Increasing H-aggregation of p-DTS(FBTTh2)2 to improve photovoltaic efficiency by solvent vapor annealing

• H-aggregation and crystallinity of p-DTS(FBTTh2)2 were improved via SVA.
• The mechanisms rely on controlling molecular motion ability and prolonging time.
• The ratio of H/J could be tuned from 0.71 to 1.47.

The proper crystallinity and a balance between H-aggregation and J-aggregation are both necessary for the best performance of small molecule organic solar cells. It is well known that J-aggregation is favorable to form more excitons due to its narrower bad-gap, leading to a high photocurrent. While H-aggregation has a higher lowest excited level which could provide stronger driving force for exciton dissociation. However, for 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (p-DTS(FBTTh2)2)/[6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) blend system, the crystallinity is very low and p-DTS(FBTTh2)2 is mainly in J-aggregation. Thus, increasing H-aggregation of p-DTS(FBTTh2)2 is an effective way to improve photovoltaic efficiency of p-DTS(FBTTh2)2/PC70BM system. In this paper, it was found that solvent vapor annealing (SVA) was an effective method to improve the crystallinity and the H-aggregation of p-DTS(FBTTh2)2 in blend film. Due to the prolonged time for self-organization, the crystallinity of p-DTS(FBTTh2)2 was enhanced. In addition, molecular motion ability of p-DTS(FBTTh2)2 was controlled and p-DTS(FBTTh2)2 could adjust its structure to form H-aggregation. As a result, the ratio of H/J could be tuned from 0.71 to 1.47 and the power conversion efficiencies (PCE) was improved from 2.64% to 6.63%.

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