Fluorinated Organic A-Cation Enabling High-Performance Hysteresis-Free 2D/3D Hybrid Tin Perovskite Transistors

2D tin-based perovskites have gained considerable attention for use in diverse optoelectronic applications, such as solar cells, lasers, and thin-film transistors (TFTs), owing to their good stability and optoelectronic properties. However, their intrinsic charge-transport properties are limited, and the insulating bulky organic ligands hinder the achievement of high-mobility electronics. Blending 3D counterparts into 2D perovskites to form 2D/3D hybrid structures is a synergistic approach that combine the high mobility and stability of 3D and 2D perovskites, respectively. In this study, reliable p-channel 2D/3D tin-based hybrid perovskite TFTs comprising 3D formamidinium tin iodide (FASnI₃) and 2D fluorinated 4-fluoro-phenethylammonium tin iodide ((4-FPEA)₂SnI₄) are reported. The optimized FPEA-incorporated TFTs show a high hole mobility of 12 cm² V⁻¹ s⁻¹, an on/off current ratio of over 10⁸, and a subthreshold swing of 0.09 V dec⁻¹ with negligible hysteresis. This excellent p-type characteristic is compatible with n-type metal-oxide TFT for constructing complementary electronics. Two procedures of antisolvent engineering and device patterning are further proposed to address the key concern of low-performance reproducibility of perovskite TFTs. This study provides an alternative A-cation engineering method for achieving high-performance and reliable tin-halide perovskite electronics.     

A Large Bandgap Organic Salt Dopant for Sn-Based Perovskite Thin-Film Transistor

Metal halide perovskite optoelectronic devices have made significant progress over the past few years, but precise control of charge carrier density through doping is essential for optimizing these devices. In this study, the potential of using an organic salt, N,N-dimethylanilinium tetrakis(pentafluorophenyl)borate, as a dopant for Sn-based perovskite devices, is explored. Under optimized conditions, the thin film transistors based on the doped 2D/3D perovskite PEAFASnI₃ demonstrate remarkable improvement in hole mobility, reaching 7.45 cm²V⁻¹s⁻¹ with a low subthreshold swing and the smallest sweep hysteresis (ΔV_hysteresis = 2.27 V) and exceptional bias stability with the lowest contact resistance (2.2 kΩ cm). The bulky chemical structure of the dopant prevents it from penetrating the perovskite lattice and also surface passivation against Sn oxidation due to its hydrophobic nature surface. This improvement is attributed to the bifunctional effect of the dopant, which simultaneously passivates defects and improves crystal orientation. These findings provide new insights into potential molecular dopants that can be used in metal halide perovskite devices.     

Fully Transparent p‐MoTe2 2D Transistors Using Ultrathin MoOx/Pt Contact Media for Indium‐Tin‐Oxide Source/Drain

Since transition metal dichalcogenide (TMD) semiconductors are found as 2D van der Waals materials with a discrete energy bandgap, many 2D‐like thin field effect transistors (FETs) and PN diodes are reported as prototype electrical and optoelectronic devices. As a potential application of display electronics, transparent 2D FET devices are also reported recently. Such transparent 2D FETs are very few in report, yet no p‐type channel 2D‐like FETs are seen. Here, 2D‐like thin transparent p‐channel MoTe₂ FETs with oxygen (O₂) plasma‐induced MoO_x/Pt/indium‐tin‐oxide (ITO) contact are reported for the first time. For source/drain contact, 60 s short O₂ plasma and ultrathin Pt‐deposition processes on MoTe₂ surface are sequentially introduced before ITO thin film deposition and patterning. As a result, almost transparent 2D FETs are obtained with a decent mobility of ≈5 cm² V^?1 s^?1, a high ON/OFF current ratio of ≈10⁵, and 70% transmittance. In particular, for normal MoTe₂ FETs without ITO, O₂ plasma process greatly improves the hole injection efficiency and device mobility (≈60 cm² V^?1 s^?1), introducing ultrathin MoO_x between Pt source/drain and MoTe₂. As a final device application, a photovoltaic current modulator, where the transparent FET stably operates as gated by photovoltaic effects, is integrated.     

