Balancing the H- and J-aggregation in DTS(PTTh2)2/PC70BM to yield a high photovoltaic efficiency

Abstract

The content and ratio of two stacking styles, namely H-aggregation and J-aggregation, of 5,5′-bis{(4-(7-hexylthiophen-2-yl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine}-3,3′-di-2-ethylhexylsilylene-2,2′-bithiophene, DTS(PTTh₂)₂, had a profound influence on the performance of solar cells based on DTS(PTTh₂)₂/[6,6]-phenyl-C71-butyric acid methyl ester (PC₇₀BM) (7/3, w/w) blend films. It was found that the H/J ratio could be tuned from 0.30 to 1.40 by controlling the boiling point of the main solvent, the content of additives and the selective dissolution by additives (such as 1,8-diiodooctane (DIO), 1,6-diiodohexane (DIH) and 1,4-diiodobutane (DIB)) of the DTS(PTTh₂)₂ side chains. The power conversion efficiency (PCE) of DTS(PTTh₂)₂/PC70BM was firstly improved, then decreased with increasing H/J ratio. The best PCE of 6.51% was achieved by adding 0.20 vol% 1,6-diiodohexane (DIO) into solutions in thiophene (Th), which improved it by 203% compared to the reference without additives, when the H/J ratio was 1.01. Balance between the two stacking styles is needed for the device to obtain best performance. On one hand, J-aggregation was favorable for forming more excitons because a lower energy was needed to excite the J-aggregation. On the other hand, the H-aggregation favored exciton dissociation for two reasons: providing excitons with a longer lifetime and stronger driving force for exciton dissociation. In addition, the complementary light absorption of the two stacking styles is advantageous for maximal light absorption.