Oxygenation and air-annealing effects on the electronic properties of Cu(In,Ga)Se-2 films and devices

Post-deposition air-annealing effects of Cu(In,Ga)Se-2 based thin films and heterojunction solar cell devices are studied by photoelectron spectroscop y and admittance spectroscopy. Ultraviolet photoelectron spectroscopy revea ls type inversion at the surface of the as-prepared films, which is elimina ted after exposure of several minutes to air due to the passivation of surf ace Se deficiencies. X-ray photoelectron spectroscopy demonstrates that air annealing at 200 degrees C leads to a decreased Cu concentration at the fi lm surface. Admittance spectroscopy of complete ZnO/CdS/Cu(In,Ga)Se-2 heter ojunction solar cells shows that the Cu(In,Ga)Se-2 surface type inversion i s restored by the chemical bath used for CdS deposition. Air annealing of t he finished devices at 200 degrees C reduces the type inversion again due t o defect passivation. Our results also show that oxygenation leads to a cha rge redistribution and to a significant compensation of the effective accep tor density in the bulk of the absorber. This is consistent with the releas e of Cu from the absorber surface and its redistribution in the bulk. (C) 1 999 American Institute of Physics. [S0021-8979(99)02813-3].