3,6‐Dialkylthieno[3,2‐b]thiophene moiety as a soluble and electron donating unit preserving the coplanarity of photovoltaic low band gap copolymers

It has been shown recently, that the presence of alkyl side chains at the 3‐positions on the thiophene rings placed next to 2,1,3‐benzothiadiazole core in the backbone of several conjugated polymers results in severe steric hindrance and prevents efficient planarity of the thiophene‐2,1,3‐benzothiadiazole‐thiophene (TBzT) segment. Both properties have a strong influence on the optoelectronic properties of the polymer and need to be considered when the polymer is to be used for organic electronics applications. In this work, we modified a previously synthesized oligothiophene copolymer, consisting of two 3,4′‐dialkyl‐2,2′‐bithiophene units attached to a 2,1,3‐benzothiadiazole unit (TBzT segment) and a thieno[3,2‐b]thiophene unit, by optimizing the lateral alkyl side chains following a density functional theory investigation. It is demonstrated that eliminating the alkyl side chains from the 3‐positions of the TBzT segment and anchoring them onto the thieno[3,2‐b]thiophene, using an efficient synthesis of the 3,6‐dihexylthieno[3,2‐b]thiophene unit, allows us to reduce the energy band gap. In addition, the chemical modification leads to a better charge transport and to an enhanced photovoltaic efficiency of polymer/fullerene blends. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

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     

非富勒烯聚合物太阳电池研究进展

综述了以p-型共轭聚合物为给体、n-型有机半导体为受体的非富勒烯聚合物太阳电池光伏材料最新研究进展,包括n-型共轭聚合物和可溶液加工小分子n-型有机半导体(n-OS)受体光伏材料,以及与之匹配的p-型共轭聚合物给体光伏材料.介绍的n-型共轭聚合物受体光伏材料包括基于苝酰亚胺(BDI)、萘酰亚胺(NDI)以及新型硼氮键连受体单元的D-A共聚物受体光伏材料,目前基于聚合物给体(J51)和聚合物受体(N2200)的全聚合物太阳电池的能量转换效率最高达到8.26%.n-OS小分子受体光伏材料包括基于BDI和NDI单元的有机分子、基于稠环中心给体单元的A-D-A型窄带隙有机小分子受体材料等.给体光伏材料包括基于齐聚噻吩和苯并二噻吩(BDT)给体单元的D-A共聚物,重点介绍与窄带隙A-D-A结构小分子受体吸收互补的、基于噻吩取代BDT单元的中间带隙二维共轭聚合物给体光伏材料.使用中间带隙的p-型共轭聚合物为给体、窄带隙A-D-A结构有机小分子为受体的非富勒烯聚合物太阳电池能量转换效率已经突破12%,展示了光明的前景.最后对非富勒烯聚合物太阳电池将来的发展进行了展望.     

Quinoxaline‐based D‐A conjugated polymers for organic solar cells: Probing the effect of quinoxaline side chains and fluorine substitution on the power conversion efficiency

We describe the successful synthesis of four novel donor‐acceptor (D‐A) type copolymers, referred to as PQxBT , PQxFBT , TQxBT , and TQxFBT . The effects of using a fluorinated bithiophene (FBT) and varying the side‐chain moieties tethered to the quinoxaline (Qx) unit (electron‐withdrawing group in the polymer backbone) on the physical properties and photovoltaic performance were investigated. Specifically, the four polymers were synthesized using either alkoxyphenyl (P) or alkylthiophene (T) units anchored to the quinoxaline in the polymer backbone. The FBT‐bearing polymers, PQxFBT and TQxFBT , displayed more redshifted absorption spectra and higher crystallinity owing to the greater planarity of their polymer backbone as compared to the non‐fluorinated polymers. The TQxFBT copolymer, equipped with both the alkylthiophene side chains and FBT, exhibited face‐on orientation in film state and a well‐mixed nanophase morphology in TQxFBT :PC₇₁BM blend films. The photovoltaic device fabricated from TQxFBT :PC₇₁BM exhibited the highest power conversion efficiency of 4.18%. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 55, 1209–1218     

Modifying the morphology via employing rigid phenyl side chains achieves efficient nonfullerene polymer solar cells

