Ruddlesden–Popper Tin‐Based Halide Perovskite Field‐Effect Transistors

Tin‐based halide perovskites garner attention as a promising semiconducting layer material for field‐effect transistors (FETs) owing to their lower effective mass than their lead‐based counterparts. However, they suffer from low ambient stability because Sn2+ is readily oxidized to Sn4+ in air. To address this issue, Ruddlesden–Popper (RP) perovskites featuring large organic cations emerge as promising materials for FETs. In this article, a comprehensive overview of the properties and advantages of RP‐phase tin‐based halide perovskites used in FETs are provided. Recent advancements in 2D and 2D/3D RP tin‐based perovskite FETs are examined, and challenges related to the fabrication of uniform large‐area films and strategies for improving ambient stability and operational durability are discussed. In this review article, the potential of RP perovskites for FET applications is emphasized and the need for further research to unlock their full potential is highlighted.


Introduction
Metal-halide perovskites (MHPs) have emerged as a leading class of semiconducting materials for optoelectronic devices, including photovoltaic cells, [1] light-emitting diodes (LEDs), [2] detectors, [3] and lasers, [4] owing to their superior properties and cost-effective processing.Recent advancements in perovskite solar cells and perovskite/Si tandem cells have yielded power conversion efficiencies exceeding 26% and 33%, respectively, fueling interest in next-generation photovoltaic materials. [5]reover, green and red perovskite LEDs have achieved external quantum efficiencies surpassing 20%. [6]Their distinct features, including defect tolerance and low effective mass, position MHPs as potential active layers in field-effect transistors (FETs).Due to the scarcity of high-performance and reliable p-type materials available in the field of semiconductors, even with the development of p-type oxide materials, the advancement of complementary circuits has been slow thus far. [7]MHPs, especially tin-based MHPs, offer a promising solution owing to their high intrinsic carrier mobility.Nevertheless, MHP FETs remain relatively underexplored compared with other applicable optoelectronic areas.
Ruddlesden-Popper (RP)-phase perovskites have recently attracted significant interest as potential platforms for FETs due to their distinct crystal structure.RP perovskites feature alternating layers of continuous corner-sharing metal-halide octahedra and bulky organic cations.These cations foster a unique-layered structure by ensuring appropriate separation between the perovskite sheets.Furthermore, introducing these cations modulates strain on the perovskite lattice, offering precise control of orbital overlap.By finely tuning the organic cations to exhibit specific functionalities, the resultant perovskite films can bestow distinct properties.The meticulous tunability of perovskite composition and crystallization enables the researchers to design a targeted approach to modulate desirable optoelectronic properties, and ultimately enhance the device performance.
In this review, we delve into the promising RP perovskites, with a special focus on tin-based perovskites.Their fundamental properties and unique advantages are introduced, with highlighted recent advancements in 2D and 2D/3D RP FETs.Through the discussion over challenges in RP tin-based perovskite FETs, we offer critical insights into their future application.

