Light-activated surface passivation for more efficient silicon heterojunction solar cells: Origin, physics and stability

Silicon heterojunction (HJT) solar cells have world-leading efficiencies due to outstanding surface passivation. Yet, maintaining their performance during the lifetime of a photovoltaic module requires excellent quality and stability of the surface regions. It is well known that HJT solar cells can show an increase or reduction in performance under illumination, and this instability has been related to changes in the surface regions. This work investigates the stability of surface passivation in HJT solar cells by modelling the injection-dependent minority carrier lifetime of a range of symmetrically a-Si passivated silicon wafers. Fixed charges and defects at the interface are varied in the model to find the best fit to the injection-dependent lifetime before and after a high-intensity illumination treatment. The results indicate that the laser process induces an increase in field effect passivation at the surface, which is then reduced upon storage in the dark. The results show that lifetime spectroscopy is a useful tool to investigate the nature of a-Si passivation degradation.