Rational design of conjugated side chains for high-performance all-polymer solar cells

Abstract

With their rapid development in recent years, organic solar cells hold the exciting potential to be a groundbreaking form of solar harvesting technology. Among these, all-polymer solar cells (all-PSCs) stand out owing to various advantages such as complementary absorption and superior stability. However, the advance of all-PSCs has been greatly impeded by energy level mismatches and unfavorable morphology of the active layer. Here, we report a molecular engineering approach featuring asymmetrical 4-methoxythiophene/thiophene as conjugated side chains of the donor polymer to fine-tune the energy level alignment and phase separation. The corresponding polymer, namely, poly{4-[5-(2-ethylhexyl)-4-methoxythiophen-2-yl]-8-[5-(2-ethylhexyl)thiophen-2-yl]benzo[1,2-b:4,5-b′]dithiophene}-alt-[bis(5-thiophene-2-yl)-5,6-difluoro-2-(2-hexyldecyl)-2H-benzo[d][1,2,3]triazole-4,7-diyl] (PMOT32) exhibited a power conversion efficiency that exceeded 8.5% in all-PSCs with poly{[N,N′-bis(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (N2200) as the acceptor. PMOT32:N2200 also maintained good photovoltaic performance when processed as a thick film or from non-halogenated solvents. Detailed comparisons with two other polymers with pure thiophene or 4-methoxythiophene as their side chains revealed how these side chains affected the photovoltaic performance via energy level alignment and phase separation.