Perovskite solar cells: Thermal stresses are the key to long-term stability

Perovskites are a wide class of materials with semiconducting properties suitable for energy conversion in a solar cell: the best of them, the metal-halide perovskites, already deliver efficiencies of up to 27%. The production of such thin-film solar cells requires particularly little material and energy, so solar energy could become considerably cheaper. However, when used outdoors, solar modules should provide a nearly stable yield for at least 20 to 30 years. And here, there is still a lot of room for improvement in perovskite materials.

Results from several years

An international research collaboration led by Prof. Antonio Abate has now published the results of several years of work in a review paper in the journal Nature Reviews Materials. Together with a team led by Prof. Meng Li, Henan University, China, and other partners in Italy, Spain, UK, Switzerland and Germany, they show that thermal stress is the decisive factor in the degradation of metal-halide perovskites.

Harsh conditions in "real life"

'When used outdoors, solar modules are exposed to the weather and the seasons,' says Abate. While encapsulation can effectively protect the cells from moisture and atmospheric oxygen, they are still exposed to quite large temperature variations day and night and throughout the year. Depending on the geographical conditions, temperatures inside the solar cells can range from minus 40 degrees Celsius to plus 100 degrees Celsius (in the desert, for example).

Effects of extreme temperature differences studied

To simulate this, the perovskite solar cells in the study were exposed to much more extreme temperature differences in several cycles: From minus 150 degrees Celsius to plus 150 degrees Celsius, and again and again. Dr Guixiang Li (then a postdoc at HZB, now a professor at Southeast University, China) investigated how the microstructure within the perovskite layer changed during the cycles and to what extent the interactions with the neighbouring layers were also affected by the temperature cycles.

Thermal stress inside the perovskite film and in between layers

Together, these factors affect the performance of the cell. In particular, the temperature cycles caused thermal stress, i.e. stress both within the perovskite thin film and between the different adjacent layers: 'In a perovskite solar cell, layers of very different materials need to be in perfect contact; unfortunately, these materials often have quite different thermal behaviours,' explains Abate. For example, plastics tend to shrink when heated, while inorganic materials tend to expand. This means that in each cycle the contact between the layers becomes worse. What is more, also local phase transitions and diffusion of elements into adjacent layers have been observed.

Most promising strategy

From this, the research teams have derived a strategy to increase the long-term stability of perovskite solar cells. 'Thermal stress is the key,' says Abate. The main thing, therefore, is to make the perovskite structures and the adjacent layers more stable against thermal stress, for example by increasing the crystalline quality, but also by using suitable buffer layers. The scientists highlight the importance of uniform test protocols for evaluating stability under temperature cycling and propose an approach to facilitate comparison between different studies.