Fundamentals of Tin-Based Halide Perovskite
The term MHP typically refers to an ABX 3 structure, where A þ represents a monovalent organic/inorganic cation (methylammonium is a divalent metal cation (typically Pb 2þ or Sn 2þ ), and X À represents a halide anion (I À , Br À , or Cl À ) (Figure 1a).The perovskite crystal features a corner-sharing (BX 6 ) 4À octahedral structure, continuously and isotropically overlapping the s orbital of the metal and the p orbital of the halogen.The overlap is maximized when the bond angle between s(metal)-p(halogen)s(metal) orbitals (θ s-pÀs ) reaches 180°, eliciting a broad dispersive energy band and reduced effective mass (Figure 1b).For instance, the effective masses of holes and electrons in the tin-based 2D perovskite phenethylammonium tin iodide (PEA 2 SnI 4 ) are 0.04 m 0 and 0.03 m 0 , respectively. [8]A low effective mass leads to high carrier mobility in electronic devices.
Specific criteria for the tolerance factor (t) and octahedral factor (μ) must be followed to form a stable ABX 3 perovskite structure.The tolerance factor is given by t , where R A , R B , and R X represent the ion radii of A, B, and X, respectively.This factor indicates the preferred ion combination within a perovskite structure depending on their relative ion sizes.For a stable structure, t should lie between 0.8 and 1, ideally near 1.When the A cation size exceeds the space within the octahedral cage layers, the A cation separates the corner-sharing octahedral network of (BX 6 ) 4À into an alternating-layered 2D structure.The octahedral factor, μ ¼ R B R X , reflects the preference of B atoms for coordination with X atoms, and should fall within the range of 0.4 ≤ μ ≤ 0.9.
When a large organic A cation is incorporated into 3D ABX 3 perovskite, the RP perovskite with 2D and 2D/3D hybrid-layered structure can be formed, following the formula R 2 A n-1 B n X 3nþ1 .In this formula, R þ represents a bulky organic cation larger than A þ and n represents the number of (BX 6 ) 4À octahedral perovskite layers separated by the bulky A-site cations.A 2D perovskite corresponds to n = 1, symbolized as R 2 BX 4 .For n > 1, the perovskite exhibits intermediate properties between 2D and 3D perovskites.We specify these to differentiate between 2D RP perovskites (n = 1) and 2D/3D RP perovskites (n > 1).As n increases, the properties of the perovskite gradually resemble those of 3D perovskites.The n value can be controlled by adjusting the stoichiometry of the precursor.Compared with 3D perovskites, 2D perovskites have higher formation energy, resulting in a more stable crystal structure.The hydrophobic nature of bulky organic cations enhances this intrinsic stability against oxygen and moisture.Moreover, bulky cations promote a template effect, where the initial 2D perovskite grows in a direction toward the substrate, leading the growth of subsequent layers for higher crystallinity. [9]e unique band structure of the valence band maximum (VBM) underpins the superior p-type behavior of the tin-based perovskites.Although n-type oxide materials like indium gallium zinc oxide (IGZO) have been successfully commercialized, p-type metal oxides still suffer from limited performance. [10]This disparity arises from the distinctive band structures of oxide materials.The conduction band minimum (CBM) is typically formed by metal ns orbitals, while the VBM is formed by oxygen 2p orbitals (Figure 1c).The spherical s orbitals of relatively large metal cations enable efficient overlap with isotropic directions, promoting a highly dispersed and delocalized CBM, ensuring high electron mobility even in amorphous structures.Conversely, the anisotropic and localized nature of the oxygen 2p orbitals, exhibiting small-sized dumbbell-like shapes, results in a large hole effective mass and less dispersive VBM.Furthermore, introducing metal-cation states through the deliberate generation of oxide vacancies can form deep hole traps within the band structure.These traps can compromise hole-transport efficiency, challenging the practical implementation of p-type oxides.
Contrastingly, MHPs exhibit an ionic bonding character due to the pronounced energy gap between the atomic orbitals of the cations and anions.The energy levels of the VBM and CBM in MHPs depend on those of the constituent anions and cations, respectively (Figure 1d).The VBM in MHP perovskites arises from the antibonding state of the s orbital of the metal and the p orbital of the halide. [11]Consequently, the trap states form near the energy-band edge, imparting the perovskite with defecttolerant properties and superior charge transport.
FET is a three-terminal electronic device using a voltage sweep to modulate electrical switching control.The primary architecture of FETs comprises of the source/drain (S/D), gate, semiconductor layer (or active layer), and dielectric layer.FET device architectures can be categorized into four distinct types; bottom-gate top contact (BGTC), bottom-gate bottom contact (BGBC), top-gate top contact (TGTC), and top-gate bottom contact (TGBC) (Figure 2a).Among these, the bottom-gate configurations have been prevalently adopted in device fabrication due to their simplicity, from direct deposition of the active layer onto the dielectric layer, reducing the risk of damage after additional dielectric deposition.In contrast, the top-gate structure, characterized by the post-deposition of the dielectric/gate layer, though more complex, confers pronounced benefits, including the refined modulation of the semiconductor/dielectric interface and improved encapsulation attributes.
The gate terminal, insulated from the channel by a thin dielectric layer, plays a pivotal role in controlling the conductivity of the carrier channel.A controlled gate voltage sweep induces the formation of a conducting channel between the source and drain terminals upon surpassing a threshold voltage, thereby facilitating the flow of charge carriers (Figure 2b).The primary charge carriers traverse at the semiconductor/dielectric interface, which quality critically influences the FET functionality.
In place of the semiconducting layers of FETs, tin-based perovskites have gained more attention and explored extensively than lead-based perovskites.A key advantage of tin-based perovskites is their lower effective mass and higher mobility at room temperature than their lead-based counterparts.This is primarily attributed to their diminished Fröhlich interaction in tin perovskites. [12]The Pb-halide bond exhibits a relatively higher polarity than the Sn-halide bond, increasing ionicity in the polar lattice. [12]This amplified ionicity enhances the electron-phonon coupling in the perovskite lattice, limiting charge-carrier mobility in Pb halide perovskites.Electron-phonon coupling significantly diminishes below a temperature aligned with the energy of the pertinent longitudinal optical (LO) phonon. [13]According to the Fröhlich approach, [13b,14] charge-carrier mobilities depend on a dimensionless parameter proportional to the LO phonon frequency.Tin-based perovskites exhibit a higher LO phonon frequency than lead-based ones since tin is lighter than lead.Moreover, the antibonding coupling with Sn 5s-I 5p is stronger than Pb 6s-I 5p near the VBM.This is attributed to the shallower and more active Sn 6s lone-pair states than those of Pb, resulting in a reduced hole effective mass. [15]Consequently, tin-based perovskites demonstrate enhanced hole mobility at room temperature with lower electron-phonon coupling and elevated LO phonon frequencies.Furthermore, tin-based perovskites are considered environmentally friendly, compared to lead-based perovskites with hazardous lead. [16]These properties establish tin-based perovskites as attractive semiconducting materials for FETs.
Despite their advantageous electrical properties, tin-based perovskites suffer from low ambient stability due to facile oxidation of Sn 2þ into Sn 4þ when exposed to air. [17]Such strong tendency comes from unique electron configurations of Sn 2þ compared to Pb 2þ , which are [Xe] 4f 14 5d 10 6s 2 and [Kr] 4d 10 5s 2 , respectively.The fully occupied 4f and 5d orbitals in Pb 2þ weaken the electron shielding effect, resulting in a strong attraction between the nucleus and the 6s 2 lone-pair electrons.Consequently, these lone-pair electrons are reluctantly lost, a phenomenon termed the lanthanide contraction.Conversely, the 5s 2 electrons in Sn 2þ experience a weaker attraction from the nucleus due to the absence of a filled 4f orbital and the strong shielding effect by the 4d orbital.This makes the 5s 2 electrons susceptible to oxidation, shifting Sn 2þ to Sn 4þ .Moreover, the redox potentials of Sn 2þ /Sn 4þ and Pb 2þ /Pb 4þ are þ0.15 and þ1.67 V, respectively, indicating tin oxidizes more easily.Thus, tin-based perovskite FETs require adequate encapsulation for ambient use to prevent defect formation and performance degradation.
The inherent instability arising from the B-site metal cation can be mitigated by A-site cations.Large organic cations in 2D perovskites are hydrophobic and bulky, serving as protective barriers against environmental moisture/oxygen that enhance air stability. [18]Moreover, organic A-site cations counteract ion migration in the perovskite lattice, diminishing hysteresis behavior and bolstering structural stability with strong van der Waals interactions between the organic cations and perovskite octahedral units. [19]However, the separation between perovskite octahedral layers introduces resistance to charge transport across the organic cation layers.Moreover, 2D perovskites inherently possess a low carrier concentration, resulting in low device mobility.Despite these challenges, the critical advantage of enhanced stability have initiated active research into 2D and 2D/3D tin-based perovskites for promising active layers of FETs.Kagan et al. first reported a 2D tin-based perovskite (n = 1) FET using PEA 2 SnI 4 spin-coated on a SiO 2 dielectric layer to construct a BGBC configuration (Figure 3a). [20]These FETs exhibited a hole mobility (μ FE ) of 0.62 cm 2 V À1 s À1 , underscoring the potential of 2D perovskite as semiconducting layers.Subsequent research employed PEA 2 SnI 4 to explore its capabilities.Mitzi et al. enhanced the μ FE to 2.6 cm 2 V À1 s À1 using a low-temperature melt-processed channel layer and polyimide substrates, enlarging the perovskite grains. [21]Matsushima et al. fabricated PEA 2 SnI 4 via vacuum vapor deposition, introducing an octadecyltrichlorosilane (OTS) buffer layer atop a SiO 2 substrate. [22]The vapor deposition method and OTS treatment improved the film morphology, and the device exhibited a μ FE of 0.78 cm 2 V À1 s À1 .After the initial studies of 2D tin-based perovskites, it took over a decade to regain its attention as active layers for FETs.
Matsushima et al. incorporated an NH 3 I self-assembly monolayer (NH 3 I-SAM) and MoO x hole-injection layers into PEA 2 SnI 4 FETs (Figure 3b). [23]NH 3 I-SAM treatment fostered the growth of well-structured perovskite crystallites and promoted their conversion into perovskite, diminishing the hole-trap density.The MoO x layer facilitated hole injection between the electrodes and the perovskite layers.The multilayered PEA 2 SnI 4 , configured in a TGTC structure, demonstrated a μ FE of up to 12 cm 2 V À1 s À1 .Subsequently, the same research group explored PEA 2 SnI 4 FETs integrated with an electron-injection layer of C 60 Reproduced with permission. [20]Copyright 1999, The American Association for the Advancement of Science.b) High-performance p-type PEA 2 SnI 4 FET with C 60 electron-injection layer in top-gate top-contact (TGTC) structure.Reproduced with permission. [23]Copyright 2016, John Wiley and Sons.c) Highperformance n-type PEA 2 SnI 4 FET with MoO x hole-injection layer in TGTC structure.Reproduced with permission. [24]Copyright 2016, AIP Publishing.and different electrode configurations (Figure 3c). [24]The lowest unoccupied molecular orbital level of C 60 is situated between the CBM of PEA 2 SnI 4 and the Fermi levels of the electrodes, enhancing electron injection through C 60 .The PEA 2 SnI 4 FETs with the C 60 layer demonstrated n-type channel operation, boasting an electron mobility (μ e ) of 2.1 cm 2 V À1 s À1 .The authors further adjusted the channel length to minimize the influence of contact resistance, achieving μ h and μ e values of up to 22 and 3.8 cm 2 V À1 s À1 , respectively. [25]They also analyzed the degradation mechanism of PEA 2 SnI 4 films, [26] and found that the oxygen source at the grain boundaries was the main cause of the degradation.The addition of ethanol intrigued dense and large grains with reduced grain boundaries, consequently preventing oxygen penetration through the films.The PEA 2 SnI 4 FETs with optimized condition and CYTOP encapsulation exhibited a μ FE of 7.9 cm 2 V À1 s À1 with good air stability.Li et al. introduced copolymer dielectric layers to construct low-operating-voltage PEA 2 SnI 4 FETs. [27]The dielectric layers consist of the combination of polymers; poly(vinylidenefluoride-co-trifluoroethylene) (PVDF-TrFE), polytetrafluoroethylene (PTFE), and cross-linked poly(4-vinylphenol) (CL-PVP).PVDF-TrFE was introduced for a high dielectric constant.PTFE and CL-PVP were used to address the issues of solvent corrosion and wettability.The ferroelectric properties of PVDF-TrFE were reduced by PTFE.The optimized PEA 2 SnI 4 FETs with double-PVDF-TrFE/PTFE and triple-PVDF-TrFE/PTFE/CL-PVP dielectric layers exhibited μ FE of 0.42 and 0.36 cm 2 V À1 s À1 with low-operating-gate voltages, respectively.
Doping is a fundamental study in semiconductor materials to modulate their work function and conductivity for higher efficiency of charge transport.Several studies have explored various doping strategies for PEA 2 SnI 4 to enhance its intrinsic lowcharge-carrier transport. [28]First of all, Liu et al. approached p doping of PEA 2 SnI 4 by introducing SnI 4 into the precursor solution (Figure 4a-d). [29]This enhanced electrical conductivity, improved film morphology, and reduced trap states.Consequently, the FET displayed a μ FE of 0.68 cm 2 V À1 s À1 .Also, Guo et al. implemented van der Waals heterojunctions (vdWHs) by layering a poly{3,6-dithiophen-2-yl-2,5-bis(2decyltetradecyl)pyrrolo [3,4-c]pyrrole-1,4-dione-alt-thienylenevinylene-w,5-yl} (PDVT-10) organic semiconductor atop PEA 2 SnI 4 through physical transfer of PDVT-10 (Figure 4e-g). [30]The transferred organic layer acted as a dopant layer for the PEA 2 SnI 4 and modified its electrical conductivity and Fermi levels, enhancing hole transfer at the heterojunction interface.The resulting vdWHs-PEA 2 SnI 4 FET demonstrated a μ FE of 0.46 cm 2 V À1 s À1 with high off currents.Once the doped PDVT-10 films were removed, the FET performance of (e-g) Reproduced with permission. [30]Copyright 2022, John Wiley and Sons.
PEA 2 SnI 4 nearly reverted to its original state, suggesting a reversible doping technique.Chao et al. employed 2D phenethylammonium iodide (PEAI) and 4-fluorophenethylammonium iodide (FPEAI) to treat the PEA 2 SnI 4 active layer. [31]These 2D organic cations triggered a p-doping effect to form a better energy-level alignment with the gold electrodes, boosting charge transport.Additionally, these organic cations contributed to surface defect passivation and grain size enlargement via surface recrystallization, ultimately bolstering ambient stability.The 2D organic cationtreated FETs exhibited μ FE values of 2.15 and 2.86 cm 2 V À1 s À1 following PEAI and FPEAI treatment, respectively.
Zhu et al. adopted multiple engineering approaches to enhance the PEA 2 SnI 4 FET performance.The authors introduced semi-carbon nanotubes (CNTs) into PEA 2 SnI 4 , resulting in hybrid FETs (Figure 5a). [32]The CNTs minimized the trapping and scattering of charge carriers on the transport channels, exhibiting a μ FE of 1.51 cm 2 V À1 s À1 .In a latter study, the authors systematically approached PEA 2 SnI 4 layer to boost its FET performance through several approaches: use of excess PEAI for self-passivation at the grain boundaries, addition of Lewis base adducts to guide grain crystallization, and minimal oxygen treatment to counteract iodine vacancies (Figure 5b-e). [33]These interventions resulted in highly reproducible and reliable FETs, achieving a μ FE of 3.51 cm 2 V À1 s À1 and an on/off current ratio of 10 6 .The authors took an alternative approach, by directly adding an antisolvent into the PEA 2 SnI 4 precursor solution. [34]he combined antisolvents, chlorobenzene (CB), and ethyl acetate (EA) promoted nucleation and cultivated perovskite films with highly aligned crystallization and comprehensive coverage.This innovation resulted in high-performance FETs boasting a μ FE of 3.8 cm 2 V À1 s À1 .Lastly, we delved into the effects of aging on the PEA 2 SnI 4 precursor solution, employing an FET device as a platform to probe the charge-transport attributes and undertake defect analyses of the perovskite films. [35]This aging process effectively eliminated aggregated or unreacted clusters in the fresh precursor, promoting uniform nucleation, crystallization, and perovskite film growth.Transistors employing the aged precursor exhibited reduced ion migration and low defect density.This boosted the charge-transport capabilities in the FET, achieving a μ FE exceeding 4 cm 2 V À1 s À1 .(b-e) Reproduced with permission. [33]Copyright 2020, John Wiley and Sons.
Additive engineering is a straightforward and effective approach to passivate grain boundaries and enhance FET performance.Copper iodide (CuI) was introduced to create a CuI-PEA 2 SnI 4 -heterostructured composite layer (Figure 6a). [36]nstead of substituting B-site metal cation within the perovskite structure, CuI compounds were strategically positioned along the grain boundaries.Given its superior hole-transport properties and ability to passivate grain boundaries, CuI refined the surface of the perovskite film and minimized film trap states (Figure 6b).The FET-incorporating CuI-PEA 2 SnI 4 demonstrated a μ FE of 2.61 cm 2 V À1 s À1 and an on/off current ratio nearing 10 7 (Figure 6c).We further examined the impact of adding sodium iodide (NaI) to PEA 2 SnI 4 . [37]The presence of Na þ mitigates ion migration due to its interaction with halides, simultaneously augmenting the hole concentration (Figure 6d).The quality of the NaI-PEA 2 SnI 4 film was enhanced, displaying larger grain sizes, improved crystallinity, and more uniform film coverage.This improvement, combined with the passivation of iodine vacancies, resulted in a μ FE of 2.13 cm 2 V À1 s À1 (Figure 6e,f ).
Another popular studies of 2D perovskite FETs are molecular engineering of A-site cations, to finely tune the chemical functionalities of organic spacers.In 2001, Mitzi et al. investigated the introduction of fluorine groups at various carbon positions on the phenyl ring of the phenethylammonium (PEA) molecule (n-FPEA, n = 2, 3, and 4) (Figure 7a). [38]This strategic modification allowed for precise control over steric constraints and chemical interactions, resulting in an exceptional μ FE of 0.51 cm 2 V À1 s À1 with (3-FPEA) 2 SnI 4 .The authors introduced a methoxy group to the PEA molecule to form 4-methoxyphenethylamine (4-MeOPEA) and (4-MeOPEA) 2 SnI 4 using a melt-processed channel layer on top of a SiO 2 /polyimide gate insulator, which exhibited a μ FE of 1.8 cm 2 V À1 s À1 . [21]Also, Wang et al. explored the incorporation of chloride (Cl) in n-butylammonium (BTA)-based 2D perovskites for the formation of BA 2 SnI 4 . [39]Cl has been incorporated into perovskite crystal lattices to induce lattice distortion, doping effects, and impurities.Furthermore, the Cl incorporation into BA 2 SnI 4 enhanced the FET performance with a μ FE of 0.1 cm 2 V À1 s À1 but a low on/ off current ratio.Moreover, Gao et al. replaced the PEA organic ligand with a π-conjugated oligothiophene ligand, 2-(3 00 , 4 0 -dimethyl-[2,2 0 :5 0 ,2 00 :5 00 ,2 00 0 -quaterthiophen]-5-yl)ethan-1ammonium (4Tm) to form (4Tm) 2 SnI 4 (Figure 7b). [40]The enlarged conjugated 4Tm ligands served as thick and dense barriers to external factors and increased the crystal formation energy through strong intermolecular interactions.This approach facilitated large grains, improved the charge injection, and stabilized the perovskite layers, thereby improving the hole mobility from 0.15 to 2.32 cm 2 V À1 s À1 .Liang et al. further developed an oligothiophene ligand by varying the number of thiophene rings (Figure 7c). [41]As the number of thiophene rings increased, the extended π conjugation and increased planarity of the ligand decreased the nucleation density by over five orders of magnitude, significantly increasing the grain size.The FETs based on TT 2 SnI 4 , where the TT ligand features one more thiophene ring than 4Tm, were demonstrated with a μ FE approaching 10 cm 2 V À1 s À1 .Another organic spacer proposed by Wang et al. is 2-thiopheneethylammonium (TEA) for grain engineering. [42]ot-casting method was used for an effective modulation of grain size and grain boundary, increasing in-plane charge-carrier transport in a perovskite film.The optimized TEA 2 SnI 4 FETs exhibited μ FE of 0.34 and 1.8 cm 2 V À1 s À1 at 295 and 100 K, respectively.The authors also incorporated pentanoic acid (PA) in TEA 2 SnI 4 films. [43]The PA additive modified the heterogeneous nucleation through the formation of hydrogen and coordination bonding.This effect reduced defect density and improved film morphology, which FET demonstrated a μ FE of 0.7 and 2.3 cm 2 V À1 s À1 at 295 and 100 K, respectively.The aforementioned 2D perovskite FETs, as well as arising dication 2D Dion-Jacobson perovskite FETs, are included in Table 1 for understanding FET research trends.