As a promising electron-deficient (acceptor) unit, fluorinated benzotriazole (TAZ) was used widely to construct wide bandgap polymers for nonfullerene polymer solar cells (NF-PSCs). However, due to the S…F noncovalent interaction, the unit show good planarity and strong aggregation, which was not beneficial for the blend with the elongated nonfullerene acceptors. Here, we tried to choose new donor polymers to match with TAZ unit, which was expected to destroy the interchain aggregation of polymer and form favorable morphology of blends. Two new wide-bandgap polymers PBDTsPhPh-T1 and PBDTsThPh-T1 based on the unsymmetrical benzodithiophene (BDT) units with a benzene ring as lever arms were synthesized. As a result, these two polymers blend well with nonfullerene acceptor (ITIC). And then, PBDTsPhPh-T1 and PBDTsThPh-T1-based devices exhibit the decent photovoltaic properties with high power conversion efficiencies of 8.85 and 9.34%, respectively. The work demonstrates that the unsymmetrical BDT units could be outstanding for building donor materials toward high-performance NF-PSCs. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2762–2770     

Donor–acceptor (donor) polymers with differently conjugated side groups at the acceptor units for photovoltaics

Four new donor–acceptor (donor) [D–A(D)], PBDT‐PTQ, PBDT‐PTTQ, PBDT‐TQ, and PBDT‐TTQ, bearing the same backbone of alternative benzodithiophene (BDT) and quinoxaline units but with phenylene thienyl, phenylene di‐thienyl, thienyl and di‐thienyl groups (other donors), respectively, at the acceptor quinoxaline units, were designed and synthesized to investigate the impacts of the conjugated side chains at the acceptor units on the photovoltaic properties of polymers. The power conversion efficiencies (PCEs) of the polymer solar cells (PSCs) based on PBDT‐TQ:[6,6]‐phenyl‐C‐70‐butyric acid methyl ester (PC70BM) and PBDT‐PTQ:PC70BM reach to 4.39 and 3.58%, respectively, which are 43 and 17% higher, respectively, than that of a reported alkylphenyl substituted polymer with the same main chain. However, the PCEs based on PBDT‐TTQ and PBDT‐PTTQ, in which an additional thiophene is added at a side chain of PBDT‐TQ and PBDT‐PTQ, respectively, decline. The mechanism how the conjugated side chains affect the performance of the PSCs is also discussed. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Effect of additive length and chemistry on the morphology of blends of conjugated thiophenes and fullerene derivative acceptor molecules

Small molecule additives have been shown to increase the device efficiency of conjugated polymer (donor) and fullerene derivative (acceptor) based organic solar cells by modifying the morphology of the device active layer. In this paper we conduct a systematic study of how additives affect the donor‐acceptor morphology using molecular dynamics simulations of blends of thiophene‐based oligomers, mimicking poly(3‐dodecylthiophene) (P3DDT) or poly(2,2′:5′,2”‐3,3”‐didocyl‐terthiophene) (PTTT), and fullerene derivatives with additives of varying length and chemical functionalization, mimicking experimentally used additives like methyl ester additives, diiodooctane, and alkanedithiols. We find that functionalization of additives with end groups that are attracted to acceptor molecules are necessary to induce increased donor‐acceptor macrophase separation. In blends where acceptors intercalate between oligomer alkyl side chains, functionalized additives decrease acceptor intercalation. Functionalized additives with shorter alkyl segments increase acceptor macrophase separation more than additives with same chemical functionalization but longer alkyl segments. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1046–1057     

Theoretical and experimental study of low band gap polymers for organic solar cells

A combined theoretical and experimental investigation of the electronic structure and optical properties of poly(3-hexylthiophene) (P3HT), poly[3-(4-octylphenyl)thiophene] (POPT) and poly[3-(4-octylphenoxy)thiophene] (POPOT) is reported. In comparison with P3HT, POPT and POPOT exhibit better stabilities and the presence of an oxygen atom and/or a phenyl ring in the side chains enhances conjugation. Quantum chemical calculations have been performed on oligomers of increasing chain length to establish the changes in the electronic and optical properties when going from P3HT to the new derivative POPOT. The knowledge of the structure of these polymers is of utmost importance in understanding their optical properties in different phases (solution and condensed phase). The calculations indicate that, in opposition to P3HT and POPT polymers where the introduction of alkyl chains and the pendant phenyl disturbs the planarity of the backbone of the conjugated segment, POPOT has a better degree of organization in both states: the conjugated chain remains planar even in the presence of the phenoxy groups. Finally, the exciton binding energy is evaluated for these polymers and allows us to conclude that the POPOT is a promising polymer for photovoltaic applications when compared to P3HT and POPT.     

