Photoconductivity of CdS-CdSe granular films: influence of microstructure

TL;DR

This study investigates how microstructure influences the photoconductivity of CdS-CdSe granular films, revealing optimal heat-treatment conditions and mechanisms similar to monocrystals, with environmental effects on performance.

Contribution

It provides new insights into the microstructure-dependent photoconductivity of CdS-CdSe films and identifies optimal heat-treatment conditions for maximum photoconductivity.

Findings

Maximum photoconductivity occurs at intermediate heat-treatment.

Charge transfer mechanism is similar to that in CdSe monocrystals.

Suppressing oxygen increases photoconductivity at high temperatures.

Abstract

We study experimentally the photoconductivity of CdS-CdSe sintered granular films obtained by the screen printing method. We mostly focus on the dependences of photoconductivity on film's microstructure, which varies with changing heat-treatment conditions. The maximum photoconductivity 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 intermediate (optimal) time. 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 peculiar nonmonotonic shape as in CdSe monocrystals, from which we conclude that the basic mechanism is also the same. It is suggested that the optimal heat-treatment time appears as a result of a competition between two 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 increases photoconductivity, especially at high temperatures.