The Effect of Thermal Annealing on Charge Transport in Organolead Halide Perovskite Microplate Field-Effect Transistors

Enhancing Stability and Performance in Tin-Based Perovskite Field-Effect Transistors Through Hydrogen Bond Suppression of Organic Cation Migration

Ion migration poses a substantial challenge in perovskite transistors, exerting detrimental effects on hysteresis and operational stability. This study focuses on elucidating the influence of ion migration on the performance of tin-based perovskite field-effect transistors (FETs). It is revealed that the high background carrier density in FASnI3 FETs arises not only from the oxidation of Sn2+ but also from the migration of FA+ ions. The formation of hydrogen bonding between FA+ and F- ions efficiently inhibits ion migration, leading to a reduction in background carrier density and an improvement in the operational stability of the transistors. The strategy of hydrogen bond is extended to fluorine-substituted additives to improve device performance. The incorporation of 4-fluorophenethylammonium iodide additives into FETs significantly minimizes the shift of turn-on voltage during cyclic measurements. Notably, an effective mobility of up to 30 cm2 V-1 s-1 with an Ion/off ratio of 107 is achieved. These findings hold promising potential for advancing tin-based perovskite technology in the field of electronics.

Evaluation of Machine Learning Interatomic Potentials for Gold Nanoparticles-Transferability towards Bulk

We analyse the efficacy of machine learning (ML) interatomic potentials (IP) in modelling gold (Au) nanoparticles. We have explored the transferability of these ML models to larger systems and established simulation times and size thresholds necessary for accurate interatomic potentials. To achieve this, we compared the energies and geometries of large Au nanoclusters using VASP and LAMMPS and gained better understanding of the number of VASP simulation timesteps required to generate ML-IPs that can reproduce the structural properties. We also investigated the minimum atomic size of the training set necessary to construct ML-IPs that accurately replicate the structural properties of large Au nanoclusters, using the LAMMPS-specific heat of the Au147 icosahedral as reference. Our findings suggest that minor adjustments to a potential developed for one system can render it suitable for other systems. These results provide further insight into the development of accurate interatomic potentials for modelling Au nanoparticles through machine learning techniques.

The important role of surface charge on a new mechanism of nitrogen reduction

The electrocatalytic nitrogen reduction reaction (NRR) is a green and sustainable approach for producing ammonia. Low-cost carbon-based materials are promising catalysts for the electrochemical NRR. Among them, Cu-N₄-graphene is a unique catalytic substrate. Its catalytic performance for the NRR has remained unclear as N₂ can only be physisorbed on such a substrate. In this work, we focus on the influence of an electronic environment on the electrocatalytic NRR. DFT computations reveal that the NN bond can be effectively activated at a surface charge density of -1.88 × 10¹⁴ e cm^-2 on Cu-N₄-graphene and further the NRR proceeds via an alternating hydrogenation pathway. This work offers a new insight into the mechanism of the electrocatalytic NRR and emphasizes the importance of environmental charges in the electrocatalytic process of the NRR.

Sn-Based Perovskite Halides for Electronic Devices

Because of its less toxicity and electronic structure analogous to that of lead, tin halide perovskite (THP) is currently one of the most favorable candidates as an active layer for optoelectronic and electric devices such as solar cells, photodiodes, and field-effect transistors (FETs). Promising photovoltaics and FETs performances have been recently demonstrated because of their desirable electrical and optical properties. Nevertheless, THP's easy oxidation from Sn²⁺ to Sn⁴⁺ , easy formation of tin vacancy, uncontrollable film morphology and crystallinity, and interface instability severely impede its widespread application. This review paper aims to provide a basic understanding of THP as a semiconductor by highlighting the physical structure, energy band structure, electrical properties, and doping mechanisms. Additionally, the key chemical instability issues of THPs are discussed, which are identified as the potential bottleneck for further device development. Based on the understanding of the THPs properties, the key recent progress of THP-based solar cells and FETs is briefly discussed. To conclude, current challenges and perspective opportunities are highlighted.

