The analysis of the degradation of tandem amorphous silicon (a-Si:H) and microcrystalline silicon ( -Si:H): Micromorph thin-film photovoltaic (TFPV) modules and its impact on the output power of a PV array under outdoor long term exposure located in Jaén (Spain) is addressed in this work. Furthermore, the evolution of main solar cell model parameters is evaluated by means of parameters extraction techniques from monitored data of the PV system in real operation of work. The degradation rate and the stabilization period of micromorph TFPV modules, as well as the results of the evolution of each of the solar cell model parameters along the outdoor long-term exposure are analysed in order to gain a better understanding of changes in performance of micromorph TFPV modules and the behaviour of the output power of the PV generator.