2D/3D RP Perovskite FETs (n > 1)
The 2D/3D RP tin-based perovskites with n > 1 have attracted attention owing to their excellent and stable optoelectronic properties and tunability.Shen et al. reported the first 2D/3D RP tinbased perovskite FETs with PEA 2 CsSn 2 I 7 (n = 2). [44]The FET with a millimeter-sized PEA 2 CsSn 2 I 7 crystal exhibited the highest μ FE of 34 cm 2 V À1 s À1 at 77 K with a high off-state current (Figure 8a).Since then, several PEA/formamidinium (FA)-based 2D/3D RP tin-based perovskite FETs have been reported.Shao et al. introduced a small amount of 2D PEA 2 SnI 4 into 3D FASnI 3 to construct PEA 2 FA n-1 Sn n I 3nþ1 (n = 1, 4, 8), enhancing the crystallinity, and reducing the trap density and hole-carrier density by one order of magnitude, exhibiting a μ FE of 0.21 cm 2 V À1 s À1 (Figure 8b). [45]Also, Kim et al. reported a new 2D/3D core-shell structure TFTs, where the 3D core FASnI 3 was separated by 2D PEA 2 SnI 4 (Figure 8c). [8]This structure offers advantages such as V th regulation by the 2D components and enhanced grain boundary properties through the 3D components.Through additional SnF 2 incorporation for preferential crystallization and vacuum treatment to eliminate residual solvent, the core-shell-structured FETs (n = 10) enhanced the linear μ FE to 25.5 cm 2 V À1 s À1 with an on/off ratio of 10 6 .
Roh et al. performed a special study to investigate the effect of ion migration on charge transport in tin perovskites using a transistor as a test platform. [46]The PEA/FA-based hybrid perovskite (n = 6) FETs with CuI as an ion source for the perovskite channel exhibited increased ion migration at a low-voltage scan rate.This induced electrochemical doping and resulted in abnormal FET behaviors, elucidating ion migration in tin-based perovskite groups.Doping study was also explored in 2D/3D RP perovskites, using N,N-dimethylanilinium tetrakis(pentafluorophenyl)borate (DTB) with a large bandgap (>3 eV) as a molecular dopant. [47]The dissociated tetrakis(pentafluorophenyl)borate anion (TPFB À ) from DTB contains abundant fluorine groups that can form hydrogen bonds with the perovskite lattice and enhance crystallization.Furthermore, the hydrophobic fluorinated groups of TPFB À could contribute to the surface passivation of the perovskite films by protecting against moisture. [48]These effects led to the improved electrical performance and operational stability of the FETs, exhibiting a μ FE of 7.45 cm 2 V À1 s À1 .Our group reported 2D/3D RP perovskite FETs with 4-fluoro-phenethylammonium (4-FPEA)/FA hybrid tin-based perovskites (n = 6) (Figure 8d). [49]Incorporating fluorinated 4-FPEA ligands into FASnI 3 -induced aligned perovskite growth on the substrates, enhancing lateral charge transport.The FPEA-based 2D/3D RP perovskite films exhibited a  μ e , electron mobility; À, not available.
smoother morphology than their analogs with increased environmental stability.These films demonstrated an FET performance with a μ FE of 12 cm 2 V À1 s À1 and an on/off current ratio exceeding 10 8 with negligible hysteresis.Recently, Zhu et al. demonstrated high-performance tin-based perovskite FETs through the incorporation of triple A cations comprising Cs, FA, and PEA (n = 8). [50]The PEA incorporation in 3D FASnI 3 to form PEA 2 FASn 8 I 25 (PEAFA) greatly improved the film morphology, due to the moderated crystallization kinetics.The large PEA cations in the precursor retarded the fast reaction between the perovskite building components, leading smooth/pin-hole-free morphology.Furthermore, the Cs substitution into the PEAFA film to form Cs 0.1 FA 0.9 PEA 2 Sn 8 I 25 (CsFAPEA) tin perovskite resulted in the full-coverage morphology and the ordered crystallinity/texture by the large stable clusters with a radius of 500 nm in the precursor solutions.This led to the easy nucleation with the subsequent more uniform crystal growth and structural stabilization in the films.The optimized CsFAPEA FETs exhibited a high μ FE of 72 cm 2 V À1 s À1 , comparable with commercially available low-temperature polysilicon transistors.The aforementioned 2D/3D RP perovskite FETs (n > 1) are summarized in Table 2. To elucidate high-dimensional FET research trends, 3D perovskite FETs (n = ∞) are also included in Table 2.

