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Keywords:

  • back-contacted silicon solar cells;
  • electrical shading;
  • overcompensation of diffused junctions

Abstract

The performance of n-type back-contacted back-junction silicon solar cells where the boron-doped emitter diffusion on the rear side is locally overcompensated by a phosphorus-doped base-type back surface field (BSF) diffusion has been analysed theoretically and experimentally. By overcompensating the emitter diffusion the noncollecting base-type region can be reduced significantly allowing electrical shading losses to be minimized. It has been found that for solar cells with a lowly doped BSF diffusion the local external quantum efficiency and the short-circuit current density Jsc could be improved significantly. For reference solar cells with an undiffused gap between emitter and BSF diffusion and a large noncollecting base-type region, a maximum Jsc of 40.9 mA/cm2 could be achieved and for solar cells with a locally overcompensated boron-doped emitter diffusion featuring a small noncollecting base-type region a maximum Jsc of 41.4 mA/cm2 has been measured. The reduction of Jsc losses caused by free carrier absorption (FCA) in highly doped silicon at near-infrared wavelengths is also shown. Furthermore, theoretical investigations are performed by one-dimensional device simulations and the influence of highly doped and lowly doped emitter and BSF diffusions on the open-circuit voltage Voc is presented. For solar cells with a locally overcompensated boron-doped emitter diffusion Voc could be improved from 629 to 652 mV when lowly-doped diffusions and thermally grown SiO2 and antireflection plasma enhanced chemical vapour deposited (PECVD) SiNx passivation stacks are applied. For the reference solar cells with an undiffused gap between the lowly doped emitter and BSF diffusions Voc of 693 mV could be achieved for a plasma enhanced atomic layer deposited (PEALD) Al2O3 passivation layer.