Electrical and Electroluminescence Evaluation of 17 Year Old Monocrystalline Silicon Building Integrated Photovoltaic Modules

Longterm reliable operation of photovoltaic modules is necessary to ensure the technology is key to reducing the cost of solar energy. However, degradation mechanisms in the field are not always consistent or predictable. We present a dataset of 72 building integrated photovoltaic (BIPV) monocrystalline silicon modules that were operated on the rooftop of a building at the National Institute of Standards and Technology (NIST) for 17 years. Electrical performance shows a majority of modules exhibit severe degradation beyond the typical ~ 1% loss per year. Xenon lamp flash solar simulator I-V curve measurements were compared to historical performance data to verify that the degradation seen in I-V measurements also existed during operation. While the open circuit voltage remained unchanged for most modules, a decrease in short circuit current and increase in series resistance led to deterioration of the short circuit current and maximum power. Hyperspectral electroluminescence (EL) imaging of three modules shows further evidence of significant series resistance increase for degraded modules. A module with a standard degradation rate has a clear EL image while a very degraded module has a very faint EL emittance at the same voltage due to series resistance losses. Further work will include calibrating the EL imaging system to quantify poor performing and current limiting regions within the module and determine the I-V curves of individual cells using the reciprocity relationship between EL and External Quantum Efficiency.