Physical mechanisms responsible for the water-induced degradation of PC61BM P3HT photovoltaic thin films

We show that [6,6]-phenyl-C61-butyric acid methyl ester (PC₆₁BM) at the surface of thin film blends of poly(3-hexylthiophene) (P3HT):PC₆₁BM can be patterned by water. Using a series of heating and cooling steps, water droplets condense onto the blend film surface. This is possible due to the liquid-like, water swollen layer of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate. Breath pattern water deformation and subsequent drying on the film surface results in isolated PC₆₁BM structures, showing that migration of PC₆₁BM takes place. This was confirmed by selective wavelength illumination to spatially map the photoluminescence from the P3HT and PC₆₁BM. Within a device, redistribution of the surface PC₆₁BM into aggregates would be catastrophic, as it would markedly alter device performance. We also postulate that repeated volume change of the poly(3,4-ethylenedioxythiophene) polystyrene sulfonate layer by water swelling may be, in part, responsible for the delamination failure mechanism in thin film solar cells devices. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 141–146     

Impact of 1,8-diiodooctane on the morphology of organic photovoltaic (OPV) devices – A Small Angle Neutron Scattering (SANS) study

The impact of the additive 1,8-diiodooctane on the morphology of bulk-heterojunction solar cells based on the systems P3HT:PC₇₁BM, PTB7:PC₇₁BM and PTB7-Th:PC₇₁BM is studied using a combination of Small Angle Neutron Scattering (SANS) and Atomic Force Microscopy (AFM). The results clearly show that while in the P3HT:PC₇₁BM system, the additive DIO promotes a slight coarsening of the phase domains (type I additive), in the systems PTB7:PC₇₁BM and PTB7-Th:PC₇₁BM, DIO promotes a large decrease in the size of the phase domains (type II additive). SANS is demonstrated as being particularly useful at detecting the minor morphological changes observed in the P3HT:PC₇₁BM system, which can be hardly seen in AFM. This work illustrates how SANS complements AFM and both techniques when used together provide a deeper insight into the nanoscale structure in thin organic photovoltaic (OPV) device films.     

Free Carrier Generation in Organic Photovoltaic Bulk Heterojunctions of Conjugated Polymers with Molecular Acceptors: Planar versus Spherical Acceptors

A comparative study of the photophysical performance of the prototypical fullerene derivative PC₆₁BM with a planar small‐molecule acceptor in an organic photovoltaic device is presented. The small‐molecule planar acceptor is 2‐[{7‐(9,9‐di‐n‐propyl‐9H‐fluoren‐2‐yl)benzo[c][1,2,5]thiadiazol‐4‐yl}methylene]malononitrile, termed K12. We discuss photoinduced free charge‐carrier generation and transport in blends of PC₆₁BM or K12 with poly(3‐n‐hexylthiophene) (P3HT), surveying literature results for P3HT:PC₆₁BM and presenting new results on P3HT:K12. For both systems we also review previous work on film structure and correlate the structural and photophysical results. In both cases, a disordered mixed phase is formed between P3HT and the acceptor, although the photophysical properties of this mixed phase differ markedly for PC₆₁BM and K12. In the case of PC₆₁BM the mixed phase acts as a free carrier generation region that can efficiently shuttle carriers to the pure polymer and fullerene domains. As a result, the vast majority of excitons quenched in P3HT:PC₆₁BM blends yield free carriers detected by the contactless time‐resolved microwave conductivity (TRMC) method. In contrast, approximately 85 % of the excitons quenched in P3HT:K12 do not result in free carriers over the nanosecond timescale of the TRMC experiment. We attribute this to poor electron‐transport properties in the mixed P3HT:K12 phase. We propose that the observed differences can be traced to the respective shapes of PC₆₁BM and K12: the three‐dimensional nature of the fullerene cage facilitates coupling between PC₆₁BM molecules irrespective of their relative orientation, whereas for K12 strong electronic coupling is only expected for molecules oriented with their π systems parallel to each other. Comparison between the eutectic compositions of the P3HT:PC₆₁BM and P3HT:K12 shows that the former contains enough fullerene to form a percolation pathway for electrons, whereas the latter contains a sub‐percolating volume fraction of the planar acceptor. Furthermore, the planar K12 co‐assembles with P3HT into a disordered, glassy phase that partly accounts for the poor electron‐transport properties, and may also enhance recombination due to the strong intermolecular interactions between the donor and the acceptor. The implication for the performance of organic photovoltaic devices with the two acceptors is also discussed.     