Recent Criterion on Stability Enhancement of Perovskite Solar Cells

Perovskite solar cells (PSCs) have captured the attention of the global energy research community in recent years by showing an exponential augmentation in their performance and stability. The supremacy of the light-harvesting efficiency and wider band gap of perovskite sensitizers have led to these devices being compared with the most outstanding rival silicon-based solar cells. Nevertheless, there are some issues such as their poor lifetime stability, considerable J–V hysteresis, and the toxicity of the conventional constituent materials which restrict their prevalence in the marketplace. The poor stability of PSCs with regard to humidity, UV radiation, oxygen and heat especially limits their industrial application. This review focuses on the in-depth studies of different direct and indirect parameters of PSC device instability. The mechanism for device degradation for several parameters and the complementary materials showing promising results are systematically analyzed. The main objective of this work is to review the effectual strategies of enhancing the stability of PSCs. Several important factors such as material engineering, novel device structure design, hole-transporting materials (HTMs), electron-transporting materials (ETMs), electrode materials preparation, and encapsulation methods that need to be taken care of in order to improve the stability of PSCs are discussed extensively. Conclusively, this review discusses some opportunities for the commercialization of PSCs with high efficiency and stability.

Review on Perovskite Semiconductor Field-Effect Transistors and Their Applications

Perovskite materials are considered as the most alluring successor to the conventional semiconductor materials to fabricate solar cells, light emitting diodes and electronic displays. However, the use of the perovskite semiconductors as a channel material in field effect transistors (FET) are much lower than expected due to the poor performance of the devices. Despite low attention, the perovskite FETs are used in widespread applications on account of their unique opto-electrical properties. This review focuses on the previous works on perovskite FETs which are summarized into tables based on their structures and electrical properties. Further, this review focuses on the applications of perovskite FETs in photodetectors, phototransistors, light emitting FETs and memory devices. Moreover, this review highlights the challenges faced by the perovskite FETs to meet the current standards along with the future directions of these FETs. Overall, the review summarizes all the available information on existing perovskite FET works and their applications reported so far.

Moisture Stability of Perovskite Solar Cells Processed in Supercritical Carbon Dioxide

Performance degradation under environmental conditions currently limits the practical utility of perovskite-based solar cells. The moisture stability of CH3NH3PbI3 perovskite films and solar cells was measured during exposure to three different levels of relative humidity. The films were crystallized at two different temperatures with and without simultaneous exposure to supercritical carbon dioxide. The film crystallinity, optical absorption, and device photoconversion efficiency was measured over time for three relative humidity levels and both crystallization methods. It was determined that film crystallization in supercritical CO2 resulted in significant improvement in moisture stability for films processed at 50 °C, but negligible improvement in stability for films processed at 100 °C.

Time-Dependent Field Effect in Three-Dimensional Lead-Halide Perovskite Semiconductor Thin Films

Charge transport in three-dimensional metal-halide perovskite semiconductors is due to a complex combination of ionic and electronic contributions, and its study is particularly relevant in light of their successful applications in photovoltaics as well as other opto- and microelectronic applications. Interestingly, the observation of field effect at room temperature in transistors based on solution-processed, polycrystalline, three-dimensional perovskite thin films has been elusive. In this work, we study the time-dependent electrical characteristics of field-effect transistors based on the model methylammonium lead iodide semiconductor and observe the drastic variations in output current, and therefore of apparent charge carrier mobility, as a function of the applied gate pulse duration. We infer this behavior to the accumulation of ions at the grain boundaries, which hamper the transport of carriers across the FET channel. This study reveals the dynamic nature of the field effect in solution-processed metal-halide perovskites and offers an investigation methodology useful to characterize charge carrier transport in such emerging semiconductors.

Oxide Passivation of Halide Perovskite Resistive Memory Device: A Strategy for Overcoming Endurance Problem

Owing to a low operation voltage, high on/off ratio, and tunable band gap, halide perovskites (HPs) are being conceived as an alternative to oxide or chalcogenide materials in resistive-switching (RS) memory devices. However, the HP-based RS memory devices face problems such as short endurance, low retention, and device stability. Herein, the oxide-passivated HP devices were fabricated by hybridizing the oxide sol-gel and halide adduct methods. The silicon oxide (SiO₂)-passivation enhanced the device properties with an endurance of 6000 cycles and retention of 1.8 × 10⁴ s. The study of activation energy using ionic conductivity and time-of-flight secondary-ion mass spectroscopy demonstrated that the migration path of the Ag ions is well-controlled by the SiO₂ passivation layer. Various oxides were used as passivation materials. Especially, the zirconium oxide-passivated devices exhibit excellent properties with an endurance of 57 000 cycles and retention of 7.8 × 10⁴ s. The high cohesive energy of oxides effectively increased the formation voltage by retarding the Ag-ion migration, leading to the improved endurance properties of the devices. This paper proposes a strategy for significantly improving the low endurance property of HP-based RS memory devices using the oxide passivation technology.

