Rational design of metal-free organic D-π-A dyes in dye-sensitized solar cells: Insight from density functional theory (DFT) and time-dependent DFT (TD-DFT) investigations

Abstract

The search for potential high efficiency dye sensitizer for dye-sensitized solar cell (DSSCs) application is still appealing at present. By manipulation of electron donor and π-bridges groups of organic dyes with multiple design strategies, we theoretically developed series of promising organic D-π-A dyes. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were used to elucidate the geometries, electronic structures, and optical features of the isolated dyes and dye-TiO2 combined system. Quantum dynamics simulation in the framework of extended Huckel theory (EHT) was carried out to simulate the interface electron transfer process across the dye/TiO2 interface. The key parameters influence the short-circuit photocurrent density and open-circuit photovoltage were analyzed in detail. Our results suggest that the dyes with further extended π-bridges has the gradually red-shifted absorption spectra and a persistent decrease of highest occupied molecular orbital (HOMO) energy. The extended π-conjugated bridges deteriorate the performance mainly because of the increased charge recombination between injected electron and electrolyte arising from the unfavorable energy alignment between dye HOMO and redox potential of electrolyte, leading to inefficient dye regeneration process. This rationalizes the experimental observation that longer π-bridge doesn't benefit the DSSCs performance. Further replacement of triphenylamine donor with carbazole or coumarin electron donor groups can shift down the HOMO level and avoid the inefficient dye regeneration issue whereas perseves other beneficial properties. The simulation performed in this work provides general design principle for experimental optimization of metal-free organic dyes towards DSSCs application.