Metal halide perovskites (MHPs) have emerged as highly promising materials for a wide range of applications, including solar cells, light-emitting diodes, and photodetectors, owing to their outstanding optoelectronic properties. Beyond conventional optoelectronic performance, MHP films also exhibit unique ferroic characteristics, such as ferroelectricity, piezoelectricity, ferroelasticity, and flexoelectricity. These properties are influenced by external mechanical stress, leading to microstructural changes in MHP films and thereby triggering notable electrical responses. However, the optical response of MHP thin films to mechanical force has received less attention. The optical response of materials to mechanical force, known as triboluminescence (TL), has also been reported in perovskite-related single crystals, such as oxide perovskites, anti-perovskites, and low-dimensional perovskites. Yet, researchers have never distinguished different types of mechanical forces. In fact, friction differs from other forms of mechanical stresses because it not only induces deformation but also leads to the transfer of surface charges between two contacting bodies. Such charge transfer induced by friction can alter the electrical properties of materials; however, its impact on optical properties has never been investigated.
In a new paper published in Light: Science & Applications, professors Xin Guo and Can Li from State Key Laboratory of Photoelectric Conversion and Utilization of Solar Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, China, and co-workers have found the TL of the MHP films based on a friction-induced charge transfer mechanism. Researchers discovered that when MHP films were scraped with metals such as copper, gold, or platinum, the photoluminescence (PL) in the friction region was significantly enhanced, accompanied by a slight blue-shift. Conversely, when aluminum, plastic, or glass was used for scraping, the PL was instead attenuated. This phenomenon was universally observed across commonly studied MHP films. Further investigation revealed that only materials with a Fermi level lower than that of the MHP can enhance the PL, whereas materials with a higher Fermi level reduce the PL of the MHP film. During the friction, the Fermi-level difference between the two materials causes electron transfer from the MHP film surface to the material with the lower Fermi level. This leads to the accumulation of positive charges on the perovskite surface, generating a positive electrostatic field. Conversely, when materials with a Fermi level higher than that of the perovskite are used for friction, negative charges are deposited, resulting in a negative electrostatic field.
“When two materials are rubbed against each other, the difference in their Fermi levels drives the transfer of surface-state electrons from the material with a higher Fermi level to that with a lower one, resulting in the development of static electricity on the frictional surfaces of both materials. In our case, metals like Cu, Ag, etc. have lower Fermi levels than that of perovskites, accepting surface electrons from perovskites, which leads to a positive electrostatic field on the surface of the perovskite film. Conversely, other materials like Al with a higher Fermi level than that of perovskites will result in a negative electrostatic field on the perovskite surface after friction”, claimed the authors.
“The friction-induced positive (by Cu) and negative (by Al) electrostatic fields on the surface of the perovskite film led to stretched and compressed perovskite lattices, respectively. The changed lattice structures can be explained by the interaction between the type of the electrostatic field and the surface charge state of the perovskite film. It has been reported that positively charged undercoordinated Pb2+ defects are abundant on the surface of the perovskite film due to the migration of halide ions in [PbX6]4- octahedra. Upon the friction by Cu, the generated positive electrostatic field forms the electrostatic repulsion interaction with the positively charged surface, leading to the stretch of the perovskite lattice. The stretched lattice structure slightly widens the band gap and reduces the trap density of the MHP film, which enhances the PL accompanied by a slight blue-shift and prolongs the lifetime of charge carriers, resulting in the TL. On the contrary, the Al friction-induced negative electrostatic field causes the lattice compression due to the coulomb attraction, which weakens the PL.”