CMOS Application of 2D and 2D/3D RP Perovskite FETs
A step beyond the potentials of tin-based perovskite FETs, 2D, and 2D/3D RP tin-based perovskite FETs have been successfully incorporated into complementary metal-oxide-semiconductor (CMOS) applications.Zhu et al. demonstrated the first perovskite-integrated complementary inverter, integrating p-channel PEA 2 SnI 4 with an n-channel IGZO (Figure 9a). [33]he inverter exhibited well-balanced combination as pand n-type channel (Figure 9b).The inverter demonstrated ideal rail-to-rail output voltage characteristics with a high gain of 30 at drain voltage (V DD )= 40 V with negligible power loss (Figure 9c-f ).A high noise margin of 14 V (70% of ideal value [V DD /2]) at V DD = 40 V ensures tolerance against variability in transistor threshold voltages.Moreover, Kim et al. demonstrated 2D/3D RP tin-based perovskite inverter with p-channel PEA 2 SnI 4 /FASnI 3 and n-channel IGZO. [8]The inverter produced distinct logic signals with high voltage gain of 200 at V DD = 20 V, low power consumption of ≈2 μA at V DD = 10 V, and the noise margin with 3.72 V (74% of V DD /2).Additionally, Zhu et al. implemented CMOS inverter and NAND/NOR logic gates with p-channel CsFAPEA tin-based perovskite and n-channel In 2 O 3 TFTs. [50]The inverter shows a high gain of 370 at Reproduced with permission. [45]Copyright 2021, John Wiley and Sons.c) Core-shell structure of PEA 2 FA 9 Sn 10 I 31 FETs.d) (4-FPEA) 2 FA 6 Sn 7 I 22 FETs.Reproduced with permission. [49]Copyright 2023, John Wiley and Sons.
V DD = 12.5 V and a noise margin of 5.16 V (82% of V DD /2).Furthermore, the complementary logic gates, namely NAND and NOR, were demonstrated with full rail-to-rail Boolean functionalities (Figure 9g-i).These advanced demonstrations based on 2D (n = 1) and 2D/3D RP (n > 1) tin-based perovskite FETs delivered the feasible technological integration of tin perovskite TFTs toward CMOS applications.

