Studying Of High-pressure Photophysical Properties Of All-inorganic Perovskite Material CsPbI₃ Nanocrystals

In recent years,All-inorganic perovskite has become a star material in the fields of photovoltaics due to its long carrier lifetime,high absorption coefficient,suitable optical band gap,and high quantum yield.However,all-inorganic perovskite materials are more sensitive to changes in the external environment.For example,some all-inorganic perovskites are easy to decompose at high temperatures,and some will undergo non-perovskite transformations.The pressure is a physical parameter independent of temperature and chemical composition,and it is an efficient cleaning tool.By increasing the pressure,the distance between atoms can be effectively shortened,and the orbital coupling of adjacent electrons can be strengthened,resulting in many new structures and new properties that are not available under ambient conditions.Therefore,exploring the optical properties of all-inorganic perovskite materials under high pressure has broad application prospects and important practical value.In this paper,structure change,band gap evolution,and carrier dynamics of CsPbI₃ nanocrystals have been studied by in-situ high pressure.The results obtained are as follows:1.The CsPbI₃ nanocrystal undergoes a phase transition under pressure.At 0.39GPa,the steady-state emission and absorption spectra change from the initial red shift to a continuous blue shift.At the same time,with the increase of pressure,the PL peak position and PL intensity of CsPbI₃ nanocrystals both have an inflection point at 0.39 GPa.Then,according to the Tauc curve extrapolation method,the change curve of the band gap of CsPbI₃ nanocrystals with pressure is obtained,which further clarifies the structural phase transition under pressure.In addition,we analyzed the structural changes of CsPbI₃ nanocrystals under pressure.The structural phase transition of CsPbI₃ nanocrystals mainly comes from the shrinkage and tilt of Pb I^6-octahedron.Due to the small initial pressure gradient,the Pb I^6-octahedral contraction is in the dominant mode,which strengthens the coupling of the Pb 6s and I 5p electron orbitals,increases the valence band energy,and reduces the band gap.When the pressure exceeds 0.39 GPa,the Pb I^6-octahedron begins to tilt.As the bond angle between Pb-I-Pb decreases,the electron orbital coupling between Pb 6s and I 5p is weakened,and the band gap is widened.2.The dynamics of CsPbI₃ nanocrystals changed under pressure.We used the combination of high-pressure technology and time-dependent single-photon counting technology to explore the carrier dynamics of CsPbI₃ nanocrystals under pressure.According to the changing trend of the PL dynamics curve with pressure,it shows that the phase change of the crystal structure also influences the carrier dynamics process.At the same time,the PL QY of CsPbI₃ nanocrystals decreased significantly with the increase of pressure,while the radiative lifetime was continuously increasing,indicating that the non-radiative channel opened further under the action of pressure and competed with the radiative channel.In addition,the carrier dynamics of CsPbI₃ nanocrystals were further studied by monomolecular recombination and bimolecular recombination,and it was found that the monomolecular recombination and bimolecular recombination rate slowed down with increasing pressure.This indicates that the non-radiative channel in the monomolecular recombination and the non-radiative channel in the bimolecular recombination act together on the radiative channel,which leads to a decrease in the photoluminescence yield of CsPbI₃ nanocrystals.