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 Voc, the short‐circuit current density Jsc, and the fill factor FF are significantly reduced by about 25 mV, 5 mA cm−2, and 4%, respectively, compared to the buried emitter design where 678 mV, 41.5 mA cm−2, 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 B‐ and P‐diffusions due to the opposing p‐n junctions are detrimental for the performance of the “floating base” solar cell.