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

摘要

Backcontacted backjunction ntype Si solar cells with locally overcompensated diffusion regions are investigated in two different designs. In the buried emitter design, borondoped (Bdoped) emitter diffusions are partially diffused and locally overcompensated by phosphordoped (Pdoped) back surface field (BSF) diffusions, leading to ntype regions that are overlapping the ptype regions. In the floating base design, the Bdiffusions are diffused on the entire rear side, so that the Pdiffusions are separated from the base by the emitter. Hence, ptype regions exist between the ntype regions, creating opposing pn junctions and a base that is floating. For the latter design with an efficiency of 17.1%, the opencircuit voltage Voc, the shortcircuit current density Jsc, and the fill factor FF are significantly reduced by about 25mV, 5mAcm2, and 4%, respectively, compared to the buried emitter design where 678mV, 41.5mAcm2, and 76.1%, respectively, and an efficiency of 21.4% can be achieved. Twodimensional 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 Pdiffusions due to the opposing pn junctions are detrimental for the performance of the floating base solar cell.