Two-Dimensional Hybrid Perovskite-Based van der Waals Heterostructures

Two-dimensional (2D) hybrid perovskites, as newly emerging materials, have become the center of attention in optoelectronic fields within a few years because of their unique optoelectronic properties, including tunable bandgap, long carrier diffusion length, and high absorption coefficient. 2D perovskite-based van der Waals heterostructures via integration of 2D perovskites with other layered materials provide new platforms for many optoelectronic devices with prominent performance, such as photodetectors, light-emitting diodes (LEDs), and phototransistors. In this Perspective, the unique properties of 2D perovskites will be first introduced to explore why this material is attractive and popular. Subsequently, various types of heterostructures based on 2D perovskites will be illustrated, including the heterostructure construction approaches as well as their optical and optoelectronic applications. Finally, potential research directions based on 2D perovskite heterostructures are also proposed.

Superior photo-carrier diffusion dynamics in organic-inorganic hybrid perovskites revealed by spatiotemporal conductivity imaging

The outstanding performance of organic-inorganic metal trihalide solar cells benefits from the exceptional photo-physical properties of both electrons and holes in the material. Here, we directly probe the free-carrier dynamics in Cs-doped FAPbI₃ thin films by spatiotemporal photoconductivity imaging. Using charge transport layers to selectively quench one type of carriers, we show that the two relaxation times on the order of 1 μs and 10 μs correspond to the lifetimes of electrons and holes in FACsPbI₃, respectively. Strikingly, the diffusion mapping indicates that the difference in electron/hole lifetimes is largely compensated by their disparate mobility. Consequently, the long diffusion lengths (3~5 μm) of both carriers are comparable to each other, a feature closely related to the unique charge trapping and de-trapping processes in hybrid trihalide perovskites. Our results unveil the origin of superior diffusion dynamics in this material, crucially important for solar-cell applications.

State of the Art and Prospects for Halide Perovskite Nanocrystals

Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

Halide Perovskites: A New Era of Solution-Processed Electronics

Organic-inorganic mixed halide perovskites have emerged as an excellent class of materials with a unique combination of optoelectronic properties, suitable for a plethora of applications ranging from solar cells to light-emitting diodes and photoelectrochemical devices. Recent works have showcased hybrid perovskites for electronic applications through improvements in materials design, processing, and device stability. Herein, a comprehensive up-to-date review is presented on hybrid perovskite electronics with a focus on transistors and memories. These applications are supported by the fundamental material properties of hybrid perovskite semiconductors such as tunable bandgap, ambipolar charge transport, reasonable mobility, defect characteristics, and solution processability, which are highlighted first. Then, recent progresses on perovskite-based transistors are reviewed, covering aspects of fabrication process, patterning techniques, contact engineering, 2D versus 3D material selection, and device performance. Furthermore, applications of perovskites in nonvolatile memories and artificial synaptic devices are presented. The ambient instability of hybrid perovskites and the strategies to tackle this bottleneck are also discussed. Finally, an outlook and opportunities to develop perovskite-based electronics as a competitive and feasible technology are highlighted.

Wide-Band-Gap Mixed-Halide 3D Perovskites: Electronic Structure and Halide Segregation Investigation

Mixed-halide organolead perovskites (MAPbX 3 ) are of great interest for both single-junction and tandem solar cells because of their wide band gap. In this study, we investigate the family of mixed iodide/bromide (I/Br) and bromide/chloride (Br/Cl) perovskites, revealing the strong influence of halide substitution on electronic properties, morphology, film composition, and phase segregation. A qualitative blue shift with the I → Br → Cl series was observed, with the resulting optical absorption ranging from 420 to 800 nm covering nearly the entire visible region. The ionization potential increases from ≈6.0 to ≈7.0 eV as the halide composition changes from I to Br. However, with Cl components, the valence band position shows little variation, while the conduction band minimum shifts to a lower value with increasing Cl concentration. By collecting XPS spectra as a function of the sputtering depth, we observed halide segregation in both I/Br and Br/Cl mixed-halide perovskite films, where the large halide ion (I in the I/Br mix or Br in the Br/Cl mix) is preferentially found on the surface of the film and the smaller halide ion (Br in the I/Br mix or Cl in the Br/Cl mix) accumulates at the bottom of the film. These differences in the band structure, electronic properties, morphology, and film composition impacted the device performance: a decreased short-circuit current density and increased open-circuit voltage were observed with the I → Br → Cl series. This study highlights the role of halides in the band structure and phase segregation in mixed-halide perovskite solar cells and provides a foundational framework for future optoelectronic applications of these materials.

