Formation of configurable uniform CdSeTe thin films by close-space sublimation deposition of multiple alternating CdSe and CdTe layers

TL;DR

Researchers developed a new method to create uniform CdSeTe thin films for solar cells by alternating CdSe and CdTe layers, but the process can create voids that need to be controlled.

Contribution

A multilayer-based deposition method for CdSeTe thin films with precise composition control and research flexibility is introduced.

Findings

Alternating CdSe and CdTe layers followed by annealing produces high-quality CdSeTe films with uniform grain size and low roughness.

The method tends to form voids due to CdTe resublimation during CdSe layer deposition, which persist after CdCl2 annealing.

The process window to avoid voiding is narrow, limiting its scalability but suitable for fundamental material studies.

Abstract

Incorporation of selenium within cadmium telluride to form the CdSexTe1-x alloy has enabled higher device efficiencies in photovoltaic applications through improved passivation and current collection. Recent investigations of this alloy have simultaneously indicated significant potential for further performance gains as well as potentially serious detrimental characteristics. Exploring how best to utilize this important material requires consistent, configurable film deposition. Absorber layers are often deposited by close-space sublimation, which can produce high-quality films at large scale. It has previously been demonstrated that close-space sublimation of CdSeTe material directly is inconvenient and inflexible; therefore, an alternative method with greater consistency and control over film composition is sought. In this work, CdSeTe films were formed by close-space sublimation of alternating CdSe and CdTe layers followed by a high-temperature annealing in the presence of CdCl2. Under the optimal deposition conditions, this technique was shown to produce high-quality films of CdSeTe with homogeneous elemental distribution, uniform grain size, and low roughness. However, this method also exhibited a significant tendency to form numerous voids which largely persist after CdCl2 annealing. The source of this porosity was investigated and determined to primarily be resublimation of CdTe during the higher-temperature deposition of the CdSe layers. The process window to prevent voiding was observed to be rather small; while this would be a significant detriment in a production setting, it is less important in the targeted application for this approach, which is fundamental material and device investigations.

This manuscript describes a multilayer-based method for depositing thin films of the photovoltaic alloy CdSeTe with precise control of elemental composition, which provides research-supporting flexibility not afforded by traditional techniques.

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