Methods to Predict the Real-World Stability of Commercial Perovskite-on-Si Tandem Photovoltaics

Perovskite silicon tandem solar cells offer high power conversion efficiencies and promise cost-effective solar energy. At Oxford PV, we have demonstrated world record efficiencies at both small-scale cell and full wafer scale, and last year, revealed a 26.9% 60-cell module, which remains a record for a residential-size photovoltaic module. In this talk we will discuss the critical aspects of reliability and scalability of these solar cells, with a focus on the recent progress made at Oxford PV. Perovskite silicon tandem solar cells are widely regarded as the next technological step after silicon solar cells reach their maximum efficiency. Tandem solar cells profit from highly efficient silicon bottom cells and can be manufactured using thin film deposition equipment. Oxford PV has an R&D facility, focused on state of the art research and development of new cell technologies, and a production facility for product manufacture and up-scaling. We will show insights into the different requirements and ways of working for both ends of our efforts. One big topic across the research community is the enhancement of the long-term stability in order to provide confidence that the technology can deliver sufficient energy yield across the expected product lifetime(s). For utility and rooftop applications, this would typically mean demonstrating stabilities sufficient to maintain the desired energy yield improvements over incumbent technologies over their respective lifetimes. At Oxford PV, we prioritise technology development from a perspective to ensure that efficiency gains can be maintained over multiple decades, and have active research on: a) material and cell technology improvements; b) the development of accelerated stress tests to mimic many years in the field; and c) outdoor testing to corroborate our stress tests and equivalency models.
We will discuss our reliability testing methodologies for accelerated stress testing and recent results for different technologies. We will highlight the necessity to use multiple stressors simultaneously, namely light and temperature, and the importance of different cell stressing conditions. To ensure a long lifetime and to confirm the shape of long-term reliability curves, multiple subsequent reliability cycles may also be performed. This allows us to check for and exclude the existence of catastrophic losses at very long stressing times. We will provide recommendations as to the stress conditions that should be used, and evaluate a range of measurement methods in their capacity to aid technology development, or in their accuracy at real-world lifetime predictions.
Ultimately, the most important result of these measurements is the predicted real world reliability. In order to accurately simulate the long term field efficiency we employ a simulation model that uses measured temperature coefficients, temperature dependent degradation rates and illumination dependencies as input. Together with real world weather and illumination data this model allows us to predict the real world efficiencies of our cells for different climates.
We benchmark our simulation results with real world efficiency data we collected with our R&D and production size cells over the last years, and find a good agreement between simulation and real world data. This shows that our accelerated stress test are relevant and allow us to test new cell stack technologies quickly.