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(PTTh 2 ) 2 had a profound influence on performance of solar cells based on DTS(PTTh 2 ) 2 /[6,6]-phenyl-C71-butyric acid methyl ester (PC 70 BM) (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 boiling point of main solvent, content of additive and the dissolving selectivity of additives (such as 1,8-diiodooctane (DIO), 1,6-diiodohexane (DIH) and 1,4-diiodobutane (DIB)) for DTS(PTTh 2 ) 2 side chain.The power conversion efficiency (PCE) of DTS(PTTh 2 ) 2 /PC70BM was improved firstly, then decreased with increasing of the 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), improved 203% compared to the reference without additives, when the H/J ratio is 1.01. A balance between the two stacking styles is needed for device to obtain best performance. On one hand, J-aggregation was favorable to form 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 longer lifetime and stronger driving force for exciton dissociation. And also, the complementary light absorption of the two stacking styles is advantageous for maximal light absorption. 4 molecules. [15][16][17] The adjacent molecules stack face-to-face to form H-aggregation, and head-to-tail to form J-aggregation. [18] As to the relationship between absorption peaks and H-/J-aggregation, there are two viewpoints. One was that the three peaks (0-0, 0-1, 0-2) taken together represent a vibronic progression, due to a symmetric vinyl stretching and/or ring breathing mode (or modes) in the region from 1200-1500cm -1 . The ratio of the 0-0 peak to the 0-1 peak is very sensitive to the exciton bandwidth. In particular, the peak ratio decreases with bandwidth for H-aggregation and increases with bandwidth for J-aggregation. [19][20][21] The other was that red-shift absorption peak (compared to absorption peak of monomer) represented J-aggregation and the blue-shift one represented H-aggregation. [16,[22][23][24] Each of the two viewpoints has its own merits.Here, we adopted the latter one due to that it was more direct and allowed us to compare H-and J-aggregation intuitively.The H-/J-aggregation state has multiple influences on many important factors concerning about the performance of solar cells. Firstly, the absorption peaks of H-aggregation and J-aggregation are blue-shifted and red-shifted compared to those in isolated state, respectively. [16,25] If the two stacking styles coexist, the absorption range could be enlarged. Secondly, H-aggregation has a higher lowest excited level [19] which provides stronger driving force for exciton dissociation, and excitons may have longer lifetime in H-aggrega...

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

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