Influence of impurities and free carriers on the optical properties of CuInSe2

As-grown n-type CuInSe2 single crystals are investigated by electrical, optical transmittance and reflectance, and photoconductivity measurements. It is established that two donor states with ionization energies of (7 +- 1) meV and (225 +- 5) meV and one deep acceptor state with an ionization energy of about 400 meV are simultaneously present in the crystals. The observed absorption due to free electrons can be understood in terms of the scattering mechanisms known to be important in n-type CuInSe2. The temperature coefficient of the gap energy is found to be dEsub(g)/dT = -(1.6 +- 0.3) x 10-4 eV/K from photovoltage measurements. (author)