Perovskite Solar Cells: The Key to Long-Term Stability

Perovskite solar cells are inexpensive to manufacture and highly efficient. However, it is still questionable how long they will remain stable when used outdoors under real weather conditions. This topic is now being addressed by an international collaboration led by Prof. Antonio Abate in the journal Nature Reviews Materials. The researchers investigated the effects of repeated thermal cycles on microstructures and interactions between the various layers of perovskite solar cells. The conclusion: the decisive factor for the degradation of metal halide perovskites is thermal stress. This can be used to derive strategies for specifically increasing the long-term stability of perovskite solar cells.

Perovskites are a class of materials with semiconducting properties that are ideal 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 extremely little material and energy, so solar energy could become significantly cheaper. However, when used outdoors, solar modules should deliver a nearly stable output for at least 20 to 30 years. And here there is still a lot of room for improvement with perovskite materials.

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

"Although encapsulation can effectively protect the cells from moisture and atmospheric oxygen, they are still exposed to large temperature fluctuations during outdoor use, day and night, and throughout the seasons," says Abate. Depending on the geographical conditions, the temperatures inside the solar cells can be between minus 40 degrees Celsius and plus 100 degrees Celsius (e.g. in the desert).

To simulate this, the perovskite solar cells in the study were exposed to even more extreme temperature differences: from minus 150 degrees Celsius to plus 150 degrees Celsius, repeatedly. 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 interactions with the neighboring layers also changed over the course of the temperature cycles.

This also reduced the cell's performance. In particular, the large temperature fluctuations caused thermal stresses, both within the perovskite thin film and between the various adjacent layers: "In a perovskite solar cell, layers made of very different materials must be in perfect contact; unfortunately, these materials often have quite different thermal behavior," explains Abate. Plastics, for example, tend to shrink when heated, while inorganic materials tend to expand. As a result, the contact between the layers becomes worse with each cycle. In addition, the team also investigated local phase transitions and the diffusion of elements into adjacent layers.

From this, the researchers derived a strategy to increase the long-term stability of perovskite solar cells. "Thermal stress is the key," says Abate. The main aim 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. Standardized test protocols are necessary to determine stability during temperature changes uniformly and correctly.