A general approach for hysteresis-free, operationally stable metal halide perovskite field-effect transistors

Perovskite-Like Liquid-Crystalline Materials Based on Polyfluorinated Imidazolium Cations

Hybrid Organic-Inorganic Halide Perovskites (HOIHPs) represent an emerging class of semiconducting materials, widely employed in a variety of optoelectronic applications. Despite their skyrocket growth in the last decade, a detailed understanding on their structure-property relationships is still missing. In this communication, we report two unprecedented perovskite-like materials based on polyfluorinated imidazolium cations. The two materials show thermotropic liquid crystalline behavior resulting in the emergence of stable mesophases. The manifold intermolecular F ⋅ ⋅ ⋅ F interactions are shown to be meaningful for the stabilization of both the solid- and liquid-crystalline orders of these perovskite-like materials. Moreover, the structure of the incorporated imidazolium cation was found to tune the properties of the liquid crystalline phase. Collectively, these results may pave the way for the design of a new class of halide perovskite-based soft materials.

NH2(CH3)2CuCl3 Organic-Inorganic Hybrid Perovskite: Consideration to Crystal Structure, Thermodynamics, and Structural Molecular Dynamics

The crystal structure of NH2(CH3)2CuCl3, an organic-inorganic hybrid perovskite, undergoes a phase transition from triclinic to monoclinic at the phase transition temperature T C of 287 K. We investigated the temperature dependencies of NMR chemical shifts and spin-lattice relaxation time T 1ρ to gain insights into the structural geometry and molecular dynamics during the transition from phase II to phase I at high temperatures. Analysis of the 1H and 13C NMR chemical shifts of the cation revealed a continuous change in the surrounding structural geometry with temperature, without any anomalous changes around T C. The sudden decrease in T 1ρ values from low to high temperatures indicated a significant variation in proton and carbon dynamics at T C, arising from the slowing motion of molecular dynamics across the phase transition. The activation energies E a obtained from the temperature dependence of T 1ρ for 1H and 13C were larger in phase I than in phase II. This suggests that molecular motions in phase II exhibit a higher degree of freedom compared to those in phase I, where they are more constrained. These findings on NH2(CH3)2CuCl3 are presented to enhance its potential applications by elucidating the crystal configuration and structural molecular dynamics of ABX3 type compound.

What Matters for the Charge Transport of 2D Perovskites?

Compared to 3D perovskites, 2D perovskites exhibit excellent stability, structural diversity, and tunable bandgaps, making them highly promising for applications in solar cells, light-emitting diodes, and photodetectors. However, the trade-off for worse charge transport is a critical issue that needs to be addressed. This comprehensive review first discusses the structure of 3D and 2D metal halide perovskites, then summarizes the significant factors influencing charge transport in detail and provides a brief overview of the testing methods. Subsequently, various strategies to improve the charge transport are presented, including tuning A'-site organic spacer cations, A-site cations, B-site metal cations, and X-site halide ions. Finally, an outlook on the future development of improving the 2D perovskites' charge transport is discussed.

High-entropy alloy screening for halide perovskites

As the concept of high-entropy alloying (HEA) extends beyond metals, new materials screening methods are needed. Halide perovskites (HP) are a prime case study because greater stability is needed for photovoltaics applications, and there are 322 experimentally observed HP end-members, which leads to more than 1057 potential alloys. We screen HEAHP by first calculating the configurational entropy of 106 equimolar alloys with experimentally observed end-members. To estimate enthalpy at low computational cost, we turn to the delta-lattice parameter approach, a well-known method for predicting III-V alloy miscibility. To generalize the approach for non-cubic crystals, we introduce the parameter of unit cell volume coefficient of variation (UCV), which does a good job of predicting the experimental HP miscibility data. We use plots of entropy stabilization versus UCV to screen promising alloys and identify 102 HEAHP of interest.