Challenges and Future Outlook
As discussed earlier, 2D and 2D/3D tin-based perovskites show promising future for FET applications.However, over the decades, the perovskite community has focused mainly on photovoltaics and LEDs, and the research on perovskite FETs has just gained its momentum.Still, we face many challenges to gain an in-depth understanding of perovskite FETs, before their potential commercial launch.

Fabrication Method
So far, the research on perovskite FETs has focused on characterizing the perovskite material itself, with scarce attention on the fabrication method.Majority of the research employs solutionbased approaches, such as spin-coating process, which have clear limitations for industrial applications.Therefore, various alternative processing methods are crucial for confirming the viability of expanding the perovskite fabrication techniques used for FETs.
We suggest alternative deposition methods that allow precise control and large-area fabrication for perovskite FETs.Thermal evaporation is an attractive industry-ready deposition method, which has already shown its success in perovksite photovoltaics. [51]This method has significant advantages over solution process.First, it is a solvent-free method that resolves solubility issues of some precursor or additive materials.For examples, tin fluoride (SnF 2 ) is a key material in tin-based perovskites that suffers from limited solubility in conventional solvents.Poor solubility restricts controlled growth of highquality and uniform thin films, resulting in poor reproducibility as well. [52]Thermal evaporation allows for precise control of the film growth in a highly reproducible manner without the use of solvent.Second, it facilitates fabrication of large-area thin films.Unlike solution process, which requires intense care and expensive treatment for high uniformity in large-scale thin films, thermal evaporation only involves simple deposition over a largescale wafer.Thus, this method is well-matured technique in semiconductor industry, as well as perovskite photovoltaic devices.Third, it alleviates concerns regarding the toxicity of the solvents, which pose risks to human health and the environment. [53]hermal evaporation, as a dry deposition method, eliminates the need for these solvents, consequently minimizing associated risks.Finally, the precise control of film thickness and composition via thermal evaporation allows specific modulation of the electrical properties of perovskite films.The properties are intricately tied to the thickness of perovskite films.Thermally deposited films can ensure excellent film quality and consistency under the control.Forward; b) linear mobility; c) backward.
However, thermal evaporation also holds several downsides.First of all, it requires substantial amount of precursor material for evaporation, increasing the overall manufacturing cost.Also, despite its simplicity in deposition technique, the deposition quality depends on multiple factors, such as the vacuum level of the chamber and reaction dynamics of the perovskite precursors on top of the substrate.22a,54] The vapor pressure is often increased drastically during the deposition of the organic salts, causing inaccuracy in detecting the exact deposition rate.Thus, we must thoroughly explore precise techniques in thermal evaporation to uncover each factor involved in thermal evaporation.22a]

