First-principles study of carbon impurities in CuIn$_{1-x}$Ga$_x$Se$_{2}$, present in nonvacuum synthesis methods

TL;DR

This study uses first-principles calculations to analyze carbon impurities in CuIn$_{1-x}$Ga$_x$Se$_{2}$, revealing their electronic behavior and suggesting they are unlikely to form during nonvacuum synthesis due to high formation energies.

Contribution

It provides the first detailed computational analysis of carbon impurities in CuIn$_{1-x}$Ga$_x$Se$_{2}$, highlighting their electronic levels and formation likelihood in nonvacuum processes.

Findings

C$_{ ext{Cu}}$ acts as a shallow donor

C$_{ ext{In}}$ and interstitial C create deep donor levels

High formation energies suggest impurities are unlikely to form

Abstract

A first-principles study of the structural and electronic properies of carbon impurities in CuIn$_{1-x}$Ga$_x$Se$_{2}$ is presented. Carbon is present in organic molecules in the precursor solutions used in nonvacuum growth methods, making more efficient use of material, time and energy than traditional vacuum methods. The formation energies of several carbon impurities are calculated using the hybrid HSE06 functional. C$_{\mathrm{Cu}}$ acts as a shallow donor, C$_{\mathrm{In}}$ and interstitial C yield deep donor levels in CuInSe$_{2}$, while in CuGaSe$_{2}$ C$_{\mathrm{Ga}}$ and interstitial C act as deep amphoteric defects. So, if present, these defects reduce the majority carrier (hole) concentration by compensating the acceptor levels and become trap states for the photogenerated minority carriers (electrons). However, the formation energies of the calculated carbon impurities are high, even under C-rich growth conditions. Therefore, these impurities are not likely to form and will probably be expelled to the intergranular region and out of the absorber layer.