All-conjugated poly(3-alkylthiophene) diblock copolymer-based bulk heterojunction solar cells with controlled molecular organization and nanoscale morphology

Abstract

Control over the ratio of two blocks in a new class of all-conjugated diblock copolymers, poly(3-butylthiophene)-b-poly(3-hexylthiophene) (P3BHT), provides a facile approach to precisely tune the molecular organization and nanoscale morphology in polymer bulk heterojunction (BHJ) solar cells. In stark contrast to the power conversion efficiency, PCE, of 1.08% in poly(3-butylthiophene) (P3BT)/[6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) and 3.54% in poly(3-hexylthiophene) (P3HT)/PC₇₁BM solar cells, an attractive, high PCE of 4.02% was achieved in a P3BHT21/PC₇₁BM BHJ device in which the molar ratio of P3BT : P3HT in P3BHT21 was 2 : 1. The ratio of P3BT and P3HT blocks was found to exert a noteworthy influence on the molecular organization of P3BHT, the film morphology of P3BHT/PC₇₁BM blend, and the final performance of P3BHT/PC₇₁BM photovoltaic devices. This enhanced performance reflected a synergy of finer phase separation of P3BHT21 and PC₇₁BM and the formation of respective percolation networks of electron donor P3BHT and electron acceptor PC₇₁BM. The P3HT block rendered the P3BHT chains with favorable chemical compatibility for the diffusion of PC₇₁BM molecules, allowing for finer phase separation between P3BHT crystalline domains and PC₇₁BM domains at the nanoscale and maximizing the interfacial area of P3BHT21/PC₇₁BM for improved charge generation. The P3BT block facilitated the self-assembly of P3BHT chains into sufficient interpenetrating pathways for efficient charge transport and collection. Moreover, a small crystalline domain with a size of 10.4 nm formed in the active layer that is comparable to the exciton diffusion length of most conjugated polymers (∼10 nm).