Environmental Stability
Tin-based perovskites show high fragility with low oxidation energy from Sn 2þ to Sn 4þ , which causes doping effect to lose  (a-f ) Reproduced with permission. [33]Copyright 2020, John Wiley and Sons.g) NAND and NOR optical images and h, i) their output characteristics with p-type Cs 0.1 FA 0.9 PEA 2 Sn 8 I 25 and n-type In 2 O 3 TFTs.their semiconducting characteristics. [55]For example, dimethyl sulfoxide (DMSO) is a commonly used solvent in perovskite community to control the crystal growth to form large grains and highly crystalline films in solution process. [33]However, DMSO can also act as an oxidation agent, due to the reaction between its carbonyl group and tin, especially during long-term storage. [56]Also, when tin-based perovskites are used in FETs, the active layer is very thin in tens of nanometer scale, making them susceptible to external conditions, such as oxygen and moisture.Their vulnerability to external environmental conditions can induce unintentional doping, increasing charge scattering and ultimately suppressing charge transport.These fundamental issues must be considered to further develop tin-based perovskites.
To develop upon the fragility of tin-based perovskites, several approaches have been explored: composition engineering, [57] molecular engineering, [40,58] additive engineering, [59] and posttreatments such as encapsulation. [60]For example, Gao et al. introduced a new organic spacer 4Tm into the A site to form (4Tm) 2 SnI 4 , which exhibited superior structural and FET stability compared to PEA-based 2D perovskite. [40]Its bulky size, hydrophobicity, and enhanced conjugation of 4Tm led to its structural and environmental stability (Figure 10a-d).To gain insights into the interaction of conjugated molecules with the inorganic layer and the stabilization mechanism, the authors conducted single-crystal synthesis and analysis of PEA  10f,h).As the N-I distance decreased, the electrostatic interactions between the negatively charged inorganic layers and the positively charged ammonium groups increased.Therefore, hydrogen bonding between the ammonium groups and iodide atoms strengthened in (4Tm) 2 SnI 4 .Large π-conjugated organic cations significantly influenced the lattice bonding features within the inorganic layers.The strong intermolecular interactions in ionic and hydrogen bonds of (4Tm) 2 SnI 4 allowed for higher intrinsic stability than PEA 2 SnI 4 The 2D/3D RP perovskites can display the best qualities from each dimension: environmental protection of the octahedral layer with large organic spacers from 2D perovskite structure, and excellent electrical properties from 3D perovskite structure.The 3D perovskites contain smaller A-site cations, such as FA, MA, and Cs, and show high efficiency in carrier transport, which is a favorable property for FETs.Furthermore, 3D perovskites often contain Pb and Sn mixed together. [52,61,62]Pb acts as a substitution dopant in tin-based perovskites for inducing antioxidation and film stabilization due to its lower Lewis acidity.However, due to its toxicity, alternative environmentally friendly candidates require consistent research, heading toward a completely tin-composed perovskites for commercially successful perovskite FETs.

Operational Stability
Operational stability is another crucial factor in device integration and complementary applications.While recent studies on tin-based perovskite FETs have primarily focused on boosting the field-effect mobility, limited studies have investigated the operational stability.
Several methodologies have been applied to validate the reliability of perovskite FET devices.Zhu et al. tested the bias stress stabilities of 2D PEA 2 SnI 4 FETs with different Lewis bases under constant bias (V GS = V DS = À40 V) for 2000 s. [33] As a figure of merit, the normalized drain current (I DD ), I D (t)/I D (0), exhibited the variation in device performance overtime.It was reduced to 40% with oxygen-containing Lewis bases such as DMSO, N-methyl-2-pyrrolidone, and γ-butyrolactone.However, it was maintained constant or increased to 30% with the nitrogencontaining Lewis bases of pyridine or acetonitrile, respectively (Figure 11a).Moreover, the authors also thoroughly analyzed the operational stability through consecutive on/off switching, cycling, and bias stresses. [61]The optimized I/Br/Cl device maintained a consistent on/off state through 1000 switching cycles.Furthermore, the transfer characteristics of the device consistently aligned across 100 cycle sweeps, highlighting the enhanced reliability of the I/Br/Cl devices (Figure 11b,c).The bias stress stability of the 3D MASnI 3 -based FETs was confirmed using V TH variation as a figure of merit (Figure 11d).Under constant bias (V GS = V DS = À12 V) for 12 h, the MASnI 3 device with I pristine exhibited V TH shifts of À2 V within 1000 s.However, the device with optimized I/Br/Cl substitution exhibited improved stability, with minor shifts of À0.52 V after biasing for 12 h.Also, Liu et al. confirmed the operational stability of 3D CsSnI 3 -based FETs using V TH variation. [62]The bias stress stability of optimized CsSnI 3 FETs exhibited V TH shifts of À5.2 V after 1 h (V GS = V DS = À40 V), but the V TH rapidly reverted to its initial value within 1 min.Thus, these studies demonstrate that device performances of various perovskite materials depend on their operational stability.
To thoroughly understand the efficacy of perovskites in devices, it is crucial to prioritize research that highlights their sustained stability, including bias stress, cycling, and switching stability, rather than solely assessing transistor performance.This wide range of understanding will ensure a solid foundation for highly reliable and high-performance perovskite FETs.
3.4.Design Protocol of RP MHP (R 2 A n-1 B n X 3nþ1 ) for High-Performance FETs

A, R-Cation Engineering
The monovalent cations in RP-phase perovskites consist of A-and R-site cations.The A-site cation is ideally small to fit within the octahedral framework, while the larger R-site cation separates the perovskite layers through van der Waals forces.Although the A-and R-site cations do not directly contribute to the electrical band structure, they indirectly affect crystal structural properties, such as tilting and distortion of the octahedral perovskite framework.Consequently, the chemical properties of A-and R-site cations effect their electrical properties, including orbital overlap, bandgap, and ultimately the charge transport.
The size of the A-site cations in the RP perovskite is constrained by the available space between the octahedral frameworks.Consequently, research has primarily focused on a narrow range of A cations, including MA þ , FA þ , guanidinium (GA þ ), Cs þ , and rubidium (Rb þ ).By considering the tolerance factor, it is possible to enhance charge transport through proper orbital overlap.This involves careful selection of A-site cations or their alloy combinations that ensure stable structural formation.Furthermore, the A-site cations can influence the stability of the perovskite layers.For instance, the MA þ -based perovskite is an ideal fit for the tolerance factor but exhibits poor thermal and ambient stability due to its hydrophilic, volatile, and acidic nature.Moisture-induced thermal decomposition can cause defects in crystals.Additionally, more acidic organic cations can actively react with halides, deprotonating perovskite crystals to produce gaseous byproducts such as hydrogen iodide.Organic cations with a less acidic and resistant nature have been explored to improve thermal stability, including replacing MA with FA or a mixture of FA and Cs.R-site cation modification enables a high degree of diversity and freedom in targeting the specific functionality of ligands.For phenylalkylammonium cations (C 6 H 5 (CH 2 ) n NH 3 þ , alkyl number (n) = 1-4), the shortest alkyl chain yields a 2D perovskite with a corner-sharing octahedral structure.As the alkyl chain length increases, the perovskite structure forms corner-and face-sharing octahedral sheets or even new compounds. [63]In other cases, introducing fluorine groups into phenethylammonium (x-FPEA, x = ortho, meta, or para) induces a substantial increase in the dipole moment of each molecule, imparting charge-separation and hole-transfer benefits. [64]Introducing fluorine at the para-position in the PEA molecule changes the orientation from edge-to-face to parallel slip stacking.This change facilitates better stacking of perovskite sheets and interlayer electronic coupling for charge transfer. [65]By careful selection and incorporation of specific functional groups as R-site cations can tailor the electrical, morphological, and stability properties of perovskite films.

