Oxygenation and air-annealing effects on the electronic properties of Cu(In,Ga)Se2 films and devices

Abstract

Post-deposition air-annealing effects of ${\mathrm{Cu(In,Ga)Se}}_2$ based thin films and heterojunction solar cell devices are studied by photoelectron spectroscopy and admittance spectroscopy. Ultraviolet photoelectron spectroscopy reveals type inversion at the surface of the as-prepared films, which is eliminated after exposure of several minutes to air due to the passivation of surface Se deficiencies. X-ray photoelectron spectroscopy demonstrates that air annealing at 200 $°$ C leads to a decreased Cu concentration at the film surface. Admittance spectroscopy of complete ${\mathrm{ZnO/CdS/Cu(In,Ga)Se}}_2$ heterojunction solar cells shows that the ${\mathrm{Cu(In,Ga)Se}}_2$ surface type inversion is restored by the chemical bath used for CdS deposition. Air annealing of the finished devices at 200 $°$ C reduces the type inversion again due to defect passivation. Our results also show that oxygenation leads to a charge redistribution and to a significant compensation of the effective acceptor density in the bulk of the absorber. This is consistent with the release of Cu from the absorber surface and its redistribution in the bulk.