Exploring the Crystal Structure, Thermodynamics, and Molecular Dynamics of NH(CH3)3CuCl3·2H2O Organic-Inorganic Hybrid Perovskite

Understanding the physical properties of the organic-inorganic hybrid NH(CH3)3CuCl3·2H2O is necessary for its potential applications. Initially, the monoclinic structure of this crystal was discussed via single-crystal X-ray diffraction. Moreover, the previously unknown phase transition temperature was 350 K, as revealed by differential scanning calorimetry and powder X-ray diffraction. Attributed to ferroelasticity, domain walls were observed between the temperatures T c (Low) (223 K) and T c (High) (350 K). Furthermore, changes in chemical shifts for 1H and 13C indicated alterations in the molecular environment, whereas a notable decrease in line width was attributed to increased molecular motion freedom. Subsequently, spin-lattice relaxation times (T 1ρ values) for 1H and 13C (indicative of energy transfer) were influenced by tumbling motions. The high activation energy barrier for molecular reorientation was associated with the tumbling motion of methyl groups around the triple symmetry axis. These foundational properties guide the development of efficient organic-inorganic hybrids suitable for practical applications.

Low-Temperature Nanosecond Laser Process of HZO-IGZO FeFETs toward Monolithic 3D System on Chip Integration

Ferroelectric field-effect transistors (FeFETs) are increasingly important for in-memory computing and monolithic 3D (M3D) integration in system-on-chip (SoC) applications. However, the high-temperature processing required by most ferroelectric memories can lead to thermal damage to the underlying device layers, which poses significant physical limitations for 3D integration processes. To solve this problem, the study proposes using a nanosecond pulsed laser for selective annealing of hafnia-based FeFETs, enabling precise control of heat penetration depth within thin films. Sufficient thermal energy is delivered to the IGZO oxide channel and HZO ferroelectric gate oxide without causing thermal damage to the bottom layer, which has a low transition temperature (<250 °C). Using optimized laser conditions, a fast response time (<1 µs) and excellent stability (cycle > 106, retention > 106 s) are achieved in the ferroelectric HZO film. The resulting FeFET exhibited a wide memory window (>1.7 V) with a high on/off ratio (>105). In addition, moderate ferroelectric properties (2·Pr of 14.7 µC cm-2) and pattern recognition rate-based linearity (potentiation: 1.13, depression: 1.6) are obtained. These results demonstrate compatibility in HZO FeFETs by specific laser annealing control and thin-film layer design for various structures (3D integrated, flexible) with neuromorphic applications.

Ferroelectricity and Thermochromism in a 2D Dion-Jacobson Organic-Inorganic Hybrid Perovskite

2D organic-inorganic hybrid perovskites (OIHPs) have become one of the hottest research topics due to their excellent environmental stability and unique optoelectronic properties. Recently, the ferroelectricity and thermochromism of 2D OIHPs have attracted increasing interests. Integrating ferroelectricity and thermochromism into perovskites can significantly promote the development of multichannel intelligent devices. Here, a novel 2D Dion-Jacobson OIHP of the formula (3AMP)PbI4 (where 3AMP is 3-(aminomethyl)pyridinium) is reported, which has a remarkable spontaneous polarization value (Ps) of 15.6 µC cm-2 and interesting thermochromism. As far it is known, such a large Ps value is the highest for 2D OIHPs recorded so far. These findings will inspire further exploration and application of multifunctional perovskites.

Surface heterojunction based on n-type low-dimensional perovskite film for highly efficient perovskite tandem solar cells

Enhancing the quality of junctions is crucial for optimizing carrier extraction and suppressing recombination in semiconductor devices. In recent years, metal halide perovskite has emerged as the most promising next-generation material for optoelectronic devices. However, the construction of high-quality perovskite junctions, as well as characterization and understanding of their carrier polarity and density, remains a challenge. In this study, using combined electrical and spectroscopic characterization techniques, we investigate the doping characteristics of perovskite films by remote molecules, which is corroborated by our theoretical simulations indicating Schottky defects consisting of double ions as effective charge dopants. Through a post-treatment process involving a combination of biammonium and monoammonium molecules, we create a surface layer of n-type low-dimensional perovskite. This surface layer forms a heterojunction with the underlying 3D perovskite film, resulting in a favorable doping profile that enhances carrier extraction. The fabricated device exhibits an outstanding open-circuit voltage (VOC) up to 1.34 V and achieves a certified efficiency of 19.31% for single-junction wide-bandgap (1.77 eV) perovskite solar cells, together with significantly enhanced operational stability, thanks to the improved separation of carriers. Furthermore, we demonstrate the potential of this wide-bandgap device by achieving a certified efficiency of 27.04% and a VOC of 2.12 V in a perovskite/perovskite tandem solar cell configuration.

