The performance of four thin-film photovoltaic modules is analyzed after an initial stabilization
period and a subsequent outdoor exposition. The seasonal variations and the degradation rates
of a single-junction hydrogenated amorphous silicon (a-Si:H) module, a tandem amorphous
microcrystalline Silicon (a-Si/𝜇c-Si) module, a heterostructure cadmium sulfide-cadmium tel luride (CdS/CdTe) module and a copper indium gallium selenide (CIGS) are examined and
correlated to spectral changes. The I-V curves have been measured every five minutes; the
electrical parameters and parasitic resistances have been identified. By exploiting a number of
experimental measurements acquired within a narrow interval of irradiance and cell temperature,
a novel mathematical model has been developed and fitted: it considers a stationary seasonal
variation component and a linear long-term degradation component. The results show annual
power degradation rates of 4.0% for the a-Si:H module, 3.4% for the a-Si/𝜇c-Si and 3.5% for the
CdS/CdTe, whereas for the CIGS module the annual degradation is not significant, i.e. 0.2%.