Uncertainty in PV Module Measurement---Part II: Verification of Rated Power and Stability Problems

Standard testing condition power of PV modules, especially thin-film modules, is not a constant value. Exposure to irradiance and temperature, as well as electrical bias and dark storage, cause changes in STC power, which complicate the interpretation of quality assurance measurements and tests. This paper presents a literature review and summarizes current knowledge of metastability and stability problems of PV modules with focus on their influence on quality assurance, especially the verification of rated module power. Three groups of thin-film PV technologies are addressed: amorphous silicon, cadmium telluride (CdTe), and chalcopyrite technologies (CIGS). Amorphous silicon is affected mostly by light-induced degradation and seasonal annealing, whereas sensitivity to dark storage is the most relevant effect for CdTe and CIGS PV modules. In conclusion, it is not possible to quantify the impact of specific (meta)stability effects on measured STC power for all modules of one technology in general, as there is a strong sensitivity to the exact composure and processing of the modules. With regard to the total uncertainty inherent in the verification of rated power of new modules, this paper introduces the concept of differentiating between measurement-related and module-stability-related uncertainty and suggests a procedure for its determination.