Research On Preparation And Photovoltaic Performance Improvement Of Two-dimensional Perovskite Solar Cells

Three-dimensional organic-inorganic metal halide perovskite materials have great photoelectric properties and the power conversion efficiency of perovskite solar cells has increased rapidly from 3.8%to the certified 25.5%.Perovskite solar cell is an outstanding candidate of emerging photovoltaics,however its stability is affected by various factors,including intrinsic material characteristics and external environmental conditions（light,moisture,temperature,ultraviolet irradiation and so on）.The instability is a fatal flaw for practical application of perovskite solar cells.Two-dimensional perovskite solar cells can block the intrusion of moisture and oxygen through the introduction of large-size organic cations,and at the same time prevent ion migration within the crystal structure,providing a way out for solving the instability problem of three-dimensional perovskite solar cells.Although the two-dimensional perovskite cell shows excellent environmental stability,its current photovoltaic performance is lagging far behind its three-dimensional counterpart,failing to meet our expectations.Firstly,the photophysical properties of the two-dimensional perovskite are determined by its structural characteristics where the conductive inorganic layer and the insulating organic layer arrange alternatively.Due to the quantum confinement effect and the dielectric confinement effect,the two-dimensional perovskite exhibits a higher exciton binding energy and limited conductivity,which will hinder the electron/hole separation and charge transfer.It is where the power conversion efficiency loss stems from.Secondly,the introduction of large-size spacer cation in the precursor solution makes its crystallization a fully complicated process.And the final film processed by solution method contains low-dimensional phases and three-dimensional phases at the same time.The content and distribution of these phases in the film relate to charge transfer and energy level matching and determines the final device performance.Therefore,for efficient and stable perovskite solar cells,more efforts are needed to solve these problems.Based on the above problems,the research contents mainly focus on the three following aspects:1.The solution method was used to prepare two-dimensional perovskite films,and the properties of the films and solar cell device were characterized to explore their photoelectric conversion performance,and the properties of two-dimensional and three-dimensional perovskite films and devices were compared.The results show that,compared with the three-dimensional perovskite film,the two-dimensional perovskite film has a stack of layered grains and a crystal orientation that is not helpful to carrier transport.The results of ultraviolet absorption spectra showed that the light absorption capacity of the two-dimensional perovskite film decreased,and an exciton absorption peak representing a low n-value phase appeared,indicating that there are multiple n-value phases coexisting inside the film.Through comparing photovoltaic performance parameter,it is found that the short-circuit current of the two-dimensional perovskite cell has dropped significantly from 20.33 to 14.15 m A·cm^-2,which is the main reason for the degradation of photoelectric conversion performance.In addition to the decline in the light absorption of the film,the photon conversion ability of the device also drops sharply.According to the property of the film and device,we have established three-dimensional and two-dimensional carrier transport models inside the perovskite film,which correspond to the uniform energy levels and blocked carrier transport by disordered energy levels,respectively.The properties of the two-dimensional perovskite film and device prepared by the solution method in this work are clarified in order to further realize its application in solar cells.2.Focusing on the large gap between the photoelectric conversion efficiency of two-dimensional and three-dimensional perovskite solar cells,the commercialized three-dimensional perovskite phase MAPb I₃ was directly introduced into the precursor solution of ACI two-dimensional perovskite.It can adjust the content and distribution of the low-dimensional phase and the three-dimensional phase in the final prepared film simultaneously.The characterization and test results of the two-dimensional perovskite film before and after thermal annealing show that the content of the low-dimensional phase which is unfavorable to the photoelectric performance in the film is reduced after the introduction of MAPb I₃,and the light absorption capacity,crystal orientation,and charge transfer are all improved.The defect density is reduced,and finally the power conversion performance of the two-dimensional perovskite solar cell is successfully improved from 10.30%to 13.82%.3.To improve the photoelectric conversion performance of the self-assembled two-dimensional perovskite,we chose the surfactant CTAB which can adjust the surface tension as the additive to improve quality of the film prepared by the hot casting method and one-step spin coating method without dripping any anti-solvent.When the concentration of the additive CTAB is 0.6 mg/m L,the open circuit voltage of the device is increased from 1.103 V to 1.153 V,and the power conversion efficiency is increased from 6.78%to 9.31%.The absorbance and crystallinity of the film are enhanced.Pinholes and defect density are also reduced.