Phototransport Properties of CuInSe2 Thin Films: The Influence of Na and Planar Defects

Abstract

The presence of in-gap states, especially deep ones, that act as recombination centers in solar-cell absorbers influence the phototransport properties of the films, and thus, strongly affect the performance of the corresponding solar cells. The type and density of such defects is influenced by the deposition process. In particular, for $\mathrm{Cu}({\mathrm{In}}_{1\text{\ensuremath{-}}x}{\mathrm{Ga}}_{x}){\mathrm{Se}}_{2}$, it is well established that the presence of a small amount of alkali metal atoms (e.g., $\mathrm{Na}$) improves the performance of the solar cell. Furthermore, in a coevaporation process, a $\mathrm{Cu}$-rich [[$\mathrm{Cu}$]/([$\mathrm{In}$] + [$\mathrm{Ga}$]) g 1] intermediate step decreases the density of extended structural defects, such as stacking faults. Here, we apply temperature-dependent and intensity-dependent photoconductivity measurements, along with Shockley-Read-Hall model calculations, to study the phototransport properties of $\mathrm{Cu}({\mathrm{In}}_{1\text{\ensuremath{-}}x}{\mathrm{Ga}}_{x}){\mathrm{Se}}_{2}$ films with and without a $\mathrm{Cu}$-rich process step, as well as with and without $\mathrm{Na}$. Our experimental and theoretical results indicate a correlation between the presence of planar defects and the formation of a shallow recombination level. Our results further suggest that $\mathrm{Na}$ eliminates deep recombination centers, and thus, increases the lifetime of the photogenerated charge carriers.