Nuclei And Crystal-growth Control Based On Temperature And Solvent Engineering For Perovskite Films

Organic–inorganic hybrid perovskite materials are ideal candidates for solar cells because of their promising features such as tunable band gap,high carrier mobility,strong optical absorption and long carrier diffusion length.Over last ten years,the power conversion efficiency（PCE）of perovskite solar cells（PSCs）had increased from 3.8%to25%.At present,various strategies have been developed to prepare high efficiency PSCs,such as solution spin-coating,vacuum deposition,doctor blading,spraying and printing.As a simple and low-cost method,the solution spin-coating attracts much more attentions for researchers than other methods.The quality of perovskite film is the key factor for high performance PSCs.The films with high surface coverage,pinhole-free and high quality crystalline grains are required for efficient PSCs.The crystallization and nucleation processes are essential for obtaining high quality perovskite films by solution spin-coating.The initial nucleation process determines the surface coverage of the final film and the crystal growth process determines the final crystallization quality of perovskite films.Therefore,the key of preparing high quality perovskite films by solution method is to control the crystallization and nucleation processes during fabrication.According to the classical nucleation theory and the previous studies of perovskite films,the crystallization and nucleation processes of perovskite films are affected by many factors,such as solution composition,solvent composition,temperature,preparation process and so on.In this work,we discussed the effects of temperature and solvent engineering process for perovskite film crystallization and nucleation processes.The main research contents are as the following:1.Nuclei position-control and crystal growth-guidance（NPCG）on frozen substrate for high performance perovskite solar cell.In this section,we introduced the Nuclei Position-Control and Growth-Guidance（NPCG）method to produce high quality perovskite films with controlled nuclei position and crystal growth orientation.The perovskite film is spin-coated on a cold substrate to form a plurality of crystal nuclei on the surface of the substrate.During the thermal annealing process,those formed nuclei will preferentially direct subsequent grain growth from the substrate side.At the same time,the residual solvent in the films due to the frozen substrate can help the recrystallization of perovskite grains and promote the crystal growth to achieve high quality perovskite film.And an efficient inverted planar solar cell（ITO/PEDOT:PSS/CH₃NH₃PbI₃/PC61BM/BCP/Ag）with the highest power conversion efficiency of 17.14%and open-circuit voltage of 1.14V has been achieved by this technique.2.The effect of solution temperature on crystallization for perovskite films and PSCs performance.In this section,we discussed the effect of MA or FA solution temperature on the crystallization process.The XRD characteristics of perovskite films shown that there was no obvious effect on the crystallinity for perovskite films as the temperature of MA solution is between 0 and 100℃.However,the crystal growth orientation of FA films has been changed as the solution temperature increased.Based on the morphology characterization of AFM,the roughness of MA films was increased as the solution temperature raised.On the contrary,the surface roughness of FA films was decreased when the temperature of solution is increased.For both PSCs based on MA and FA perovskite films prepared by anti-solvent method,the change of solution temperature in the range of 0-80℃has no obvious effect on their performance.3.The proposed anti-solvent and thermal annealing free（AAF）method for fabricating PSCs.Spin-coating method is a widely used solution process to prepare perovskite solar cell.For films fabricated from precursors with conventional solvents of DMF or DMSO,stable intermediate phase is formed in the films after spin-coating.Thermal annealing or other processes are required to transform this intermediate phase into perovskite crystal.Herein,we prepared perovskite films by a single spin-coating process without anti-solvent and thermal annealing,which is called anti-solvent and thermal annealing free（AAF）method here,based on perovskite precursor with solvent of 2-methoxyethanol（2-M）.For the film obtained by this simple AAF fabrication method,a completely transformed perovskite film could been obtained without any intermediate phase immediately after spin-coating.By using AAF method with precursor prepared by 2-M solvent,power conversion efficiency of 12.95%has been achieved at room temperature for the planar perovskite solar cell（ITO/PEDOT:PSS/perovskite/PCBM/BCP/Ag）without any assistance processes by just single one-step spin-coating.4.The nucleation and crystallization control for AAF perovskite films based on solvent engineering.In section 3,we prepared the PSCs with AAF method from 2-M perovskite solution.Due to the rapid evaporation of 2-M solvent during spin coating,the nucleation process is completed in a short time and the perovskite grain have no chance to growth.Finally,we obtained a perovskite film with small grains and rough surface from AAF process,which would block the charge transfer.To improve the crystal growth,high boiling point solvent（DMF,DMSO,GBL）were mixed with 2-M solution to try to delay the solvent evaporation for enhanced perovskite grain size and better surface morphology.Based on this solvent engineering,the surface morphology of the films has been improved from GBL and 2-M mixed solution by AAF process.5.The effect of low temperature on the crystallization process for AAF perovskite films.Beside high boiling point solvent doping,lowering temperature could also reduce the solvent evaporation rate.Based on section 1 and 2,herein,we controlled the nucleation and crystallization process of AAF perovskite films by decreasing the temperature of substrate or solution.When the substrate temperature decreased,heterogeneous nucleation of AAF films has be promoted and homogeneous nucleation has be inhibited,which makes the surface morphology of perovskite films much smooth.