A Study On Inorganic Sn-Pb Halide Perovskite Films And Their Optoelectronic Properties

In recent years,inorganic perovskite materials CsPb I₂Br have attracted widespread attention because of their excellent thermal stability.Due to the large band gap（E_g～1.91 e V）and poor phase stability of CsPb I₂Br perovskite,the maximum power conversion efficiency（PCE）of CsPb I₂Br（17.45%）perovskite solar cells（PSC）currently reported is still far smaller than that of organic-inorganic hybrid PSC（25.7%）.The tolerance factor of CsPb_1-xSn_xI₂Br perovskite can be adjusted by partial substitution of Pb²⁺with Sn²⁺to improve phase stability and the band gap of CsPb_1-xSn_xI₂Br perovskite can be regulated.However,Sn²⁺is easily oxidized to Sn⁴⁺in the tin-based perovskite,resulting in high defect density,which leads to large nonradiative recombination energy loss and limits the improvement of photovoltaic performance of devices.This study has optimized Sn/Pb ratio in CsPb_1-xSn_xI₂Br perovskite through composition regulation,and studied the reduction of Sn²⁺oxidation by introducing additive into the perovskite precursor solution in order to improve the PCE and stability of the device.The specific research contents are as follows:（1）The ratio of Sn/Pb in CsPb_1-xSn_xI₂Br（0<x<1）perovskite material was explored and the technological parameters of high quality perovskite film preparation are determined Firstly,the ratio of Sn/Pb was determined to be 1:1 by a comprehensive comparison of crystal structure stability,band gap and device performance.CsPb_0.5Sn_0.5I₂Br has good phase stability with a band gap of 1.5 e V.Subsequently,the effects of the ratio of solvent,the amount of Sn F₂ and the annealing method on the CsPb_0.5Sn_0.5I₂Br perovskite film quality were systematically investigated.Consequently,the key regulation mechanism of nucleation crystallization was indentified.The PCE of CsPb_0.5Sn_0.5I₂Br PSC was achieved at 6.54%.（2）The introduction of multifunctional additives in CsPb_0.5Sn_0.5I₂Br precursor solution is an effective way to reduce the defect density in thin films and improve the photovoltaic performance of devices.In this study,thiobenzamide（TBA）and 4-fluorothiobenzamide（F-TBA）were used as model molecules to study the effects of their addition to CsPb_0.5Sn_0.5I₂Br precursor solution on the performance of perovskite films and devices,respectively.The results show that the C=S and NH₂ functional groups in thiobenzamide（TBA）can coordinate with metal ions,which effectively inhibits the oxidation of Sn²⁺.The content of Sn⁴⁺in CsPb_0.5Sn_0.5I₂Br perovskite film decreases from 47%to 37%,along with a decreased defect density from 2.62×10¹⁶ cm^-3 to 1.66×10¹⁶ cm^-3 after adding TBA additive.Upon that,the PCE of CsPb_0.5Sn_0.5I₂Br perovskite solar cells increaseed from 6.54%to 8.11%.Further study shows that F-TBA can not only effectively reduce the defect density but also improve the carrier transport ability due to its larger dipole moment（3.80 Debye）.Using F-TBA as an additive,the PCE was improved to 10.73%.The device also exhibited good stability.After 1000 h storage in an inert gas glove box,the PCE could maintain over 90%of the initial power conversion efficiency.