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.

Kinetic Process with Anti-Frenkel Disorder in a CsPbI3 Perovskite

Halide perovskites are rich in ionic diffusion phenomena due to their low activation energy. The soft lead iodide lattice can, in theory, endow the system with more complex defect collaborative motions. In this work, we systematically investigated the hopping mechanics of iodide interstitials with respect to various defect behaviors, such as anti-Frenkel disorder creation and annihilation. We found that the existence of iodide vacancies and interstitials can effectively lower the creation barrier of additional anti-Frenkel disorder in the halide perovskite. The free energy barriers for generating additional Frenkel defect pairs vary from 0.25 to 0.43 eV, in the proximity of those of the original iodide defects at 300 K. This finding suggests that the spontaneous creation of a specific level of anti-Frenkel disorder facilitates long-range annihilation and defect hopping processes.

Unveiling the role of linear alkyl organic cations in 2D layered tin halide perovskite field-effect transistors

Two-dimensional (2D) tin halide perovskites are promising semiconductors for field-effect transistors (FETs) owing to their fascinating electronic properties. However, the correlation between the chemical nature of organic cations and charge carrier transport is still far from understanding. In this study, the influence of chain length of linear alkyl ammonium cations on film morphology, crystallinity, and charge transport in 2D tin halide perovskites is investigated. The carbon chain lengths of the organic spacers vary from propylammonium to heptanammonium. The increase of alkyl chain length leads to enhanced local charge carrier transport in the perovskite film with mobilities of up to 8 cm2 V-1 s-1, as confirmed by optical-pump terahertz spectroscopy. A similar improved macroscopic charge transport is also observed in FETs, only to the chain length of HA, due to the synergistic enhancement of film morphology and molecular organization. While the mobility increases with the temperature rise from 100 K to 200 K due to the thermally activated transport mechanism, the device performance decreases in the temperature range of 200 K to 295 K because of ion migration. These results provide guidelines on rational design principles of organic spacer cations for 2D tin halide perovskites and contribute to other optoelectronic applications.

Hole Mobility Enhancement in Benzo[1,2-b:4,5-b']Dithiophene-Based Conjugated Polymer Transistors through Directional Alignment, Perovskite Functionalization and Solid-State Electrolyte Gating

Tunability in electronic and optical properties has been intensively explored for developing conjugated polymers and their applications in organic and perovskite-based electronics. Particularly, the charge carrier mobility of conjugated polymer semiconductors has been deemed to be a vital figure-of-merit for achieving high-performance organic field-effect transistors (OFETs). In this study, the systematic hole carrier mobility improvement of benzo[1,2-b:4,5-b']dithiophene-based conjugated polymer in perovskite-functionalized organic transistors is demonstrated. In conventional OFETs with a poly(methyl methacrylate) (PMMA) gate dielectric, improvements in hole mobility of 0.019 cm2 V-1 s-1 are measured using an off-center spin-coating technique, which exceeds those of on-center counterparts (0.22 ± 0.07 × 10-2 cm2 V-1 s-1). Furthermore, the mobility drastically increases by adopting solid-state electrolyte gating, corresponding to 2.99 ± 1.03 cm2 V-1 s-1 for the control, and the best hole mobility is 8.03 cm2 V-1 s-1 (average ≈ 6.94 ± 0.59 cm2 V-1 s-1) for perovskite-functionalized OFETs with a high current on/off ratio of >106. The achieved device performance would be attributed to the enhanced film crystallinity and charge carrier density in the hybrid perovskite-functionalized organic transistor channel, resulting from the high-capacitance electrolyte dielectric.

