the highest occupied molecular orbital (HOMO) energy levels of the three copolymers are deep‐lying, which implies that these copolymers have good stability in the air and the relatively low HOMO energy level assures a higher open‐circuit potential when they are used in photovoltaic cells. Bulk‐heterojunction photovoltaic cells were fabricated with these polymers as the donors and PC₇₁BM as the acceptor. The cells based on the three copolymers exhibited power conversion efficiencies of 0.22, 0.74, and 3.11% with large open‐circuit potential of 1.01, 0.99, and 0.90 V under one sun of AM 1.5 solar simulator illumination (100 mW/cm²). © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Intramolecular donor–acceptor copolymers containing side‐chain‐tethered perylenebis(dicarboximide) moieties for panchromatic solar cells

A series of side‐chain‐tethered copolymers containing the N‐(2‐ethylhexyl)‐N′‐(thiophene‐3‐yl)‐3,4:9,10‐perylenebis(dicarboximide) (thiophene‐PDI) moieties and 4,4‐diethylhexyl‐cyclopenta[2,1‐b:3,4‐b′]dithiophene unit were synthesized via Grignard metathesis polymerizations. With the incorporation of pendent perylenebis(dicarboximide) (PDI) moieties as acceptor side chains and thiophene as the donor backbone, the copolymers exhibited the intramolecular donor–acceptor characteristic and displayed a panchromatic absorption ranging from 290 to 1100 nm and ideal bandgaps of 1.49 to 1.52 eV. Due to the coplanarity of PDI moieties, the charge separation and transfer process were more effective and enhanced after photoexcitation. When increased the weight ratio of PC₆₁BM:polymer to 3, the J_sc could be raised significantly. The value of bandgap decreased slightly, and both V_oc and J_sc showed an upward trend with the increase of molar ratio of thiophene‐PDI unit from 50% (the copolymer P11) to 75% (the copolymer P13). The polymer/PC₆₁BM devices have shown a significant improvement from 0.45 to 1.66% with a judicious modulation. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1978–1988     

Synthesis of phenanthro[1,10,9,8‐cdefg]carbazole‐based conjugated polymers for organic solar cell applications

Low bandgap polymers with dithienylquinoxaline moieties based on 6H‐phenanthro[1,10,9,8‐cdefg]carbazole were synthesized via the Suzuki coupling reaction. Alkoxy groups were substituted at two different positions on the phenyl groups of the quinoxaline units of these polymers: in the para‐position (PPQP) and in the meta‐position (PPQM). The two polymers showed similar physical properties: broad absorption in the range of 400–700 nm, optical bandgaps of ～1.8 eV, and the appropriate frontier orbital energy levels for efficient charge transfer/separation at polymer/PC₇₁BM interfaces. However, the PPQM solar cell achieved a higher PCE due to its higher J_sc. Our investigation of the morphologies of the polymer:PC₇₁BM blend films and theoretical calculations of the molecular conformations of the polymer chains showed that the polymer with the meta‐positioned alkoxy group has better miscibility with PC₇₁BM than the polymer with the para‐positioned alkoxy group because the dihedral angle of its phenyl group with respect to the quinoxaline unit is higher. This higher miscibility resulted in a polymer:PC₇₁BM blend film with a better morphology and thus in a higher PCE. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 796–803     

Comprehensive study of pyrido[3,4‐b]pyrazine‐based D–π–a copolymer for efficient polymer solar cells

Two D–π–A copolymers, based on the benzo[1,2‐b:4,5‐b′]‐dithiophene (BDT) as a donor unit and benzo‐quinoxaline (BQ) or pyrido‐quinoxaline (PQ) analog as an acceptor (PBDT‐TBQ and PBDT‐TPQ), were designed and synthesized as a p‐type material for bulk heterojunction (BHJ) photovoltaic cells. When compared with the PBDT‐TBQ polymer, PBDT‐TPQ exhibits stronger intramolecular charge transfer, showing a broad absorption coverage at the red region and narrower optical bandgap of 1.69 eV with a relatively low‐lying HOMO energy level at ?5.24 eV. The experimental data show that the exciton dissociation efficiency of PBDT‐TPQ:PC₇₁BM blend is better than that in the PBDT‐TBQ:PC₇₁BM blend, which can explain that the IPCE spectra of the PBDT‐TPQ‐based solar cell were higher than that of the PBDT‐TBQ‐based solar cell. The maximum efficiency of PBDT‐TPQ‐based device reaches 4.40% which is much higher than 2.45% of PBDT‐TBQ, indicating that PQ unit is a promising electron‐acceptor moiety for BHJ solar cells. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1822–1833