Ethynylene‐containing donor–acceptor alternating conjugated polymers: Synthesis and photovoltaic properties

Four ethynylene‐containing donor‐acceptor alternating conjugated polymers P1 – P4 with 2,5‐bis(dodecyloxy) substituted phenylene or carbazole as the donor unit and benzothiadiazole (BTZ) as the acceptor unit were synthesized and used as donor polymers in bulk heterojunction polymer solar cells. The optical, electrochemical, and photovoltaic properties of these four polymers with the ethylene unit located at different positions of the polymer chains were systematically investigated. Our results demonstrated that absorption spectra and the HOMO and LUMO energy levels of polymers could be tuned by varying the position of the ethynylene unit in the polymer chains. Photovoltaic devices based on polymer/PC₇₁BM blend films spin coated from chloroform and dichlorobenzene solutions were investigated. For all four polymers, open circuit voltages (V_oc) higher than 0.8 V were obtained. P4 , with ethynylene unit between BTZ and thiophene, shows the best performance among these four polymers, with a V_oc of 0.94 V, a J_sc of 4.2 mA/cm², an FF of 0.40, and a PCE of 1.6%. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Synthesis and characterization of new sulfone‐derivatized phenylenevinylene‐based conjugated copolymers with evolving energy levels and gaps

A new series of stable, processable, and chain end functionalizable sulfone‐derivatized phenylenevinylene‐based conjugated polymers (SFPVs) containing different donor type comonomers have been synthesized and characterized. The polymer main chains are consisted of a sulfone‐phenylene electron accepting unit coupled with an electron donating unit which is derived from one of the dialdehyde comonomers based on benzene, thiophene, and pyrrole (with or without alkoxy side chains). The optical energy gaps (E_g) of the new polymers (in solvent) are in a range of 1.9–2.3 eV, with the lowest energy gap obtained from the polymer containing pyrrole as the donor unit. By using a combination of strong donor unit (such as pyrrole) and a relatively weak but stable acceptor unit (sulfone‐substituted benzene), E_g of the conjugated polymers can be tailored to below 2 eV, while the vinylene bonds on the polymer main chain are still chemically stable to survive strong basic conditions as compared with the S,S‐dioxo‐thiophene‐based PTV polymers developed earlier for potential supra‐molecular block copolymer systems. The lowest energy gap P(Pyrrole‐SFPV) exhibited 10 times better photoelectric power conversion efficiency than P(TV‐SFPV). © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Novel two‐dimensional donor–acceptor conjugated polymers containing quinoxaline units: Synthesis,characterization, and photovoltaic properties

Novel two‐dimensional donor–acceptor (D–A) structured conjugated polymers, P1–P4, were designed and synthesized by introducing electron‐deficient quinoxaline as core and electron‐rich alkoxyl‐phenylenevinylene in side chains and p‐phenylenevinylene, triphenylamine, or thiophene in main chain. Benefited from the D–A structures, the polymers possess low bandgaps of 1.75 eV, 1.86 eV, 1.59 eV, and 1.58 eV for P1, P2, P3, and P4, respectively, and show broad absorption band in the visible region: the shorter wavelength absorption peak at ～400 nm ascribed to the conjugated side chains and the longer wavelength absorption peak between 500 nm and 750 nm belonging to the absorption of the conjugated main chains. Especially, the absorption band of P4 film covers the whole visible range from 300 nm to 784 nm. The power conversion efficiencies of the polymer solar cells based on P1–P4 as donor and PCBM as acceptor are 0.029%, 0.14%, 0.46%, and 0.57%, respectively, under the illumination of AM 1.5, 100 mW/cm². The polymers with the low bandgap and broad absorption band are promising photovoltaic materials. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4038–4049, 2008     

Thiophene π bridge effect on bulky side‐chained benzodithiophene‐based photovoltaic polymers

Recently, we have used terthiophene side chain to modify benzo[1,2‐b:4,5‐b′]dithiophene (BDT) to form novel building block for BDT polymers. In this paper, this building block is used to copolymerized with thieno[3,4‐c]pyrrole‐4,6‐dione (TPD) and thieno[3,4‐b]thiophene (TT). This building block and TPD‐ or TT‐based polymers (P1 and P3) show high open circuit voltage (V_OC) (ca. 0.9–0.95 V) and low energy loss (E_g–eV_OC) in solar cells devices compared with similar polymers without bulky side chain. We further introduce thiophene π bridge into these polymers backbone to form two other polymers (P2 and P4). We find this thiophene π bridge does contribute to this bulky side chained benzodithiophene polymer photovoltaic performances, especially for power conversion efficiencies (PCEs). The polymer solar cells (PSCs) performances are moderate in this article due to the serious aggregation in the PSCs active layer. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1615–1622     

Synthesis and characterization of highly conjugated side‐group‐substituted benzo[1,2‐b:4,5‐b′]dithiophene‐based copolymer for use in organic solar cells