Magnetic-biased chiral molecules enabling highly oriented photovoltaic perovskites

The interaction between sites A, B and X with passivation molecules is restricted when the conventional passivation strategy is applied in perovskite (ABX3) photovoltaics. Fortunately, the revolving A-site presents an opportunity to strengthen this interaction by utilizing an external field. Herein, we propose a novel approach to achieving an ordered magnetic dipole moment, which is regulated by a magnetic field via the coupling effect between the chiral passivation molecule and the A-site (formamidine ion) in perovskites. This strategy can increase the molecular interaction energy by approximately four times and ensure a well-ordered molecular arrangement. The quality of the deposited perovskite film is significantly optimized with inhibited nonradiative recombination. It manages to reduce the open-circuit voltage loss of photovoltaic devices to 360 mV and increase the power conversion efficiency to 25.22%. This finding provides a new insight into the exploration of A-sites in perovskites and offers a novel route to improving the device performance of perovskite photovoltaics.

High-Efficiency and Emission-Tunable Inorganic Blue Perovskite Light-Emitting Diodes Based on Vacuum Deposition

The fabrication of perovskite light-emitting diodes (PeLEDs) with vacuum deposition shows great potential and commercial value in realizing large-area display panel manufacturing. However, the electroluminescence (EL) performance of vacuum-deposited PeLEDs still lags behind the counterparts fabricated by solution process, especially in the field of blue PeLEDs. Here, the fabrication of high-quality CsPbBr3- x Clx film through tri-source co-evaporation is reported to achieve high photoluminescence quantum yield (PLQY). Compared with the conventional traditional dual-source co-evaporation, the tri-source co-evaporation method allows for freely adjustable elemental ratios, enabling the introduction of the lattice-matched Cs4 Pb(Br/Cl)6 phase with the quantum-limited effect into the inorganic CsPb(Br/Cl)3 emitter. By adjusting the phase distribution, the surface defects of the emitter can be effectively reduced, leading to better blue emission and film quality. Further, the effects of Cs/Pb ratio and Br/Cl ratio on the PLQY and carrier recombination dynamics of perovskite films are investigated. By optimizing the deposition rate of each precursor source, spectrally stable blue PeLEDs are achieved with tunable emission ranging from 468 to 488 nm. Particularly, the PeLEDs with an EL peak at 488 nm show an external quantum efficiency (EQE) of 4.56%, which is the highest EQE value for mixed-halide PeLEDs fabricated by vacuum deposition.

Photoinduced Nonvolatile Memory Transistor Based on Lead-Free Perovskite Incorporating Fused π-Conjugated Organic Ligands

Perovskites field-effect transistors (PeFETs) have been intensively investigated for their application in detector and synapse. However, synapse based on PeFETs is still very difficult to integrate excellent charge carrier transporting ability, photosensitivity, and nonvolatile memory effects into one device, which is very important for developing bionic electronic devices and edge computing. Here, two-dimensional (2D) perovskites are synthesized by incorporating fused π-conjugated pyrene-O-ethyl-ammonium (POE) ligands and a systematic study is conducted to obtain enhanced performance and reliable PeFETs. The optimized (POE)2 SnI4 transistors display the hole mobility over 0.3 cm2  V-1  s-1 , high repeatability, and operational stability. Meanwhile, the derived photo memory devices show remarkable photoresponse, with a switching ratio higher than 105 , high visible light responsivity (>4 × 104  A W-1 ), and stable storage-erase cycles, as well as competitive retention performance (104  s). The photoinduced memory behavior can be benefiting from the insulating nature of quantum-well in 2D perovskite under dark and its excellent light sensitivity. The excellent photo memory behaviors have been maintained after 40 days in a N2 atmosphere. Finally, a 2D perovskite-only transistors with a multi-level memory behavior (16 distinct states) is described by controlling incident light pulse. This work provides broader attention toward 2D perovskite and optoelectronic application.

In Situ Real-Time Observation of Formation and Self-Assembly of Perovskite Nanocrystals at High Temperature

All-inorganic cesium lead halide perovskite nanocrystals (NCs) have received much attention due to their outstanding optical and electronic properties, but the underlying growth mechanism remains elusive due to their rapid formation process. Here, we report an in situ real-time study of the growth of Cs4PbBr6 NCs under practical synthesis conditions in a custom-made reactor. Through the synchrotron-based small-angle X-ray scattering technique, we find that the formation of Cs4PbBr6 NCs is accomplished in three steps: the fast nucleation process accompanied by self-focusing growth, the subsequent diffusion-limited Ostwald ripening, and the self-assembly of NCs into the face-centered cubic (fcc) superlattices at high temperature and the termination of growth. The simultaneously collected wide-angle X-ray scattering signals further corroborate the three-step growth model. The influence of superlattice formation is also elucidated, which improves the uniformity of the final NCs.

