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

Abstract

Post-deposition air-annealing effects of Cu(In,Ga)Se2 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 ZnO/CdS/Cu(In,Ga)Se2 heterojunction solar cells shows that the Cu(In,Ga)Se2 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.

Defect Spectroscopy on Damp-Heat Treated ZnO/CdS/Cu(In,Ga)(S,Se)2/Mo Heterojunction Solar Cells

“…The experimental difference is that DH testing is performed at a relatively low temperature (85 • C) combined with humidity treatment, whereas air (or oxygen) annealing happens at temperatures of above 200 • C without being subject to water vapor. The two major effects of oxygenation are a passivation of Se vacancies at the CdS/chalcopyrite interface and Cu migration into the absorber bulk [7]. We emphasize that both explanations are model pictures proposed to capture various experimental findings, but have been observed directly neither for air annealing nor for DH treatment.…”

“…Let us discuss the relation of the DH exposure and air annealing (oxygenation [6,7]) phenomena to each other. The experimental difference is that DH testing is performed at a relatively low temperature (85 • C) combined with humidity treatment, whereas air (or oxygen) annealing happens at temperatures of above 200 • C without being subject to water vapor.…”

“…In particular, the influence of water vapor and oxygen on the charge transport properties of the heterostructure 0932-0784 / 03 / 1200-0691 $ 06.00 c 2003 Verlag der Zeitschrift für Naturforschung, Tübingen · http://znaturforsch.com cells is the focus of our paper. Oxygen-related phenomena have been discussed in the literature for more than a decade [6], and extensive work covering oxygenation [7] and related topics [8] is available. The impact of water vapor on the cell characteristics goes beyond the magnitude of the influence of oxygen [9], even though both effects are closely related to each other.…”

“…Such metastability most likely originates from the absorber layer [9]. In this context (also concerning the differences of DH exposure to oxygenation [7]), it should be mentioned that the effect of dry air annealing (85 • C, 2000 h) on the electronic properties is much smaller than that of DH exposure [9].…”

Time-of-flight studies on TiO2/CuInS2 heterojunctions

“…When the interface state concentration is high, Fermi-level pinning may occur, which affects the internal electric field at the junction. 10,11 Not much is known about the influence of interface states on the charge carrier transport in chalcopyrite-based solar cells. The present study is directed to elucidate the relationship between the density of interface states and the charge carrier dynamics in TiO 2 / CuInS 2 heterojunctions using the time-offlight ͑TOF͒ technique.…”