Fluorine Treatment of TiO2 for Enhancing Quantum Dot Sensitized Solar Cell Performance

Abstract

Surface treatments of TiO2 nanostructure in semiconductor quantum dot sensitized solar cells (QDSCs) aimed to increase the photovoltaic conversion efficiencies of the solar cells are analyzed. A fluorine treatment, with NH4F or HF, on the TiO2 electrodes leads to a general increase of QDSCs performance in a range of QDSCs using different light absorbing materials: CdS, CdSe, and PbS/CdS. In contrast, no significant effect on QDSC performance has been observed after a TiCl4 treatment conventionally used for high performance dye sensitized solar cells (DSCs). Surface and photoelectrochemical characterization of treated electrodes and full solar cells was carried out by means of X-ray photoelectron spectroscopy (XPS), impedance spectroscopy (IS), and applied bias voltage decay (ABVD), to understand the origin of the beneficial effect of fluorine. It was found that the origin of the enhancement is different depending on the semiconductor material (CdS, CdSe, and PbS/CdS). For CdS and CdSe, the recombination of photoinjected carrier is reduced after F treatment. On the other hand, for PbS/CdS, the treatment accelerates the deposition kinetics of PbS by successive ionic layer adsorption and reaction (SILAR), increasing the amount of deposited material and consequently the light harvesting. Our study indicates, in general, that treatments different from those conventionally used in DSCs are required and, specifically, that F treatment can be systematically used in QDSCs to increase the solar cell performance.