Surface And Interface Regulation And Photoelectric Properties Of Inorganic Perovskite Solar Cell Materials

The optoelectronic performance of perovskite solar cells depends not only on the composition,properties and morphology of each layer of the materials,but also on the composition,structure and properties of the surface and interface,because the surface and interface are the main places for charge extraction,transfer and recombination.The perovskite battery materials prepared under mild experimental conditions are far from the equilibrium state,rich in surfaces and interfaces,structural disorders and crystal defects,resulting in poor reproducibility of experimental results and poor device stability.In recent years,researchers have become more aware of the harmful surface-interface effects caused by component disorders and structural defects at the surfaces and interfaces of the battery materials,which become the major obstacle in improving the conversion efficiency of the battery,and have proposed several control methods.Due to the difficulty of directly observing the surface and interface structure at the atomic scale in experiments,even if the high resolution transmission electron microscope is used,it only provides average result,and it is difficult to give detailed information of local lattice structure.Therefore,there is no complete surface-interface（local）structure model of perovskite battery materials so far,which makes the theoretical research far behind the experimental research and development.The intersection of computer,physics,materials and other disciplines has accelerated the progress of material research and development,and computer simulation can obtain comprehensive and basically reliable scientific data that is not easy or impossible to obtain in experiments at a small cost and in a short time.Therefore,the microscopic surface-interface model of perovskite solar cells was constructed to understand the microstructure,function and mechanism of the surface-interface that has been proposed or may exist in experiments.Thus,it can predict some passivation measures that can reduce the surface states and interface states and improve the device performance.It has the dual significance of theory and engineering to study the interface local structure and interface optoelectronic properties,and to seek effective interface regulation methods.Based on above reasons,in this paper,for the purpose of regulating the surface-interface structure of inorganic perovskite solar cells,several typical surface-interface structures and passivation materials were selected.We regulated the crystal and surface-interface structure of inorganic perovskite materials through composition changes,and designed reasonable heterojunction of perovskite solar cells.The change rule of photoelectric performance,surface-interface regulation and passivation mechanism were explored.The main research results obtained are as follows:（1）Using first-principles calculations,the crystal structure of the bulk CsPbI₃phase was optimized,and the electronic properties of the bulk CsPbI₃ phase were calculated.The calculation results show that there are surface states on the surface of CsPbI₃（110）,and the surface states is optimized by modifying its surface with Cl,F and H atoms.By calculating and analyzing the density of states（DOS）,charge density difference and Bader charge of the modified surface,it is found that the surface states near the Fermi level are weakened to varying degrees.According to adsorption energy and the weakening degree of surface states,the passivation effect of H atom is the best,followed by the passivation effect of F atom,and the weakest of Cl atom.（2）The electronic properties of the bulk CsPbI₂Br and the passivation of the surface states of CsPbI₂Br（110）terminated with I-Br atoms were calculated by the first-principles calculations.It is found that the band gap of CsPbI₂Br calculated by the GGA-PBE functional is 1.42 e V,the calculated band gap value of HSE06 is about 1.96e V,which is closer to the experimental value of 1.92 e V.The valence band top of CsPbI₂Br bulk is mainly contributed by I-5p orbital and Br-4p orbital,and the conduction band bottom is mainly contributed by Pb-6p orbital.Through the calculation of the surface states of CsPbI₂Br（110）passivated by Cl,F and H atoms,it is found that H atom has the best passivation effect.Its adsorption energy value fluctuates less,it is less sensitive to the adsorption position,and the adsorption is stable.Followed by F atom,its passivation effect is worse than that of H atom,but better than that of Cl atom.Although the passivation position has a certain influence on it,it has little effect.The Cl atom is most affected by the passivation position.The position of the passivation atoms is different,the adsorption energy fluctuates greatly,the adsorption stability is poor,and the passivation effect is also poor.Through the charge density difference and Bader charges,it is found that the H atom gets more electrons from the I atom,which is beneficial to the passivation of the surface states.The electron-getting ability of the F atom is inferior to that of the H atom and is superior to that of the Cl atom.The electron-acquiring ability and passivation ability of Cl atom are the weakest among the three elements.（3）The PEA⁺-CsPbI₃（110）adsorption model iswas reasonably constructed by adopting an organic cation PEA⁺passivation strategy.The calculation results show that the model in which the CsPbI3（110）surface with the I atom as the termination end bound to the PEA+ molecule is the most stable,and the CsPbI3（110）surface is the most stable model.The electronic states of the surface at the Fermi level are mainly contributed by the I-5p orbital.Comparing the change results of the DOS near the Fermi level between the clean surface and the system after passivation,it is found that the surface states of the CsPbI3（110）surface optimized by organic cation PEA+ has a significant weakening effect,with a decrease rate of 38.8%.（4）The surface of CsPbI2 Br was optimized by PEA+ ions,and it is found that the introduction of PEA+ ions did not produce new surface states,and at the same time could effectively suppress the original surface states,so that the value of the DOS at the original Fermi level is changed from the original 16.8 e/e V dropped to 10.5 e/e V,a drop rate of 37.5%.Calculations of charge density difference,Bader charge and partial charge density indicate that charge transfer follows the path of N（PEA+）→I and Br（CsPbI2Br（110））from the PEA+ layer to the CsPbI2 Br layer,showing the formation of N-I between organic ligands and inorganic layers,N-Br,N-Pb electronic coupling can provide an effective channel for carrier transport,thereby achieving a substantial enhancement of the carrier transport capability.（5）The interfacial heterojunction structure was constructed by the inorganic surface CsPbI3（Pb I）/Sn O2（O）,and the lattice mismatch degree is 4.45%,indicating that the CsPbI3（100）and Sn O2（110）surfaces can be well matched to form a co-lattice plane.The interfacial binding energy is-2.86 J/m2,and indicating that the interfacial heterostructure is stable.Through the analysis of the photoelectric properties of the CsPbI3（100）/Sn O2（110）interface,it is found that the interface states appearing near the Fermi level are derived from I-5p,O-2p,Pb-6p and Pb-6s orbitals contribution,and interface states inhibit carrier transport and affect solar cell efficiency.Through the calculation and analysis of the dielectric constant with the interface,it is found that compared with the bulk CsPbI3 phase,the static dielectric constant of the heterojunction decreases,and the absorption coefficient shows a decreasing trend in the visible light range.In the UV region between 7.18 and 12.23 e V,the absorption intensity increases with photon energy.Affected by the interface effect,the light absorption ability of the heterojunction is decreasing.