Solar cells made with sunlight-absorbing, semiconducting perovskite materials have been photovoltaic rock stars in recent years. These metallo-organic perovskites such as formamidinium lead trihalides (FAPbX3) are less expensive to make and process than crystalline silicon, a conventional photovoltaic material, yet they offer comparable performance. But top-performing perovskites—α-FAPbI3, for example—are not stable. They decompose and undergo structural changes when exposed to heat and bright sunlight, which ruin device performance and have impeded commercialization.
Straining a semiconductor by stretching or compressing its lattice can stabilize the crystal and alter its properties. Researchers have successfully used pressure and other complex physical means to strain perovskites, but have had little success with a simpler chemical method known as epitaxy, a process in which a crystal product grows on top of a crystalline substrate.
That method is what Sheng Xu of the University of California San Diego and coworkers now have used with α-FAPbI3. The team grew the compound from solution on top of other, more stable perovskites that have different lattice dimensions. The process yielded crystals with lattices compressed by up to 2.4% (Nature 2020, DOI: 10.1038/s41586-019-1868-x). Compared with unstrained crystals, the strained ones exhibited improved charge-carrier mobility—a key semiconductor property—and did not undergo device-ruining structural change even after 1 year of storage at room temperature.
This study “provides an extremely accessible and practical avenue through which to explore and use the physical properties of strained halide perovskites,” says materials specialist Jian Shi of Rensselaer Polytechnic Institute.