For emerging photovoltaic technologies to become commercially and technically viable, it is important to understand how performance in the laboratory translates to the field. A new study analyses the yearly changes in the energy yield of perovskite solar cells under simulated realistic temperature and irradiance conditions.
Perovskite solar cells are one of the most promising next-generation photovoltaic (PV) technologies. Their power conversion efficiency can reach 28% when used in a tandem configuration with silicon1, which is greater than the current silicon record efficiency of 27.6%2. Measurement of the efficiency takes place under standard test conditions (STC) — that is, using the air-mass 1.5 spectrum, an intensity of 100 mW cm–2 (also known as 1 sun of illumination) and a cell temperature of 25 °C. Such testing conditions can be vastly different to real-world conditions, where temperature, the light intensity level and its spectrum constantly vary with time. Wolfgang Tress and colleagues from the École Polytechnique Fédérale de Lausanne, Switzerland, now present in Nature Energy a comprehensive report on the simulated outdoor performance of perovskite solar cells3. The researchers show that perovskite technology exhibits a very promising performance, comparable to the silicon solar cell used as a reference device, when tested in the relevant weather conditions. They recorded weather data from Switzerland — namely solar irradiance, ambient temperature and wind speed — from 24 days distributed across one year, and reproduced these conditions in the laboratory to investigate their effect on the efficiency and stability of multi-cation double-halide perovskite solar cells.
In order to be commercially viable, perovskite solar cells need to fulfil three requirements — long lifetime, high efficiency and low cost — otherwise they will be limited to niche markets. While indoor stability studies are more prevalent in the literature, outdoor tests have been considered to be of major importance to more mature PV technologies, such as organic and dye-sensitized solar cells. Outdoor testing provides an opportunity to understand materials and device degradation under field conditions. In some cases, outdoor tests corroborate failure modes observed in indoor testing and relate them to potential failures in real-world conditions4. However, other failure modes are reduced or increased in severity under such conditions and failure modes that are observed outdoors are not observed indoors5. Furthermore, the case of organic and dye-sensitized solar cells has highlighted that standards issued by the International Electrotechnical Commission can overstress the devices, leading to failure modes that do not necessarily occur in the field6. Therefore, specific tests are needed to properly estimate the potential failure rates in emerging PV technologies, including perovskite.
Indeed, for perovskite solar cells, outdoor tests have raised awareness of issues such as reversible degradation, temperature dependence of solar cell performance and low-light behaviour, which were not taken into consideration in indoor testing7,8. In this respect, Tress and colleagues show that the highest performance is achieved in the morning and decreases later on in the day as a consequence of reversible degradation, leading to a reduction in energy yield (Fig. 1a). This phenomenon is not observed when the device is measured continuously at STC, but it is of critical importance to the deployment of PV technology. In fact, the energy demand from the grid is typically highest in the early evening9, indicating that future studies need to address reversible degradation to minimize disruption. The researchers have also examined the temperature coefficient of their solar cells. While previous reports in perovskite solar cells have shown that this coefficient can be positive or negative depending on the chemical composition of the perovskite absorber layer8, the researchers have found that the device efficiency does not increase or decrease monotonically with temperature for their multi-cation double-halide perovskite solar cells, illustrating a lack of clear trend for these record-efficiency and long-term stable devices (Fig. 1b). Measuring low-light behaviour is also important in order to predict energy yield from perovskite modules as this is a more common lighting condition than STC in many of the world’s populous locations. For example, based on weather station data from Bangor University in the UK from 2018, there were just 51.4 hours over the whole year where the solar irradiance was equal to or greater than 1 sun and the mean irradiance during daylight hours was just 0.226 sun. Interestingly, Tress and colleagues report that the highest energy yields are actually obtained at low-light conditions (~0.2 sun, 20 ˚C), suggesting that perovskite solar cells are well-suited to northern climates or low-light indoor conditions.
For perovskite solar cells to be commercially successful, investors need to be to be able to confidently forecast future energy yield. This cannot be accurately achieved with data only obtained at STC. Furthermore, as electricity infrastructure transforms towards smart grids, understanding variability in PV energy production will become a major component in the design and optimization of energy exchange and storage at all temporal and spatial scales. Within this framework, the findings on the different energy yields obtained under STC and real-life temperature and irradiance data are of crucial importance.
While the work is a step in the right direction, other factors come into play when discussing the commercial viability of perovskite solar cells. In particular, large-scale devices should be considered in outdoor testing. At present, the vast majority of devices reported on are small-sized laboratory-scale devices with an active area of 0.2 cm2 or less, including those reported by Tress and colleagues. One of the next issues that the community needs to address is how larger area and module sizing affect energy yield.
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Kettle, J. Ready cells for large-scale systems. Nat Energy 4, 536–537 (2019). https://doi.org/10.1038/s41560-019-0422-2
ACS Materials Letters (2020)