| Perovskite solar cell,as a new type of solar cell with great potential,has attracted more and more attention due to its increasing photoelectric conversion efficiency.However,the light absorbing layer material of methylamine lead iodine?CH3NH3PbI3,or MAPbI3?used in high-efficiency perovskite solar cells is extremely unstable,especially in humid environment,which causes the device's photoelectric conversion efficiency to decay violently in a short time.This indicates that water molecules play an important role in the degradation process of MAPbI3,however,the state of adsorption of water molecules on the surface of MAPbI3 is not yet clear.In addition,experiments reported that the addition of graphene improves the stability and photoelectric performance of perovskite solar cells,but the interpretation at the micro level is still lacking.To this end,the density functional theory based on the first-principle is used to study the adsorption behavior of water molecules on the surface of MAPbI3?001?in order to understand the initial degradation of MAPbI3.At the same time,the mechanism of graphene in improving the stability of MAPbI3 in a humid environment was studied,and the carrier separation efficiency at the interface of MAPbI3/graphene heterostructure was improved through interface control.The following are the specific research contents and conclusions:1.The surface,electrical properties of MAPbI3?001?surface and the adsorption of water molecules were studied.The results show that the non-stoichiometric ratio of MAPbI3?001?surface with the lowest surface energy at the terminal of PbI2.In humid environment,water molecules tend to adsorb on the top of MAPbI3?001?in molecular form without decomposition.When water molecules are adsorbed on the surface of MAPbI3?001?,the surface polarization increases,and the band gap increases with the increase of the amount of water molecules adsorbed.2.The protective effect of graphene on the surface of MAPbI3 in humid environment was studied.The results of the adsorption energy and transition state search indicate that the system is the most stable when water molecules are adsorbed outside the heterostructure interface in the undissociated form.This shows that graphene can effectively prevent external water molecules from reacting with MAPbI3,thereby realizing the protection of MAPbI3.In addition,the adsorption of water molecules has little effect on the band structure and interface charge distribution of the MAPbI3/graphene heterostructure.3.The interface interaction and electrical properties of MAPbI3/graphene heterostructure were studied.The results show that van der Waals heterostructures with similar work functions can be formed between the MAPbI3?001?surface and graphene with different stacking angles.The energy band structure shows that the energy bands of graphene and MAPbI3 remain intact in the heterostructure,and the Schottky barrier at the interface between the two can be adjusted by the layer spacing.In addition,the built-in electric field between interfaces promotes the effective separation of photogenerated electron hole pairs.In summary,we deeply analyzed the adsorption of water molecules on the surface of MAPbI3?001?and the role of graphene in protecting MAPbI3 from the humid environment.In addition,the excellent carrier separation at the interface was realized by adjusting the Schottky barrier of MAPbI3/graphene heterostructure.The research results provide theoretical guidance for the design of efficient and stable perovskite solar cells. |
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