High efficiency a-Si/a-Si tandem solar cells

The authors report results on a-Si/a-Si tandem solar cells fabricated using niobium oxide (NbO_x) as the recombination layer in the tunneling junction and an advanced doping gas, trimethylboron (B(CH₃)₃), in the p⁺ layer. The tunneling junction has a low series resistance and minimizes optical loss. The advantage of the NbO_x layer is its high transparency; hence, a relatively thick (~10-nm) layer can be used. The ability to use a thick oxide layer is important for the implementation of the tunneling junction in the production of large-area panels. The best 1 cm² area a-Si/a-Si tandem cell reached 10.4% conversion efficiency, as confirmed by SERI. Comparison of the effects of extended light soaking on the performance of single junction and tandem devices indicates that the tandem cells are more stable. B(CH₃)₃ appears to be a better doping gas than B ₂H₆ for the p layers in the tandem cells