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.