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-mu c-Si:H) seed layer on the optoelectronic properties and crystallisation behaviour of n-type hydrogenated microcrystalline silicon oxide (n-mu c-SiOx:H) front contact layers. The presence of a non-oxidic n-mu c-Si:H seed layer can reduce the thickness and refractive index of the n-mu c-SiOx:H front layer significantly while maintaining a high degree of crystallisation and excellent conductivity. This leads to increase in short-circuit current density (J(sc)) by 2.64% and open-circuit voltage (V-oc) 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 V-oc 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-mu c-SiOx:H front layer yields lower internal recombination losses. In conjunction with an optimised n-mu c-Si:H seed layer and n-mu c-SiOx:H front layer, we obtained a high conversion efficiency value of 21.8% with V-oc of 727 mV, J(sc) of 39 mA/cm(2), and FF of 77% among the fabricated cells in laboratory.