The influence of fluorinated position on the performance of isoindigo-based polymer solar cells

Two novel donor-acceptor (D-A) polymers based on two-dimensional conjugated alkylthienyl substituted benzo[1,2-b:4,5-b′]dithiophene and difluorine substituted isoindigo derivatives were synthesized with the structures as poly{6-(4,8-bis(4,5-dioctylthiophen-2-yl)-benzo[1,2-b:4,5-b′]-dithiophen-2-yl)−5,5′-difluoro-1,1′-dioctyl-isoindigo} (P1) and poly{5-(4,8-bis(4,5-dioctyl-thiophen-2-yl)-benzo[1,2-b:4,5-b′]-di-thiophen-2-yl)−4,6-difluoro-1,1′-dioctyl-isoindigo} (P2), respectively. Thermal stability, absorption properties, energy levels and photovoltaic properties of the polymers were intensively investigated. The symmetric polymer P1 showed broad absorption in comparison with the asymmetric polymer P2 both in solution and solid film, corresponding to their band gaps of 1.45 (P1) and 1.59 (P2) eV, respectively. The copolymers possessed similar high-lying highest occupied molecular orbital (HOMO) levels of – 5.34 (P1) and – 5.32 eV (P2). The performance of conventional solar cells based on copolymers/PCBM (PC61BM and PC71BM) films were investigated. All of P2-based PSCs delivered superior power conversion efficiencies (PCEs) and higher short circuit current (Jsc) than P1-based devices, where a best PCE up to 4.23% was obtained from P2/PC71BM device by THF solvent-vapor-annealing (SVA) treatment with an open-circuit voltage (Voc) of 0.88 V, a Jsc of 8.27 mA/cm2 and a fill factor (FF) of 0.578. The encouraging performance was attributed to its good miscibility, preferable nanophase separation and high mobilities, as well as efficient exciton dissociation with weak bimolecular and trap recombination. In this study, the incorporated position of fluorine atoms on IID-based copolymer influences not only the thin-film structure such as phase separation structure but also photophysical/electrochemical properties and photovoltaic performance, which highlights a new strategy for the design and synthesis of fluorinated D-A polymers. [Display omitted] •Two novel D-A polymers based on BDT and difluorinated isoindigo were designed.•Symmetric polymer P1 showed larger absorption than asymmetric P2 with similar HOMO.•The performance of polymers fabricated with PC61BM and PC71BM was studied.•P2 showed higher performance than P1 in all the devices with a best PCE of 4.23%.•F position affects absorption, morphology, exciton dissociation and recombination.