Ambient-Stable 2D Dion-Jacobson Phase Tin Halide Perovskite Field-Effect Transistors with Mobility over 1.6 Cm2 V-1 s-1

Solution-processed metal halide perovskites hold immense potential for the advancement of next-generation field-effect transistors (FETs). However, the instability of perovskite-based transistors has impeded their progress and practical applications. Here, ambient-stable high-performance FETs based on 2D Dion-Jacobson phase tin halide perovskite BDASnI4 , which has high film quality and excellent electrical properties, are reported. The perovskite channels are established by engineering the film crystallization process via the employment of ammonium salt interlayers and the incorporation of NH4 SCN additives within the precursor solution. The refined FETs demonstrate field-effect hole mobilities up to 1.61 cm2 V-1 s-1 and an on/off ratio surpassing 106 . Moreover, the devices show impressive operational and environmental stability and retain their functional performance even after being exposed to ambient conditions with a temperature of 45 °C and humidity of 45% for over 150 h.

Direct Observation of Contact Reaction Induced Ion Migration and its Effect on Non-Ideal Charge Transport in Lead Triiodide Perovskite Field-Effect Transistors

The migration of ionic defects and electrochemical reactions with metal electrodes remains one of the most important research challenges for organometal halide perovskite optoelectronic devices. There is still a lack of understanding of how the formation of mobile ionic defects impact charge carrier transport and operational device stability, particularly in perovskite field-effect transistors (FETs), which tend to exhibit anomalous device characteristics. Here, the evolution of the n-type FET characteristics of one of the most widely studied materials, Cs0.05 FA0.17 MA0.78 PbI3, is investigated during repeated measurement cycles as a function of different metal source-drain contacts and precursor stoichiometry. The channel current increases for high work function metals and decreases for low work function metals when multiple cycles of transfer characteristics are measured. The cycling behavior is also sensitive to the precursor stoichiometry. These metal/stoichiometry-dependent device non-idealities are correlated with the quenching of photoluminescence near the positively biased electrode. Based on elemental analysis using electron microscopy the observations can be understood by an n-type doping effect of metallic ions that are created by an electrochemical interaction at the metal-semiconductor interface and migrate into the channel. The findings improve the understanding of ion migration, contact reactions, and the origin of non-idealities in lead triiodide perovskite FETs.

Modification of Two-Dimensional Tin-Based Perovskites by Pentanoic Acid for Improved Performance of Field-Effect Transistors

Understanding and controlling the nucleation and crystallization in solution-processed perovskite thin films are critical to achieving high in-plane charge carrier transport in field-effect transistors (FETs). This work demonstrates a simple and effective additive engineering strategy using pentanoic acid (PA). Here, PA is introduced to both modulate the crystallization process and improve the charge carrier transport in 2D 2-thiopheneethylammonium tin iodide ((TEA)₂ SnI₄ ) perovskite FETs. It is revealed that the carboxylic group of PA is strongly coordinated to the spacer cation TEAI and [SnI₆ ]^4- framework in the perovskite precursor solution, inducing heterogeneous nucleation and lowering undesired oxidation of Sn²⁺ during the film formation. These factors contribute to a reduced defect density and improved film morphology, including lower surface roughness and larger grain size, resulting in overall enhanced transistor performance. The reduced defect density and decreased ion migration lead to a higher p-channel charge carrier mobility of 0.7 cm² V^-1 s^-1 , which is more than a threefold increase compared with the control device. Temperature-dependent charge transport studies demonstrate a mobility of 2.3 cm² V^-1 s^-1 at 100 K due to the diminished ion mobility at low temperatures. This result illustrates that the additive strategy bears great potential to realize high-performance Sn-based perovskite FETs.