Microfluidic Approach for Lead Halide Perovskite Flexible Phototransistors

Lead halide perovskites possess outstanding optical characteristics that can be employed in the fabrication of phototransistors. However, due to low current modulation at room temperature, sensitivity to the ambient environment, lack of patterning techniques and low carrier mobility of polycrystalline form, investigation in perovskite phototransistors has been limited to rigid substrates such as silicon and glass to improve the film quality. Here, we report on room temperature current modulation in a methylammonium lead iodide perovskite (MAPbI3) flexible transistor made by an extremely cheap and facile fabrication process. The proposed phototransistor has the top-gate configuration with a lateral drain–channel–source structure. The device performed in the linear and saturation regions both in the dark and under white light in different current ranges according to the illumination conditions. The transistor showed p-type transport characteristics and the field effect mobility of the device was calculated to be ~1.7 cm2 V−1 s−1. This study is expected to contribute to the development of MAPbI3 flexible phototransistors.

Electron tunneling at the molecularly thin 2D perovskite and graphene van der Waals interface

Quasi-two-dimensional perovskites have emerged as a new material platform for optoelectronics on account of its intrinsic stability. A major bottleneck to device performance is the high charge injection barrier caused by organic molecular layers on its basal plane, thus the best performing device currently relies on edge contact. Herein, by leveraging on van der Waals coupling and energy level matching between two-dimensional Ruddlesden-Popper perovskite and graphene, we show that the plane-contacted perovskite and graphene interface presents a lower barrier than gold for charge injection. Electron tunneling across the interface occurs via a gate-tunable, direct tunneling-to-field emission mechanism with increasing bias, and photoinduced charge transfer occurs at femtosecond timescale (~50 fs). Field effect transistors fabricated on molecularly thin Ruddlesden-Popper perovskite using graphene contact exhibit electron mobilities ranging from 0.1 to 0.018 cm²V⁻¹s⁻¹ between 1.7 to 200 K. Scanning tunneling spectroscopy studies reveal layer-dependent tunneling barrier and domain size on few-layered Ruddlesden-Popper perovskite.

Insulating molecular layers on the basal plane of 2D perovskite is a major bottleneck for charge injection that limiting device performance. Here, the authors show that plane-contacted graphene functions as a low barrier and gate-tunable contact to overcome this limitation.

Perovskite Core-Shell Nanowire Transistors: Interfacial Transfer Doping and Surface Passivation

While halide perovskite electronics are rapidly developing, they are greatly limited by the inferior charge transport and poor stability. In this work, effective surface charge transfer doping of vapor-liquid-solid (VLS)-grown single-crystalline cesium lead bromide perovskite (CsPbBr₃) nanowires (NWs) via molybdenum trioxide (MoO₃) surface functionalization is achieved. Once fabricated into NW devices, due to the efficient interfacial charge transfer and reduced impurity scattering, a 15× increase in the field-effect hole mobility (μ_h) from 1.5 to 23.3 cm²/(V s) is accomplished after depositing the 10 nm thick MoO₃ shell. This enhanced mobility is already better than any mobility value reported for perovskite field-effect transistors (FETs) to date. The photodetection performance of these CsPbBr₃/MoO₃ core-shell NWs is also investigated to yield a superior responsivity (R) up to 2.36 × 10³ A/W and an external quantum efficiency (EQE) of over 5.48 × 10⁵% toward the 532 nm regime. Importantly, the MoO₃ shell can provide excellent surface passivation to the CsPbBr₃ NW core that minimizes the diffusion of detrimental water and oxygen molecules, improving the air stability of CsPbBr₃/MoO₃ core-shell NW devices. All these findings evidently demonstrate the surface doping as an enabling technology to realize high-mobility and air-stable low-dimensional halide perovskite devices.

Pushing Absorption of Perovskite Nanocrystals into the Infrared

To date, defect-tolerance electronic structure of lead halide perovskite nanocrystals is limited to an optical feature in the visible range. Here, we demonstrate that IR sensitization of formamidinium lead iodine (FAPI) nanocrystal array can be obtained by its doping with PbS nanocrystals. In this hybrid array, absorption comes from the PbS nanocrystals while transport is driven by the perovskite which reduces the dark current compared to pristine PbS. In addition, we fabricate a field-effect transistor using a high capacitance ionic glass made of hybrid FAPI/PbS nanocrystal arrays. We show that the hybrid material has an n-type nature with an electron mobility of 2 × 10^-3 cm² V^-1 s^-1. However, the dark current reduction is mostly balanced by a loss of absorption. To overcome this limitation, we couple the FAPI/PbS hybrid to a guided mode resonator that can enhance the infrared light absorption.