Effect of side‐chain positions on morphology and photovoltaic properties of phenazine‐based donor–acceptor copolymers

Two phenazine donor–acceptor‐conjugated copolymers (P1 and P2) with the same polymer backbone but different anchoring positions of alkoxy chain on the phenazine unit were investigated to identify the effect of changing the position of alkoxy chains on their optical, electrochemical, blend film morphology, and photovoltaic properties. Although the optical absorption and frontier orbital energy levels were insensitive to the position of alkoxy chains, the film morphologies and photovoltaic performances changed significantly. P1/PC₇₁BM blend film showed the formation of phase separation with large coarse aggregates, whereas P2/PC₇₁BM blend film was homogeneous and smooth. Accordingly, power conversion efficiency (PCE) of photovoltaic devices increased from 1.50% for P1 to 2.54% for P2. In addition, the PCE of the polymer solar cell based on P2/PC₇₁BM blend film could be further improved to 3.49% by using solvent vapor annealing treatment. These results clearly revealed that tuning the side‐chain position could be an effective way to adjust the morphology of the active layer and the efficiency of the photovoltaic device. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2910–2918     

聚(3-己基噻吩)作为光谱增感层在有机太阳电池光谱响应增强中的应用

报道了利用聚(3-己基噻吩)(P3HT)作为前置缓冲层来弥补(4,8-双-(2-乙基己氧基)-苯并[1,2-b:4,5-b']二噻吩)-(4-氟代噻并[3,4-b]噻吩(PBDT-TT-F):[6,6]-苯基-C₆₁-丁酸甲酯(PC₆₁BM)共混体相异质结(BHJ)电池对450-600 nm处光谱响应不足的新的器件结构设计思路. 光谱带隙为1.8 eV的PBDT-TT-F 在550-700 nm处有很强的光谱吸收, 在有机太阳电池器件上有很好的应用潜能. 但其在350-550 nm处的吸收不强, 影响了器件对太阳光谱的利用效率. 与此相比, P3HT薄膜的光谱吸收主要在450-600 nm范围内, 同PBDT-TT-F 形成良好的互补关系. 新设计的器件外量子效率(EQE)研究结果表明, 利用P3HT 作为前置缓冲层可以与PBDT-TT-F:PC₆₁BM薄膜中的PC₆₁BM形成平面异质结, 从而拓展了器件在450-600 nm处的光谱响应范围,实现光谱增感作用. 优化P3HT的厚度为20 nm左右, 器件对外输出的短路光电流密度从11.42 mA·cm^-2提高到12.15 mA·cm^-2, 达到了6.3%的提升.     

Silole‐containing polymers for high‐efficiency polymer solar cells

Silole‐containing conjugated polymers ( P1 and P2 ) carrying methyl and octyl substituents, respectively, on the silicon atom were synthesized by Suzuki polycondensation. They show strong absorption in the region of 300–700 nm with a band gap of about 1.9 eV. The two silole‐containing conjugated polymers were used to fabricate polymer solar cells by blending with PC₆₁BM and PC₇₁BM as the active layer. The best performance of photovoltaic devices based on P1 /PC₇₁BM active layer exhibited power conversion efficiency (PCE) of 2.72%, whereas that of the photovoltaic cells fabricated with P2 /PC₇₁BM exhibited PCE of 5.08%. 1,8‐Diiodooctane was used as an additive to adjust the morphology of the active layer during the device optimization. PCE of devices based on P2 /PC₇₁BM was further improved to 6.05% when a TiO_x layer was used as a hole‐blocking layer. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Positioning lithium ions by host–guest chemistry combined with self‐assembly using a thiophene‐based all‐conjugated amphiphilic block copolymer

Self‐assembly of poly(3‐hexylthiophene) ( P3HT) driven by π–π stacking, combined with “Host‐Guest Chemistry” of ethylene glycol oligomer and lithium ion is demonstrated using a thiophene‐based all conjugated amphiphilic block copolymer, containing 93 mol % of P3HT and 7 mol % of poly(3‐(2‐(2‐{2‐[2‐(2‐methoxy‐ethoxy)‐ethoxy]‐ethoxy}‐ethyl))thiophene), P3EGT blocks. An ion chelating ability of ethylene glycol oligomers with lithium ions in the P3EGT block is confirmed using ¹H‐NMR spectrometry. This method could allow positioning lithium ions at the interface between P3HT domains and PC₆₁BM clusters, confirmed using XRD and photoluminescence quenching experiments. The compact lamellar P3HT domains by side repulsion driven self‐assembly of amphiphilic block copolymer and the molecular engineering of the interface with an optimized lithium contents are resulted in the improvement of photovoltaic performance in an organic solar cell (2.1–3.0%). © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1068–1074     

