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The influence of atmospheric species on the degradation of aluminum doped zinc oxide and Cu(In,Ga)Se2 solar cells

Aluminum doped zinc oxide (ZnO:Al) layers were exposed to the atmospheric gases carbondioxide (CO2), oxygen (O2), nitrogen (N2) and air as well as liquid H2O purged with these gases, in order to investigate the chemical degradation behavior of these layers. The samples were analyzed by electrical, compositional and optical measurements before, during and after exposure to these conditions in order to follow the degradation behavior of these layers in time. We have shown that ZnO:Al layers degraded in the presence of a mixture of H2O and CO2. Individually, CO2 does not impact the degradation at all during the tested period, while the individual impact of H2O is small. However, when CO2 is also present, the concentration of OH increases greatly in the bulk and even more at the air/ZnO:Al and the ZnO:Al/glass interfaces. Carbon based species are then also present, indicating that Zn5(OH)6(CO3)2 is also formed at the grain boundaries. The degradation of ZnO:Al was accompanied by the occurrence of holes in the ZnO:Al layer near the ZnO:Al/glass interface. The impact of gaseous O2 as well as water purged with N2 and O2 on ZnO:Al degradation is very small. Complete Cu(In,Ga)Se2 solar cells were also exposed to unpurged liquid H2O and H2O purged with CO2, O2, N2 and air. The samples exposed to H2O purged with air and CO2 showed a rapid decrease in efficiency after approximately 180 hours of exposure. This efficiency decrease is mainly driven by a very rapid decrease in current density and an increase in series resistance.