Solution-Processed Mixed-Dimensional Hybrid Perovskite/Carbon Nanotube Electronics

Benefiting from their extraordinary physical properties, methylammonium lead halide perovskites (PVKs) have attracted significant attention in optoelectronics. However, the PVK-based devices suffer from low carrier mobility and high operation voltage. Here, we utilize sorted semiconducting single-walled carbon nanotubes (95% s-SWCNTs) to enhance the performance of thin-film transistors (TFTs) based on the mixed-cation perovskite (MA_1-xFA_x)Pb(I_1-xBr_x)₃, enabling mixed-dimensional solution-processed electronics with high mobility (32.25 cm²/(V s)) and low voltage (∼3 V) operation. The resulting mixed-dimensional PVK/SWCNT TFTs possess ON/OFF ratios on the order of 10⁷, enabling the fabrication of high-gain inverters.

Organic–inorganic hybrid perovskite electronics

Organic–inorganic hybrid perovskite is a leading successor for the next generation of (opto)electronics.

Electrical Methods to Elucidate Charge Transport in Hybrid Perovskites Thin Films and Devices

The panchromatic light absorption and excellent charge carrier transport properties in organo lead halide perovskites allowed to achieve an unprecedented power conversion efficiency in excess of 25 % for thin film photovoltaics fabrication. To understand the underlying phenomena, various comprehensive set of optical and electrical techniques have been employed to investigate the charge carrier dynamics in such devices. In this perspective, we aim to summarize the electrical transport properties of perovskite thin films by using (i) impedance spectroscopy (IS), (ii) space charge limited current (SCLC), (iii) field-effect transistors (FETs) and (iv) time-of-flight (TOF) methods. We have deliberated various equivalent circuit used to model the perovskite solar cells by means of IS. The SCLC technique provide vital electrical parameters such as mobility, activation energy, traps density and distribution, carrier concentration, density of states, etc. The TOF technique measures mobility as a primary parameter while the FETs configuration provide a valuable insight into the in-plane charge transport in perovskites thin films. We believe that these notable understanding will provide insights into charge carrier dynamics in perovskite materials and devices.

Contrasting Effects of Organic Chloride Additives on Performance of Direct and Inverted Perovskite Solar Cells

Perovskite solar cells (PSCs) have demonstrated encouraging progress in recent years. Additive engineering, where diverse additives are incorporated into the perovskite layer, has been widely adopted to tune the perovskite grains, reduce defect density and charge recombination. Here, we observe a universal phenomenon that organic chloride additives enhance the open circuit voltage (V_OC) and power conversion efficiencies (PCEs) of direct PSCs but decrease the V_OC, short-circuit current (J_SC), and PCE of inverted PSCs, regardless of the choice of charge transport materials. The polyTPD-based direct device incorporating trimethylammonium chloride (TACl) additive delivery improved PCE from 17.8 to 20.0%, arising from the enhanced V_OC from 1.03 to 1.12 V. With the same content of TACl, the best PCE of the polyTPD-based inverted device decreased from 20.2 to 18.5% because of the reduced V_OC (1.05-1.01 V) and J_SC (23.2-22.5 mA/cm²). Our investigation confirms that organic chloride will p-dope perovskites and elevate the work functions, which lead to favorable/unfavorable charge transfer between perovskite films and its upper transport layers in direct and inverted devices. This work provides an insight into the rational design of the device structure when applying additives which can dope the perovskite to affect charge transfer at the perovskite/charge transport layer interface.

Investigation of Electrode Electrochemical Reactions in CH3 NH3 PbBr3 Perovskite Single-Crystal Field-Effect Transistors

Optoelectronic devices based on metal halide perovskites, including solar cells and light-emitting diodes, have attracted tremendous research attention globally in the last decade. Due to their potential to achieve high carrier mobilities, organic-inorganic hybrid perovskite materials can enable high-performance, solution-processed field-effect transistors (FETs) for next-generation, low-cost, flexible electronic circuits and displays. However, the performance of perovskite FETs is hampered predominantly by device instabilities, whose origin remains poorly understood. Here, perovskite single-crystal FETs based on methylammonium lead bromide are studied and device instabilities due to electrochemical reactions at the interface between the perovskite and gold source-drain top contacts are investigated. Despite forming the contacts by a gentle, soft lamination method, evidence is found that even at such "ideal" interfaces, a defective, intermixed layer is formed at the interface upon biasing of the device. Using a bottom-contact, bottom-gate architecture, it is shown that it is possible to minimize such a reaction through a chemical modification of the electrodes, and this enables fabrication of perovskite single-crystal FETs with high mobility of up to ≈15 cm² V^-1 s^-1 at 80 K. This work addresses one of the key challenges toward the realization of high-performance solution-processed perovskite FETs.