High‐Performance,Air‐Stable,Top‐Gate,p‐Channel WSe2 Field‐Effect Transistor with Fluoropolymer Buffer Layer

High‐performance, air‐stable, p‐channel WSe₂ top‐gate field‐effect transistors (FETs) using a bilayer gate dielectric composed of high‐ and low‐k dielectrics are reported. Using only a high‐k Al₂O₃ as the top‐gate dielectric generally degrades the electrical properties of p‐channel WSe₂, therefore, a thin fluoropolymer (Cytop) as a buffer layer to protect the 2D channel from high‐k oxide forming is deposited. As a result, a top‐gate‐patterned 2D WSe₂ FET is realized. The top‐gate p‐channel WSe₂ FET demonstrates a high hole mobility of 100 cm²_­ V^?1 s^?1 and a I_ON/I_OFF ratio > 10⁷ at low gate voltages (V_GS ca. ?4 V) and a drain voltage (V_DS) of ?1 V on a glass substrate. Furthermore, the top‐gate FET shows a very good stability in ambient air with a relative humidity of 45% for 7 days after device fabrication. Our approach of creating a high‐k oxide/low‐k organic bilayer dielectric is advantageous over single‐layer high‐k dielectrics for top‐gate p‐channel WSe₂ FETs, which will lead the way toward future electronic nanodevices and their integration.     

Low temperature,solution-processed ambipolar field-effect transistors based on polymer/self-assembled monolayer modified InOx hybrid structures for balanced hole and electron mobilities exceeding 1 cm2 V−1 s−1

Ambipolar field-effect transistors (FETs) based on solution-processed organic-inorganic bilayer structures were investigated. An amorphous indium oxide (InO_x) film, as the n-type semiconducting layer, was prepared with an environmentally friendly method and annealed at a low temperature; and a low band-gap (LBG) donor–acceptor (D–A) conjugated polymer, FBT-Th₄(1,4), was spin-coated on the InO_x film as the p-type semiconducting layer. To improve the p-type mobility, a self-assembled monolayer (SAM) of octadecyl-phosphonic acid was introduced to modify the surface of InO_x. The ambipolar FETs showed high and well-balanced hole and electron mobilities of 1.1 and 1.5 cm² V⁻¹ s⁻¹, respectively. Furthermore we found that ambipolar FETs could be integrated into functional complementary metal oxide semiconductor (CMOS)-like inverters.     

Orientation Regulation of Tin‐Based Reduced‐Dimensional Perovskites for Highly Efficient and Stable Photovoltaics

Tin‐based perovskites have exhibited high potential for efficient photovoltaics application due to their outstanding optoelectrical properties. However, the extremely undesired instabilities significantly hinders their development and further commercialization process. A novel tin‐based reduced‐dimensional (quasi‐2D) perovskites is reported here by using 5‐ammoniumvaleric acid (5‐AVA⁺) as the organic spacer. It is demonstrated that by introducing appropriate amount of ammonium chloride (NH₄Cl) as additive, highly vertically oriented tin‐based quasi‐2D perovskite films are obtained, which is proved through the grazing incidence wide‐angle X‐ray scattering characterization. In particular, this approach is confirmed to be a universal method to deliver highly vertically oriented tin‐based quasi‐2D perovskites with various spacers. The highly ordered vertically oriented perovskite films significantly improve the charge collection efficiency between two electrodes. With the optimized NH₄Cl concentration, the solar cells employing quasi‐2D perovskite, AVA₂FA_n?1Sn_nI_3n+1 (<n> = 5), as light absorbers deliver a power conversion efficiency up to 8.71%. The work paves the way for further employing highly vertically oriented tin‐based quasi‐2D perovskite films for highly efficient and stable photovoltaics.     

