Improving performance of organic solar cells by supplying additional acceptors to surface of bulk-heterojunction layers

Abstract

In this study, we improved the performance of 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]:[6,6]-phenyl-C₇₀-butyric acid methyl ester (PTB7-Th:PC₇₁BM)-based organic solar cells (OSCs) by thermally evaporating additional acceptors PC₇₁BM or 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC) on the surface of bulk-heterojunction layers. We found that the surface of the bulk-heterojunction layers showed a shortage of acceptor materials due to vertical phase separation, which was detrimental to the power conversion efficiency (PCE). The acceptor addition (PC₇₁BM or ITIC) was beneficial for charge transport and suppressed charge recombination. Addition of a 9 nm PC₇₁BM layer on the surface of the bulk-heterojunction layers significantly improved the performance of rigid OSCs (on glass substrates), and the maximum PCE increased from 8.99% to 10.25%, which was higher than that (9.78%) of the OSCs using ITIC. In addition, the degradation of the OSCs during long-term stability testing reduced from 21% to 14% Furthermore, the PCE of flexible OSCs (on polyethylene naphthalate substrate) improved from 7.65% to 8.52%. Our results indicated that evaporating PC₇₁BM on the surface of bulk-heterojunction layers is a simple and effective method to fabricate high-performance rigid and flexible OSCs.