【作者】 刘鑫；

【导师】 娄志东；

【作者基本信息】 北京交通大学 ， 光学工程， 2023， 博士

【摘要】 二维（2D）Ruddlesden-Popper（R-P）相A₂Sn X₄（A⁺:单铵有机阳离子;X^-:卤素阴离子）锡基钙钛矿具有独特的层状晶体结构,载流子在其无机八面体[Sn X₆]^4-层中的横向输运特性和晶体管的二维输运类似,因此其可作为有机-无机杂化卤化物钙钛矿薄膜场效应晶体管而被广泛研究。A₂Sn X₄钙钛矿的电子能带结构及其光电性质主要由[Sn X₆]^4-层决定,但A位阳离子也有一定作用。目前,关于X位卤素离子和A位有机离子对二维锡基钙钛矿材料特性的影响研究及其在场效应晶体管中的潜在应用仍是不充分的。本博士论文围绕不同卤素离子和不同链长的脂肪族单铵阳离子的二维R-P锡基钙钛矿的薄膜特性及其场效应晶体管的性能而开展了研究工作,取得了如下结果:（1）对芳香族混合卤素（PEA）₂Sn(I_1-nX_n)₄（PEA为苯乙胺,n=0、0.15、0.3、0.5,X=Br、Cl）钙钛矿薄膜的晶体结构、表面形貌、电子能带结构等进行了一系列研究。证明了I^-、B^-和Cl^-是互相取代存在于（PEA）₂Sn(I_1-nX_n)₄钙钛矿晶格中的,并且混合卤素钙钛矿薄膜和（PEA）₂Sn I₄薄膜一样,呈二维层状R-P结构。因为少量Br的熟化作用,使得（PEA）₂Sn(I_0.85Br_0.15)₄具有比（PEA）₂Sn I₄更光滑的表面以及更小的粗糙度（8.487 nm）。证明了Br^-和Cl^-对I^-的替代会导致钙钛矿的VBM（价带顶）的大幅下移,从而影响其能带结构。制备了以（PEA）₂Sn(I_1-nX_n)₄薄膜作为活性层,双聚合物PVA/CL-PVP为介电层的场效应晶体管。氮气环境室温下,与（PEA）₂Sn I₄场效应晶体管相比,（PEA）₂Sn(I_0.85Br_0.15)₄器件的载流子迁移率由0.068 cm²V^-1s^-1提高到0.089 cm²V^-1s^-1,阈值电压由21.7 V降低到7.1 V,电流开关比从1200增加到1600。这是由于（PEA）₂Sn(I_0.85Br_0.15)₄薄膜中Br的熟化作用,使其具有较高的结晶度和较好的形貌,这有利于晶体管沟道中载流子的输运,从而提高了晶体管的整体性能。（2）以N,N-二甲基甲酰胺（DMF）/二甲基亚砜（DMSO）（体积比70/30）作为溶剂,利用溶剂工程和N₂处理共同作用,通过系统的优化结晶和成膜工艺,合成了高质量的脂肪族（BA）₂Sn I₄钙钛矿薄膜。XRD测试和DFT计算结果表明（BA）₂Sn I₄薄膜具有二维层状结构,有利于横向电荷传输。理论计算还表明,[Sn I₆]^4-八面体决定了（BA）₂Sn I₄的电子能带结构,其中导带底由Sn-5p轨道组成,价带顶主要由I-5p态与少量Sn-5s态杂化构成。同时,发现有机BA⁺阳离子对价带和导带边缘没有直接贡献,但可以通过影响八面体的扭曲程度来影响电子结构。以PVA/CL-PVP作为介电层和以优化的（BA）₂Sn I₄薄膜作为活性层,制备出薄膜场效应晶体管。其空气中室温下最大空穴迁移率为0.030 cm² V^-1 s^-1,平均空穴迁移率为0.015 m² V^-1 s^-1,电流开/关比高于10²。并且发现,（BA）₂Sn I₄薄膜中垂直方向的离子迁移被大BA⁺阳离子抑制,但存在横向离子迁移。（3）采用一步旋涂法制备了脂肪族锡基钙钛矿(C_nH_2n+1NH₃)₂Sn I₄（n=3、4、5）薄膜,并对其晶体结构和形貌、光学特性和电子能带结构、激子和电子-声子相互作用及稳定性进行了研究。结果发现,在结晶方面,随着A位脂肪族阳离子链长的增加,(C_nH_2n+1NH₃)₂Sn I₄薄膜的层间距逐渐变大,晶粒变小、晶界逐渐模糊,粗糙度也逐渐减小。(C_nH_2n+1NH₃)₂Sn I₄钙钛矿的光学特性差别较小,这是由于钙钛矿的带隙主要由无机八面体决定,烷基链长对带隙影响也不明显。通过变温光致发光（PL）光谱探究了(C_nH_2n+1NH₃)₂Sn I₄薄膜的相变、激子性质、陷阱和电子-声子相互作用的机制。结果表明,(C_nH_2n+1NH₃)₂Sn I₄钙钛矿均存在结构相变,室温PL峰Peak 1和低温PL峰Peak A和自由激子发射相关,而Peak 2和Peak B是由束缚激子引起。计算了(C_nH_2n+1NH₃)₂Sn I₄钙钛矿相变前后的激子结合能,通过对Peak 1和Peak A的带隙以及Peak A的半高全宽（FWHM）与温度的关系曲线的拟合,发现在(C_nH_2n+1NH₃)₂Sn I₄薄膜中纵向光学（LO）声子通过Fr?hlich相互作用控制电荷载流子和激子的散射。在对(C_nH_2n+1NH₃)₂Sn I₄钙钛矿的稳定性研究中发现,更长的烷基链具有更好的覆盖性以及更强的疏水性。成功地制备了由PVA/CL-PVP作为底栅介电层的(C_nH_2n+1NH₃)₂Sn I₄薄膜晶体管。和（BA）₂Sn I₄一样,空气中室温下（PA）₂Sn I₄和（AA）₂Sn I₄晶体管均表现出p型性能,平均迁移率分别为6.4×10^-4和6.0×10^-3 m² V^-1 s^-1。（BA）₂Sn I₄因其更好的形貌具有最大平均空穴迁移率。本论文全面而深入地理解不同X位卤素阴离子和不同链长的A位脂肪族阳离子的二维锡基钙钛矿薄膜的基本物理性质,并为钙钛矿场效应晶体在有机介电层上的应用管提供了新思路。本论文包含图74幅,表12个,参考文献276篇。更多还原

【Abstract】 Two-dimension（2D）Ruddlesden-Popper（R-P）Sn-based perovskites with a chemical formula of A₂Sn I₄,where A is a large organic cation with monoammonium and X is halogen,are widely investigated due to their unique layered structure that enables the horizontal transport of charge carriers in metal-halide[Sn X₆]^4-octahedra,resembling the transport behavior in the semiconducting channel of field-effect transistors.Their electronic band structures are mainly dominated by the metal-halide octahedra,which eventually determine the optical and electrical characteristics.Although organic cations have no direct