Accelerated ageing in damp heat (DH) can reduce the stability of chalcopyrite solar modules. The decrease in lateral conductivity of the transparent Al-doped zinc oxide (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. After 1000 h of exposure to DH, 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 thus conclude, that the substrate morphology is the dominant factor on the extent of ZAO degradation. Due to the significant difference in the decrease of conductivity on rough and smooth substrates, we postulate that the extended grain boundaries are the prevailing source of this degradation. (C) 2008 Elsevier B.V. All rights reserved.