Investigation into fill factor and open-circuit voltage degradations in silicon heterojunction solar cells under accelerated life testing at elevated temperatures

There are many reports on open-circuit voltage (VOC) degradation but limited reports on fill factor (FF) degradation in silicon heterojunction (SHJ) solar cells which are either subjected to an accelerated testing condition or to a field condition. The factors contributing to FF loss and VOC loss due to aging required a thorough investigation to gain insight into potential loss mechanisms and the approaches to mitigate them. In this context, we subjected SHJ solar cells of the same structure to varying accelerated testing conditions (i.e. exposure temperature of 90 °C versus 120 °C, under 1-sun illumination) in experiment 1 to 3 of this work. The SHJ cells exposed to a temperature of 90 °C were nearly stable in performance even after 2000 h while those exposed to 120 °C degraded significantly in FF. Device measurements and analysis revealed that the FF degradation was due to an increase in series resistance (Rs) and hole collection barrier. In experiment 4, the effect of device structure was investigated by subjecting cells with only front-side indium tin oxide (ITO) and those with both-sides ITO to the same accelerated testing conditions at a temperature of 90 °C under 1-sun illumination. The result obtained revealed that SHJ cells with both sides ITO were nearly stable in electrical performance while those with only front ITO degraded in performance, dominated by VOC loss. The VOC loss was perhaps due to loss of passivation at the rear-side of the cells.