Research On High-Performance Tin Perovskite Solar Cells

Perovskite solar cell devices(PSCs)have experienced very rapid growth in both power conversion efficiency(PCE)and long-term stability over the past few years,showing great promise for future photovoltaic energy systems.However,until now,this development has been mainly based on the optimization and improvement of lead halide perovskite materials,in which toxic lead(Pb)restricts its further promotion and application,although lead leakage has been extensively studied recently,but lead leakage must be avoided in many cases,such as distributed photovoltaic applications,and the use of toxic lead also increases the difficulty and cost of PSCs fabrication.Therefore,it is urgent to find lead-free halide perovskites as lead-based perovskites alternatives.Tin-based halide perovskites are by far the most promising candidates.Compared with lead-based perovskites,tin-based perovskites have more suitable band gaps and higher carrier mobility.Through the improvement of device structure and perovskite films,the current tin-based perovskites have the highest efficiency has reached 14.81%.Due to the low formation energy of Sn(IV)in tin-based perovskite films,a large amount of Sn(II)spontaneously transforms into Sn(IV)to form vacancy defect states,resulting in p-type doping,which leads to an increase in the density of defect states.To improve the performance of tin-based perovskite solar cells,the quality of perovskite films was improved by doping Zn²⁺at the B site,and the effects of different doping ratios on device performance were discussed.The lower density of defect states in the perovskite thin film is beneficial to the transport and extraction of carriers,so that the device has almost no hysteresis effect,and finally achieves a device efficiency of 10.33%,while the device still retains 90%initial efficiency of the device after 500 h in N₂ atmosphere.In order to solve the p-type self-doping problem of Sn(IV)in tin-based perovskite,a molecular layer of formamidine hydrochloride(FACl)was grown in situ on the surface of unannealed tin-based perovskite film.FACl interacts with the spontaneously formed Sn(IV)ions in the film to form Sn I₄·x FACl complexes.Thermogravimetric analysis(TGA)tests show that its volatilization temperature is lower than that of elemental Sn I₄.During the annealing process,Sn I₄·x FACl complexes are formed.The volatilization of the Sn I₄·x FACl complex achieves the purpose of Sn(IV)de-doping in the film.At the same time,in order to research the distribution of tin(IV)content in the synthesized tin-based perovskite films with the film depth,the depth profile of tin element was analyzed by in-situ etching X-ray photoelectron spectroscopy.The results show that the content of Sn(IV)in the control film drops sharply from 23.3%mol%at the surface to 10.2%mol%at a depth of 10.9nm from the surface,confirming that Sn(IV)self-doping mainly accumulates in the surface area of the film.After surface de-doping,the Sn(IV)content near the surface decreased most significantly,and the Sn(IV)content decreased gradually with the increase of the probing depth to only 3.4 mol%.Space charge limited current(SCLC)measurements show that the trap density of the in situ grown 3 nm FACl dedoped film is 4.16×10¹⁶ cm^-3,which is much lower than that of the control film(1.05×10¹⁷cm^-3).Surface dedoping increases the photogenerated carrier lifetime to 13.4 ns.Finally,a device efficiency of 14.7%was achieved.The short-circuit current of the device was 24.9 m A cm^-2,the open-circuit voltage was 0.77 V,the fill factor was0.767,and the device had a small hysteresis effect.At the same time,the device also has good stability.After being stored in nitrogen atmosphere for 1000 h,it still has an initial efficiency retention rate of 92%.This research provides new ideas for solving Sn(IV)self-doping in tin-based perovskite films and improving the power conversion efficiency of tin-based perovskite devices.