Effect of loading on long term performance of single junction amorphous silicon modules

The long-term behaviour of photovoltaic modules governs their economic feasibility. Amorphous silicon devices experience an initial degradation which is partially reversible due to thermal annealing. This degradation is commonly investigated using indoor light-soaking setups, typically leaving the devices open circuited. Alternatively, devices would be placed outdoors and in most cases are left open circuited as well. The outdoor exposure is closer to realistic performance as the devices will experience continuously changing environments as well as seasonal annealing and degradation cycles. However in both cases, the loading is not as it would be in realistic operation, where devices are operating at the maximum power point. The aim of this study is to verify if the practice of keeping modules at Voc is appropriate for outdoor measurements. The differences in the long term behaviour of the devices due to different loading strategies are investigated for open circuit, short circuit, fixed resistor loading, and MPPT. It is investigated for single junctions with more than 1 year outdoor exposure. It is shown that one needs to at least load the modules resistively to achieve realistic degradation rates, as the degradation at Voc overestimates by 23% and at Isc underestimates by 7.69% relative to the MPPT condition.