Spatially resolved analysis and minimization of resistive losses in high-efficiency Si solar cells

This paper presents an improved method for measuring the total lumped series resistance (R_s) of high-efficiency solar cells. Since this method greatly minimizes the influence of non-linear recombination processes on the measured R_s values, it is possible to determine R_s as a function of external current density over a wide range of illumination levels with a significantly improved level of accuracy. This paper furthermore explains how resistive losses in the emitter, the base, the metal/silicon contacts and the front metal grid can be separately determined by combining measurements and multi-dimensional numerical simulations. A novel combination of device simulation and circuit simulation is introduced in order to simulate complete 2x2 cm² PERL ('passivated emitter and rear locally-diffused') silicon solar cells. These computer simulations provide improved insight into the dynamics of resistive losses, and thus allow new strategies for the optimization of resistive losses to be developed. The predictions have been experimentally verified with PERL cells, whose resistive losses were reduced to approximately half of their previous values, contributing to a new efficiency world record (24.0%) for silicon solar cells under terrestrial illumination. The measurement techniques and optimization strategies presented here can be applied to most other types of solar cells, and to materials other than silicon.