The Role of Chloride Incorporation in Lead-Free 2D Perovskite (BA)2SnI4: Morphology, Photoluminescence, Phase Transition, and Charge Transport

The incorporation of chloride (Cl) into methylammonium lead iodide (MAPbI3) perovskites has attracted much attention because of the significantly improved performance of the MAPbI3-based optoelectronic devices with a negligible small amount of Cl incorporation. It is expected that the Cl incorporation in 2D perovskites with layered nature would be much more efficient and thus can greatly alter the morphology, optical properties, phase transition, and charge transport; however, studies on those aspects in 2D perovskites remain elusive up to date. Here, a one-pot solution method to synthesize the Cl-doped lead-free 2D perovskite (BA)2SnI4 with various Cl incorporation concentrations is reported and how the Cl incorporation affects the morphology change, photoluminescence, phase transition, and charge transport is investigated. The Cl element is successfully incorporated into the crystal lattice in the solution-processed perovskite materials, confirmed by X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy measurements. The temperature-dependent photoluminescence studies indicate that the emission properties and phase transition behavior in (BA)2SnI4- x Cl x can be tuned by varying the Cl incorporation concentration. Electrical measurement suggests that the charge transport behavior can also be greatly altered by the Cl doping concentration and the electrical conductivity can be significantly improved under a higher Cl incorporation concentration.

Two-Dimensional Lead-Free Perovskite (C6H5C2H4NH3)2CsSn2I7 with High Hole Mobility

Two-dimensional (2D) organic-inorganic perovskites have recently undergone rapid development because of their unique optical properties, layered nature, and better environmental stability. Here, we report on a new type of 2D lead-free perovskite (C₆H₅C₂H₄NH₃)₂CsSn₂I₇ crystals with millimeter size and high field-effect hole mobility synthesized by a solution method. The excellent crystalline quality and phase purity have been verified by X-ray diffraction and low-temperature photoluminescence studies, while the X-ray photoelectron spectroscopy and energy-dispersive spectrometry measurements reveal the successful incorporation of Cs element into the resultant crystals and the absence of Sn⁴⁺ in the as-synthesized crystals. The as-synthesized crystals exhibit a high electrical conductivity and a high hole mobility up to 34 cm²·V^-1·s^-1 at 77 K. The crystals also show a high photoresponse resulting from the excellent optical properties and high electrical conductivity. Our findings show that the lead-free (C₆H₅C₂H₄NH₃)₂CsSn₂I₇ crystals with excellent optoelectronic properties would be a promising material for electronic and optoelectronic applications.

Single crystal hybrid perovskite field-effect transistors

The fields of photovoltaics, photodetection and light emission have seen tremendous activity in recent years with the advent of hybrid organic-inorganic perovskites. Yet, there have been far fewer reports of perovskite-based field-effect transistors. The lateral and interfacial transport requirements of transistors make them particularly vulnerable to surface contamination and defects rife in polycrystalline films and bulk single crystals. Here, we demonstrate a spatially-confined inverse temperature crystallization strategy which synthesizes micrometre-thin single crystals of methylammonium lead halide perovskites MAPbX₃ (X = Cl, Br, I) with sub-nanometer surface roughness and very low surface contamination. These benefit the integration of MAPbX₃ crystals into ambipolar transistors and yield record, room-temperature field-effect mobility up to 4.7 and 1.5 cm² V⁻¹ s⁻¹ in p and n channel devices respectively, with 10⁴ to 10⁵ on-off ratio and low turn-on voltages. This work paves the way for integrating hybrid perovskite crystals into printed, flexible and transparent electronics.

The methylammonium lead halide perovskites have shown excellent optoelectronic properties but the field-effect transistors are much less studied. Here Yu et al. synthesize micrometer-thin crystals of perovskites with low surface contamination and make ambipolar transistor devices with high mobilities.

Controllable Epitaxial Growth of Core-Shell PbSe@CsPbBr3 Wire Heterostructures

1D semiconductor core-shell wire heterostructures are crucial for high-performance optical and optoelectronic device applications, but they are limited to the traditional semiconductor families. Here, the conformal epitaxy of CsPbBr₃ shell on PbSe wire core is realized to form the core-shell PbSe@CsPbBr₃ wire heterostructures via a chemical vapor deposition route. The Pb-particle catalysts at the tips of the PbSe wires grown by vapor-liquid-solid provide the nucleation sites for the in situ rapid growth of CsPbBr₃ cube crystals, which serve as the adatom collector for the following shell growth due to the faster adsorption of the evaporated source atoms on them than on the sidewalls of PbSe wires. This determines the directional growth of the shell along the PbSe wires from the tip to bottom. The spectral and transient photoluminescence reveals the efficient photogenerated carrier transfer from the shell to the core. Importantly, the photodetectors (PDs) based on the heterostructures show responsivity up to 4.7 × 10⁴ A W^-1 under 405 nm light illumination, and a wavelength-dependent photocurrent polarity with the excitation of the light from near- to mid-infrared (IR), which indicates potential applications in IR PDs and novel optoelectronic logical circuits.