Thiazolothiazole‐containing polythiophenes with low HOMO level and high hole mobility for polymer solar cells

Two regiochemically defined polythiophenes containing thiazolothiazole acceptor unit were synthesized by palladium(0)‐catalyzed Stille coupling reaction. The thermal, electrochemical, optical, charge transport, and photovoltaic properties of these copolymers were examined. Compared to P1 with head‐to‐head coupling of two middle thiophenes, P2 with head‐to‐tail coupling of two middle thiophenes exhibits 40 nm red shift of absorption spectrum in film and 0.3 eV higher HOMO level. Both polymers exhibit field‐effect hole mobility as high as 0.02 cm² V^?1 s^?1. Polymer solar cells (PSCs) were fabricated based on the blend of the polymers and methanofullerene[6,6]‐phenyl C71‐butyric acid methyl ester (PC₇₁BM). The PSC based on P1 :PC₇₁BM (1:2, w/w) exhibits a power conversion efficiency of 2.7% under AM 1.5, 100 mW cm^?2, two times of that based on P2 :PC₇₁BM. The higher efficiency is attributed to lower HOMO (?5.6 eV) and smaller phase separation scale in P1 :PC₇₁BM blend. Tiny change in thiophene connection of P1 and P2 lead to great difference in HOMO, phase separation scale, and efficiency of their photovoltaic devices. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Copolymers of fluorene and thiophene with conjugated side chain for polymer solar cells: Effect of pendant acceptors

Two copolymers of fluorene and thiophene with conjugated side‐chain pending acceptor end group of cyanoacetate ( P2 ) and malononitrile ( P3 ) were synthesized. Both polymers exhibit good thermal stability and low highest occupied molecular orbital level (?5.5 eV). In comparison with P2 , P3 exhibits stronger UV–vis absorption and higher hole mobility. Polymer solar cells based on P3 :PC₇₁BM exhibits a power conversion efficiency of 1.33% under AM 1.5, 100 mW/cm², which is three times of that based on P2 :PC₇₁BM. The higher efficiency is attributed to better absorption, higher hole mobility, and the reduced phase separation scale in P3 :PC₇₁BM blend. The aggregate domain size in P3 :PC₇₁BM blend is 50 nm, much smaller than that in P2 :PC₇₁BM blend (200 nm). Tiny difference in the end groups on side chains of P2 and P3 leads to great difference in phase separation scale, charge transport, and efficiency of their photovoltaic devices. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Significance of miscibility in multidonor bulk heterojunction solar cells

Ternary organic blends have potential in realizing efficient bulk heterojunction (BHJ) organic solar cells by harvesting a larger portion of the solar spectrum than binary blends. Several challenging requirements, based on the electronic structure of the components of the ternary blend and their nanoscale morphology, need to be met in order to achieve high power conversion efficiency in ternary BHJs. The properties of a model ternary system comprising two donor polymers, poly(3-hexylthiophene) (P3HT) and a furan-containing, diketopyrrolopyrrole-thiophene low-bandgap polymer (PDPP2FT), with a fullerene acceptor, PC₆₁BM, were examined. The relative miscibility of PC₆₁BM with P3HT and PDPP2FT was examined using diffusion with dynamic secondary ion mass spectrometry (dynamic SIMS) measurements. Grazing incidence small and wide angle X-ray scattering analysis (GISAXS and GIWAXS) were used to study the morphology of the ternary blends. These measurements, along with optoelectronic characterization of ternary blend solar cells, indicate that the miscibility of the fullerene acceptor and donor polymers is a critical factor in the performance in a ternary cell. A guideline that the miscibility of the fullerene in the two polymers should be matched is proposed and further substantiated by examination of known well-performing ternary blends. The ternary blending of semiconducting components can improve the power conversion efficiency of bulk heterojunction organic photovoltaics. The blending of P3HT and PDPP2FT with PC₆₁BM leads to good absorptive coverage of the incident solar spectrum and cascading transport energy levels. The performance of this ternary blend reveals the impact of the miscibility of PC₆₁BM in each polymer as a function of composition, highlighting an important factor for optimization of ternary BHJs. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 237–246     

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     

Novel conjugated copolymers based on dithiafulvalene moiety for bulk heterojunction solar cells

