Temperature dependence of the optical energy gap and Urbach's energy of CuIn5Se8

The optical properties of the ordered defect compound CuIn5Se8, which cryst allizes in a hexagonal structure, have been studied by the absorption techn ique. The analysis of the data shows that the band gap energy E-G varies fr om 1.23 to 1.13 eV between 10 and 300 K. It is found that the variation of E-G with temperature is due to the contribution of both acoustic and optica l phonons with a characteristic phonon energy of about 14 meV. The optical absorption coefficient just below the absorption edge varies exponentially with photon energy indicating the presence of Urbach's tail. The phonon ene rgy h nu (p) associated with Urbach's tail, which is found to be 53 meV, is higher than the highest optical phonon mode reported for this compound, wh ich is about 29 meV. The origin of the additional energy is attributed to t he contribution of localized modes produced by structural disorder of low e nergy formation. An empirical relation, also used earlier in the case of 1: 1:2 and other ordered defect compounds of the 1:3:5 phases, explains very w ell the variation of Urbach's energy with temperature. (C) 2001 American In stitute of Physics.