Lead-tin mixed perovskites possess tunable band gaps as low as 1.2 eV, making them highly promising for applications in photovoltaics, infrared photodetectors, and infrared monochromatic light sources. Although lead-tin mixed perovskites have been widely used in the preparation of bottom cells for perovskite tandem solar cells due to their narrow band gap, the tunability of their optical band gap makes the design of lead-tin mixed perovskite materials very important and promising. However, a comprehensive understanding of lead-tin mixed perovskites remains limited.
As typical alloyed semiconductors, lead-tin mixed perovskites exhibit unique electronic properties. Theoretical calculations indicate that the valence band maximum (VBM) is primarily composed of Sn-5s/I-5p hybridized orbitals, while the conduction band minimum (CBM) consists of Pb-6p/I-5p hybridized orbitals, leading to a band gap lower than that of the constituent components. As a result, the band gap exhibits a bowing curve characteristic with respect to the Sn/Pb ratio in the perovskite alloy.
Despite this theoretical understanding, there is still a lack of comprehensive experimental validation of the band gaps across the entire Sn/Pb ratio range. The imbalance in crystallization rates of lead and tin perovskites can easily lead to compositional and phase separation issues, making it challenging to obtain lead-tin perovskite films with accurate ratios.
To address this challenge, Professor Zhubing He’s research group from the Department of Materials Science and Engineering at the Southern University of Science and Technology (SUSTech) has revealed the key mutation mechanism in the variation of the optical band gap of lead-tin mixed perovskites with changing lead-tin ratios. Their work provides a complete panoramic picture of the material’s lattice structure. The team successfully synthesized lead-tin mixed perovskite nanocrystals (FAPb1-xSnxI3) with full compositional ratios, ensuring uniform phase structures. Using cryo-electron microscopy, they achieved the first successful high-resolution atomic-level lattice imaging of these nanocrystals.
Their findings, titled “Disorder-order transition–induced unusual bandgap bowing effect of tin-lead mixed perovskites”, have been published in Science Advances. It has also been featured on Science Net, one of China’s most prominent online platforms dedicated to science, technology, and academic research.
Figure 1. Composition, Structure, Band Gap, and Optoelectronic Devices of Perovskite Nanocrystals
Based on the optical band gap of high-quality single crystals with various Sn/Pb ratios, a typical bowing effect was observed. The band gap of FAPb0.5Sn0.5I3 significantly deviated from this bowing curve. This phenomenon has been mentioned to varying degrees in previous experimental and computational literature but has not received focused attention.
The research group measured the microscopic lattice strain of nanocrystals with different compositions, starting from the arrangement of lead and tin atoms. Their findings showed that lattice strain increases as the composition ratio moves from the ends to the center, reaching a maximum at Pb0.6Sn0.4 and Pb0.4Sn0.6. In contrast, the lattice strain of Pb0.5Sn0.5 showed a sudden drop, comparable to that of pure lead and pure tin.
This result supports the idea of a disorder-to-order transition in the arrangement of lead and tin at Pb0.5Sn0.5. The variation in atomic column contrast in the lattice images also corroborates this phenomenon. The abrupt change in lattice strain aligns well with the band gap variation. Density functional theory calculations based on cluster expansion methods also revealed that at the Sn/Pb ratio of Pb0.5Sn0.5, the formation enthalpy shows a significant negative value, indicating the potential for an ordered phase. Additionally, the study discovered that the strain energy was caused by the size difference between lead and tin ions, and an ordered arrangement of lead and tin reduces strain, corroborating the experimentally measured strain variation. Meanwhile, studies on Coulomb energy indicated that the magnitude of Coulomb energy in the ordered structure reached a maximum, leading to an energy gain in formation enthalpy.
This work reveals the disorder-to-order phase transition phenomenon and physics present in lead-tin mixed perovskite lattices, completing a key piece of the optical band gap “bowing” mechanism. This has important implications for the future design of compositions and phase structures in lead-tin mixed perovskite materials. The nanocrystals synthesized in this study can be used to create near-infrared LED devices with a light emission wavelength of 930 nm, marking the longest wavelength reported for perovskite-based LED devices.
Master’s student Han Gao is the first author of this paper, with Professor Zhubing He and Professor Su-Huai Wei from the Eastern Institute of Technology serving as the corresponding authors. Other contributors from SUSTech include Ph.D. student Dong He from the Department of Materials Science and Engineering, postdoctoral fellow Peili Gao from the Department of Electronic and Electrical Engineering, Dr. Dongsheng He from the Core Research Facilities, Professor Shuming Chen from the Department of Electronic and Electrical Engineering, and Chair Professor Shu-Hong Yu, along with postdoctoral fellow Zehua Chen from the Beijing Computational Science Research Center. SUSTech is the first affiliated institution of the paper.
Paper link: https://www.science.org/doi/10.1126/sciadv.ads4038
Related link: https://news.sciencenet.cn/htmlnews/2025/1/537216.shtm
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