Thin p++ μc-Si layers for use as back surface field in p-type silicon heterojunction solar cells

A p++ μc-Si:H layer for the use as deposited back surface field (BSF) has to be highly doped in order to achieve a truly functioning BSF. We optimized a p++ μc-Si:H deposited on glass in terms of activation energy by varying the deposition parameters. The activation energy of the 30 nm thick μc p++ layer on glass reaches an optimum at Ea = 0.15 ± 0.01 eV at a flow ratio TMB/SiH4 of 0.016. To obtain a measure of the value of the activation energy of the p++ μc-Si:H layer on wafers, the layers were deposited on highly crystalline intrinsic μc-Si:H layers due to which the incubation layer of the μc p-layer can be avoided. The optimum TMB flow is higher on the microcrystalline i-layer, simulating the wafer, and the activation energy is greatly reduced to Ea = 0.08 ± 0.01 eV. The best bifacial silicon heterojunction cell (SHJ) using such a low activation energy layer as the deposited BSF has an active area efficiency of 14.87%. This is high compared to the efficiency of 11.86% for the cell without BSF, due to a 22.4 mV higher Voc and a 4.6 mA/cm2 higher Jsc, demonstrating that the deposited BSF performs successfully.