A First-principles Study On The Stability And Self-trapped Excitons Of Lead Halide Perovskites

Organic-inorganic hybrid perovskites have attracted extensive research interests in the field of photovoltaic and light emitting fields due to their excellent optoelectronic properties.Despite the power conversion efficiency of perovskite solar cells has developed rapidly,device stability issues seriously hinder their commercial application.The device structure,interfacial engineering,and chemical component tuning are effective methods to enhance the stability.Although it has been widely reported that the materials stability can be enhanced via tuning their cationic components,the underlying physical mechanisms are still unclear.The self-trapped exciton in perovskites has a significant effect on its optoelectronic properties.Studies have shown that factors like dopants,chemical composition,and structural dimensionality may all affect the self-trapping of excitons,a systematic understanding of the mechanism is lacking.Therefore,in order to better improve the performance of perovskite optoelectronic devices,this paper studies the phase diagram of a series of mixed cation perovskites,and elucidates the microscopic mechanism of the effect from cations on the perovskite stability at different temperatures,and further investigates the properties of self-trapped excitons and phonon vibration modes in perovskites of different dimensions.A universal mechanism for the formation of self-trapped excitons is proposed.The main research contents are as follows:Mixed-cation perovskite phase diagram and stability studies.A series of studies on mixed-cation FA-based perovskites showed that,for Cs and Rb doping,the phase separation by-product is non-perovskite phase(δ-phase),and the by-product is iodide for K and MA doping;The phase diagram shows that only a small amount of MA,Cs,and Rb can be uniformly doped into the FA-based perovskite,and K cannot enter the host lattice at all.A doping ratio of about 10 mole percent favors the transition from the δ-phase to the perovskite phase(α-phase).The thermodynamic properties of the hybrid perovskites were further investigated by generalized quasi-chemical approximation method,a phase diagram of composition versus temperature was drawn,and the critical temperature for phase separation and the ratio of cations for the formation of stable and metastable perovskites were determined.We further studied the ternary cation perovskite system in which MA and Cs were doped with FA group,and successfully established the regular cognition of "component versus stability" of ternary cation perovskite.The microscopic mechanism of the stability of mixed cation perovskites affected by temperature is studied.Experiments show that at high aging temperatures,an increase in the content of organic cations MA(or a decrease in the content of inorganic cations Cs)is detrimental to the stability of hybrid perovskites;while at low aging temperatures,the opposite is true.To elucidate the physical mechanism of the temperature-induced stability inversion,we investigated the decomposition behavior of perovskites at different temperatures.Thermodynamic analysis at high temperature shows that the incorporation of MA changes the perovskite decomposition from an endothermic reaction to an exothermic reaction,resulting in low stability.The kinetic decomposition process at low temperature shows that the protonation of iodine atoms plays a key role in the surface decomposition,MA inhibits the further desorption of organic molecules after the formation of iodine vacancies,which is beneficial to the device stability,and Cs promotes the desorption process.These results provide a strong theoretical basis for understanding the phenomenon of "temperature-induced reversal of perovskite stability".Formation mechanism study of self-trapped excitons in perovskites of different dimensions.Free excitons in one-dimensional(1D)perovskites are easily trapped by the lattice to form self-trapped excitons,and some two-dimensional(2D)and threedimensional(3D)perovskites can also induce the formation of self-trapped excitons by introducing defects or doping.Although self-trapped excitons are widespread in perovskites,the mechanism of their formation is not well understood.We investigate the properties of self-trapped excitons in intrinsic and defective 3D,2D,and 1D perovskites.The self-trapping energy of the three-dimensional intrinsic structure is extremely small,indicating that the structure is stable and difficult to form local distortion.However,after the introduction of halogen vacancies,the self-trapping energy of 3D and 2D perovskites increases significantly.For the one-dimensional structure,the intrinsic structure has a large self-trapping energy due to the soft lattice characteristics.The distorted positions and modes of the self-trapped excitons were determined by the partial charge density.Then,by calculating the strength of the electron-phonon coupling,the three-dimensional perovskite intrinsic structure vibration modes are obtained.Based on the distortion modes of self-trapped excitons and the vibrational modes of three-dimensional perovskites,we further investigate the formation mechanism of the self-trapped excitons.The results suggest that the formation of self-trapped excitons in perovskites may depend on the competition between the band gap and lattice deformation energy.When the energy of band gap reduction is greater than the energy of lattice deformation,excitons are more likely to self-trapped;otherwise,excitons are difficult to self-trapped.Introducing halogen vacancies and reducing dimensionality can increase the bandgap change or decrease the lattice deformation energy,leading to exciton self-trapping.In this paper,the phase diagrams and stability of a series of cationic mixed perovskites are analyzed in detail,and the physical mechanism of the effect of temperature on the stability of mixed cationic perovskites is revealed;the self-trapping process is explored from the atomic scale,and the formation mechanism is clarified.These results deepen the understanding of perovskites and provide a feasible way to experimentally enhance the performance of perovskite optoelectronic devices.