Correlating the photovoltaic performance of alumina modified dye-sensitized solar cells with the properties of metal-free organic sensitizers

Abstract

Low photovoltage for metal-free organic dye-sensitized solar cells (DSSCs) has been restricting them surpassing the equivalent cells based on ruthenium complex sensitizers (typically N719). In this study, an alumina (Al2O3) layer was employed to cover on the titania (TiO2) surface before dye loading in order to improve the photovoltage of the organic dye based DSSCs. The open-circuit voltage (V-oc) of the solar cell was found to increase by 0.02-0.06 V for a variety of organic sensitizers by the presence of the Al2O3 overcoat, while the photocurrent (J(sc)) was observed highly dependent on the property of the sensitizer. Results show that for the sensitizers with a low Low-Unoccupied-Molecular-Orbital (LUMO) level or a large molecular size, the DSSC yielded a decreased J(sc) due to two factors, (i) an insufficient driving force for electron injection due to the upshift of the conduction band level of the TiO2/Al2O3 electrode, and (ii) a decrease of dye adsorption amount for the TiO2/Al2O3 electrode in comparison with the referenced TiO2 electrode because of the reduced pore size and porosity by Al2O3 coating. However, for the sensitizers with a high (more negative) LUMO level and a small molecular size, the J(sc) of the DSSCs based on the TiO2/Al2O3 electrode was very close to that composed of only TiO2 electrode. Using such organic dyes sensitizing on the TiO2/Al2O3 electrode, over 8% in the efficiency of the DSSCs was facilely attained and particularly an efficiency of 8.72% was obtained for a cocktail solar cell with two organic dyes. (C) 2011 Elsevier B.V. All rights reserved.