Effect of synthesis conditions on large and persistent photoconductivity in chemical bath deposited cadmium sulfide thin films

Abstract

Persistent photoconductivity (PPC) has been observed in numerous semiconductors and dielectrics. It refers to the phenomenon that photoconductivity can last for a long time after photo-excitation is terminated. In most cases, the causes of PPC are unknown, and the connections between synthesis conditions, composition, morphology, and PPC are unexplored. Here, we study the effect of chemical bath deposition parameters on PPC in CdS thin films, including bath temperature, concentration of reactants, deposition time, stirring speed and cation source. By tuning the chemical conditions during deposition, we achieved near stoichiometric CdS thin films exhibiting large photoconductive responses (up to 109 times conductivity increase in simulated solar light compared to the dark) that persist for a long time (decay time constant up to 10 hours) after illumination is turned off. We offer experimental support for the hypothesis that sulfur vacancies are responsible for PPC in CdS, and we discuss the role of trap states in PPC are discussed. We also discuss the implications of giant and persistent photoconductivity for thin film solar cells in which CdS is used as the n-type buffer layer.