Influence of nonstoichiometry on the Urbach's tails of absorption spectra for CuInSe₂ single crystals

Optical absorption spectra of CuInSe₂ single crystals were measured for the samples with -0.150≤x≤0.053, where x represents a degree of nonstoichiometry in the formula Cu_1-xIn_1+xSe₂. The Urbach's tail was observed for all samples between 90 K and room temperature. The Urbach's energy, which represents an arbitrary intensity of exciton-phonon interaction, was almost constant for the Cu-rich samples (x<0), while it increased with increasing In composition for the In-rich ones (x>0). Such an increase of the Urbach's energy was explained to be due to enhanced electronic distortion caused by the compositional deviation from stoichiometry in terms of simultaneous influence of electron-phonon interaction and structural disorder.