Optical parameters of Cu(In,Ga)Se <sub>2</sub> thin films in polyimide-based solar cells
The structure and optical properties of Cu(In,Ga)Se <sub>2</sub> thin films and cells deposited on Mo­coated polyimide at temperatures 430 and 450 °C are studied by the experimental techniques. The band gap energy E <sub>g</sub> of the Cu(In,Ga)Se <sub>2</sub> thin films at 4.2 K is determined by employing photoluminescence excitation spectroscopy. The spectral positions of the photoluminescence bands are about 190 - 270 meV below the band gap of E <sub>g</sub>. Such spectral positions and excitation power dependences of the band suggest that these bands have a deep donor­acceptor (DA) recombination nature influenced by a strong spatially fluctuating potential. The most likely defects, involved in the DA recombination, are the Cu vacancy (V <sub>Cu</sub>) as acceptors and the Se vacancy (V <sub>Se</sub>) or In <sub>Cu</sub> substitutional defects as donors.