Climate change is a race against time, and the need for more efficient and cheaper solar energy is a high priority. Perovskite-silicon tandem solar cells are among the most promising options that can significantly contribute to the decarbonization of the global energy system through their increased power output. Two main sustainability considerations for any renewable technology that dictate whether it can fulfil its promises are efficiency and lifetime.

Perovskite-silicon tandem solar cells have made unprecedented strides in the field of photovoltaics (PV) regarding efficiency. At Oxford PV, we have demonstrated world-record efficiencies at both small-scale and full wafer scale. Last year, we revealed a 26.9% 60-cell module, which remains a record for a residential-size photovoltaic module. Nevertheless, long-term stability remains a major concern across the research community. Established PV technologies such as silicon (c-Si) and cadmium telluride (CdTe) have set the long-term stability benchmark at a 0.25-0.5% relative loss per year, with an expected lifetime of up to 25-30 years.

We are confident that perovskite PV can maintain high performance over multiple decades. In this talk, we will discuss the major R&D approaches we are following to achieve this goal. Unlike incumbent PV technologies, where outdoor performance data and knowledge of real-world degradation modes and their characteristics are available, it is imperative to deploy perovskite PV in the field before having performed outdoor testing for decades to demonstrate the reliability of the solar panels. Accelerated stress testing is designed to expose PV cells and modules to a variety of stressors that can mimic expected real-world conditions – the combined light and heat exposure test is a common route to evaluate the cumulative affect of these stressors in the field, and typically elevated temperatures are chosen to compress ‘years of operation’ into week-to-month long experiments. In this way, relevant degradation modes can be triggered and rapid iteration of technology can be made, However, it is crucial to both: a) choose the relevant parameter ranges that activate the degradation processes prominent under field-deployment conditions whilst avoiding the triggering of degradation artifacts unique to the stress conditions; and b) have consideration as to the means in which cell parameters are determined, either during, or following, stress tests.

By selecting the right stressing conditions and employing suitable models, we can calculate an acceleration factor and estimate real-world operation lifetime, which can be validated by comparing with corresponding outdoor performance data, providing reliable lifetime predictions. We will present activity at Oxford PV for each of these stages. The range of degradation modes in perovskites are well known, and have been reviewed extensively in the literature. A key consideration for enhancing cell performance longevity however, is to  identify the critical degradation modes, those that dominate the measured power output. dAt Oxford PV, we focus on identifying the link between material properties and changes with device losses for the critical degradation modes, aim to introduce mitigating routes, and move to the next critical mode. Representative examples of optimized device performance and stability will be discussed, Lastly, looking towards scalability, being able to repeatedly obtain high-efficiency and stable devices is of paramount importance. By continuously tracking our device stability, we gain more insights into the correlation of process capability and reproducibility. This allows us to identify process windows and parameters affecting performance and stability, informing the design of scalable processes that can be transferred from R&D to high-throughput manufacturing.