Development of thin film a-SiO:H/a-Si:H double-junction solar cells and their temperature dependence

Hydrogenated amorphous silicon oxide (a-SiO:H)/hydrogenated amorphous silicon (a-Si:H) double-junction solar cells with a high open-circuit voltage (Voc) and a low temperature coefficient (TC) were developed using a wide bandgap a-SiO:H film as the intrinsic (i) layer of the top cell. It was found that with an increasing carbon dioxide (CO2)/silane (SiH4) ratio, the optical bandgap (Eopt) of the a-SiO:H films increased remarkably while the photogain tended to decrease. By employing an optimized a-SiO:H film as the i top layer of the a-SiO:H/a-Si:H solar cell, an initial conversion efficiency (η) of 10.2% was obtained. This solar cell showed a higher η than the conventional a-Si:H/a-Si:H structure, a result of incremental improvements in the Voc and short-circuit current density (Jsc), which were attributed to the wider bandgap of the intrinsic top layer. It was found that the TC for η of the a-SiO:H/a-Si:H solar cell was − 0.10%/°C, slightly lower than that of the a-Si:H/a-Si:H solar cell, whose TC value is about − 0.15%/°C. The light-induced degradation (LID) ratio for η of the a-SiO:H/a-Si:H solar cell was approximately 19%, which was 2% lower than that of the a-Si:H/a-Si:H solar cell. These results have demonstrated the great potential of the i-a-SiO:H films as absorber layers of top cells in multi-junction silicon-based thin-film solar cells. The a-SiO:H/a-Si:H solar cells with low TCs and low LID ratios are attractive for their potential use in high-temperature environments or tropical regions.