Band gap narrowing and prolongation of carrier lifetime in solution-processed CeO2/CdS thin films for application as photoanodes in quantum dot sensitized solar cells

Abstract

Band gap energy of more than 3.5 eV for CeO2 nanostructures restricts their efficacies in visible light harvesting. But, reduction in the optical band gap and efficient separation of photogenerated charge carriers can be achieved by forming heterojunction of CeO2 with suitable narrow band gap semiconductors such as CdS and CdSe. In this way, the energy application of CeO2 nanostructured thin films can be extended to visible region of the electromagnetic spectrum. For the above purpose, we deposited CeO2/CdS nanocomposite thin films on fluorine-doped tin oxide (FTO) glass substrates by sol-gel spin coating followed by chemical bath deposition (CBD) process. The structure, phase, morphology, surface topography, as well as interface charge transfer dynamics of the nanocomposites were systematically investigated by various characterization techniques. The CeO2/CdS thin films with CdS bath deposition time of 60 min showed a polydispersity index of 0.19 with an average particle size of 8.8 nm. The observed band gap for one of the CeO2/CdS thin films was 2.19 eV which was well below the bulk band gap for CeO2. The prolongation of carrier lifetime (~8.24 μs) in CeO2/CdS thin film was established from photoluminescence decay analysis. The dominance of radiative recombination in the nanocomposite implies a reduction in the rate of non-radiative re-combinations. This situation favors the separation of more number of e--h+ pairs. The interface formed between the two phases facilitates transportation of separated carriers. An enhancement in the electrical conductivity of the CeO2 occurred when a surface CdS layer was added. A narrowed band gap, prolonged carrier lifetime, and improved electrical conductivity of CeO2/CdS films may lead to enhanced charge transportation ability. The significant values of Jsc and Voc obtained from the photoelectrochemical solar cell measurements prove that CeO2/CdS thin films are suitable for photovoltaic applications.