Ultra-thin stack of n-type hydrogenated microcrystalline silicon and silicon oxide front contact layer for rear-emitter silicon heterojunction solar cells

Abstract We demonstrate the clear advantage of a n-type hydrogenated microcrystalline silicon (n-µc-Si:H) seed layer on the optoelectronic properties and crystallisation behaviour of n-type hydrogenated microcrystalline silicon oxide (n-µc-SiOx:H) front contact layers. The presence of a non-oxidic n-µc-Si:H seed layer can reduce the thickness and refractive index of the n-µc-SiOx:H front layer significantly while maintaining a high degree of crystallisation and excellent conductivity. This leads to increase in short-circuit current density (Jsc) by 2.64% and open-circuit voltage (Voc) by 0.56% in comparison to that of a device without the seed layer. The enhancement in Jsc can be attributed to the reduction in parasitic absorption loss in the extremely thin front layer. In addition, the improvement in Voc can result from enhanced surface passivation of the wafer due to seed layer growth in very high hydrogen plasma environment which can play a role as the hydrogen post-plasma treatment. The low thickness of the n-µc-SiOx:H front layer yields lower internal recombination losses. In conjunction with an optimised n-µc-Si:H seed layer and n-µc-SiOx:H front layer, we obtained a high conversion efficiency value of 21.8% with Voc of 727 mV, Jsc of 39 mA/cm2, and FF of 77% among the fabricated cells in laboratory.