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

Abstract

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 Cu1−xIn1+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.