Side chain engineering of naphthalene diimide–bithiophene-based polymer acceptors in all-polymer solar cells

Two novel polymeric acceptors based on naphthalene diimide (NDI) and 2.2′-bithiophene, named as P(NDI2THD-T2) and P(NDI2TOD-T2), were designed and synthesized for all polymer solar cells application. The structural and electronic properties of the two acceptors were modulated through side-chain engineering of the NDI units. The optoelectronic properties of the polymers and the morphologies of the blend films composed of the polymer acceptors and a donor polymer PTB7-Th were systemically investigated. With thiophene groups introduced into the side chains of the NDI units, both polymers showed wider absorption from 350 nm to 900 nm, compared with the reference polymer acceptor of N2200. No redshift of absorption spectra from solutions to films indicated reduced aggregation of the polymers due to the steric hindrance effect of thiophene rings in the side chains. The photovoltaic performance were characterized for devices in a configuration of ITO/PEDOT:PSS/PTB7-Th:acceptors/2,9-bis(3-(dimethylamino)propyl)anthra[2,1,9-def:6,5,10-def]diisoquinoline-1,3,8,10(2H,9H)-tetraone (PDIN)/Al. With the addition of diphenyl ether as an additive, the power conversion efficiencies (PCEs) of 2.73% and 4.75% for P(NDI2THD-T2) and P(NDI2TOD-T2) based devices were achieved, respectively. The latter showed improved J_sc, Fill Factor (FF), and PCE compared with N2200 based devices.