As an inexhaustible clean energy source, solar energy has attracted the attention of researchers, and solar cells that convert solar energy into electric energy have also been favored by many research groups in the world. In recent years, organic-inorganic hybrid perovskite solar cells have attracted extensive attention in academia and industry because of their high efficiency and low cost. In just a few years, their photoelectric conversion efficiency has rapidly increased to 24.2%, making them the most efficient thin-film solar cells in a single cell.
However, the poor stability of this kind of battery is the main factor that seriously hinders its commercial application. Compared with traditional inorganic photovoltaic materials, organic-inorganic hybrid perovskite materials are ionic crystals with soft lattice, which are prone to ion migration and form a large number of vacancy defects under the interference of external environment, thus inducing lattice collapse and component decomposition, thus making them no longer have excellent photoelectric conversion ability.
Among many vacancy defects, halogen anion and organic cation vacancies generally exist on the surface and grain boundary of perovskite because of their low defect formation energy. These two vacancy defects will not only affect the working efficiency of solar cells, but also induce further degradation of perovskite crystals and form more bulk defects. Previous reports on these two defects mainly focused on the passivation of a single defect, that is, organic cation or halide vacancy, which could not achieve "both fish and bear's paw". How to eliminate these two defects at the same time and achieve higher efficiency and stability of perovskite solar cells is the biggest problem of perovskite materials at present.
In view of the above-mentioned important problems, Zhou's research group proposed a brand-new elimination mechanism, that is, by introducing sodium fluoride into the perovskite active layer, fluoride can simultaneously form strong hydrogen bonds with organic cations and strong ionic bonds with lead ions by taking advantage of the extremely high electronegativity of fluorine. Based on the chemical bond modulation of ionic bonds and hydrogen bonds, organic cations and halogen anions in perovskite components can be fixed, thus eliminating the corresponding vacancy defects and significantly improving the efficiency and stability of the battery. The highest efficiency of the battery device with sodium fluoride is 265,438 0.92% (certified value is 265,438 0.7%), and there is no obvious delay. At the same time, the devices containing sodium fluoride show excellent thermal stability and light stability. After 1000 hours of continuous solar irradiation or heating at 85℃, the device can still maintain 95% and 90% of its original efficiency respectively, and after continuous operation at the maximum power point 1000 hours, it can still maintain 90% of its original efficiency. This method solves two important factors that limit the stability of titanium solar cells-organic cation and halogen anion vacancy, and can be extended to other perovskite photoelectric devices. The method of chemical bond modulation is also of great reference significance to other inorganic semiconductor devices facing similar problems.
The first author of this paper is Li, a doctoral student of grade 20 17 of Zhou's research group, and Zhou's invited researcher is a correspondent. Collaborators include Tao from Eindhoven University of Technology and Hong Jiawang from Beijing Institute of Technology, Yang Shihe from Hong Kong University, Xie Haipeng from Central South University and Geert Brocks from Twente University. This work is supported by National Natural Science Foundation of China, Ministry of Science and Technology, Beijing Natural Science Foundation, Beijing Science and Technology Commission and Beijing Key Laboratory of Advanced Battery Materials Theory and Technology.
Zhou's research group has been working hard to improve the efficiency and stability of perovskite solar cells recently, and has made a series of important progress in Science (doi:10.126/science.aau5701). Natural energy (doi:10.1038/S41560-019-0382-6) and natural communication (doi:10./kloc-0) Doi:10.1038/s 41467-019-08507-4 and doi:10.1038/s 4/. Advanced Materials (doi:10.1002/adma.201900390), Journal of the American Chemical Society (doi:10.102/kloc)