New conjugated copolymers, P1‐P3 , based on dithiafulvalene‐fused entity and different conjugated segments have been synthesized. Incorporation of electron‐deficient conjugated segments into the conjugated copolymers results in red shifting the absorption band and lowering the hole mobility. Bulk heterojunction solar cells using on these polymers as the donor and [6,6]‐phenyl‐C61 ‐butyric acid methyl ester (PC₆₁BM) as the acceptor were fabricated by solution process. The cells based on the blend of P1‐P3 /PC₆₁BM (1:1, w/w) have power conversion efficiencies (PCEs) ranging from 0.53 to 0.93%. Among these, the cell of P1 /PC₆₁BM exhibited the highest open‐circuit voltage at 0.85 V, and the cell of P3/PC₆₁BM exhibited the best PCE at 0.93% with the short‐circuit current (J_SC) of 4.88 mA/cm². © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Direct arylation synthesis of thienoisoindigo‐based low‐band‐gap polymer from asymmetric donor–acceptor monomer

Thienoisoindigo (TIG) moiety has been paid numerous attentions as an excellent acceptor building block in low‐band‐gap polymers. Herein, a new TIG‐dithiophene alternating copolymer (PTIG2T) was successfully synthesized from an asymmetric TIG‐based donor–acceptor (D‐A) monomer via the self‐condensation‐type direct arylation polymerization. PTIG2T exhibited the light absorption over 1000 nm owing to the intramolecular charge transfer in the thin film state, which corresponded to an optical band gap of 1.24 eV. The HOMO and LUMO levels of PTIG2T were determined to be −5.08 and −3.60 eV, respectively. Furthermore, the organic photovoltaic (OPV) with a PTIG2T/PC₆₁BM active layer achieved a power conversion efficiency (PCE) of 3.19%, which is one of the highest PEC achieved by OPVs with TIG‐based materials. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 430–436     

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     

Exploring the influence of acceptor content on semi‐random conjugated polymers

A family of diketopyrrolopyrrole (DPP)‐incorporated P3HT based semi‐random copolymers was synthesized and their optical, electronic and photovoltaic properties were investigated. For the first time, the influence of acceptor content on semi‐random copolymers was explored in the broad range of 10–40% acceptor. A mixture of DPP acceptor units with different side chains (ethylhexyl and decyltetradecyl) was incorporated into each copolymer to improve solubility and film quality. Increased DPP content in the polymer backbone resulted in broadened absorption between 350 and 900 nm, resulting in a monotonic decrease in optical band gap (E_g) of the polymers from 1.49 to 1.37 eV. Highest occupied molecular orbital (HOMO) energy levels showed an increase from 10% DPP to 20–30% DPP, while decreasing for 40% DPP. V_oc values followed a consistent trend with HOMO energy levels. Semi‐random copolymers showed significantly improved photovoltaic properties compared with P3HT. Bulk heterojunction solar cells fabricated from the semi‐random copolymers blended with PC₆₁BM exhibited high short‐circuit current densities (J_sc) up to 10.29 mA/cm² and efficiencies up to 4.43%. A new method of methanol treatment was developed and applied to the semi‐random copolymers resulting in high fill factors approaching 0.70 under ambient conditions. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3884–3892     

Bay‐annulated indigo based near‐infrared sensitive polymer for organic solar cells

A new conjugated polymer (PBAIIDTT) based on bay‐annulated indigo and indacenodithieno[3,2‐b]thiophene was designed, synthesized, and characterized. PBAIIDTT shows strong absorption in 400–500 and 600–800 nm, and its HOMO and LUMO energy levels are −5.45 eV and −3.65 eV, respectively. In organic field‐effect transistors, the polymer exhibits a relatively high hole mobility of 0.39 cm² V⁻¹ s⁻¹. PBAIIDTT was added to poly(3‐hexylthiophene) (P3HT) and phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) based organic solar cells. Ternary blend solar cells with 10% PBAIIDTT show an increased short circuit current density due to the broadened photocurrent generated in the near‐infrared region, and a power conversion efficiency of 3.78%, which is higher than that of the P3HT:PC₆₁BM binary control devices (3.33%). © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 213–220     

An acrylated fullerene derivative for efficient and thermally stable polymer solar cells

The continuous microstructure evolution occurring in active layers of polymer-fullerene solar cells is one of the main causes for their device instability. With aim to tackle it, this work developed a new polymerizable fullerene acceptor, [6,6]-phenyl-C₆₁-butyl acrylate (PC₆₁BA). It was found that PC₆₁BA has similar light-absorption properties and HOMO and LUMO energy levels as [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM), and can be converted into insoluble oligomers upon heating at 150 °C. Polymer-fullerene solar cells using poly(3-hexylthiophene) (P3HT) as donor and PC₆₁BA as acceptor exhibited an optimized efficiency of 3.54%, the performance comparable to P3HT/PC₆₁BM cells (optimized efficiency: 3.70%). But, the former possess much better thermal stability than the latter owing to aggregation suppression by the polymerized PC₆₁BA. These results indicate that PC₆₁BA, unlike most previous reported, is a unique polymerizable fullerene derivative that can be used alone as acceptor to achieve both efficient and thermally stable polymer solar cells.     

