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)GaX)(SYSe(1-Y))2-based thin-film solar cells. To avoid stability issues, the fraction of metastable Zn(OH)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.
Journal of Applied Physics
- Pub Date:
- September 2005
- Photoelectric conversion: solar cells and arrays;
- Surface states band structure electron density of states;
- Adsorbed layers and thin films;
- Clean metal semiconductor and insulator surfaces