Understanding the role of the dye/oxide interface via SnO₂-based MK-2 dye-sensitized solar cells

To understand the role of the dye/oxide interface, a model system using a nanocrystalline SnO₂ and 3-hexyl thiophene based MK-2 dye is proposed. A thin interfacial TiO₂ blocking layer (IBL) is introduced in between SnO₂ and MK-2 and its effects on photocurrent-voltage, electron transport-recombination, and density of states (DOS) are systematically investigated. Compared to the bare SnO₂ film, the insertion of IBL leads to a 14-fold improvement in the power conversion efficiency (PCE) despite little change in the dye adsorption amount, which is due to the 7-fold and 2-fold increase in the photocurrent density and voltage, respectively. The charge collection efficiency is substantially improved from 38% to 96% mainly due to the increase in the electron lifetime. The IBL is also found to enhance the dye regeneration efficiency as confirmed by the 15-fold faster dye bleaching recovery dynamics. The recombination resistance increases and the DOS decreases after surface modification of SnO₂, which is responsible for the doubly increased voltage. This study suggests that the interfacial layer between the oxide and the dye plays a crucial role in retarding recombination, improving charge collection efficiency, increasing diffusion length, accelerating dye regeneration and narrowing the density of states.