A new donor–acceptor (D–A) conjugated copolymer based on benzo[1,2‐b:4,5‐b′]dithiophene (BDT) and thieno[3,4‐c]pyrrole‐4,6‐dione (TPD) was synthesized via a Stille cross‐coupling reaction. A highly conjugated thiophene‐based side group, tris(thienylenevinylene) (TTV), was incorporated into each BDT unit to generate the two‐dimensional D–A copolymer (PBDT‐TTV). An alkoxy‐substituted BDT‐based TPD copolymer (PBDT‐OR) was synthesized using the same polymerization method for comparison. PBDT‐TTV thin films produced two distinct absorption peaks. The shorter wavelength absorption (458 nm) was attributed to the BDT units containing the TTV group, and the longer wavelength band (567–616 nm) was attributed to intramolecular charge transfer between the BDT donor and the TPD acceptor. The highest occupied molecular orbital energy levels of PBDT‐OR and PBDT‐TTV were calculated to be −5.53 and −5.61 eV, respectively. PBDT‐TTV thin films harvested a broad solar spectrum covering the range 300–700 nm. A comparison with the PBDT‐OR films revealed stronger interchain π–π interactions in the PBDT‐TTV films and, thus, a higher hole mobility. A polymer solar cell device prepared using PBDT‐TTV as the active layer was found to exhibit a higher power conversion efficiency than a device prepared using PBDT‐OR under AM 1.5 G (100 mW/cm²) conditions. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 653–660     

Thieno[3,2‐b]thiophene‐substituted benzodithiophene in donor–acceptor type semiconducting copolymers: A feasible approach to improve performances of organic photovoltaic cells

Thieno[3,2‐b]thiophene‐substituted benzo[1,2‐b:4,5‐b′]dithiophene donor units (TTBDT) serve as novel promising building blocks for donor–acceptor (D‐A) copolymers in organic photovoltaic cells. In this study, a new D‐A type copolymer (PTTBDT‐TPD) consisting of TTBDT and thieno[3,4‐c]pyrrole‐4,6‐dione (TPD) is synthesized by Stille coupling polymerization. A PTTBDT‐TPD analog consisting of TTBDT and alkylthienyl‐substituted BDT (PTBDT‐TPD) is also synthesized to compare the optical, electrochemical, morphological, and photovoltaic properties of the polymers. Bulk heterojunction photovoltaic devices are fabricated using the polymers as p‐type donors and [6,6]‐phenyl C₇₁‐butyric acid methyl ester (PC₇₁BM) as the n‐type acceptor. The power conversion efficiencies of the devices fabricated using PTTBDT‐TPD and PTBDT‐TPD are 6.03 and 5.44%, respectively. The difference in efficiency is attributed to the broad UV–visible absorption and high crystallinity of PTTBDT‐TPD. The replacement of the alkylthienyl moiety with thieno[3,2‐b]thiophene on BDT can yield broad UV–visible absorption due to extended π‐conjugation, and enhanced molecular ordering and orientation for organic photovoltaic cells. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3608–3616     

Acceptor–acceptor conjugated copolymers based on perylenediimide and benzothiadiazole for all‐polymer solar cells

Donor–acceptor (D–A) conjugated copolymers are one of known classes of organic optoelectronic materials and have been well developed. However, less attention has been paid on acceptor–acceptor (A–A) conjugated analogs. In this work, two types of A–A conjugated copolymers, namely P1‐Cn and P2‐Cn (n is the carbon number of their alkyl side chains), were designed and synthesized based on perylenediimide ( PDI ) and 2,1,3‐benzothiadiazole ( BT ). Different from P1‐Cn , P2‐Cn polymers have additional acetylene π‐spacers between PDI and BT and thus hold a more planar backbone configuration. Property studies revealed that P2‐Cn polymers possess a much red‐extended UV–vis absorption spectrum, stronger π–π interchain interactions, and one‐order larger electron mobility in their neat film state than P1‐Cn . However, all‐polymer solar cells using P1‐Cn as acceptor component and poly(3‐hexyl thiophene) or poly(2,7‐(9,9‐didodecyl‐fluoene)‐alt?5,5′‐(4,7‐dithienyl‐2‐yl‐2,1,3‐benzothiadiazole) as donor component exhibited much better performance than those based on P2‐Cn . Apart from their backbone chemical structure, the side chains were found to have little influence on the photophysical, electrochemical, and photovoltaic properties for both P1‐Cn and P2‐Cn polymers. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1200–1215     

A vinylene‐linked benzo[1,2‐b:4,5‐b']dithiophene‐2,1,3‐benzothiadiazole low‐bandgap polymer

