Study of the Efficiency of Photovoltaic Modules of Monocrystalline (Mono-Si), Polycrystalline Silicon (Poly-Si) and Cadmium Telluride (CdTe) Thin-Film Modules After 15 Years of Operation in the Conditions of Southeastern Europe, Bulgaria

The long-term degradation of photovoltaic (PV) modules under real operating conditions remains a critical factor for accurately assessing lifetime energy yield and economic performance. In this study, the field performance of thin-film cadmium telluride (CdTe), polycrystalline silicon (poly-Si), and monocrystalline silicon (mono-Si) photovoltaic modules was evaluated after 15 years of continuous outdoor operation in a temperate continental climate in Southeastern Europe (Sliven region, Bulgaria). All modules were installed at the same site and operated under identical mounting and environmental conditions, enabling a direct comparison between technologies. The results after electrical characterization reveal a total efficiency reduction of 12.3% for CdTe modules, corresponding to an average degradation rate of approximately 0.82% per year. In contrast, polycrystalline silicon modules exhibit a significantly lower efficiency loss of 1.2% (≈0.08% per year), while monocrystalline silicon modules show the lowest degradation, with only a 0.4% decrease over the 15-year period (≈0.03% per year). Visual inspection identified localized degradation features, including surface alterations and defects, which serve as qualitative indicators of long-term material aging. The findings indicate that crystalline silicon technologies demonstrate substantially higher long-term stability than thin-film CdTe modules under moderate continental climatic conditions. The presented long-term field data contribute to improved understanding of climate-specific degradation behavior and support more accurate lifetime performance modeling for photovoltaic systems in similar environments.