B-Cation Selection
Selecting the B-site cation also requires careful consideration involving the overall electrical property and the environmental impact.Divalent metal cations with ns 2 orbitals, such as lead (Pb 2þ ), tin (Sn 2þ ), germanium (Ge 2þ ), bismuth (Bi 3þ ), and antimony (Sb 3þ ), are potential candidates for B sites that should fit within the proper tolerance and octahedral factor ranges with A-and X-site elements.Until now, high-performance electronic and optoelectronic devices frequently included lead, despite its toxicity.For commercial use of perovskite materials, there is an urgent need to substitute lead with safer alternatives.
The substitution of Pb in perovskite materials can be achieved with another 14-group element, such as Sn or Ge.Although Sn is a remarkable candidate for p-type perovskites with high hole density, its high tendency for oxidation remains as a significant issue.Moreover, the energy of the 5s orbital in Sn is higher than the 6s orbital in Pb, rendering the Sn-I bond more prone to breaking than the Pb-I bond. [66]Consequently, Sn vacancies are more likely to form in Sn-based perovskites.The higher Lewis acidity of SnI 2 compared to PbI 2 contributes to a rapid crystallization process, resulting in inhomogeneous film morphologies.Therefore, further research is required to enhance the stability and control the crystallization process of Sn introduction.For example, Liu et al. successfully improved the crystalline quality of CsSnI 3 -based perovskite films by introducing excess CsI and substituting 10 mol% of Sn with Pb to retard the conversion and crystallization of the perovskite phase. [62]The earth-abundant and less toxic Ge is another environmentally friendly B-site metal candidate. [67]However, similar to Sn-based perovskites, their stability issues are the primary obstacle hindering the widespread exploration of Ge-based perovskites.These should be addressed and investigated to enable the development of Ge-based perovskite FETs.
The 15-group elements Bi and Sb are potential candidates for perovskite B-site metal cations. [68]Bi and Sb possess three valence electrons in the outer shell and exhibit a trivalent form.These metals demonstrate diverse phase structures, including A 3 B 2 X 9 with 0D or 2D structures, A 2 B I B III X 6 (double perovskites) with 3D corner-sharing structure, and A a B b A aþ3b (rudorffite) with 3D edge-shared configurations of [AX 6 ] and [BX 6 ] octahedra.Bi and Sb demonstrate high chemical stability and low toxicity in their trivalent states.Furthermore, Bi and Sb ions have similar ionic radii and ns 2 np 0 electronic configurations to Pb and Sn, making them attractive potential metal elements.For charge transfer, the 2D A 3 B 2 X 9 and double-perovskite structures offer potential advantages for FETs due to their corner-sharing configurations.This promotes favorable orbital overlap between the s and p orbitals of the metal and halide atoms, respectively.Despite their promising characteristics, the exploration of FETs utilizing these materials remains limited.There is a single report on FETs based on double-perovskite structures derived from 15-group elements, specifically Cs 2 AgBiBr 6 . [69]No further investigations have been conducted on FETs based on perovskite structures composed of 15-group elements in the RP phase.

Effect of Halide Atoms for Charge Transport
Perovskite materials are fabricated using halide ions, predominantly iodide (I), bromide (Br), and chloride (Cl) ions.Incorporating fluoride (F) ions complicates stable perovskite phase formation owing to their small size, which hinders their proper integration within the perovskite structure.For example, SnF 2 is a ubiquitous additive in tin-based perovskites. [17]By introducing SnF 2 , detrimental effects, such as excess p doping, can be mitigated by tin compensation without directly integrating F ions into the perovskite crystal lattices.Moreover, SnF 2 contributed to the crystallization process, facilitating the formation of perovskite films with improved morphologies.This fluorinebased strategy enables exploring fluorine chemistry in perovskite materials to improve film formation.
Different halide species possess unique electronegativities and ionic sizes.These distinctions lead to varied bonding patterns and strengths when these halides interact with B-site metal cations in the perovskite structure.Such interactions can trigger structural relaxation or distortions, introducing strains into the lattice.Since the VBM consists of an antibonding state between the s orbital of the metal and the p orbital of the halide, an enhanced orbital overlap increases the valence band energy level by destabilizing this antibonding state.Conversely, the conduction band, Figure 12.Orientational analysis with different n-number based on n-butylammonium (BTA) and PEA RP-phase perovskite films.Grazing-incidence wide-angle X-ray scattering (GIWAXS) images of hot-cast/spin-cast a,d,e,h,i,l,m) PEA-based and b,c,f,g,j,k) BTA-based RP-phase perovskite in four different size regimes.Reproduced with permission. [70]Copyright 2018, Springer Nature.derived from the bonding state of p orbitals from metal and halide, likely reacts less to lattice structural changes than the valence band, which emerges from the s and p orbitals.Therefore, any structural shift in perovskite that boosts the orbital overlap between metal and halide reduces the bandgap, and the opposite also holds.Typically, the bandgap size adheres to the I < Br < Cl order among halide groups.Strain variations also influence orbital overlap, which adjusts the routes for charge movement within the lattice.Pairing specific halide attributes with B-site metal cations imparts distinct electronic and structural qualities to perovskite, shaping its charge-transport behavior.
Halide engineering using combinations of multiple halides has also been explored.Zhu et al. studied triple-halide MASn(I/Br/Cl) 3 perovskites. [61]They discovered that perovskite films with a mix of I and Cl anions struggle to integrate Cl anions into the perovskite octahedral structure due to the significant size difference between I and Cl ions.However, introducing Br anions to the mixture is pivotal, as Br anions act as hosts and enable the integration of Cl anions into the perovskite structure via the bridging effect of Br.Density-functional theory calculations on mixed-halide perovskites revealed that Br and Cl anions interact more strongly with iodine vacancies in the perovskite lattice than the I anion.Consequently, the introduction of small amounts of Br/Cl in iodine-based perovskites effectively passivates the vacancy defects in the films, leading to a reduction in the hysteresis characteristics of the FETs.

