A Study On Optoelectronic Properties Regulation And Stability Of Sn-based Halide Perovskite Films

Metal halide perovskite solar cells（PSCs）have become the focus of photovoltaic research due to their high power conversion efficiency.The highest-performing perovskite materials for PSCs contain lead（Pb）,which is regulated worldwide as a hazardous material.Therefore,one major research direction for PSCs is to explore eco-friendly lead-free perovskite materials to replace Pb.Tin（Sn）has become the ideal alternative element due to its similar crystal and electronic structure to Pb.However,the poor film quality,and Sn²⁺oxidation severely impede the development of Sn-based PSCs.In this thesis,the relationship between the film quality of three-dimensional（3D）and two-dimensional（2D）Sn-based perovskites and device performance was investigated from the perspectives of crystallization regulation,doping control,and forming low-dimensional perovskites,to improve the device performance and stability of Sn-based optoelectronic devices.（1）Pb S quantum dots（QDs）were used to realize crystallization regulation by severing as a nucleation site.The solubility difference between Pb and Sn-based components in the Cs Pb_0.5Sn_0.5I₂Br perovskite results in the inhomogeneous Pb/Sn elemental distribution and the accompanying phase segregation（shown as double photoluminescence emission peaks,additional emission peaks blue-shifted relative to the bandgap emission）.Pb S QDs enlarge the colloidal size in the precursor solution and act as a template for crystal growth,preventing the segregation of Pb/Sn elements and its induced phase segregation.Therefore,the carrier diffusion length increases from 156 nm to 419 nm as the single-alloyed phase forms.Upon that,an 8.0%planar device was obtained,and the performance deterioration was less than 10%after 400 h of storage in an inert atmosphere.Modulating the colloidal chemistry of the perovskite precursor solution is the key to realizing crystallization regulation of Sn-based perovskite films.（2）Electron transfer doping was applied to suppress the Sn²⁺oxidation in the Sn-based perovskite films.Cobaltocene donates its extra electrons to Cs Sn I₃ to create a reducing environment in which the high-energy state Sn 5s² can exist stably,increasing the free electron concentration and decreasing the doping concentration of Cs Sn I₃ perovskite film.As a result,the efficiency of Cs Sn I₃ PSCs based on a fully-printed Ti O₂/Al₂O₃/Ni O/C framework was significantly increased to 3.0%with a saturation-current density reduced by two orders of magnitude.Above this,the prerequisites for electron transfer doping have been established,namely that the donor’s HOMO energy level is near the bottom of the conduction band minimum of perovskite and the reduction potential is less than that of Sn⁴⁺/Sn²⁺.Electron transfer doping is a novel,non-destructive,and plausible solution to develop efficient and stable Sn-based PSCs.（3）Dimensionality engineering was utilized to further reduce the doping concentration of Sn-based perovskite films.The transient photovoltage/photocurrent decay method was used to investigate the relationship between dimensionality,stability,and optoelectronic properties of Sn-based perovskites,where the interfacial capacitance accumulation caused by the multiple phase distribution impedes the development of 2D Sn-based PSCs.Phenethylammonium was replaced by 4-fluorophenethylammonium（FPEA⁺）to deposit（FPEA）₂（MA）₃Sn₄I₁₃perovskite film,which introduces interlayer electrostatic interaction between the bulky organic cations.The electrostatic interaction promotes narrow phase distribution,induces vertical orientation,facilitates charge transport,and improves the antioxidant activity of perovskite films.Consequently,the efficiency of 2D Sn-based PSCs has been tripled and the external quantum efficiency of pure-red（630 nm）perovskite light-emitting diodes has increased from 0.09%to 0.42%.Then,the low-dimensional Cs Sn I₃perovskite was formed by introducing FPEA⁺cations into the precursor solution to boost Cs Sn I₃ solar cell efficiency up to 4.94%.Reducing the dimensions of Sn-based perovskites is an effective way to balance the efficiency and stability of Sn-based PSCs.