First-principles framework for bifacial dye-sensitized solar cells: electronic structure, optical properties and...

The bifacial dye@TiO₂ architectures have recently emerged as a promising strategy to enhance light harvesting in dye-sensitized systems. However, their fundamental electronic and optical properties remain largely unexplored from a theoretical perspective. Despite growing experimental interest, a comprehensive first-principles understanding of bifacial dye-semiconductor interfaces is still missing. In this work, a systematic density functional theory (DFT) and linear-response time-dependent density functional theory (LR-TDDFT) investigation of monofacial and bifacial dye@TiO₂ systems is presented for three representative organic dyes. The impact of bifacial functionalization on electronic structure, excitation pathways, and directional optical response is analyzed in detail. The results reveal that bifaciality is not an intrinsic property of all dyes, but instead depends critically on the spatial distribution of frontier molecular orbitals and on the exchange treatment within the chosen functional. This study provides the first consistent theoretical framework for interpreting bifacial effects in the dye-sensitized architectures and offers design principles for future bifacial dye systems.