Effect of Perovskite Layer Number (n)
In solution process, the number n of the perovskite films can be adjusted by altering the precursor ratio for 2D and 3D perovskites.Increasing the proportion of 3D components favors the creation of high-n-number phases.Typically, perovskite films form with multiple phases of different compositions instead of a single phase with a specific stoichiometry, particularly evident in high-n-number phases. [70]Achieving precise control over the homogeneity of perovskite films with phase-pure RP-layered MHPs is challenging.
In RP perovskites, the number of layers balances electrical performance and stability.Typically, as the number of perovskite layers, n, grows, the area for charge carriers to move increases, leading to greater orbital overlap vertically.Additionally, with fewer organic cations at the R sites, charge transport encounters less resistance, enhancing charge flow.However, this reduction in R-site organic cations compromises the environmental stability of the perovskite materials under ambient conditions.Therefore, finding the optimal n to balance electrical performance with stability is crucial.
The orientation of perovskite films can differ based on the number of layers.At low n values, organic cations typically align parallel to the substrate.As n increases, the orientation of the cations becomes more random, leading to a mixture of perovskite sheets with different n values.The degree of this tendency varies based on the conformation of the A-site cation.Quintero-Bermudez et al. analyzed the orientation of RP perovskite films with varying n numbers, using BTA and PEA as A-site cations (Figure 12). [70]With BTA and PEA as cations, both perovskite films exhibited a parallel alignment with n = 1, forming a pure quantum well structure.The perovskite films with n = 2-4 were highly monodispersive, aligning mostly parallel to the substrate mixed with a random orientation.Films with n = 5-10 formed large wells perpendicular to the substrate.However, small wells with smaller n values parallel to the substrate were still observed.Finally, the films with n > 10 exhibited a preferred orientation perpendicular to the substrate with anisotropic bulk-like grains.Under these conditions, the PEA-based RP perovskite films no longer exhibit a small-n quantum well structure, while the BTAbased films still do.This is attributed to their smaller and less bulky structures, which nature can give high possibility of forming low n-wells and easily redistributing within the solvent better than with PEA.To enhance charge transport in FETs, it is crucial to consider the tendency of perovskite layers to align with a specific n number.Deposition methods that orderly arrange the perovskite layers can be optimized to achieve a desired pure phase.Solar cells and LED devices function well with a relatively random n distribution. [71]However, FET devices require higher phase purity, and optimizing phase and morphology control is crucial for superior performance.

Conclusion
This review has covered the fundamentals of RP perovskites, with a special focus on tin-based perovskites, including an indepth discussion of their chemical properties, recent studies, existing challenges, and outlook on their future applications.Among those, RP tin-based perovskite with n > 1 shows exceptional potential for p-type FETs, due to their tunability in chemical and electrical properties.Consequently, their excellent device performance and stability show high compatibility with commercial n-type oxide FETs in logic circuits, as recent studies have shown success in 2D and 2D/3D RP FETs, as well as their CMOS applications.To step further, the film deposition techniques should be explored in detail, and fundamental issues of environmental and operational stability should be challenged.
The most attractive feature of RP perovskite is its versatile designs.This review covers the design protocol of RP perovskites, discussed in terms of A, R cations, B cations, X halides, and n-layer numbers.Through precise modulation of R-site cation chemistry by incorporating specific functional groups, we can tailor desirable properties of the resultant perovskite film.The interaction strength and structural strain between the octahedral layers should be considered for optimal material design.Furthermore, exploration of the "n" layer numbers in the channel layer is pivotal for high-performance FETs.As the charge transport in FETs predominantly takes place at the dielectric/ semiconductor interface, instead of the bulk channel, the accurate "n" value has a significant correlation with the overall chargetransport layer.Identifying the optimal "n" value can facilitate the integration of R cations into perovskite structure for boosting device performance and stability.Finally, we emphasize thermal evaporation as a promising deposition method for RP perovskite FETs, offering enhanced homogeneity with precise control of the "n" value, for a clear understanding from "n" to overall charge transport.
Thus, there is an exciting future ahead, uncovering the correlation between chemical design-film growth-charge-carrier transport of the RP perovskites for successful electronic applications.

Figure 1 .
Figure 1.a) Basic structure of Ruddlesden-Popper (RP) perovskite.b) Orbital overlap between metal s orbital and halogen p orbital.Band structure of conventional c) oxide and d) tin/lead-based halide perovskites.

Figure 9 .
Figure 9. Complementary inverter of tin perovskite thin-film transistors (TFTs).a) Circuit diagram of the inverter with p-type PEA 2 SnI 4 and n-type indium gallium zinc oxide (IGZO).b) Transfer characteristics of PEA 2 SnI 4 and IGZO TFTs.c) Voltage-transfer curve, d) gain, and e) I DD of the complementary inverter with different V DD values (V DD = 20, 30, 40 V).f ) Noise margin extraction using maximum equal criterion method.(a-f)Reproduced with permission.[33]Copyright 2020, John Wiley and Sons.g) NAND and NOR optical images and h, i) their output characteristics with p-type Cs 0.1 FA 0.9 PEA 2 Sn 8 I 25 and n-type In 2 O 3 TFTs.

Figure 10 .
Figure 10.Environmental stability of 2D perovskite.a) Transfer curves of PEA 2 SnI 4 devices overtime.b) Transfer curves of (4Tm) 2 SnI 4 devices overtime.c) Evolution of hole mobilities of the BGTC FETs based on PEA 2 SnI 4 and (4Tm) 2 SnI 4 over long storage time.d) Evolution of X-Ray diffraction patterns of thin films in air.Top view of single inorganic layer structure in e) PEA 2 SnI 4 and g) (4Tm) 2 SnI 4 (bond lengths of Sn-I bond angles of Sn-I-Sn are marked).N-I distances marked with green lines in the crystal structures of f ) PEA 2 SnI 4 and h) (4Tm) 2 SnI 4 (purple dots: iodine, gray dots: tin).

Figure 11 .
Figure 11.Characterization of operational stability.a) Bias stress stabilities of PEA 2 SnI 4 FETs with different Lewis bases.Normalized I DD I D (t)/I D (0) (t is time and I D (0) is pristine I DD ) under a constant bias stress (V GS = V DS = À40 V for 2000 s).b) On/off switching sweep of the I pristine and I/Br/Cl FETs.c) Cycling test with transfer characteristics of I pristine and I/Br/Cl FETs (V DS = À12 V). d) Bias test with transfer characteristics of I pristine and I/Br/Cl FETs (V GS = V DS = À12 V).