Photoconductivity of CdS-CdSe granular films: influence of microstructure

We study experimentally the photoresistance of CdS-CdSe sintered granular films obtained by the screen printing method. We mostly focus on the dependences of photoresistance on film's microstructure, which varies with changing heat-treatment conditions. The minimum photoresistance is found for samples with compact packing of individual grains, which nevertheless are separated by gaps. Such a microstructure is typical for films heat-treated during an optimal time of 30 min at a temperature of 823 K. In order to understand whether the dominant mechanism of charge transfer is identical with the one in monocrystals, we perform temperature measurements of photoresistance. Corresponding curves have the same nonmonotonic shape as in CdSe monocrystals. Namely, photoresistance first increases with the growth of temperature, and then starts to decrease. Thus we conclude that the basic mechanism is also the same, as in monocrystals, and it is based on two types of centers in the forbidden gap. We suggest that the optimal heat-treatment time depends on two competing mechanisms: improvement of film's connectivity and its oxidation. Photoresistance is also measured in vacuum and in helium atmosphere, which suppress oxygen and water absorption/chemisorption at intergrain boundaries. We demonstrate that this suppression decreases photoresistance, especially at high temperatures.