High Performance β-Ga2O3 Schottky Barrier Transistors with Large Work Function TMD Gate of NbS2 and TaS2

Gallium trioxide, β-Ga₂O₃, has been recently studied due to its promising semiconducting properties as active material in transistors or Schottky diodes. Transistors with β-Ga₂O₃ channels are mostly metal oxide field effect transistors (MOSFET), and they show very negative threshold voltages (V_th) in general. Metal semiconductor field effect transistors (MESFETs) with top gate are also reported with less negative V_th. Still, β-Ga₂O₃ MESFETs are only a few. Here, bottom gate architecture β-Ga₂O₃ MESFETs using transition metal dichalcogenide (TMD) NbS₂ and TaS₂ are reported. Due to the large work functions of those metallic TMDs, the MESFETs display minimum subthreshold swing of 61 mV dec⁻¹, small V_th of −1.2 V, minimum OFF I_D of ≈100 fA, and maximum ON/OFF current ratio of ≈10⁸. Both β-Ga₂O₃ Schottky diodes with TaS₂ and NbS₂ display good junction stability even after 300 °C measurements in 10 mTorr vacuum. When the β-Ga₂O₃ MESFET with TaS₂ gate is integrated as a switching FET into an organic light emitting diode (OLED) circuit, it demonstrates long-term leakage endurance performance, maintaining an OLED brightness higher than 58% of the initial intensity after 100 s passes since the ON-switching point, which is even superior to the performance of conventional a-IGZO MOSFET switch.     

Fully Printed High-Performance Quasi-Two-Dimensional Perovskite Solar Cells via Multifunctional Interfacial Engineering

Planar n–i–p carbon perovskite solar cells (PSCs) with a hole transport layer that can be fabricated at low temperatures at low-cost exhibit great potential for large-scale manufacturing. Moreover, 2D perovskites have attracted considerable attention owing to their higher stability. In this work, scalable and highly efficient fully printed large-area carbon electrode-based 2D perovskite modules are reported through the insertion of a thin naphthaleneimide derivative (CATNI)-based interfacial layer between tin (IV) oxide and the perovskite layer. The results show that this facilitates the formation of the interfacial contact, suppresses energy losses, and substantially improves the performance parameters of the PSCs, especially their V_OC value. A significantly enhanced V_OC of 1.13 V is achieved resulting in the device PCE value reaching over 18%, which is one of the highest reported for fully printed PSCs so far. It is found that with the deployment of this CATNI-based interfacial layer, a more efficient carrier extraction is achieved. This ultimately contributed to enhanced spectral response as well as improved V_OC for these carbon electrodes based on fully printed devices. Finally, the carbon-perovskite solar modules (carbon-PSMs) are fabricated on ITO glass substrates with dimensions of 5.0 × 5.0 cm. These prepared modules exhibited outstanding photovoltaic performance with the highest PCE value of over 14.6%.     

Probing into Reverse Bias Dark Current in Perovskite Photodiodes: Critical Role of Surface Defects

Minimizing reverse bias dark current density (J_dark) while retaining high external quantum efficiency is crucial for promising applications of perovskite photodiodes, and it remains challenging to elucidate the ultimate origin of J_dark. It is demonstrated in this study that the surface defects induced by iodine vacancies are the main cause of J_dark in perovskite photodiodes. In a targeted way, the surface defects are thoroughly passivated through a simple treatment with butylamine hydroiodide to form ultrathin 2D perovskite on its 3D bulk. In the passivated perovskite photodiodes, J_dark as low as 3.78 × 10^-10 A cm^-2 at -0.1 V is achieved, and the photoresponse is also enhanced, especially at low light intensities. A combination of the two improvements realizes high specific detectivity up to 1.46 × 10¹² Jones in the devices. It is clarified that the trap states induced by the surface defects can not only raise the generation-recombination current density associated with the Shockley–Read–Hall mechanisms in the dark (increasing J_dark), but also provide additional carrier recombination paths under light illumination (decreasing photocurrent). The critical role of surface defects on J_dark of perovskite photodiodes suggests that making trap-free perovskite thin films, for example, by fine preparation and/or surface engineering, is a top priority for high-performance perovskite photodiodes.     

