MA（Pb/Sn/Cu）I₃ Alloyed Semiconductors:Optoelectronic Conversion Devices,Properties And Stability

Due to their excellent photophysical properties,organic-inorganic hybrid perovskite materials have broad application prospects in the fields of photoelectric conversion devices,such as perovskite solar cells（PSCs）and light-emitting diodes（Pe LEDs）.To date,the power conversion efficiency（PCE）of small-area PSCs has exceeded 25%,a value that exceeds that of commercial CIGS solar cells（～23%）and is close to the highest efficiency of monocrystalline Si solar cells（～26%）.The external quantum efficiency（EQE）of Pe LEDs（green light）has exceeded 28%in recent years.However,organic-inorganic hybrid perovskite materials are easily decomposed under the influence of water molecules,oxygen molecules and temperature in the atmospheric environment,resulting in a sharp decline in device performance.In addition,the toxicity of elements such as Pb also limits their commercialization.In view of the above problems,this thesis aims to obtain stable,green and environmentally friendly perovskite optoelectronic devices based on the basic theory of semiconductor alloy preparation and physical properties.From the aspects of material composition and preparation process optimization,MA（Pb/Sn/Cu）I₃ semiconductor alloy-type perovskite films are prepared,and two typical photoelectric conversion devices,PSCs and Pe LEDs,were assembled.The photophysical properties and stability were systematically studied.（1）A strategy to release microstrain by doping with small-sized alkali metal cations（Cs⁺,Rb⁺,K⁺）is proposed,and high-quality alloyed MAPb_1-xSn_xI₃ perovskite films and high-efficiency near-infrared Pe LEDs are obtained.The band gap of MAPb I₃ semiconductor can be effectively reduced to the near-infrared region by Sn²⁺doping,but the lattice microstrain caused by the radius mismatch between Sn²⁺and Pb²⁺seriously affects the film quality,resulting in a large number of defects in the film.By substituting MA⁺cations with alkali metal cations（Cs⁺,Rb⁺,K⁺）with smaller radii,the microstrain in the lattice of MAPb_1-xSn_xI₃ perovskite films is reduced,resulting in low defect state density and high carrier density.The MAPb_1-xSn_xI₃ alloyed perovskite film with higher mobility and higher luminous efficiency was assembled,and the near-infrared MAPb_1-xSn_xI₃ alloyed Pe LEDs with the device structure of ITO/Poly-TPD/MAPb_1-xSn_xI₃/TPBi/Li F/Al were assembled.Among them,the EQE of MAPb_0.8Sn_0.2I₃ based Pe LEDs reaches 9.6%,which is the highest recorded value of Sn-based near-infrared Pe LEDs.In addition,the K⁺doped MAPb_0.8Sn_0.2I₃ alloyed Pe LEDs device showed better working stability,and after 45 days of nitrogen storage,the K⁺doped Pe LEDs still maintained about 90%of their initial EQE.By further adjusting the stoichiometric ratio of Pb/Sn(MAPb_1-xSn_xI₃,x=20%,40%,60%,80%),Pe LEDs with tunable EL emission peaks were prepared.For the adjustment of the near-infrared region from 868 nm to 917 nm,the corresponding EQE is increased from about 0.2%to 9.6%,7.03%,3.34%and0.65%,respectively,but the stability of Sn²⁺perovskite materials in air is still a challenge.（2）In order to overcome the problem that MAPb_1-xSn_xI₃ alloyed devices are easily oxidized(Sn²⁺/⁴⁺),Cu²⁺was used instead of Sn²⁺to prepare MAPb_1-xCu_xI₃ alloyed perovskite films,and high-efficiency PSCs based on MAPb_1-xCu_xI₃ alloyed films were assembled.The preparation of MAPb_1-xCu_xI₃ alloyed perovskite by replacing Pb²⁺with Cu²⁺can not only effectively reduce the content of toxic Pb elements,but also have better material stability than Pb/Sn alloys.However,the organic amine vacancies(V_MA)formed during the preparation of perovskite films by solution methods can lead to the formation of defects such as halide dangling bonds and halide interstitials（I_i）,which in turn lead to reduced PSCs efficiency.By introducing Gua⁺with strong hydrogen bonding into the MAPb_1-xCu_xI₃ alloyed film,it can combine with halogen atoms to passivate the halogen defects existing in the film.Pb-less MAPb_1-xCu_xI₃ alloyed perovskite films with low density of defect states and strong fluorescence intensity were obtained.Then,inverted PSCs with the device structure of ITO/PTAA/MAPb_1-xCu_xI₃/PCBM/BCP/Ag were assembled.When the Cu content was 5%,10%and 20%,the PCE of the PSCs reached 9.69%,9.05%and 8.81%,respectively.This efficiency is one of the highest reported so far for similar alloyed perovskite materials.At the same time,the storage stability of MAPb_1-xCu_xI₃ alloyed PSCs in the environment has also been significantly improved thanks to the hydrogen bond passivation of the defects in the film.（3）Taking advantage of the structural properties of two-dimensional（2D）perovskites and the hydrophobic properties of the surface of fluorinated materials,a fluorinated 2D/3D perovskite composite structure was designed to further improve the stability of MAPb_1-xCu_xI₃alloyed PSCs in the working environment.The MA⁺cations in 3D organo-inorganic halide perovskites are volatile and hygroscopic,making such materials highly susceptible to decomposition in the environment.Selecting 4-trifluorophenylmethylamine cation（CF3PMA⁺）as the organic spacer,an ultrathin fluorinated 2D perovskite layer was formed in situ on the surface of the 3D MAPb_1-xCu_xI₃ alloyed perovskite film,forming a film with a2D/3D composite structure.Compared with the pure MAP_b1-xCu_xI₃ film,the composite film has stronger fluorescence intensity and lower defect state density.The unencapsulated MAPb I₃ perovskite film has better stability in the environment and can still maintain the perovskite black phase after 22 days.When the Cu content was 5%,10%and 20%,the PCE of the corresponding cells reached 7.82%,7.35%and 7.34%,respectively.Especially when the Cu content is 20%,after 20 days of storage in the environment,the efficiency is increased from decay to 20%of the initial efficiency to more than 60%.（4）Starting from the preparation process,large-area MAPb_1-xCu_xI₃ alloyed perovskite films and PSCs devices were prepared by blade-coating in the atmospheric environment using an environmentally friendly ionic liquid methylamine acetate（MAAc）as a solvent.In addition to Pb toxicity,the instability of perovskite precursor solutions and the use of toxic and harmful solvents will also limit their applications.In addition,the traditional spin-coating method is still unable to achieve the preparation of large-area uniform perovskite films,and the blade-coating method is expected to achieve the preparation of large-area perovskite films.At present,the preparation of large-area perovskite films by the blade-coating method usually requires the addition of surfactants,which makes the preparation process cumbersome.In this paper,green ionic liquid MAAc is selected as the solvent for the blade-coating method.The MA⁺in MAAc can form N-H...I hydrogen bonds with Pb I₂ to protect I-from being oxidized.And MAAc has a relatively large viscosity,which helps to further protect the perovskite precursor solution from being affected by water molecules in the environment.Using MAAc as solvent and without adding any surfactant,the Pb-less large-area（1.5×2 cm²）MAPb_1-xCu_xI₃ alloyed films were prepared by blade-coating in the air environment,and the Pb-less PSCs with the device structure of FTO/Ni O/MAPb_1-xCu_xI₃/PCBM/BCP/Ag were assembled.When the Cu content is 5%,the PCE of the small-area device reaches more than10%,and when the Cu content is 10%,the PCE of the small-area device reaches more than6%.The device efficiencies are comparable to those fabricated by conventional spin-coating methods.In addition,the PCE of the large-area（1cm²）battery device reaches 3.1%（x=5%）and 1.8%（x=10%）,respectively.The unpackaged device has good stability in the environment,and its PCE can still maintain more than 80%of its initial PCE after 20 days of storage.