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

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-C-70-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.