2D [small pi]-conjugated benzo[1,2-b:4,5-b']dithiophene- and quinoxaline-based copolymers for photovoltaic applications

Two medium gap semiconducting polymers, P(1)-Q-BDT-4TR and P(2)-FQ-BDT-4TR, based on alternate units of alkyl-dithiophene substituted benzodithiophene (BDT) and quinoxaline units (without or with fluorine substitution), are synthesized and fully characterized. The polymers exhibit optical and electrical properties favorable for being employed as donors in BHJ OPV devices, such as: absorption spectra extending up to around 720 nm for a high solar spectrum coverage, deep lying HOMO energy levels for a high device open circuit voltage and LUMO energy levels higher than those of PC61BM and PC71BM for an efficient exciton dissociation. In particular, the presence of alkyl-dithiophene side chains allows us to obtain a high 2D π-conjugation which promotes red shifted absorption profiles, low HOMO energy levels (<−5.6 eV) and enhanced environmental and thermal stability. Moreover, the introduction of the fluorine atom in the polymer backbone allows us to obtain efficient OPV devices, based on as-cast P(2)-FQ-BDT-4TR:PC61BM blend, showing a JSC of −10.2 mA cm−2, VOC of 0.90 V, FF of 58% and PCE of 5.3%, without the need for any additional thermal treatment