Metal Halide Perovskites: Synthesis, Ion Migration, and Application in Field-Effect Transistors

The past several years have witnessed tremendous developments of metal halide perovskite (MHP)-based optoelectronics. Particularly, the intensive research of MHP-based light-emitting diodes, photodetectors, and solar cells could probably reform the optoelectronic semiconductor industry. In comparison, in spite of the large intrinsic charge carrier mobility of MHPs, the development of MHP-based field-effect transistors (MHP-FETs) is relatively slow, which is essentially due to the gate-field screening effect induced by the ion migration and accumulation in MHP-FETs. This work mainly aims to summarize the recent important work on MHP-FETs and propose solutions in terms of the development bottleneck of perovskite-based transistors, in an attempt to boost the research of MHP transistors further. First, the advantages and potential applications of MHP-FETs are briefly introduced, which is followed by a detailed description of the MHP crystalline structure and various material fabrication techniques. Afterward, MHP-FETs are discussed, including transistors based on hybrid organic-inorganic perovskites, all-inorganic perovskites, and lead-free perovskites.

A general printing approach for scalable growth of perovskite single-crystal films

Perovskite single-crystal films, which exhibit exceptionally low trap density and nearly perfect translational symmetry, are believed to achieve the highest performance of perovskite-based optoelectronic devices. However, fabrication of these perovskite single-crystal films is quite difficult because of the uncontrollable nucleation caused by the rapid reaction of two perovskite precursors. We report a facile seed printing approach to selectively create millimeter-sized perovskite single-crystal films with controlled thickness and high yield. We show that perovskite single-crystal films can be perfectly transferred to almost arbitrary substrates through the printing process. The as-grown perovskite single-crystal films have excellent crystalline quality and morphology. We further demonstrate that perovskite single-crystal films can be directly printed for scalable fabrication of photodetectors and effective image sensors. This strategy allows high-yield fabrication of large perovskite single-crystal films for functional devices and may extend to other solution-processed materials for wide applications.

High- k Gate Dielectrics for Emerging Flexible and Stretchable Electronics

Recent advances in flexible and stretchable electronics (FSE), a technology diverging from the conventional rigid silicon technology, have stimulated fundamental scientific and technological research efforts. FSE aims at enabling disruptive applications such as flexible displays, wearable sensors, printed RFID tags on packaging, electronics on skin/organs, and Internet-of-things as well as possibly reducing the cost of electronic device fabrication. Thus, the key materials components of electronics, the semiconductor, the dielectric, and the conductor as well as the passive (substrate, planarization, passivation, and encapsulation layers) must exhibit electrical performance and mechanical properties compatible with FSE components and products. In this review, we summarize and analyze recent advances in materials concepts as well as in thin-film fabrication techniques for high- k (or high-capacitance) gate dielectrics when integrated with FSE-compatible semiconductors such as organics, metal oxides, quantum dot arrays, carbon nanotubes, graphene, and other 2D semiconductors. Since thin-film transistors (TFTs) are the key enablers of FSE devices, we discuss TFT structures and operation mechanisms after a discussion on the needs and general requirements of gate dielectrics. Also, the advantages of high- k dielectrics over low- k ones in TFT applications were elaborated. Next, after presenting the design and properties of high- k polymers and inorganic, electrolyte, and hybrid dielectric families, we focus on the most important fabrication methodologies for their deposition as TFT gate dielectric thin films. Furthermore, we provide a detailed summary of recent progress in performance of FSE TFTs based on these high- k dielectrics, focusing primarily on emerging semiconductor types. Finally, we conclude with an outlook and challenges section.

