Investigation On The Solvent-assisted Method For Controlling Two-dimensional Perovskite Crystallization To Improve Solar Cells Efficiency

Two-dimensional（2D）perovskite materials,are more stable than three-dimensional（3D）perovskite materials,which can solve the stability issue of perovskite solar cells（PSCs）.However,PSCs based on 2D perovskite exhibit poor power conversion efficiency（PCE）due to the introduction of bulky organic cations.The main reason is that the bulky organic cations are insulating,which affects the charge transport in the vertical direction of the 2D perovskite materials.Controlling the crystal growth of 2D perovskite materials is an effective strategy.When the inorganic layers of 2D perovskites grow perpendicular to the substrate,charge transport occurs directly in the inorganic layers.In this way,the adverse effects of bulky organic cations will be greatly reduced,thereby effectively enhancing the optoelectronic properties of the device.This paper mainly explores the improvement of 2D perovskite crystal orientation:（1）In order to accurately control the top-down crystallization process of 2D perovskite to improve its orientation,a solvent-assisted method was proposed to prepare 2D perovskite films.The solvent-assisted method was proposed to prepare 2D perovskite films,which could accurately control the top-down crystallization process of 2D perovskite,and improve the crystal orientation of 2D perovskite and cell efficiency.Compared with the control method,the perovskite films prepared by the solvent-assisted method had higher crystallinity,higher vertical orientation and fewer trap states.In order to explore the reason why the solvent-assisted method could effectively improve the quality of 2D perovskite films,the mechanism of its influence on 2D perovskite crystallization was analyzed.When using the solvent-assisted method,the assistant solvent DMSO could be distributed in a gradient（less on the top and more on the bottom）in the precursor solution.This allowed it to react with more Pb²⁺at the bottom to form intermediates,and reduce the crystallization rate of the bottom perovskite to a greater extent.During the top-down crystallization of 2D perovskites,due to the gradient distribution of DMSO,the bottom Pb²⁺was gradually released and crystallized vertically with the preferentially crystallized large-n phase at the top as a template.Therefore,the solvent-assisted method could accurately control the crystallization of 2D perovskite,making it have a high of crystallinity and orientation.It was beneficial to the transport and extraction of charges in the device,which resulted in a significantly increased J_sc than that of control devices in the current density-voltage（J-V）test,resulting in 12.42%PCE and 48%increase more than the control device.（2）When the 2D perovskite crystallization was controlled by the solvent-assisted method,Pb²⁺coordinates with the assistant solvents to form an intermediate phase,and then escaped from the intermediate phase to participate in the crystallization in the later stage of crystallization.Among them,the coordination between Pb²⁺and the assistant solvents had a great influence on the crystal morphology of 2D perovskites.Therefore,the properties of the assistant solvents in the solvent-assisted method are the main factors affecting the crystal morphology of 2D perovskites.The results showed that the order of the quality of the 2D perovskite films and the photovoltaic performance of the device was consistent with the order of the Lewis basicity of the solvent.This indicated that the Lewis basicity of the assistant solvent plays more important roles in the crystallization process of 2D perovskites than the boiling point and polarity.This was because that the stronger the Lewis basicity was,the slower the crystallization rate was and the more obvious the accurate control effect on the top-down crystallization process of 2D perovskite would be.When DMPU with the strongest Lewis basicity was used as the assistant solvent,the 2D perovskite films exhibited higher crystallinity and orientation,higher photophysical properties,and lower density of trap states.The device based on DMPU assistant solvent had higher charge transport and extraction,and exhibited higher PCE（13.69%）and better environmental stability（retains 85%of the initial PCE after1512 h storage in air）.The solvent-assisted method is an effective method to improve the crystal orientation of2D perovskite to enhance the PCE of PSCs.In order to control the crystallization of 2D perovskite more accurately,an assistant solvent with strong Lewis basicity should be selected for solvent-assisted 2D perovskite crystallization.