Control Of The Properties Of Two-dimensional ?PEA?₂Sn I₄ Perovskite Thin Films And The Performances Of The Field-effect Transistors

Two dimensional?2D?organic-inorganic hybrid Sn-based perovskite field-effect transistors?FETs?are currently being developed and the 2D perovskite phenylethylammonium tin iodide??PEA?₂SnI₄?thin films have been most widely explored so far in 2D perovskite FETs.In these FETs,the oxidation of Sn²⁺in air and the compatibility of 2D Sn-based perovskites with inexpensive and commercially available polymer dielectrics are still great challenges.In this dissertation we report on the investigations of controlling the characteristics of?PEA?₂SnI₄thin films and the performances of the corresponding field-effect transistors.Based on systematic experiments and mechanism analysis the main results are included the following four parts:1.Fabrication of the 2D?PEA?₂SnI₄ perovskite thin film field-effect transistors with poly?vinyl alcohol??PVA?modified by cross-linking poly?4-vinylphenol??PVP?as gate dielectrics?PVA/CL-PVP?in a bottom-gate configuration by a spin coating method.?1?The PVA/CL-PVP gate dielectric layers not only have uniform and smooth surfaces with high solvent resistance and low polarity,but maintain relatively high capacitances and low leakage currents simultaneously.?2?The transistors has a hole accumulation-depletion mode and can operate at room temperature in air,showing a p-channel characteristic with mobilities and threshold voltages of 0.28 cm²V^-1s^-1 and 21 V in the forward gate voltage scan and 0.33 cm²V^-1s^-1 and 20 V in the reverse one,respectively.In particular,the hysteresis in the devices can be neglected because of suppressed ion migration in the?PEA?₂SnI₄ films,high-quality surfaces of the PVA/CL-PVP layers,and the good compatibility of the perovskite with the polymer gate dielectric.?3?The interface trap densities are estimated from the subthreshold slope and the capacitance–voltage characteristics,which further confirms that the gate dielectric layers offer a high-quality surface and are highly compatible with the?PEA?₂SnI₄ films.2.Optimization of the characteristics of the?PEA?₂SnI₄ thin films by aromatic polymer additives and the performances of their field-effect transistors with PVA/CL-PVP as gate dielectrics.?1?Aromatic polymers PVP and poly?vinyl pyrrolidone??PVPD?are utilized as additives to tune the?PEA?₂SnI₄ film properties.Based on the proposed mechanisms of the film formation,the functional groups in PVP and PVPD interact with?PEA?₂SnI₄,such as hydrogen bonding,?-?interaction,coordination interaction,etc.,which help the polymer molecules distribute in the organic layers and at the grain boundaries as well as the surfaces of ?PEA?₂SnI₄ during the film formation,thereby improving the morphology,crystallization,oxidation resistance,and stability of the perovskite film.In particular,the presence of Sn⁴⁺has not been found in the two aromatic polymer-assisted perovskite films,which indicates that the use of PVP and PVPD can effectively inhibit the oxidation of Sn²⁺.?2?The aromatic polymer-assisted perovskite?PEA?₂SnI₄:ARP thin film field-effect transistors are constructed on the PVA/CL-PVP dielectric,where ARP stands for PVP or PVPD.Compared with the?PEA?₂SnI₄ transistor,the?PEA?₂SnI₄:PVP devices is still in a hole accumulation-depletion mode and the mobility and threshold voltage of remain almost unchanged,but the current on-off ratio is increased from 10² to 10³ and the subthreshold slope and interface trap density are reduced.In the contrast,the?PEA?₂SnI₄:PVPD devices exhibit performance degradation.These results show that different interactions between the aromatic polymers and the?PEA?₂SnI₄can effectively affect the performances of the transistor devices.?3?Analysis of the physical processes of the?PEA?₂SnI₄:ARP transistor performances.The results of the lateral and vertical leakage currents and AC impedance spectra prove that the addition of the aromatic polymers can effectively reduce the leakage currents in the perovskite films and improve quality of the perovskite/dielectric interface for improving the performances of the transistors.3.Optimization of the characteristics of the?PEA?₂SnI₄ thin films by aliphatic polymer additives and the performances of their field-effect transistors.?1?The?PEA?₂SnI₄:ALP films areprepared by adding a small amount of aliphatic polymers poly?ethylene oxide??PEO?,PVA and poly?methyl methacrylate? ?PMMA?,where ALP are the abbreviation of aliphatic polymers,and a mechanism model for the film formation is proposed.The interactions between the functional groups in PEO,PVA,and PMMA and?PEA?₂SnI₄,such as hydrogen bonding,coordination interaction,nucleophilic addition,etc.,result in the distributions of the polymer molecules at the grain boundaries and surfaces of the perovskite during the film formation,which improve the morphology,crystallization,oxidation resistance, and stability of the?PEA?₂SnI₄:ALP perovskite films.In particular,the coordination and nucleophilic addition reactions can effectively passivate the defects in the films and prolong carrier lifetime.?2?The three?PEA?₂SnI₄:ALP field-effect transistors exhibit obviously different performances.The?PEA?₂SnI₄:PEO devices exhibit a transition from a hole accumulation-depletion mode to an accumulation mode.More interestingly,they possess ambipolar transport with balanced hole and electron mobilities of 2.1×10^-3 and 1.3×10^-3 cm²V^-1s^-1.The threshold voltages are also reduced by an order of magnitude,which are 3.2 V for the p-channel and-2.9 V for the n-channel,due to the effective defect passivation of PEO in the perovskite film.The?PEA?₂SnI₄:PVA and?PEA?₂SnI₄:PMMA devices still operate in a single hole accumulation-depletion mode with little changes in the mobility and threshold voltage compared to the?PEA?₂SnI₄ transistors.However,the current on-off ratio increases to more than 10⁴.The results of the subthreshold slope and the interface trap density suggest that PVA with the hydroxyl groups increases the interface traps and PMMA reduces the interface trap density owing to the defect passivation.?3?The lateral and vertical leakage currents and the AC impedance spectroscopy are employed to investigate the physical processes in the?PEA?₂SnI₄:ALP transistors.The results indicate that the aliphatic polymers can effectively reduce the leakage currents of the perovskite films.The defect passivation of PEO is the crucial factor in shifting the operation mode and achieving the ambipolar transport of the ?PEA?₂SnI₄ transistors.4.Investigations of the decomposition processes and mechanisms of the?PEA?₂SnI₄ thin films under the conditions of water,oxygen,light,and heat.It is proved that the ?PEA?₂SnI₄ film finally degrades into PEAI and SnO₂ in an environment of water and oxygen in the dark.In nitrogen,the?PEA?₂SnI₄ film does not decompose significantly upon light illumination.However,light seems to serve as a"catalyst"in air for accelerating the decomposition of the perovskite film,and the larger light intensity leads to the more rapid degradation.It is also found that higher temperatures?>370K?cause significant decomposition of the?PEA?₂SnI₄ film.The decomposition processes of the?PEA?₂SnI₄ film under different conditions provides effective evidences for comprehensive understanding of the physical mechanism of?PEA?₂SnI₄.Our work provides a feasible strategy for selecting additives to control the charateristies of Sn-based perovskites,and also opens up a new path to integrate solution-processed perovskites and low-cost and commercially available polymer dielectrics in a bottom-gate transistor structure for future applications in solution-based flexible optoelectronics.