contributions,they would impact the electronic structures indirectly.At present,the studies on the effects of X halogen ions and A organic ions on the properties of 2D Sn-based perovskite materials and their potential applications in field effect transistors are relatively rare.This dissertation focuses on the thin film properties of two-dimensional R-P Sn-based perovskite with different halogen ions and aliphatic monoammonium cations of different chain lengths and the performance of corresponding perovskite field-effect transistors.The main innovative results obtained in this dissertation are as follows:（1）Comprehensive studies have been conducted on the crystal structure,surface morphology,and energy band structure of aromatic mixed halogen perovskite（PEA）₂Sn(I_1-nX_n)₄（n=0,0.15,0.3,0.5,X=Br,Cl）thin films.It has been proved that I^-,B^-,and Cl^-are mutually substituted in the perovskite lattice.The mixed halogen perovskite films,like the（PEA）₂Sn I₄ film,exhibit a 2D layered R-P structure.Due to the ripening effect of Br,the（PEA）₂Sn(I_0.85Br_0.15)₄ film has better morphology and a smaller roughness（8.487 nm）than（PEA）₂Sn I₄.It is proved that the substitution of Br^-and Cl^-for I^-can lead to to a downward shift in the VBM of the perovskites,thereby affecting their energy band structures.The field-effect transistors with（PEA）₂Sn(I_1-nX_n)₄ thin films as the semiconductor layers and PVA/CL-PVP as the gate dielectrics are fabricated.At room temperature in a nitrogen atmosphere,compared to the（PEA）₂Sn I₄field-effect transistors,the average carrier mobility of the（PEA）₂Sn(I_0.85Br_0.15)₄ devices increase from 0.068 cm² V^-1s^-1 to 0.089 cm² V^-1s^-1,and the average threshold voltage decreases from 21.7 V to 7.1 V.The current on-off ratio also increases from 1200 to1600.The higher crystallinity and better morphology of the（PEA）₂Sn(I_0.85Br_0.15)₄ film,which is conducive to the transport of carriers,are believed to improve the overall performance of the transistors（PEA）₂Sn(I_0.85Br_0.15)₄ transistors.（2）We have synthesized the（BA）₂Sn I₄ thin films with DMF/DMSO（volume ratio70/30）as the solvent through the systematic optimizations of the crystallization and film formation processes.The XRD and DFT results reveal that the film possesses a 2D layered perovskite structure,facilitating lateral charge transport.The theoretical calculations also suggest that the[Sn I₆]^4-octahedra determine the electronic band structure of（BA）₂Sn I₄,where the lower conduction band is derived from Sn-5p orbitals,and the upper valence band is mainly composed of I-5p states hybridized with a small amount of Sn-5s states.The organic BA⁺cations are found to have no direct contributions to the construction of valence and conduction band edges,but influence the electronic structure via distorting the octahedra.It is found that the vertical ion migration in the（BA）₂Sn I₄ film is suppressed by the large BA⁺cations,but the ion migration exists in the lateral direction.The thin film field-effect transistors are fabricated using PVA/CL-PVP as the dielectric layer and optimized（BA）₂Sn I₄ thin film as the semiconductor layer.The maximum hole mobility in air at room temperature is0.030 cm² V^-1 s^-1.The average hole mobility is 0.015 cm² V^-1 s^-1,and the current on/off ratio is higher than 10².