All-polymer solar cells performance enhanced via side-chain engineering of the polymer acceptor

A series of novel donor–acceptor copolymers (PNDI-TVT-xOD) with different ratios of monomers was synthesized and used in all-polymer solar cells (all-PSCs). The copolymers were synthesized via random copolymerization between two dibromo-naphthalene tetracarboxylic diimides with different branched alkyl side chains and (E)-1,2-bis(5-(trimethylstannyl)thiophen-2-yl)-ethene. The effects of the changing monomer ratio on the crystallinity, charge transport, thin film, and photovoltaic properties of the copolymers were investigated. All-PSCs based on a blend of the donor polymer poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate] and random copolymer acceptors exhibited a maximum power conversion efficiency of 4.55%, which was markedly higher than that of devices containing an alternating copolymer acceptor. This study provides insight into the molecular design of conjugated polymers and the effects of their alkyl side chains on the performance of all-PSCs, leading to the fabrication of highly efficient all-PSCs.