Effects Of Crystallization,interface Modification And Ion Migration On The Efficiency And Stability Of Perovskite Solar Cells

Organic-inorganic perovskite solar cells（PSCs）are considered to be the successor of the next generation of solar cells due to their remarkable photoelectric conversion efficiency and inexpensive preparation process.The key to enhancing the efficiency and stability of PSCs is to improve the quality of thin films,optimize the interface and understand the degradation mechanism of devices.In this paper,the regulation of perovskite crystal growth,the optimization of the interface between electron transport layer and perovskite,and the quantitative characterization of ion behavior in perovskite were studied.The main conclusions and achievements of this paper are as follows:（1）An improved post-processing method for film preparation by dissolution and recrystallization was proposed.Methylamine hydrochloride（Dimethyl sulfoxide）（MACl（DMSO））composite reagent was prepared by taking advantage of MACl’s ability to induce vertical crystallization and DMSO’s solubility.It was used for post-processing of perovskite film,which could effectively improve its crystallization quality and photoelectric properties of the film,and photovoltaic response and stability of the cell.Its average fill factor（FF）is over 82%,and the power conversion efficiency（PCE）remained more than 80%of the original efficiency after 420 hours of continuous illumination.（2）An improved"hybrid two-step sequential deposition method"was proposed,in which a small amount of Cs I and Rb I were added into the precursor solution of Pb I₂ in the first step,and Cs⁺and Rb⁺were successfully infiltrated into the crystal lattice of Pb I₂,and Rb⁺was embedded into perovskite crystal for the first time by this method.It is found that the introduction of Cs⁺to generate a small amount of-Cs Pb I₃ crystal phase can reduce the crystal size of Pb I₂ and increase the grain boundaries,which is conducive to the infiltration of organic salts in the second step.Instead of affecting the crystallization of Pb I₂,Rb⁺ions make the film surface smoother.By adjusting the ratio of Cs⁺and Rb⁺ions,the monolayer and columnar perovskite grains can be obtained along the（100）orientation.The performance of the optimized device is improved comprehensively,and a champion device with a PCE of 21.49%and a certification efficiency of 20.9%were obtained.（3）Zirconium acetylacetonate（Zr（acac）₄）was introduced into the n-i-p PSCs based on the Sn O₂ electron transport layer（ETL）,and the Zr（acac）₄/Sn O₂ composite ETL was constructed,which successfully realized the preparation of high quality and high coverage Sn O₂ ETL.It is found that the conduction band energy levels of Zr（acac）₄ can construct an ideal cascade energy level arrangement between Sn O₂ and indium tin oxide（ITO）layers to improve the electron extraction ability.Zr（acac）₄ had a deep valence band top（-8.45 e V）,which enhanced the hole-blocking ability.In addition,the Zr（acac）₄ layer improves the film quality and coverage of Sn O₂ layer,reduces the interfacial ion accumulation,and improves the device stability.The battery achieved a FF of 80.99%,an optimal PCE of22.44%and a certified efficiency of 21.4%.After 500 hours of continuous lighting,it remains 80%of the original efficiency.（4）Two methods for measuring ion properties are proposed,which can quantify ion concentration and ion conductivity in electron-ion mixed conductor.Based on these two methods,the ion concentration and ion conductivity in perovskite materials were measured,and the effects of light and electric field on ion concentration and ion conductivity in perovskite materials,as well as the effects of ion behavior on the stability of perovskite photovoltaic devices were studied.The results show that the ion concentration of CH₃NH₃Pb I₃ perovskite at room temperature is 5.30×10¹⁹ cm^-3,while the ion concentration of Cs_0.05(（CH（NH₂）₂）_0.83（CH₃NH₃）_0.17)_0.95Pb(I_0.83Br_0.17)₃ perovskite decreases to 1.55×10¹⁸ cm^-3.It is found that the ion concentration of perovskite material can be significantly increased by light and electric field.The carrier transport layer in perovskite photovoltaic devices can significantly reduce the density of ion aggregation at the interface and the interface capacitance.Light and electric field significantly affect the performance of perovskite devices and lead to the degradation of device performance,which is related to the change of ion concentration at the interface of the material.If the interface ion concentration can always be kept at a lower value,the device will have a longer working stability.