Charge transfer and photophysical properties of DSSCs based on different π-conjugated bridges: DFT and TD-DFT study

Dye-sensitized solar cells (DSSCs) are photovoltaic devices that directly convert solar radiation into current, in which the dye sensitizer serves as a critical structure that plays a significant role in determining the performance and conversion efficiency of DSSCs devices. In this paper, a series of D-A-π-A type dyes are designed, in which 2,2'-bithiophene, 1,1'-biphenyl as well as C=C and azo groups are used as π-conjugated bridges to transport electrons, respectively. The ground and excited state properties of several molecules were analyzed by density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The results show that the thiophene-based π-bridges have smaller frontier molecular orbital energy gaps and more red-shifted UV-vis absorption peaks, which are mainly attributed to the higher electron cloud density in the thiophene ring. At the same time, the addition of azo groups could further redshift the absorption peaks, mainly due to the existence of a large number of π-electrons in the azo bonds, which can reduce the energy required for electron transitions. In addition, it is also equally concluded that better photophysical properties are obtained by combining π-bridges of thiophenes with azo groups by means of transition density matrix, chemical reactivity parameters, photovoltaic parameter calculations and so on.