In this study, a general stripping technology of perovskite polycrystalline films was developed. The prepared perovskite quasi-photovoltaic device is immersed in the anti-solvent chlorobenzene, which will dissolve the bottom polymer transport layer without affecting the perovskite polycrystalline film. At the same time, the top metal electrode as a template can ensure the integrity of the whole film, and finally the whole perovskite film is peeled off to expose its bottom surface.
Figure 1. Stripping technology of perovskite polycrystalline films. A: monovalent organic/inorganic cation, b: Pb2+ and x: halogen anion.
A completely exposed bottom sample of perovskite polycrystalline film was obtained by stripping technique. Further morphological characterization shows that the bottom surface of perovskite polycrystalline films has larger grain size than the top surface, and the residual lead halide crystals of the films are flaky at the bottom and small particles at the top. Combined with the chemical composition and potential distribution characterization of the top and bottom, it shows that the bottom presents more serious lateral inhomogeneity than the top, and also reflects the longitudinal inhomogeneity of perovskite polycrystalline films during solution growth.
Figure 2. Morphology, composition and potential distribution of upper and lower surfaces of perovskite polycrystalline films.
In addition, the fluorescence imaging test of the top and bottom of perovskite shows that the fluorescence of the bottom is weaker than that of the top, and there are a large number of non-radiation recombination regions-fluorescent dark regions, in which a large number of dark regions are distributed near the fluorescent regions of lead halide, indicating that besides the non-radiation recombination dark regions caused by serious intrinsic defects at the bottom of perovskite particles, a large number of lead halide regions at the bottom will also aggravate the non-radiation recombination near perovskite particles. In-situ observation also found that there was a time difference between the signal of lead halide and the disappearance of fluorescent dark area during passivation.
Figure 3. Fluorescence imaging of the upper and lower surfaces of perovskite polycrystalline films. Red: fluorescence in the range of 700-790 nm; Blue: Fluorescence in the range of 500-570 nm.
Figure 4. Sample preparation for in-situ bottom * * * focused fluorescence imaging and perovskite bottom time-resolved in-situ fluorescence imaging.
In addition, as we all know, the surface treatment method of heterogeneous ammonium halide is the most effective means to realize high-quality perovskite thin films and high-efficiency perovskite photovoltaic devices in recent years, but its mechanism is often considered as passivation of defects near the top of perovskite thin films. Based on the in-depth understanding and characterization of the surface properties of the bottom, the researchers found that the upper surface treatment of heterogeneous ammonium halide will also have a significant impact on the bottom, changing the bottom morphology, composition and crystal structure, thus improving a large number of non-radiation composite areas at the bottom. Therefore, the researchers defined this new mechanism as molecular-assisted microstructure reconstruction, which explained the source of high efficiency of heterogeneous ammonium halide surface treatment more comprehensively, and also confirmed the soft lattice properties of polycrystalline perovskite films, thus allowing molecules to permeate and diffuse from the top to the bottom of the films, thus realizing the whole film.
Figure 5. Up-and-down characteristics of perovskite films passivated by ammonium halide.
In this work, the micro-area morphology, chemical composition, electronic structure and photophysical properties of the bottom interface of perovskite polycrystalline films are comprehensively analyzed. The developed polycrystalline film peeling technology and in-situ focused fluorescence imaging technology will also provide a general platform for studying the bottom characteristics of polycrystalline films in the future. It is found that the bottom of the film is more uneven than the top, which further reveals the main source of a large number of non-radiation recombination zones at the bottom of the film. Finally, the real mechanism of passivation strategy on the upper surface of ammonium halide is clarified, which subverts the traditional cognition and provides guidance for the development of device optimization methods and the design of related molecules in the future.
During the research, Professor Samuel D. Stranks from Cambridge University, Professor Thomas P. Russell from Lawrence Berkeley University, Professor Shao and Dr. Shen Yonglong from Zhengzhou University, Academician Huang Wei and Associate Professor Tu Yongguang from Northwestern Polytechnical University, and professors from South University of Science and Technology gave a lot of support and help to the smooth development of this research.
This work has also been supported by the National Natural Science Foundation of China, Peking University State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, Nano-Photoelectric Frontier Science Center, Peking University Yangtze River Delta Photoelectric Science Research Institute, Extreme Optics Collaborative Innovation Center, "201Plan" Quantum Materials Science Collaborative Innovation Center, and British Engineering and Natural Science Research Council (EPSRC).
Paper link:
/doi/full/ 10. 1002/adma . 53663667