n⁺-Microcrystalline-Silicon Tunnel Layer in Tandem Si-Based Thin Film Solar Cells

In this study, the p-SiC/i-Si/n-Si cell and the p-SiC/i-SiGe/n-Si cell deposited using plasma-enhanced chemical vapor deposition were cascaded for forming the tandem Si-based thin film solar cells to absorb the wide solar spectrum. To further improve the performances of the tandem Si-based thin film solar cells, a 5-nm-thick n⁺-microcrystalline-Si (n⁺-μc-Si) tunnel layer deposited using the laser-assisted plasma-enhanced chemical vapor deposition was inserted between the p-SiC/i-Si/n-Si cell and the p-SiC/i-SiGe/n-Si cell. Since both the plasma and the CO₂ laser were simultaneously utilized to efficiently decompose the reactant and doping gases, the carrier concentration and the carrier mobility of the n⁺-μc-Si tunnel layer were significantly improved. The ohmic contact formed between the p-SiC layer and the n⁺-μc-Si tunnel layer with low resistance was beneficial to the generated current transportation and the carrier recombination rate. Therefore, the conversion efficiency of the tandem solar cells was promoted from 8.57% and 8.82% to 9.91% compared to that without tunnel layer and with 5-nm-thick n⁺-amorphous-Si tunnel layer.