Zn(O,OH) layers in chalcopyrite thin-film solar cells: Valence-band maximum versus composition

Zn(O,OH) layers deposited by the ion layer gas reaction (ILGAR) technique have the potential to replace the conventionally used CdS buffer layer in Cu ( In ( 1 − X ) Ga X ) ( S Y Se ( 1 − Y ) ) 2 -based thin-film solar cells. To avoid stability issues, the fraction of metastable Zn ( O H ) 2 should be reduced in the final buffer layer. However, hydroxide-poor or -free ZnO "buffers" result in noncompetitive devices. We have therefore investigated the impact of different oxide/hydroxide ratios on the electronic band alignment at the absorber/buffer heterointerface. The surface composition as well as the position of the valence-band maximum (VBM) of respective ILGAR-Zn(O,OH) samples was determined by photoelectron spectroscopy. The position of the conduction-band minimum (CBM) was estimated using optical band gaps determined from optical reflection/transmission measurements. From the comparison of these VBM and CBM values with the respective values of the absorber surface, predictions are made in terms of valence- and conduction-band offsets at the crucial absorber/buffer interface. The results are compared with previous findings, and the drawn conclusions are correlated with the performance of respective solar cell devices.