Non‐Lithographic Fabrication of All‐2D α‐MoTe2 Dual Gate Transistors

As one of the emerging new transition‐metal dichalcogenides materials, molybdenum ditelluride (α‐MoTe₂) is attracting much attention due to its optical and electrical properties. This study fabricates all‐2D MoTe₂‐based field effect transistors (FETs) on glass, using thin hexagonal boron nitride and thin graphene in consideration of good dielectric/channel interface and source/drain contacts, respectively. Distinguished from previous works, in this study, all 2D FETs with α‐MoTe₂ nanoflakes are dual‐gated for driving higher current. Moreover, for the present 2D dual gate FET fabrications on glass, all thermal annealing and lithography processes are intentionally exempted for fully non‐lithographic method using only van der Waal's forces. The dual‐gate MoTe₂ FET displays quite a high hole and electron mobility over ≈20 cm² V^?1 s^?1 along with ON/OFF ratio of ≈10⁵ in maximum as an ambipolar FET and also demonstrates high drain current of a few tens‐to‐hundred μA at a low operation voltage. It appears promising enough to drive organic light emitting diode pixels and NOR logic functions on glass.     

Self-catalyzed molecular beam epitaxy growth and their optoelectronic properties of vertical GaAs nanowires on Si(111)

Self-assembled GaAs nanowires were grown by molecular beam epitaxy (MBE) on un-pretreated Si(111) substrates under different As₄/Ga flux ratios (V/III ratios). It has been found that the fraction of vertical wires would be nearly 100% when the As₄/Ga ratio arrives 90. The transmission electron microscopy (TEM) and micro-photoluminescence (PL) spectra results have indicated that the GaAs nanowires grown under a larger V/III ratio (90) have a pure ZB structure. Field-effect transistors (FET) based on single nanowire were fabricated with GaAs nanowires grown under the larger V/III ratio (90). The characteristics of the FET reveal a hole concentration of 3.919×10¹⁷ cm⁻³ and a hole mobility of 0.417 cm² V⁻¹s⁻¹. Photodetectors based on single nanowire and multiple nanowires structure with a metal-semiconductor-metal (MSM) electrode configuration have been proposed and demonstrated. All the photodetectors operating at room temperature exhibit good photoconductive performance, excellent stability, reproducibility and superior peak responsivity (87.67 A/W under 5 V for single nanowire photodetector).     

Anti-Dissociation Passivation via Bidentate Anchoring for Efficient Carbon-Based CsPbI2.6Br0.4 Solar Cells

Molecular passivation on perovskite surface is an effective strategy to inhibit surface defect-assisted recombination and reduce nonradiative recombination loss in perovskite solar cells (PSCs). However, the majority of passivating molecules bind to perovskite surface through weak interactions, resulting in weak passivation effects and susceptible to interference from various factors. Especially in carbon-based perovskite solar cells (C-PSCs), the molecular passivation effect is more susceptible to disturbance in subsequent harsh preparation of carbon electrodes via blade-coating route. Herein, bidentate ligand 2,2′-Bipyridine (2Bipy) is explored to passivate surface defects of CsPbI_2.6Br_0.4 perovskite films. The results indicate that compared with monodentate pyridine (Py), bidentate 2Bipy shows a stronger chelation with uncoordinated Pb(II) defects and exhibits a greater passivation effect on perovskite surface. As a result, 2Bipy-modified perovskite films display a significantly boosted photoluminescence lifetime, accompanied by excellent anchoring stability and anti-dissociation of passivating molecules. Meanwhile, the moisture resistance of the 2Bipy-modified perovskite films is also significantly enhanced. Consequently, the efficiency of C-PSCs is improved to 16.57% (J_sc = 17.16 mA cm⁻², V_oc = 1.198 V, FF = 80.63%). As far as it is known, this value represents a new record efficiency for hole transport material-free inorganic C-PSCs.     

Degradation of n-channel a-Si:H/nc-Si:H bilayer thin-film transistors under DC electrical stress

Bottom-gated n-channel thin-film transistors (TFTs) were fabricated, using as channel material hydrogenated amorphous silicon (a-Si:H)/nanocrystalline silicon (nc-Si:H) bilayers, deposited at 230 °C by plasma-enhanced chemical vapor deposition, and SiN_x as gate dielectric. The stability of these devices is investigated under three bias stress conditions: (i) gate bias stress (V_G = 25 V, V_D = 0), (ii) on-state bias stress (V_G = 25 V, V_D = 20 V) and (iii) off-state bias stress (V_G = −25 V, V_D = 20 V). It is found that the TFT degradation mechanisms are strongly dependent on the bias stress conditions, involving generation of deep and tail states in the active area of the channel material, carrier injection (electrons or holes) within the gate insulator and generation of donor trap states at the gate insulator/channel interface. The common features and the differences observed in the degradation behaviour under the different bias stress conditions are discussed.     

