Back‐Contacted Back‐Junction Si Solar Cells with Locally Overcompensated Diffusion Regions – Comparison of Buried Emitter and Floating Base Design

Abstract

Back-contacted back-junction n-type Si solar cells with locally overcompensated diffusion regions are investigated in two different designs. In the "buried emitter" design, boron-doped (B-doped) emitter diffusions are partially diffused and locally overcompensated by phosphor-doped (P-doped) back surface field (BSF) diffusions, leading to n-type regions that are overlapping the p-type regions. In the "floating base" design, the B-diffusions are diffused on the entire rear side, so that the P-diffusions are separated from the base by the emitter. Hence, p-type regions exist between the n-type regions, creating opposing p-n junctions and a base that is floating. For the latter design with an efficiency of 17.1%, the open-circuit voltage V oc , the short-circuit current density J sc , and the fill factor FF are significantly reduced by about 25 mV, 5 mA cm À2 , and 4%, respectively, compared to the buried emitter design where 678 mV, 41.5 mA cm À2 , and 76.1%, respectively, and an efficiency of 21.4% can be achieved. Two-dimensional numerical device simulations reveal that, besides junction and/or shunt leakage currents, recombination at the surface of the emitter and electron transport in the Band P-diffusions due to the opposing p-n junctions are detrimental for the performance of the "floating base" solar cell.