Two-dimensional Layered (PEA)₂Sn_xPb_1-xI₄ Perovskite Thin Films For Field-effect Transistors

Two-dimensional(2D)layered organic-inorganic perovskite phenylethylammonium tin iodide((PEA)₂Sn I₄)with charge transport anisotropy are considered as promising channel materials for field-effect transistors and is currently the most widely used perovskite channel material in field-effect transistors,which possesses high carrier mobility.However,Sn²⁺can be easily oxidized to Sn⁴⁺,leading to p-type self-doping and reducing device performance and repeatability.In contrast,the homologous Pb is less susceptible to oxidation and more stable than Sn-based perovskite.While Sn-based perovskites have been extensively studied in this field,there is no reportsfor 2D(PEA)₂Pb I₄perovskite field-effect transistors.It is known that the optoelectronic properties of organic-inorganic perovskites can be tuned by mixing different metal cations in different ratios to form solid solutions.Since Pb²⁺and Sn²⁺are similar in electronic structures and radii(PEA)₂Sn I₄and(PEA)₂Pb I₄can be mixed in any ratio.Therefore,in this thersis,systematic and in-depth studies on the physical properties of of(PEA)₂Sn_xPb_1-xI₄thin films including charge carrier transport,as well as their applications in field-effect transistors are carried out.The results show that the properties of the 2D mixed Sn-Pb perovskite thin films and the performance of the corresponding field-effect transistors can be modulated by the Sn-Pb ration.The thesis includes the following two parts:1.The(PEA)₂Sn_xPb_1-xI₄(x=0,0.3,0.5,0.7,1)perovskite films were prepared by one-step solution spin-coating method.Their crystal structure,morphology,optical properties,energy band structure and stability were characterized.The mixed Sn-Pb perovskite films still maintain a 2D layered structure because Sn²⁺and Pb²⁺can be substituted with each other in the lattice,and the crystallinity of the mixed films is better than that of the pure films.Combined with XRD and AFM and SEM morphological characterizations,it is clear that the(PEA)₂Pb I₄film exhibits relatively weak crystallinity with small grain size.The addition of Sn enhances the crystallinity and increases the grain sizein spite of a slight increase in roughness.The(PEA)₂Sn_0.7Pb_0.3I₄film exhibit the best morphology,including relatively large grains and a smooth surface.The absorption and luminescence spectra of the films reveal that the Sn-Pb ratio also has an effect on the optical properties of the films.With the PL peaks showing a red shift and the energy band of the films not varying linearly with the Sn-Pb ratio,as reported in 3D mixed Sn-Pb films.The lifetime of the mixed films is also increased due to the improved crystallinity and morphology.In addition,the stability of the films under water,oxygen,and light conditions was investigated.The cause of the poor environmental stability of(PEA)₂Sn I₄is the oxidation of Sn²⁺,which is accelerated by water or light,and eventually the pure Sn perovskite degrades into PEAI and Sn O₂in ambient air.(PEA)₂Pb I₄is found to react with water to generate hydrates,and decompose under irradiation of light,while the stability of the mixed films is greatly improved.2.Carrier transport properties of the(PEA)₂Sn_xPb_1-xI₄(x=0,0.3,0.5,0.7,1)perovskite films and the corresponding field-effect transistor.(1)Charge transport was investigated by temperature-depedent galvanostatic characterizations.The temperature dependence of the resistivity of the perovskite films reveal that this series of perovskite films have the normal and thermally-activated semiconducting behavior.The resistance versus time curves at different temperatures indicate the presence of lateral ion migration in the(PEA)₂Pb I₄film.The temperature-dependent ionic conductivity of the perovskite films suggests that the in-plane ion migration can be largely affected by the Sn-Pb ratio,which is important for transistor performance.The ion activation of(PEA)₂Sn_0.7Pb_0.3I₄is as high as 173 me V so that the lateral ion migration in the film is effectively suppressed.(2)Furthermore,we demonstrate the 2D layered mixed Sn-Pb perovskite field-effect transistors constructed on cheap and commercially available polymer dielectrics(PVA/CL-PVP)and a signature of field-effect characteristics in the(PEA)₂Pb I₄film.The device performance improves with the increase of Sn content,and(PEA)₂Sn_0.7Pb_0.3I₄transistor exhibits a hole mobility of 0.02 cm²V^-1s^-1when operating in air at room temperature.The depletion in this device is weaken relative to the pure Sn perovskite film,and the low values of subthreshold swing and maximum interface trap density also indicate good device interface quality of the(PEA)₂Sn_0.7Pb_0.3I₄transistor.Our work not only provide a strategy for exploring the new functionalities of existing perovskite materials and widening the range of their applications in diverse optoelectronic technologies,but also improve the understanding of the influence mechanisms of ion migration in 2D layered perovskite field-effect transistors.