In 1933, to explain the color center formation inalkali halides, Lev Landau proposedthat even in intrinsic lattices, there might be new quasiparticles of dressed carriers with lattice vibration. This opens the door to the study of polarons in condensed matter physics. In recent years, polarons have been used to explain physical effects such as the pyroelectric effect, multiferroic effect, and colossal magnetoresistance, which are of great application value. Since the 21st century, the photoelectric conversion efficiency of organic-inorganic lead halide perovskite photovoltaic devices has been advanced to above 26% by material and device scientists. The superior photoelectric properties may come from the formation of polarons in perovskites, which can shield off defect and impurity recombination. However, the formation of polarons has yet to be directly observed and verified.

In response to this frontier problem,Professor Du Juan's research group from Leng Yuxin's Studio at the School of Physics and Optoelectronic Engineering, Hangzhou Institute for Advanced Study, UCAS (hereinafter referred to as "HIAS School of Physics and Optoelectronic Engineering")investigated the A-site cation mediation of polaron mobility using time-resolved THz spectroscopy (Journal of Energy Chemistry 76, 175-180, 2023). On this basis, they proposed the concept that there is anharmonic coupling on the sub-picosecond time scale (10^-12seconds) during the formation of polarons, i.e., during the strong coupling between lattice vibration and photogenerated carriers, and according to Maxwell equations, electromagnetic waves of corresponding frequencies are radiated in the process. Based on this, Professor Du Juan's research group and Professor Sun Kuan's research group from Chongqing University jointly realized the direct observation and analysis of polaron formation in 𝛼-FAPbI3 perovskite films through the innovative method of THz emission spectroscopy,providing direct evidence for the formation of polarons in perovskites on the sub-picosecond time scale for the first time in the world.

It is found that the polaron P1 (~1 THz) is in connection with the vibrational mode of Pb-I coupled with the conduction band bottom carrier, and the other polaron P2 (~0.4 THz) is formed by the strong electron-phonon coupling between hot carriers excited to CB2 and the A-site cation rotation mode. The research not only provided real-time observation of the polaron formation process in perovskites but alsorevealed a new polaron mode related to A-site cation rotation, providing new physical interpretation and research support for the utilization of hot carriers and residual energy in perovskite solar cells.

The research findings were published inNature Communications, 14, 917 (2023). Yue Xingyu, a master's student enrolled in 2020 at the HIAS School of Physics and Optoelectronic Engineering, is the first author of the paper; associate research fellow Zhang Zeyu and Professor Du Juan from the HIAS School of Physics and Optoelectronic Engineering and Professor Sun Kuan from Chongqing University are the corresponding authors of the paper.

Figure (a) Schematic diagram of THz radiation by Dember current;

(b) THz spectrum;

(c) Schematic diagram of THz radiation resulting from polaron formation;

(d) Schematic diagram of FAPbI3 energy band structure;

(e) THz time-domain spectra at different pumped photon energies.

The research was supported by theKey R&D Program of the Ministry of Science and Technology, the Major Research Plan on New Optical Field Modulation Physics and Application, the General Program and the Young Scientists Fund of the National Natural Science Foundation of China, and the Basic Research Program of the Science and Technology Innovation Action Plan of Shanghai.

Publication

Yue, X., Wang, C., Zhang, B. et al.Real-time observation of the buildup of polaron in α-FAPbI3. Nat Commun 14,917(2023).https://doi.org/10.1038/s41467-023-36652-4

Prospect

The researchcarves out a way for the exploration of the frontier mechanism of time-resolved THz spectroscopy and time-resolved THz emission spectroscopy in new physics, chemistry and materials science.It has broad prospects regarding the research and application of THz spectroscopy technology for frontier problems in semiconductor physics, including the existence of polarons in new 2D materials such as antimony selenide and antimony sulfide as well as their mobility and their impact on photoelectric properties.

Source | School of Physics and Optoelectronic Engineering

Typesetter | Xiong Yanyijia

Executive Editor | Jiang Xuchen