On the characteristics of the doping profile under local metal contacts

Abstract

In many solar cell concepts, the recombination at local contacts is a bottleneck for the efficiency. Therefore, an optimized doping profile underneath the metal contact would improve the cell performance. We investigate the saturation current density (J0e,met) value of various doping profiles by TCAD simulation and showed that lowest J0e,met values are obtained for profiles with a surface concentration Ns > 5·1020 cm-3 as a consequence of the Pauli blocking and almost independently of the junction depth xj. For profiles with lower Ns we could show an approximate proportionality between J0e,met and the sheet resistance (Rsheet) making the recombination performance of these profiles quasi-independent of the profile shape. Therefore, profiles with even lower value of Rsheet as presently used, typically <10 Ω/sqr while keeping an Ns > 1020 cm-3 could allow to reach even lower J0e,met, typically <100 fA/cm2. In general Auger recombination is very low (< 10 fA/cm2 for Rsheet > 5 Ω/sqr) and does not play a role in the optimal profile shape of the emitter.