Improved conversion efficiency of CdS quantum dot-sensitized TiO₂ nanotube-arrays using CuInS₂ as a co-sensitizer and an energy barrier layer

A thin layer of CuInS₂ and CdS quantum dots (QDs) is deposited on TiO₂ nanotube arrays (TNTs) to form CdS/CuInS₂/TNTs photoelectrodes. The CuInS₂ layer is prepared by a successive ionic layer absorption and reaction method, and the CdS QDs are deposited by a chemical bath deposition method. The CuInS₂ layer acts as both a co-sensitizer and an energy barrier layer between TNTs and CdS QDs. The deposited CuInS₂ layer significantly extends the visible-light response of CdS-sensitized TNTs into 500-700 nm wavelength range. As a consequence, the photoelectrochemical response of the CdS/CuInS₂/TNTs electrodes is much improved compared with CdS sensitized TNTs. The CdS/CuInS₂/TNTs electrodes exhibit a maximum power conversion efficiency of 7.3%, which is a 120% improvement compared with the highest efficiency of 3.3% for CdS/TNTs electrodes in our study. The improved efficiency is mainly due to the increased absorbance and the reduced recombination between the photoinjected electrons and the redox ions from the electrolyte, resulting from the formation of a CuInS₂ layer.