New Conjugated Molecules with Two and Three Dithienyldiketopyrrolopyrrole (DPP) Moieties Substituted at meta Positions of Benzene toward p‐ and n‐Type Organic Photovoltaic Materials

Two conjugated molecules, TADPP3 and TADPP2‐TT , are reported, in which three and two dithienyldiketopyrrolopyrrole (DPP) moieties, respectively, are substituted at the meta positions of benzene. Based on cyclic voltammetry and absorption data, TADPP3 and TADPP2‐TT possess similar HOMO and LUMO energies of about ?5.2 and ?3.4 eV, respectively. Thin films of TADPP3 and TADPP2‐TT exhibit p‐type semiconducting behavior with hole mobilities of 2.36×10^?3 and 3.76×10^?4 cm² V^?1 s^?1 after thermal annealing. Molecules TADPP3 and TADPP2‐TT were utilized as p‐type photovoltaic materials to fabricate organic solar cells after blending with phenyl C₇₁ butyric acid methyl ester ( PC₇₁BM ) and phenyl C₆₁ butyric acid methyl ester ( PC₆₁BM ). The relatively low J_SC and fill factor values can be attributed to poor film morphologies based on AFM and XRD studies. A solar cell with a thin film of TADPP3 with PC₇₁BM in a weight ratio of 1:2 exhibits a high open‐circuit voltage (V_OC) of 0.99 V and a power conversion efficiency (PCE) of 2.47 %. Interestingly, TADPP3 can also be employed as an n‐type photovoltaic material. The blended thin film of TADPP3 with P3HT in a weight ratio of 1:2 gave a high V_OC of 1.11 V and a PCE of 1.08 % after thermal annealing.     

Random poly(3‐hexylthiophene‐co‐3‐cyanothiophene‐co‐3‐(2‐ethylhexyl)thiophene) copolymers with high open‐circuit voltage in polymer solar cells

For the purpose of developing poly(3‐hexylthiophene) (P3HT) based copolymers with deep‐lying highest occupied molecular orbital (HOMO) levels for polymer solar cells with high open‐circuit voltage (V_oc), we report a combined approach of random incorporation of 3‐cyanothiophene (CNT) and 3‐(2‐ethylhexyl)thiophene (EHT) units into the P3HT backbone. This strategy is designed to overcome CNT content limitations in recently reported P3HT‐CNT copolymers, where incorporation of more than 15% of CNT into the polymer backbone leads to impaired polymer solubility and raises the HOMO level. This new approach allows incorporation of a larger CNT content, reaching even lower‐lying HOMO levels. Importantly, a very low HOMO level of ?5.78 eV was obtained, representing one of the lowest HOMO values for exclusively thiophene‐based polymers. Lower HOMO levels result in higher V_oc and higher power conversion efficiencies (PCE) compared to the previously reported P3HT‐CNT copolymers containing only 3‐hexylthiophene and CNT units. As a result, solar cells based on P3HT‐CNT‐EHT(15:15) , which contains 70% of P3HT, 15% of CNT and 15% of EHT, yield a V_oc of 0.83 V in blends with PC₆₁BM while preserving high fill factor (FF) and high short‐circuit current density (J_sc), resulting in 3.6% PCE. Additionally, we explored the effect of polymer number‐average molecular weight (M_n) on the optoelectronic properties and solar cell performance for the example of P3HT‐CNT‐EHT(15:15). The organic photovoltaic (OPV) performance improves with polymer M_n increasing from 3.4 to 6.7 to 9.6 kDa and then it declines as M_n further increases to 9.9 and to 16.2 kDa. The molecular weight study highlights the importance of not only the solar cell optimization, but also the significance of individual polymer properties optimization, in order to fully explore the potential of any given polymer in OPVs. The broader ramification of this study lies in potential application of these high band gap copolymers with low‐lying HOMO level in the development of ternary blend photovoltaics as well as tandem OPV. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1526–1536