Influence of TiO₂ film/substrate contact on photovoltaic performance and improved efficiency in dye-sensitized solar cells

The influence of the nano-porous TiO₂ film/FTO substrate contact on the photovoltaic performance of dye-sensitized solar cells is investigated by controlling film/FTO substrate contact through the use of insulating Al₂O₃ clusters and TiO₂ underlayer at the interface. The electrochemical properties of the DSCs are examined to correlate the photovoltaic performance and the film/FTO substrate contact condition. Results show that the short-circuit current density and thereby the photo-to-electric energy conversion efficiency of the DSCs are significantly decreased by increasing Al₂O₃ clusters coverage ratio. A modified equivalent circuit model is proposed to understand the relationship between the electric contact resistance and short-circuit current density. Although the increased electric contact resistance itself presents no significant effect on the photovoltaic performance, the decreased performance is mainly attributed to the decreased electron collection in the cell due to the presence of Al₂O₃ clusters at the film/FTO substrate interface. The result reveals that the film/substrate contact should not be negligible, even though the electric contact resistance of this interface can be neglected. The mechanism by which the film/FTO substrate contact affects cells performance reveals that the porous film cannot be considered as independent column structure but as a network structure being able to transfer electron both perpendicularly and laterally. A spray-deposited TiO₂ underlayer was applied to improve the film/FTO substrate interface contact resulting in an increased J_sc and thereby a higher energy conversion efficiency.