Preparation Of Lead-Tin Mixed Perovskite Single Crystal And Its Optoelectronic Application

Organic-inorganic metal halide perovskite materials have excellent properties such as high absorption coefficient,high carrier mobility,low trap density,and low exciton binding energy.Thus,they have good application prospects in optoelectronic devices.Currently,the most widely used perovskite materials is lead-based perovskite materials.However,the toxicity of lead will pose a certain threat to the ecological environment and human health,resulting in the commercial application of its devices being greatly limited.Therefore,it is urgent to find an element with low toxicity which can replace lead or partially replace lead.Tin is a non-toxic metal element and has similar physical and chemical properties to lead.In terms of optical properties,tin perovskite materials exhibit narrower band gap,wider light absorption range and higher charge mobility than lead-based perovskites.Therefore,they have been widely studied in narrow band gap optoelectronic devices.However,the instability of Sn²⁺,which will be easily oxidized to Sn⁴⁺,will lead to the degradation of device performance.Lead-tin mixed perovskite materials’bandgap is narrower than tin-based perovskite and the presence of Pb can keep tin-containing perovskite structure stable.Thus,lead-tin mixed perovskite materials have attracted the attention of researchers in recent years.At present,most of the research are based on the study of lead-tin perovskite polycrystalline films,while there are few studies focus on the basic properties of lead-tin perovskite single crystal materials.Therefore,it is meaningful to study single crystal materials which have no grain boundary,low defects and longer carrier diffusion length.This paper focuses on the use of antioxidants to solve the oxidation of Sn²⁺,and optimizes lead-tin perovskite single crystals to improve device performance.Since the formamidinium（FA）based perovskite single crystal is more stable than the methylammonium（MA）based perovskite single crystal,we selected the FA-based lead-tin perovskite single crystal as the research object.Firstly,Cs ions and Br ions were introduced into the A-site FA cation and the X-site halogen anion to inhibit the phase transition of the single crystal and improve the stability of the crystal.Meanwhile,a large-size guanidine cation（GA⁺）was introduced into the A-site to reduce the formation of iodine vacancies in the perovskite and reduce the generation of defects.After the composition was determined,N,N-methylenebisacrylamide（MBAA）was added to the lead-tin perovskite precursor solution pretreated with phenylhydrazine hydrochloride（PHCl）reducing agent to inhibit the oxidation of Sn²⁺during crystallization.High-quality Pb-Sn mixed perovskite single crystals were prepared by inverse temperature crystallization.By forming coordination bonds between-NH and-CO lone pair electrons in MBAA and unpaired Sn²⁺,the electron cloud density around Sn²⁺is increased,which can inhibit the oxidation of Sn²⁺and reduce the internal defect density of the crystal.The device performance is improved.The experimental results show that the addition of MBAA can inhibit the oxidation of Sn²⁺to a certain extent and improve the crystallinity of the crystal.In order to study the effect of Pb/Sn ratio on the properties of single crystals,we compared the lead-tin perovskite single crystals with Sn content of 20%,30%and 40%.The experimental results show that the Pb/Sn ratio adjustment has little effect on the oxidation degree and thermodynamic stability of the crystal on the basis of MBAA addition.Considering that the increase of Sn content,the internal defects in the obtained single crystal will increase,which will affect the performance of the device,we finally chose Cs_0.1FA_0.87GA_0.03Pb_0.7Sn_0.3I_2.85Br_0.15（Cs FAGAPb Sn IBr）perovskite single crystal to prepare a near-infrared photodetector.The results show that the addition of MBAA can improve the performance of the device.Secondly,we dissolve the obtained Cs FAGAPb Sn IBr perovskite single crystal as a raw material in a mixed solvent by pre-crystallization method and prepared a perovskite film by spin coating.The pre-crystallization scheme of crystal re-dissolution can optimize the stoichiometric proportion of the components in the precursor and reduce the generation of impurities and defects.The dissolution and recrystallization of lead-tin perovskite single crystal optimized by MBAA can improve the crystal quality of polycrystalline thin films.Its effect on improving the crystallization performance of lead-tin mixed perovskite polycrystalline thin films and the photoelectric performance of solar cells was studied.The experimental results show that the surface grains of the perovskite film obtained by this method are arranged more close and have fewer defects.The power conversion efficiency（PCE）of the polycrystalline thin film solar cell can reach 18.65%,which is 3.7%higher than that of the single crystal device without MBAA.It can also maintain 94%of the original PCE after being placed in ambient air for 300 h.