Analysing the lateral series resistance of high-performance metal wrap through solar cells

Abstract

In back-contact solar cells, both external polarities are located at the back surface of the device, which allows for higher photocurrent generation on cell level and reduced series resistance on module level, leading to higher energy conversion efficiencies compared to conventional solar cells and modules. However, the majority charge carriers, which are generated near the back emitter, have to flow laterally e.g. through the base in order to reach the external majority carrier contact. In the present work, we analyse the lateral series resistance by means of measurement and simulation for high-performance metal wrap through (HIP-MWT) solar cells. We compare theoretical models and experimental methods to extract the effective series resistance from simulated and measured current–voltage characteristics and show that lateral voltage variations significantly increase the local recombination current. If the width of the gap between the external majority carrier contacts is reduced from the typical value of 3.5mm to ideally 0mm, we expect an increase of the energy conversion efficiency of approximately 0.1%abs. for cells with three continuous rear emitter contacts on 125mm×125mm large silicon wafers. In a simulation study, the bulk doping concentration NA and the bulk lifetime are varied yielding an optimal base resistivity of 0.6Ωcm–1.5Ωcm for HIP-MWT solar cells based on Czochralski-grown silicon in the degraded state of the boron–oxygen defect and an optimal resistivity of less than 1.0Ωcm for the case of bulk lifetimes larger than ~300µs.