CdS quantum dot sensitized anatase TiO2 hierarchical nanostructures for photovoltaic application

We report on solar cells based on CdS QD sensitized anatase TiO2 hierarchical nanostructures prepared via a facile and controlled hydrothermal process. To adjust the size of CdS QDs, the TiO2 surfaces were first treated with thiolactic acid which provides a binding site for Cd2+ ions. CdS QDs with controllable size are grown onto the TiO2 hierarchical nanostructures. The CdS QDs/TiO2 hierarchical heterojunction nanocomposites display a red shift from the ultraviolet to the visible light region with a longer wavelength for the absorption edge. The matching of band edges between CdS and TiO2 to form a semiconductor heterojunction plays an important role in effective separation of light induced electrons and holes, providing a promising photoanode for its wide absorption spectrum, high electron injection efficiency, and fast electron transference. Under UV-vis irradiation, both CdS and TiO2 are excited. Under visible light irradiation, photons are captured by CdS QDs, with the photogenerated electron-hole pairs rapidly separated into electrons and holes at the interface between TiO2 and CdS QDs. In addition to this, more photoelectrons are generated from TiO2 by harvesting UV photons. TiO2 collects the photoelectrons from CdS and passes them through to the back contact. The holes (from both CdS and TiO2) generated in the process are transferred to the solid-liquid interface. Under one sun illumination, the solar cells based on CdS QDs/TiO2 hierarchical heterojunction nanocomposites with a smaller average size of 7.6 nm for CdS QDs show a maximum short circuit current density of 6.09 mA cm(-2) and a conversion efficiency of 3.06%. The performance improvement of the solar cells based on CdS QDs/TiO2 hierarchical nanostructures can be attributed to the unique microstructure characteristics and the bandgap energy matching between the CdS QDs and TiO2.