A new heteroarylene‐vinylene donor–acceptor polymer P(BDT‐V‐BTD) with reduced bandgap has been synthesized and its photophysical, electronic and photovoltaic properties investigated both experimentally and theoretically. The structure of the polymer comprises an unprecedented combination of a strong donor (4,8‐dialkoxy‐benzo[1,2‐b:4,5‐b']dithiophene, BDT), a strong acceptor (2,1,3‐benzothiadiazole, BTD) and a vinylene spacer. The new polymer was obtained by a metal‐catalyzed cross‐coupling Stille reaction and fully characterized by NMR, UV–vis absorption, GPC, TGA, DSC and electrochemistry. Detailed ab initio computations with solvation effects have been performed for the monomer and model oligomers. The electrochemical investigation has ascertained the ambipolar character of the polymer and energetic values of HOMO, LUMO and bandgap matching materials‐design rules for optimized organic photovoltaic devices. The HOMO and LUMO energies are consistently lower than those of previous heteroarylene‐vinylene polymer while the introduction of the vinylene spacer afforded lower bandgaps compared to the analogous system P(BDT‐BTD) with no spacer between the aromatic rings. These superior properties should allow for enhanced photovoltages and photocurrents in photovoltaic devices in combination with PCBM. Preliminary photovoltaic investigation afforded relatively modest power conversion efficiencies of 0.74% (AM 1.5G, 100 mW/cm²), albeit higher than that of previous heteroarylene‐vinylene polymers and comparable to that of P(BDT‐BTD). © 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 and properties of polythiophenes with conjugated side‐chains containing carbon–carbon double and triple bonds

Two soluble side‐chain conjugated polythiophenes, poly{3‐[2‐(4‐octyloxy‐phenyl)‐vinyl]‐thiophene} (P3OPVT) and poly{3‐(4‐octyloxy‐phenylethynyl)‐thiophene} (P3OPET) have been synthesized successfully. In P3OPVT and P3OPET, substituted benzene rings are connected with the polythiophene backbone through trans carbon–carbon double bond and carbon–carbon triple bond, respectively. Absorption spectra of the P3OPVT and P3OPET both show two absorption peaks located in UV and visible region, respectively. The results of optical and electrochemical measurements indicate that the conjugated side‐chains can reduce the bandgap effectively. This type of side‐chain conjugated polythiophenes may be promising for the applications in polymer photovoltaic cells and field effect transistors. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2206–2214, 2006     

Synthesis and photovoltaic properties of copolymers based on benzo[1,2‐b:4,5‐b′]dithiophene and thiophene with electron‐withdrawing side chains

Six alternating conjugated copolymers ( PL1 – PL6 ) of benzo[1,2‐b:4,5‐b′]dithiophene (BDT) and thiophene, containing electron‐withdrawing oxadiazole (OXD), ester, or alkyl as side chains, were synthesized by Stille coupling reaction. The structures of the polymers were confirmed, and their thermal, optical, electrochemical, and photovoltaic properties were investigated. The introduction of conjugated electron‐withdrawing OXD or formate ester side chain benefits to decrease the bandgaps of the polymers and improve the photovoltaic performance due to the low steric hindrance of BDT. Bulk heterojunction polymer solar cells (PSCs) were fabricated based on the blend of the as‐synthesized polymers and the fullerene derivative [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) in a 1:2 weight ratio. The maximum power conversion efficiency of 2.06% was obtained for PL5 ‐based PSC under the illumination of AM 1.5, 100 mW/cm². © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Effects of alkyl side chains positioning and presence of fused aromatic units in the backbone of low‐bandgap diketopyrrolopyrrole copolymers on the optoelectronic properties of organic solar cells

The systematic optimization of the chemical structure of low‐bandgap (LBG) donor‐acceptor polymeric semiconductors is a challenging task for which accurate guidelines are yet to be determined. Several different structural and molecular parameters are crucial ingredients for obtaining LBG polymers that simultaneously possess high power conversion efficiencies, good processability in common organic solvents, and enhanced stability in organic photovoltaic devices. In this work, we present an extensive structure–optoelectronic properties–solar cell performance study on the emerging class of diketopyrrolopyrrole‐based LBG polymers. In particular, we investigate alkyl side chain positioning by introducing linear alkyl side chains into two different positions (α‐ and β‐), and the distance of the electron rich and electron deficient monomers within the repeat units of the polymer chain. We demonstrate that anchoring linear alkyl side chains to the α‐positions and introducing fused moieties into the polymer backbone, can be beneficial toward maintaining photocurrents similar to the unsubstituted derivative, and concurrently exhibit better processabiliy in common organic solvents. These results can provide a design rationale towards further optimization of semiconducting polymers. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 138–146