This paper characterizes and compares the degradation mechanisms of Cu(In,Ga)Se-2 (CIGS)-based thin film photovoltaic (PV) modules during exposure to damp heat (85 degrees C/85% RH) for 1000 h and thermal cycling (-40 degrees C/85 degrees C) for 1000 cycles. After damp heat (DH) exposure, the efficiency of the PV modules was reduced from the initial value of similar to 12.5 +/- 0.1% to 10.5 +/- 0.2% due to increase of the resistivity in the AZO and CIGS layers. The optical band gap was also decreased from the initial value of 3.60 eV-3.54 eV after 1000-h DH exposure. This behavior was associated with oxygen adsorption and the generation of hydroxides in the AZO layer. The efficiency of the PV modules after subjection to thermal cycling (TC) was decreased to 11.4 +/- 0.2% due to increase of the resistivity of the AZO and CIGS layers. The increase in the resistivity was interpreted as being due to oxygen adsorption, as well as the formation of micro-cracks in the AZO films. (C) 2014 Elsevier B.V. All rights reserved.