Performance increase of tandem amorphous/microcrystalline Si PV devices under variable illumination and temperature conditions

Wear out in the maximum power point condition of tandem microcrystalline Si/amorphous Si thin film PV modules due to extended light soaking is a well-known effect due to the Staebler-Wronski mechanism [1]. However, several studies [2-8] on a-Si:H single junction and tandem solar cells, have shown that, under suitable conditions of electric field in reverse bias, illumination, especially at well-defined wavelengths (700-800 nm), and temperature conditions, an improvement of the cell main electric parameters is possible. In this work, we report on new stress tests performed on tandem amorphous Silicon PV devices (both non-stabilized and stabilized), highlighting the improvement of their electrical performances, founded both in the tests performed on non-stabilized samples (essentially due to the reverse bias stresses carried out in these tests) and in the outdoor tests on stabilized samples (not subjected to reverse bias stresses) going from morning to afternoon illumination. In the latter tests, the efficiency improvement observed in the afternoon is stable overnight and observed also in the early stages of the morning outdoor test in the following day. Possible causes of such effect are discussed.