Abstract

In the development of kesterite copper zinc tin selenide (CZTSe) thin-film solar cells, the realization of absorber layers with large grains and suppression of unwanted secondary phases have always been important aspects. Incorporation of silver (Ag) in the CZTSe absorber can improve grain growth but also affects both the bulk and interface electronic properties. In this work, the Ag layer was incorporated at the back contact of vacuum-processed CZTSe solar cells and its impact on the absorber growth and overall device performance was investigated. The concentration of Ag in the absorber was varied by changing the thickness of the Ag layer at the bottom of the copper-zinc-tin (CZT) precursor. We found that the grain size in the absorber significantly increased with increasing Ag layer thickness. We further investigated the phase and morphology evolution during annealing of the CZT precursor in selenium atmosphere, both with and without the Ag layer present. Using X-ray diffraction, Raman spectroscopy, and scanning electron microscopy, we demonstrated that Ag incorporation induced a fast formation of larger grains in the early stages, which could be attributed to the formation of a silver selenide (Ag–Se) liquid phase. Time-of-flight secondary ion mass spectrometry depth profiles revealed a homogeneous distribution of Ag across the absorber. An improved power conversion efficiency was obtained for very thin (10 nm) Ag layers – due to an increase in the open-circuit voltage and fill-factor – while thicker layers deteriorate the device properties, mainly due to an increased series resistance. Therefore, the incorporation of small amounts of Ag is suggested to be useful to improve the absorber morphology of CZTSe solar cells while the electronic properties suffer from too high quantities.

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