The Preparation Of High-efficiency Perovskite Solar Cells Based On Additive Passivation Strategy

Due to the suitable band positions,low exciton binding energy,high carrier mobility,long carrier diffusion length,and the ability to transport both electrons and holes,organic-inorganic hybrid metal halide perovskite materials have been considered as ideal light-absorbing materials for solar cell.The power conversion efficiency（PCE）of perovskite solar cells（PSCs）has reached 25.8%.However,due to the ionic lattice nature of perovskite materials and the low-temperature solution-based fabrication process,a large number of defects(such as vacancies,low-coordinated Pb²⁺and I^-ions)exist in the perovskite films,which lead to unnecessary non-radiative recombination of photogenerated carriers and seriously affect the PCE of devices.Low-coordinated I^-ions can also trigger the migration of I^-,Pb²⁺and organic cations in perovskite films during device operation,resulting in a large amount of charge accumulation at the interface with the charge transport layer and the decomposition of perovskite crystals,thereby affecting the stability of devices.Defect passivation strategy is one of the effective methods to reduce the defect density of perovskite films.Usually,some electron-rich groups are combined with uncoordinated Pb²⁺to reduce the deep-level defects of the film,while some electron-deficient groups are combined with free I^-to passivate shallow-level defects.In order to simultaneously reduce the deep-level and shallow-level defects density,Taurine and Sulphanilic acid（SA）,which have both electron-rich and electron-deficient groups,were into the perovskite film.By combining experimental and theoretical simulation methods,the defect passivation abilities of the two passivators on perovskite films are comprehensively investigated.It is found that the dual-functional passivators with both sulfonic acid（-SOOOH）and amino（-NH₂）groups can significantly reduce the defect density of perovskite film,effectively suppress the non-radiative recombination of carriers,and greatly improve the PCE and stability of perovskite solar cell.The main research content and results are:1.Taurine molecule with-SOOOH and-NH₂ groups was introduced into the CH₃NH₃PbI₃（MAPbI₃）perovskite precursor solution.The-SOOOH and-NH₂ groups in taurine can bind with the uncoordinated Pb²⁺and I^-in MAPbI₃,significantly reducing the defect density of perovskite film and suppressing the carriers non-radiative recombination.The PSCs with FTO/TiO₂/perovskite/C structure based on Taurine passivated MAPbI₃ film assembled in air surroundings exhibited a PCE of 13.2%under 1 sun illumination（AM 1.5 G）,which was 14.4%higher than that of the non-passivated device with a PCE of 11.3%.In addition,the stability of the device was also significantly improved.The unencapsulated devices were stored in air environment（Temp.～25℃,RH～25%）can kept 58.74%of its initial PCE after 720 hours,while the control device could only maintain 33.98%of its original PCE.2.Based on the understanding of the-SOOOH and-NH₂ groups can effectively passivate the defects in the perovskite film,in order to further enhance the passivation effect of the-NH₂ group to anchored with the uncoordinated I^-,SA was introduced into the perovskite precursor solution.SA molecule contains a benzene ring that can form a conjugatedπbond with-NH₂,making it more polar and forming a stable bond with the uncoordinated I^-.At the same time,the conjugatedπbond can also create a steric hindrance effect,which can affect the distribution of spatial electron density,and achieving a synergistic passivation effect with the-SOOOH and-NH₂ groups in anchoring the uncoordinated ions to passivate the defects in the film.After passivating with SA,the defect density of the perovskite film was reduced from 1.03×10¹⁶ cm^-3 to8.25×10¹⁵ cm^-3.The HTL-free PSCs based on SA passivated perovskite film got a PCE of 12.6%.The stability of the SA-passivated device was also significantly improved,the unencapsulated devices could maintain 67.9%of its initial PCE after 720 h in an air environment（Temp.～25℃,RH～25%）.The impact of perovskite film defect density on the device performance was simulated using SCAPS software.The influence of defect density in perovskite film on the device performance was revealed,which confirming that focusing on defect passivation is still an important strategy to improve the photovoltaic performance of PSCs.When the defect density in the perovskite layer exceeds 10¹⁴ cm^-3,which will significantly affect the fill factor（FF）and PCE of the device.When the defect density increases to 5×10¹⁵ cm^-3,the open-circuit voltage(V_OC)and short-circuit current density(J_SC)of the device will seriously be decreased,which severely affecting the photovoltaic performances of PSCs.Considering that the defect density of actual perovskite film mostly exceeds the order of 10¹⁵ cm^-3,passivating the defects still is an important method to improve the photovoltaic performance of PSCs.