Rear-Junction n-Type Cell Concept Utilizing PERC Process Sequence on Epitaxially-grown Base and Emitter

PERC is the most common cell type in today’s fabrication of crystalline Si solar cells. However, the cell design is often limited by the compromise for the front side between lateral conductivity, contact resistivity and recombination. In this work, we propose the application of the same PERC process sequence on epitaxially-grown n-type material that has a lowly-doped, deep emitter integrated at the rear side during the wafer growth. Using 3D device simulation, we investigate the influence of the doping concentration and depth of this emitter. Deep (up to 50 μm), lowly concentrated (e.g. 1×1017 cm-3) emitters are expected to be beneficial for the overall cell performance. At the best set of simulated parameters, the reference PERC is exceeded by 0.7 %abs in efficiency. As first steps towards a realization of the cell concept, epitaxially grown cell precursors with a variation of emitter depth of up to 20 μm and doping concentration in the range of 8×1016 to 1×1019 cm-3 are presented. An effective minority charge carrier lifetime τeff ≈ 3 ms was determined for these precursors if the emitter doping concentration was below 1018 cm-3, even for the thickest emitter of 20 μm indicating that the emitter at low doping concentrations does not significantly impact τeff. This highlights the potential and high purity of epitaxially grown Si with stable doping concentration and sharp p/n-transitions for cell fabrication.