Oriented Crystallization of Mixed‐Cation Tin Halides for Highly Efficient and Stable Lead‐Free Perovskite Solar Cells

As the most promising lead‐free branch, tin halide perovskites suffer from the severe oxidation from Sn²⁺ to Sn⁴⁺, which results in the unsatisfactory conversion efficiency far from what they deserve. In this work, by facile incorporation of methylammonium bromide in composition engineering, formamidinium and methylammonium mixed cations tin halide perovskite films with ultrahighly oriented crystallization are synthesized with the preferential facet of (001), and that oxidation is suppressed with obviously declined trap density. MA⁺ ions are responsible for that impressive orientation while Br^‐ ions account for their bandgap modulation. Depending on high quality of the optimal MA_0.25FA_0.75SnI_2.75Br_0.25 perovskite films, their device conversion efficiency surges to 9.31% in contrast to 5.02% of the control formamidinium tin triiodide perovskite (FASnI₃) device, along with almost eliminated hysteresis. That also results in the outstanding device stability, maintaining above 80% of the initial efficiency after 300 h of light soaking while the control FASnI₃ device fails within 120 h. This paper definitely paves a facile and effective way to develop high‐efficiency tin halide perovskites solar cells, optoelectronic devices, and beyond.     

Modeling and Understanding the Compact Performance of h-BN Dual-Gated ReS2 Transistor

In this study, high-performance few-layered ReS₂ field-effect transistors (FETs), fabricated with hexagonal boron nitride (h-BN) as top/bottom dual gate dielectrics, are presented. The performance of h-BN dual gated ReS₂ FET having a trade-off of performance parameters is optimized using a compact model from analytical choice maps, which consists of three regions with different electrical characteristics. The bottom h-BN dielectric has almost no defects and provides a physical distance between the traps in the SiO₂ and the carriers in the ReS₂ channel. Using a compact analyzing model and structural advantages, an excellent and optimized performance is introduced consisting of h-BN dual-gated ReS₂ with a high mobility of 46.1 cm² V⁻¹ s⁻¹, a high current on/off ratio of ≈10⁶, a subthreshold swing of 2.7 V dec⁻¹, and a low effective interface trap density (N_t,eff) of 7.85 × 10¹⁰ cm⁻² eV⁻¹ at a small operating voltage (<3 V). These phenomena are demonstrated through not only a fundamental current–voltage analysis, but also technology computer aided design simulations, time-dependent current, and low-frequency noise analysis. In addition, a simple method is introduced to extract the interlayer resistance of ReS₂ channel through Y-function method as a function of constant top gate bias.     

Quinoxaline-Based X-Shaped Sensitizers as Self-Assembled Monolayer for Tin Perovskite Solar cells

Four X-shaped quinoxaline-based organic dyes, PQx (1), TQx, (2), PQxD (3), and TQxD (4) (D = dye sensitizers) are developed and served as p-type self-assemble monolayer (SAM) for tin perovskite solar cells (TPSC). Thermal, optical, and electrochemical properties of these SAMs are thoroughly investigated and characterized. Tin perovskite layers are successfully deposited on these four SAM surfaces according to a two-step approach and the devices exhibit power conversion efficiency in the order of TQxD (8.3%) > TQx (8.0%) > PQxD (7.1%) > PQx (6.1%). With thiophene π-extended conjugation unit in SAM structure, TQxD (4) exhibits the highest hole extraction rates, greatest hole mobilities, and slowest charge recombination to achieve great device performance of 8.3%, which is the current best result for SAM-based TPSC ever reported. Furthermore, all devices except PQx shows great enduring stability for the performance retaining ≈90% of their original values for shelf storage over 1600 h.     

