Quantitative measurement of transport properties: Ag-doped nanocrystalline CdS thin films

Abstract

This work highlights the transport properties of undoped and Ag doped nc-CdS thin films for optoelectronic devices. nc-CdS, nc-CdS:Ag 1% and nc-CdS:Ag 5% thin films were prepared by the thermal vacuum evaporation method. X-ray diffraction and scanning electron microscope studies show that the Ag doping affects the structural morphology of the films and the average grain size decreases at low Ag doping concentration whereas it increases at high Ag doping concentration in nc-CdS thin films. We have presented a study for the evaluation of the transport parameters like the drift mobility, recombination lifetime of carriers and mobility–lifetime product of charge carriers. Time-of-flight, steady-state photo-conductivity, and steady-state photocarrier Grating techniques are utilized to measure the mobility, mobility–lifetime product and ambipolar diffusion length of charge carriers. The mobility of electrons (µ_e) and holes (µ_h), diffusion length (L) and mobility–lifetime (µτ)_h product of holes varied strongly and systematically with grain-size. The mobility–lifetime (µτ)_e product of majority carriers has been estimated from the temperature dependence of the photoconductivity, which increases with increasing temperature and doping. For nc-CdS:Ag 5%, µ_e, µ_h increases over two orders of magnitude and (µτ)_e increases over an order of magnitude compared to that of nc-CdS. The recombination lifetime of carriers increased significantly in doped and undoped nc-CdS as compared to bulk CdS. We observe that the transport properties increase significantly with higher Ag doping concentration in nc-CdS thin films.