Long-term test shows: Efficiency of perovskite cells varies with the season

Scientists at HZB run a long-term experiment on the roof of a building at the Adlershof campus. They expose a wide variety of solar cells to the weather conditions, recording their performance over a period of years. These include perovskite solar cells, a new photovoltaic material offering high efficiency and low manufacturing costs. Dr Carolin Ulbrich and Dr Mark Khenkin evaluated four years of data and presented their findings in Advanced Energy Materials. This is the longest series of measurements on perovskite cells in outdoor use to date. The scientists found that standard perovskite solar cells perform very well during the summer months, even over several years, but decline in efficiency during the darker months.

Small perovskite solar cells on a laboratory scale can now achieve an efficiency of up to 26.95% under standard testing conditions. They are inexpensive and easy to manufacture and first solar cells, based on perovskites are being sold already. However, it is important to understand the long-term behaviour of perovskite solar cells when used outdoors in order to better predict energy yields and service life.

At HZB, Dr Carolin Ulbrich and her team, supported by the Helmholtz-funded TAPAS project with the University of Ljubljana, have set up a large outdoor test station: racks equipped with solar cells and measurement technology are installed on the roof. They are exposed to wind and weather all year round. Measurement data from the past four years from small perovskite solar cells encapsulated in glass are now available. The cells were manufactured at HZB by Eva Unger's team (details on the structure: ITO | 2PACz | Cs_0.15FA_0.85PbI_2.55Br_0.45 (band gap of 1.65 eV) | C₆₀ | SnO₂ | Cu. ).

The results are encouraging: the peak power remained almost the same in the first two summers, and decreased by only about 2% in absolute terms between the first and fourth summers. However, efficiency dropped by around 30% during the winter months.

The team identified several reasons for this. At higher latitudes, such as at the Berlin site, the spectral distribution of sunlight changes, with a greater proportion of ‘blue’ components in summer and a greater proportion of ‘red’ components in winter. However, perovskite solar cells are primarily capable of converting blue light into electrical energy. In locations closer to the equator, these spectral shifts are less pronounced, meaning perovskite solar cells are likely to deliver a more consistent yield throughout the year. ‘What distinguishes perovskite solar cells from more mature PV technologies is that they often change their efficiency reversibly during the day-night cycle. This property significantly contributes to the large seasonal fluctuations observed,’ says Mark Khenkin.

The evaluation of the data was performed by doctoral student Marko Remec. Together, the team has made an important contribution to understanding the ‘real-world behaviour’ of perovskite solar cells and how it is affected by external conditions.