Fabrication of single crystal field-effect transistors with phenacene-type molecules and their excellent transistor characteristics

Single crystal field-effect transistors (FETs) using [6]phenacene and [7]phenacene show p-channel FET characteristics. Field-effect mobilities, μs, as high as 5.6 × 10^?1 cm² V^?1 s^?1 in a [6]phenacene single crystal FET with an SiO₂ gate dielectric and 2.3 cm² V^?1 s^?1 in a [7]phenacene single crystal FET were recorded. In these FETs, 7,7,8,8-tetracyanoquinodimethane (TCNQ) was inserted between the Au source/drain electrodes and the single crystal to reduce hole-injection barrier heights. The μ reached 3.2 cm² V^?1 s^?1 in the [7]phenacene single crystal FET with a Ta₂O₅ gate dielectric, and a low absolute threshold voltage |V_TH| (6.3 V) was observed. Insertion of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ) in the interface produced very a high μ value (4.7–6.7 cm² V^?1 s^?1) in the [7]phenacene single crystal FET, indicating that F₄TCNQ was better for interface modification than TCNQ. A single crystal electric double-layer FET provided μ as high as 3.8 × 10^?1 cm² V^?1 s^?1 and |V_TH| as low as 2.3 V. These results indicate that [6]phenacene and [7]phenacene are promising materials for future practical FET devices, and in addition we suggest that such devices might also provide a research tool to investigate a material’s potential as a superconductor and a possible new way to produce the superconducting state.     

Characteristics of tetracene-based field-effect transistors on pretreated surfaces

Tetracene-based organic thin-film transistors (OTFTs) were prepared using a neutral cluster beam deposition (NCBD) method. The effect of surface modification with an amphiphilic surfactant, octadecyltrichlorosilane (OTS), on the formation of thin films and the geometric influence of channel length and width on the transistor characteristics were systematically examined. The estimated trap density and temperature-dependence of the field-effect mobility in the range of 10–300 K demonstrated that surfactant pretreatment decreased the total trap density and activation energy for hole-transport by reducing structural disorder in the active layer. In particular, the room-temperature hole mobilities of 0.162 and 0.252 cm²/Vs for untreated and OTS-pretreated devices were among the best to date for polycrystalline tetracene-based transistors using SiO₂ gate dielectric layers without any thermal post-treatment.     

Spatial Localization of Carrier Traps in 4H-SiC MOSFET Devices Using Thermally Stimulated Current

Carrier traps in 4H-SiC metal–oxide–semiconductor (MOS) capacitor and transistor devices were studied using the thermally stimulated current (TSC) method. TSC spectra from p-type MOS capacitors and n-channel MOS field-effect transistors (MOSFETs) indicated the presence of oxide traps with peak emission around 55 K. An additional peak near 80 K was observed due to acceptor activation and hole traps near the interface. The physical location of the traps in the devices was deduced using a localized electric field approach. The density of hole traps contributing to the 80-K peak was separated from the acceptor trap density using a gamma-ray irradiation method. As a result, hole trap density of N _t,hole = 2.08 × 10¹⁵ cm⁻³ at 2 MV/cm gate field and N _t,hole = 2.5 × 10¹⁶ cm⁻³ at 4.5 MV/cm gate field was extracted from the 80-K TSC spectra. Measurements of the source-body n ⁺–p junction suggested the presence of implantation damage in the space-charge region, as well as defect states near the n ⁺ SiC substrate.     

All-solid solar cells with hybrid perovskite absorbers and graphene electron transport layers

This work reports the perovskite/titanium dioxide (TiO₂)heterojunction solid state solar cells (SSSCs) with a hole transport material (HTM) and graphene electron transport layer. The effects of a nanostructure CH₃NH₃PbI₃ perovskite thin film, the HTM, and graphene electron transport layer in SSSC structure were examined. The SSSCs prepared with the optimal parameter exhibited a short-circuit current density (J_SC), open-circuit voltage (V_OC), and power conversion efficiency (η) of 17.89 mA/cm², 0.89 V, and 6.91%, respectively. Obvious improvements in power conversion efficiency of the SSSCs were observed by using the HTM and graphene electron transport layer. The HTM and graphene thin films provide a great hole and electron transfer channel for the photogenerated carriers to external circuit, respectively.