Greiner, D.; Papathanasiou, N.; Pflug, A.; Ruske, F.; Klenk, R.: Influence of damp heat on the optical and electrical properties of Al-doped zinc oxide (ZAO). Thin Solid Films 517 (2009), p. 2291-2294
10.1016/j.tsf.2008.10.107
Abstract:
Accelerated ageing in damp heat (DH) can reduce the stability of chalcopyrite solar modules. The decrease in lateral conductivity of the transparent ZAO front contact contributes significantly to this effect. We present a study on the optical and electrical properties of ZAO without encapsulation on transparent quartz glass substrates with smooth and rough morphology before and after DH. Measured transmission/reflection curves are evaluated numerically to obtain the Drude mobility and carrier concentration. Comparison of optical and electrical data is helpful in evaluating the carrier transport in inhomogeneous ZAO layers, in particular the contributions of grains, grain boundaries and what we have termed extended grain boundaries. The latter are perturbations in the ZAO growth caused by the substrate microstructure. The lateral electrical conductivity of ZAO on smooth quartz decreases only by a factor of two, whereas it changes up to two orders of magnitude on rough quartz. Carrier concentration and mobility derived from the optical analysis show a systematic but only minor degradation within the grains. We conclude that the degradation depends mostly on the morphology of the substrate. Due to the significant difference in the decrease of conductivity on rough and smooth substrates, we postulate that the extended grain boundaries are the main cause of ZAO degradation.