Degradation of perovskite has been a well-developed topic on this blog. Actually, it represents one of the greatest challenges to address. Scientists try to tackle the degradation issues from many approach angles. Here I showcase two articles I’ve read in the past months about how the lattice evolves and how it leads to cell degradation: “Effects of Small Polar Molecules on Degradation Processes of Perovskite Solar Cells”, from Prof. Chun-Sing Lee et al. and “Study on degradation mechanism of perovskite solar cell and their recovering effect by introducing CH₃NH₃I layers” from Prof Masanori Ozaki et al. I find important links between those two articles, that is actually what I wanted to present.
As we all know, degradation of perovskite materials can be caused by humidity, oxygen, light, or heat. After being exposed to humidity, the perovskite’s colour changes, making it a perfect moisture sensitive material. In this very interesting article, Prof. Eugene A. Katz and co-workers found that the decomposition of perovskite is induced by illumination in the presence of water. In fact, concentrated sunlight experiments have shown that light triggers degradation and the mechanism depends highly on the temperature and the composition of the materials.
Water has been considered as the main cause of the instability of the perovskite, causing hydrolysis reactions that degrade the solar cells. Prof. Aron Walsh and co-workers have shown in 2014 that upon illumination and heating, hydrated MAPI (methylammonium lead triiodide) perovskite phases, such as MAPbI₃.H2O and MA₄PbI₆.2H₂O are formed and are followed by the release of a gas phase, composed of MAI. Those hydrated phases present no optoelectronic properties, therefore, this effect is considered responsible for the degradation along which the cell proceeds.
The release of methylammonium ignites the degradation
Prof. Nam-Gyu Park and co-workers reported last year that mobile MA⁺ ions appear in the perovskite film during degradation. Here Prof. Lee’s article is of particular importance. It appears that polar molecules, such as MA⁺ or H₂O are closely related to the degradation processes. Knowing that there are mobile MA⁺ ions in the perovskite film during degradation, the authors investigated the degradation processes of MAPI perovskite solar cells with different PbI₂/MAI ratios. They show with X-ray diffraction that the perovskite lattice is expanded by incorporation of water molecules. This mechanism is followed by a crystal breakage, in which MA⁺ and I⁻ ions are released. Those ions diffuse to form PbI₂ crystals.
The formed lead iodide crystals do not present electronic properties thus they are blocking the conducting channels. Hence the ion diffusion coefficient is lowered with the formation of those crystals. Later on, the scientists have demonstrated that the adsorption of water leads to a decrease in the Jsc along with a bulk degradation that accompanies an ion diffusion process. It becomes important when putting in relation with Prof. Ozaki’s article.
Effects of MAI in recovering the performances of degraded structures
In this interesting article, MAPI perovskite materials are evaluated before and after degradation due to moisture and sunlight. It has been reported that during degradation, the crystalline structure changed with peaks assigned to planes of hexagonal PbI₂ and orthorhombic I₂. Simultaneously, the colour of the perovskite changed, indicating its degradation. Because they observed such a phenomenon, Ozaki et al. tried to add MAI layers on top of the perovskite. They have shown that with this layer addition, the MAPI solar-cell performance recovered. The hypothesis developed is that PbI₂ formed with the degradation allows recrystallization of perovskite after introducing MAI layers.
Thus, with the assumption that evaporated MAI cannot be recycled, the authors tried to add it directly to the surface. That allows the cell to be “refurbished”. This article shows an example of what can be done with MAI to reverse the degradation process. This could have many derived applications in the future.
Those two examples show how scientists have been able to understand, in a very basic way, how degradation happens. These understandings have led them to ideas of better, more stable and more efficient cells. That was to say that the perovskite field is rapidly evolving and we will rapidly have a better understanding of the degradation processes that will allow us to develop more stable and more efficient solar cells.
The cover photo has been taken from Swansea University on Flickr