Chemical states and local structure in Cu-deficient Cu_xInSe_∼2 thin films: insights into engineering and bandgap narrowing

The Cu-deficient Cu_xInSe_∼2 (x > 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 In^3+δ 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.