Perovskite Quantum Dots with Near Unity Solution and Neat-Film Photoluminescent Quantum Yield by Novel Spray Synthesis

In this study, a novel perovskite quantum dot (QD) spray-synthesis method is developed by combining traditional perovskite QD synthesis with the technique of spray pyrolysis. By utilizing this new technique, the synthesis of cubic-shaped perovskite QDs with a homogeneous size of 14 nm is demonstrated, which shows an unprecedented stable absolute photoluminescence quantum yield ≈100% in the solution and even in the solid-state neat film. The highly emissive thin films are integrated with light emission devices (LEDs) and organic light emission displays (OLEDs). The color conversion type QD-LED (ccQD-LED) hybrid devices exhibit an extremely saturated green emission, excellent external quantum efficiency of 28.1%, power efficiency of 121 lm W^-1 , and extraordinary forward-direction luminescence of 8 500 000 cd m^-2 . The conceptual ccQD-OLED hybrid display also successfully demonstrates high-definition still images and moving pictures with a 119% National Television System Committee 1931 color gamut and 123% Digital Cinema Initiatives-P3 color gamut. These very-stable, ultra-bright perovskite QDs have the properties necessary for a variety of useful applications in optoelectronics.

Controllable growth of two-dimensional perovskite microstructures

Controllable synthesis of 2D (C₄H₉NH₃)₂(CH₃NH₃)_n−1Pb_nI_3n+1 microstructures with various shapes and sizes for functional optoelectronics.

Two-dimensional halide perovskite nanomaterials and heterostructures

Novel two-dimensional halide perovskite nanomaterials and heterostructures enable next generation high performance electronics and photonics.

Direct Vapor Growth of Perovskite CsPbBr3 Nanoplate Electroluminescence Devices

Metal halide perovskite nanostructures hold great promises as nanoscale light sources for integrated photonics due to their excellent optoelectronic properties. However, it remains a great challenge to fabricate halide perovskite nanodevices using traditional lithographic methods because the halide perovskites can be dissolved in polar solvents that are required in the traditional device fabrication process. Herein, we report single CsPbBr₃ nanoplate electroluminescence (EL) devices fabricated by directly growing CsPbBr₃ nanoplates on prepatterned indium tin oxide (ITO) electrodes via a vapor-phase deposition. Bright EL occurs in the region near the negatively biased contact, with a turn-on voltage of ∼3 V, a narrow full width at half-maximum of 22 nm, and an external quantum efficiency of ∼0.2%. Moreover, through scanning photocurrent microscopy and surface electrostatic potential measurements, we found that the formation of ITO/p-type CsPbBr₃ Schottky barriers with highly efficient carrier injection is essential in realizing the EL. The formation of the ITO/p-type CsPbBr₃ Schottky diode is also confirmed by the corresponding transistor characteristics. The achievement of EL nanodevices enabled by directly grown perovskite nanostructures could find applications in on-chip integrated photonics circuits and systems.

Field-Effect Transistors Based on van-der-Waals-Grown and Dry-Transferred All-Inorganic Perovskite Ultrathin Platelets

Nowadays, the research on perovskite transistors is still in its infancy, despite the fact that perovskite-based solar cells and light-emitting diodes have been widely investigated. Two major hurdles exist before obtaining reliable perovskite-based transistors: the processing difficulty for their sensitivity to polar solvents and unsatisfactory perovskite quality on the transistor platform. Here, for the first time, we report on high-performance all-inorganic perovskite FETs profiting from both van der Waals epitaxial boundary-free ultrathin single crystals and completely dry-processed transfer technique without chemical contaminant. These two crucial factors ensure the unprecedented high-quality perovskite channels. The achieved FET hole mobility and on-off ratio reach 0.32 cm² V^-1 s^-1 and 6.7 × 10³, respectively. Moreover, at the low temperature, the mobility and on-off ratio can be enhanced to be 1.04 cm² V^-1 s^-1 and 1.3 × 10⁴. This work could open the door for the FET applications based on perovskite single crystals.

Gate-Induced Insulator to Band-Like Transport Transition in Organolead Halide Perovskite

Understanding the intrinsic charge transport in organolead halide perovskites is essential for the development of high-efficiency photovoltaics and other optoelectronic devices. Despite the rapid advancement of the organolead halide perovskite in photovoltaic and optoelectronic applications, the intrinsic charge-carrier transport in these materials remains elusive partly due to the difficulty of fabricating electrical devices and obtaining good electrical contact. Here we report the fabrication of organolead halide perovskite microplates with mono- or bilayer graphene as low barrier electrical contact. Systematic charge-transport studies reveal an insulator to band-like transport transition. Our studies indicate that the insulator to band-like transport transition depends on the orthorhombic-to-tetragonal phase-transition temperature and defect densities of the organolead halide perovskite microplates. Our findings not only are important for the fundamental understanding of charge-transport behavior but also offer valuable practical implications for photovoltaics and optoelectronic applications based on the organolead halide perovskite.

Insights into charge carrier dynamics in organo-metal halide perovskites: from neat films to solar cells

Various transport measurements for perovskites are reviewed with profound insights into charge dynamics from neat films to solar cells.