| In recent years,organic-inorganic hybrid perovskites(ABX3)have drawn substantial interest due to their outstanding performance in solar energy conversion and optoelectronic applications.Organic-inorganic hybrid perovskites have high absorption coefficients,adjustable direct bandgap,long carrier dispersion length,low exciton binding energy,and well-balanced electron hole mobility.At present,through component engineering;development of electron and hole transport materials and optimization of device structure,power conversion efficiency(PCE)of organic-inorganic hybrid perovskite solar cells has exceeded 22%.Although the PCE of perovskite-based solar cells is growing rapidly,there are still many key factors that limit their further development,such as device stability,hysteresis behavior of the ?-? curve,expensive hole transport material(Spiro-OMeTAD),large scale fabrication of perovskite film,material toxicity and so on.In order to further improve their PCE,these problems have to be trackled.Along with the development of experimental techniques,a large number of theoretical studies have also been carried out to understand the extraordinary properties of pcovskites.Theoretically.the most widely used method is density functional theory(DFT).Electronic structure calculations based on the DFT method have deepened our understanding of the properties of these materials,such as optimal bandgap,controllable doping,defect tolerance,ferroelectricity,high absorption coefficient,bipolar carrier transport,ion migration,interface and surface,and geometrical properties.Despite the previous efforts,there are still many elusive properties,due to the diversity and complexity of organicinorganic hybrid perovskites.The density functional tight binding(DFTB)method is an approximated DFT method which allows simulations of very large systems.In addition,the non-equilibrium Green's function method(NEGF)is a powerful tool for studing electron transport in open systems.Therefore,DFTB combined with NEGF method has opened new avenues for understanding the mechanism of electron transport in organic-inorganic hybrid perovskite solar cells.In this thesis,we systematically explored the influence of organic cations on the crystal structures and photoelectric properties of lead halogen perovskites.Organic cations have noncentrosymmetic structures with permanent dipole moments.Theoretical studies have shown that their rotational energy barriers are very small and thus easy to rotate and migrate within the inorganic framework.Therefore,ferroelectric dipole orderings are formed upon application of biased voltage.In addition,the mixing of different organic cations also has an important influence on the stability of organic lead halogen perovskites.Based on previous results of,we try to unveil the important role of organic cations in the optoelectronic properties of organic leadhalogen perovskites.The main contents are summarized as follows:(1)The presence of ferroelectric domain walls in these materials has shown to have a profound effect on their electronic structure.Here,we use density-functional tight-binding model,coupled to non-equilibrium Green's function method to investigate the effects of ferroelectric domain walls on electronic transport properties and charge carrier recombination in methylammonium lead-iodide perovskite.MAPb13.With the presence of-ferroelectric domain walls,segregation of transport channels for electrons and holes is observed and the conductance of perovskitcs is substantially increased due to the reduced band gap.In addition,by taking into account interactions with photons in the vacuum environment,it is:found that electron-hole recoimbination in perovskites with ferroelectric domain walls is drastically suppressed due to the segregation of carrier transport paths,which could enhance photovoltaic performance.(2)Compositional engineering,mixing different organic cations(A)or halide anions(X)in APbX3,i.s one of the most effective ways to enhance organic perovskite stability.The Imix;ing of-f'ormamidinium(FA)and methylammonium(MA)cations has gained significant interest.Here,we performed first-principles calculation to systematically investigate the strLuctures and optical properties of FA1-xMAxPbI3.By analyzing the optimized structures of FA1-xMAxPbI3,we revealed that the lattice parameters decrease linearly with increasing proportion of MA cations.In this work,the formation energy(AE)is calculated and our results show that mixing of FA and MA cations can increase the thermodynamic stability as compared to pure FAPbI3 and MAPbI3.Among different composition,FA0.5MA0.5PbI3 is found to be the most stable.Furthermore,we found that the band gap increases with increasing the proportion of FA cations.The detailed optical absorption coefficients of all the composition are calculated,and the results show the absorption spectra blue shift with increasing proportion of MA cation.These results demonstrate the possibility of controlling optoelectronic properties by mixing FA and MA cations in organometallic lead halide perovskites and hence further improve the efficiency of perovskite solar cells. |
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