（3）Aliphatic Sn-based perovskite(C_nH_2n+1NH₃)₂Sn I₄（n=3,4,5）thin films are prepared by a one-step spin coating method.The crystal structure,morphology,optical properties,energy band structure,exciton,and electron-phonon interactions,and stability of the films are studied.The results show that with the increase of the aliphatic cations chain length,the interlayer spacing of the(C_nH_2n+1NH₃)₂Sn I₄ films gradually increases,the grain size becomes smaller,the grain boundaries gradually become blurred,and the roughness decreases.The differences in the optical properties of the(C_nH_2n+1NH₃)₂Sn I₄are small because the band gaps of the perovskites are mainly determined by the inorganic octahedra,and the effect of the alkyl chain length on the band gap is not significant.The phase transitions,exciton properties,traps,and electron-phonon interactions in the(C_nH_2n+1NH₃)₂Sn I₄ thin films are investigated through temperature-dependent photoluminescence（PL）spectroscopy.The results show that the(C_nH_2n+1NH₃)₂Sn I₄ perovskites have structural phase transitions.Peak 1 at room temperature and Peak A at low temperature are related to free exciton emission,while Peak 2 and Peak B are caused by bound excitons.The values of the exciton binding energy of the(C_nH_2n+1NH₃)₂Sn I₄ perovskites are calculated.By fitting the band gaps of Peak 1 and Peak A,as well as the temperature dependence curve of the full width at half height（FWHM）of Peak A,it is found that LO-phonons control the scattering of charge carriers and excitons through the Fr?hlich interactions in the(C_nH_2n+1NH₃)₂Sn I₄ thin films.It is also found that the perovskite films with longer alkyl chains have better coverage and stronger hydrophobicity.The(C_nH_2n+1NH₃)₂Sn I₄ thin film transistors with PVA/CL-PVP as the bottom gate dielectric layer are successfully fabricated.Similar to the（BA）₂Sn I₄ transistors,both（PA）₂Sn I₄ and（AA）₂Sn I₄ transistors exhibit p-type transport at room temperature in air with the average mobilities of 6.4×10^-4 and 6.0×10^-3 m² V^-1 s^-1,respectively.The（BA）₂Sn I₄ device has the maximum average hole mobility due to its better morphology.In a word,our work not only offers a deep insight into the physical characteristics of two-dimensional R-P phase Sn-based perovskite thin films with various X halogen anions and A aliphatic cations of various chain lengths,but also provide important experimental and theoretical guidance for their potential applications in lateral-type flexible optoelectronic devices.This dissertation includes 74 figures,12 tables,and 276 references.更多还原