Chemical States and Local Structure in Cu-Deficient CuInSe2 Thin Films: Insights into Engineering and Bandgap Narrowing

TL;DR

This study investigates how Cu deficiency in CuInSe2 thin films affects their local structure and bandgap, revealing potential for tunable, stable, and non-toxic photovoltaic applications.

Contribution

It provides detailed insights into the chemical states and local structural changes in Cu-deficient CuInSe2, linking these to bandgap narrowing and photovoltaic material optimization.

Findings

Cu deficiency stabilizes chalcopyrite structure in thin films.

Bandgap narrows from 1.51 eV to 1.4 eV with Cu loss.

In3+d 5s orbital reconstruction causes bandgap narrowing.

Abstract

The Cu-deficient CuxInSe2 (x larger than 0.3) phase can be stabilized as a thin film. A uniform Cu-deficient composition with a chalcopyrite structure was obtained by the precision engineering of a two-step synthesis process involving electron-beam evaporation and Se vapor deposition. Detailed structural and chemical analyses were performed employing various X-ray and microscopic techniques to demonstrate that the chemical states and local structure in the Cu-Se-In tetrahedral networks change with the loss of Cu, the In-Se bond becomes shorter, and the In ions become excessively oxidized without phase separation. Moreover, the results indicate that the bandgap narrowing is primarily attributed to the reconstruction of In3+d 5s orbital states. The bandgap narrows from 1.51 eV to 1.4 eV, which is optimal for the photon absorber. Therefore, cation-deficient selenide is promising for stable nontoxic photovoltaics with tunable bandgaps.