22% efficient dopant-free interdigitated back contact silicon solar cells

In this study, we present dopant-free back contact heterojunction silicon solar cells employing MoOx and MgFx based stacks as hole-and electron-selective contacts deposited using a thermal evaporation process at low temperature. Only two masking steps and one alignment are required in this simple process flow. We investigate the effect of varying the MgFx film thickness as the electron contact layer on the rear side on IBC Si solar cells and define an optimal thickness 1.5 nm of MgFx for high VOC and FF. We compare different electron-selective contact materials including Mg-based and fluoride materials and discuss the suitable combinations. We fabricate dopant-free back contact solar cells by applying a stack of 1.5 nm MgF2/70 nm Al/800 nm Ag films on intrinsic a-Si:H, maintaining excellent passivation and show efficient carrier extraction. A 4.5-cm2 dopant-free back contact solar cells fabricated with these layers enables high VOC up to 709 mV and FF up to 75.6% still limited by series resistance due to too thin metal layers, a pseudo FF of 84.2% is yet measured. The cell exhibits very low front reflection and has outstanding collection efficiency, the IQE reach 98.2% - 99% ranging from 600 to 900-nm due to low recombination of MoOx and MgFx contacts results in a high JSC of 41.5 mA/cm2.