Molecular Engineering of Indoline Dyes and Their Application in Dye-Sensitized Solar Cells: Effect of Planarity and Side Chain on Interfacial Charge-Transfer Processes

Interfacial charge-transfer is the critical process during the operation of dye-sensitized solar cells, determining the photovoltaic performance of the cell. Interfacial charge-transfer processes are affected by many factors, such as the energy level and side chain. According to our previous work, the planarization of the carbazole segment can significantly improve the interfacial charge-transfer performance. However, whether the planarization strategy is universally effective in various dye systems is still uncertain. Moreover, the planarization of the molecule also leads to unfavorable π-aggregation. Thus, herein, the planarization strategy is further applied on indoline dyes while the alkyl chain is also introduced to investigate the effect of the above two factors on interfacial charge-transfer processes. With the improvement of the planarity and the introduction of an alkyl chain, interfacial charge-transfer processes, including charge injection, dye regeneration, and charge recombination, are efficiently optimized, suggesting a certain universality of our planarization strategy. It is noteworthy that the overlong alkyl chain still negatively affects these processes. Accordingly, the improvement of donor planarity as well as the introduction of a suitable alkyl chain can finely optimize interfacial charge-transfer processes, providing a promising strategy for the development of an efficient organic sensitizer.