Self-assembled ultrathin titania nanosheets as blocking layers for significantly enhanced photocurrent and photovoltage of dye-sensitized solar cells

Abstract

To retard charge recombination efficiently in dye-sensitized solar cells (DSSCs), an ultrathin film of single-crystal titania nanosheets is pre-coated on a conductive substrate as a blocking layer through electrostatic layer-by-layer self-assembly. The marked effect of the self-assembled titania nanosheet film is the remarkable enhancements of short-circuit photocurrent by 45%, from 12.57 to 18.24 mA cm⁻², and open-circuit photovoltage by 16%, from 690 to 798 mV. As a consequence, the power conversion efficiency is improved notably by 61% from 6.50% to 10.48% when a 50 nm thick titania nanosheet film is pre-coated as a blocking layer, which is also superior to that obtained by using a pyrolyzed blocking layer (7.79%) and the well-known TiCl₄ treatment (efficiency of 7.98%). Controlled intensity modulated photovoltage/photocurrent spectra reveal that the self-assembled blocking layer of titania nanosheets not only retards charge recombination but also increases the charge collection efficiency, accounting for the remarkable increase in open-circuit photovoltage and short-circuit photocurrent, respectively. This finding implies that pre-deposition of an ultrathin TiO₂ nanosheet layer on the conductive substrate would be an ideal strategy to remarkably increase short-circuit photocurrent and open-circuit photovoltage simultaneously and thus to enhance the power conversion efficiency of DSSCs remarkably.