Kinetic Processes and surfactant design of Group I elements on CZTS (1-1-2-) surface

TL;DR

This study uses first-principles calculations to explore how Group I elements like Na, K, and Cs act as surfactants on CZTS surfaces, revealing their effects on defect suppression and growth control for improved solar cell materials.

Contribution

It provides a systematic analysis of the surfactant effects of Group I elements on CZTS surfaces, offering design principles for optimizing thin-film solar cell growth.

Findings

Group I elements protect Cu sites but can form metastable phases.

Diffusion barriers around 400 meV influence growth conditions.

Systematic comparison of Na, K, Cs surfactant effects.

Abstract

Cu2ZnSnS4 (CZTS) is a promising thin-film solar-cell material consisted of earth abundant and nontoxic elements. Yet, there exists a fundamental bottle neck that hinders the performance of the device due to complexed intrinsic defects properties and detrimental secondary phases. Recently, it was proven experimentally that Na and K in co-evaporation growth of CZTS can enlarge the grain size and suppress formation of ZnS secondary phase near surface, but the reasons are not well understood. We used first principle calculations to investigate the kinetic processes on CZTS (1-1-2-) surface involving Group I elements, including Na, K, and Cs, to demonstrate their surfactant effects. Both the structure of the reconstructed surfaces involving Group I elements and various diffusion paths of a Zn ad-atom in these reconstructed surfaces were explored. The advantages and concerns of the surfactant effects of Na, K, Cs, were systematically compared and discussed. Although Group I elements protect Cu sites on the subsurface layer, a disordered metastable configuration with a diffusion barrier of about 400meV was found. Therefore, a precise control of growth condition is essential to avoid the metastable phase. In addition, our studies provide a systematical design principle for surfactant effects during the growth.