Pressure-Regulated Crystal Structure And Photoelectric Properties Of Typical Organic-Inorganic Hybrid Perovskites

The organic-inorganic hybrid perovskites?OIHPs?with ABX₃ structure have become a star material in the fields of physics,chemistry,materials and energy due to its long carrier life,high absorption coefficient,diverse chemical composition and crystal structure,which has extensive application potential in the many material fields such as photovoltaics and photonics.Therefore,it attracts scientists from all over the world for in-depth and systematic research.On the one hand,perovskite materials show amazing prospects in the field of solar cell.The energy conversion efficiency of perovskite solar cells have rapidly increased from the initial 3.8%to the current 25.2%,and the lifetime has also increased to thousands of hours.On the other hand,the unique structure of OIHPs make them have excellent light emitting characteristics.Among them,2D OIHPs with high-dielectric inorganic layers and low-dielectric organic layers alternately have natural quantum well structures and dielectric confinements,and show excellent light-emitting properties.So far,breakthroughs have been made in the research of perovskites,but there still are some key challenges and issues:The structural stability is not high enough,the bandgap does not reach the Schottley-Quiserin limit?1.34 eV?,the conductivity is poor,the photoluminisence?PL?performance needs to be further improved,and the internal mechanism of the relationship between crystal structure and characteristic is not clear.Therefore,obtaining new phases or structures with excellent optoelectronic properties have become an important scientific issue in this field.As an independent thermodynamic parameter,pressure can effectively reduce the distance between atoms and increase the orbital coupling between adjacent electrons,thereby regulating the crystal and electronic structure of materials,forming the state of materials that cannot be formed under normal conditions.Unlike chemical doping,pressure does not introduce other influencing factors when applied to the material,so it is considered as a"clean"regulation method,which is expected to obtain new structures and new properties of perovskites.Therefore,we selected typical 3D all-inorganic hybrid perovskite cesium lead iodine?CsPbI₃?and 2D perovskite propylamine lead iodine[PA₈Pb₅I₁₈],using in-situ high-pressure electricity,fluorescence,ultraviolet-visible-infrared absorption,synchrotron XRD combined with first-principles calculations,have achieved pressure regulation of the structure and properties of these two perovskites,and achieved innovative results.1.Under high pressure,the 3D all-inorganic hybrid perovskite CsPbI₃ crystalstructure,band gap,and resistivity have changed,and a new ordered metal phase hasbeen obtained for the first time.It is a completely new substance,and the discoveryof the ordered metal phase of CsPbI₃ adds a new member to the perovskite family.The specific results are as follows:?1?.A new high-pressure phase of CsPbI₃ was obtained.In situ high-pressureXRD of CsPbI₃ shows that it has completely transformed from the Pnma phaseinto C2/m phase with an ordered structure at 18.1 GPa,accompanied by severedistortion of the PbI₆ octahedron configuration and 12%volume collapse.Thecollapse is currently the largest among hybrid perovskites.?2?.The regulation of the Shockley-Queisser limit band gap was successfullyachieved.Under pressure,the optical bandgap of CsPbI₃ decreases linearly andreached to the Shockley-Queisser limit?1.34 eV?at 13.5 GPa.The higherpressure infrared reflection indicates CsPbI₃ shows the metallicity dominated byfree electrons at 42.6 GPa.?3?.The inherent mechanism of band gap reduction and metallization transitionwas revealed.First-principles calculations of electronic energy bands andelectronic local functions determined that this transition was caused by abnormalchanges in the PbI₆ octahedra structure and revealed the physical mechanism ofchanged Pb-I bond length and Pb-I-Pb bond angle in bandgap reduction andmetallization of CsPbI₃.2.Study on the structural stability,bandgap and PL of 2D Hybrid Perovskite PA₈Pb₅I₁₈ under high pressure.Under pressure,we obtained nearly one hundred-fold PL enhancement by regulating the octahedra configuration,revealing the physical mechanism of its PL enhancement,and provided a scientific basis for the application design of 2D perovskite.The specific results are shown as follows:?1?.For the first time,we found that the severely tilted PbI₆ octahedra caused?PA?₈Pb₅I₁₈8 to undergo an isostructural phase transition at 10.7 GPa,accompanied bya 6%lattice volume collapse.?2?.Under pressure,the continuously changed Pb-I bond length and Pb-I-Pb bondangle of 2D OIHP PA₈Pb₅I₁₈ decrease the it's own bandgap.Reduced from 2.33eV at atmospheric pressure to 1.26 eV at 35.0 GPa.?3?.At 3.5 GPa,the PL of PA₈Pb₅I₁₈ showed a nearly one hundred-foldenhancement?80-fold?.During the pressurization,the PL of PA₈Pb₅I₁₈ changedfrom weak green at 1 atm to dazzling yellow at 3.5 GPa.High-pressure PL spectrashowed that the broad and weak low-energy peak at ambient condition rapidlyincreased nearly 80 times at 3.5 GPa.?4?.The main reason for PL enhancement is explained.In situ high-pressureexperiments showed that under pressure,the increasing hydrogen bonds betweenthe inorganic PbI₆ octahedra layer and propylamine molecules can drive PbI₆octahedra tilt out-of-plane along the I2-I4 axis,and the strong coulpling betweenthe distorted inorganic lattice and excitons deepens the trapped state and promotesfree excitons to form trapped excitons.The increasing number of trapped excitonsincreases the probability of their own radiative transitions,achieving nearly a hundredfold increase in PL.Our research results show that high pressure can effectively optimize the crystal structures and optoelectronic properties of hybrid perovskites,and provide new ideas and ways to solve problems such as stability and performance improvements.