Targeted and selective HOMO energy control by fine regulation of molecular planarity and its effect on the interfacial charge transfer process in dye-sensitized solar cells

In terms of the in-depth development of organic dyes, targeted and selective energy control is becoming a more and more important objective. Herein, four indoline sensitizers based on D–π–A–π–A construction were designed and synthesized with exactly the same donor and acceptor segments. Their molecular planarity was regulated by introducing various side chains into donor bridges. Interestingly, along with an improvement of planarity at a donor bridge, the HOMO levels of the dyes lift gradually, and more importantly, their LUMO levels remain at around the same value. Besides, better molecular planarity is obviously preferred to obtain higher charge injection efficiency but, an overly planar molecule may cause an overly high HOMO level, leading to poor dye regeneration efficiency. Furthermore, an appropriate side chain also restrains charge recombination to some extent, while an overly large side chain gives more chance for I3− to recombine with charge in the conduction band. Accordingly, our results demonstrated that regulation of planarity at a donor bridge not only provides targeted and selective control of the HOMO of the dye, but also enable fine adjustment with multiple interfacial charge transfer processes. Molecular planarity deserves to play an important role in the design of organic dyes, providing a significant strategy for the further development of organic sensitizers.