Insights into the Degradation of Amorphous Silicon Passivation Layer for Heterojunction Solar Cells

Heterojunction (HJ) crystalline silicon (c-Si) solar cells outstanding performance relies heavily on the excellent passivation provided by the amorphous Si (a-Si:H) layer. However, recombination at the a-Si:H/c-Si interface can vary over time and become particularly detrimental for HJ modules performance making the full understanding of the recombination mechanisms at play of paramount importance. In this work, the variation of effective lifetime for high-quality n-type FZ c-Si substrates coated with a-Si:H(i) layer after several processing steps and over a period of 28 months is tracked. The root cause for degradation is identified by experimentally evaluating the surface recombination velocity (SRV) temperature- and injection-dependence before and after degradation has occurred. By applying a model for the recombination at the a-Si:H/c-Si interface to temperature- and injection-dependent SRV data, the authors are able to assess the lifetime decay as entirely ascribed to a loss of chemical passivation. Upon re-annealing the samples, only a partial recovery of lifetime is obtained suggesting that effusion of hydrogen from the a-Si:H layer has occurred. These results indicate that the usage of a capping layer is needed whereas a thorough engineering of the a-Si:H(i) layer thickness may be necessary to avoid the loss of performance of a-Si based heterojunction structures and modules.