Optoelectronic Devices of Large-Scale Transferred All-Inorganic Lead Halide Perovskite Thin Films

We report the large-scale transfer process for monocrystalline CsPbBr₃ thin films prepared by chemical vapor deposition (CVD) with excellent optical properties and stability. The transfer process is robust, simple, and effective, in which CsPbBr₃ thin films could be transferred to several substrates and effectively avoid chemical or physical fabrication processes to damage the perovskite surface. Moreover, the transfer process endows CsPbBr₃ and substrates with atomically clean and electronically flat interfaces. We utilize this transfer process to realize several optoelectronic devices, including a photonic laser with a threshold of 61 μJ/cm², a photodetector with a responsivity of 2.4 A/W, and a transistor with a hole mobility of 11.47 cm² V^-1 s^-1. High device performances mainly originate from low defects of high-quality single-crystal perovskite and seamless contact between CsPbBr₃ and target substrates. The large-scale nondestructive transfer process provides promising opportunities for optoelectronic applications based on monocrystalline perovskites.

Crystal structure and optical characterization of a new hybrid compound, C6H9N2FeCl4, with large dielectric constants for field-effect transistors

Due to remarkable dielectric features, such as a large dielectric constant, strong electrical conductivity, high capacitance, and low dielectric loss, hybrid materials have lately seen a huge number of applications in the field of optoelectronics. These are critical characteristics that qualify the performance of optoelectronic devices, particularly field-effect transistor components (FETs). Here, the hybrid compound 2-amino-5-picoline tetrachloroferrate(iii) (2A5PFeCl₄) was synthesised by using the slow evaporation solution growth method at room temperature. Structural, optical, and dielectric properties have been investigated. The 2A5PFeCl₄ compound crystallises in the monoclinic system (P2₁/c space group). Its structure can be described as a successive layering of inorganic and organic parts. [FeCl₄]^- tetrahedral anions and 2-amino-5-picolinium cations are connected by N-H⋯Cl and C-H⋯Cl hydrogen bonds. The optical absorption measurement confirms the semiconductor nature with a band gap of around 2.47 eV. Additionally, the structural and electronic properties of the title compound have been investigated theoretically through DFT calculations. At low frequencies, this material has significant dielectric constants (ε ∼10⁶). Furthermore, the high electrical conductivity, low dielectric loss at high frequencies, and high capacitance show that this new material has great dielectric potential in FET technologies. Due to their high permittivity, these compounds can be employed as gate dielectrics.

Inorganic Perovskite Quantum Dot-Mediated Photonic Multimodal Synapse

Artificial synapse is the basic unit of a neuromorphic computing system. However, there is a need to explore suitable synaptic devices for the emulation of synaptic dynamics. This study demonstrates a photonic multimodal synaptic device by implementing a perovskite quantum dot charge-trapping layer in the organic poly(3-hexylthiophene-2,5-diyl) (P3HT) channel transistor. The proposed device presents favorable band alignment that facilitates spatial separation of photogenerated charge carriers. The band alignment serves as the basis of optically induced charge trapping, which enables nonvolatile memory characteristics in the device. Furthermore, high photoresponse and excellent synaptic characteristics, such as short-term plasticity, long-term plasticity, excitatory postsynaptic current, and paired-pulse facilitation, are obtained through gate voltage regulation. Photosynaptic characteristics obtained from the device showed a multiwavelength response and a large dynamic range (∼10³) that is suitable for realizing a highly accurate artificial neural network. Moreover, the device showed nearly linear synaptic weight update characteristics with incremental depression electric gate pulse. The simulation based on the experimental data showed excellent pattern recognition accuracy (∼85%) after 120 epochs. The results of this study demonstrate the feasibility of the device as an optical synapse in the next-generation neuromorphic system.

Ferroelectric Wide-Bandgap Metal Halide Perovskite Field-Effect Transistors: Toward Transparent Electronics

Transparent field-effect transistors (FETs) are attacking intensive interest for constructing fancy "invisible" electronic products. Presently, the main technology for realizing transparent FETs is based on metal oxide semiconductors, which have wide-bandgap but generally demand sputtering technique or high-temperature (>350 °C) solution process for fabrication. Herein, a general device fabrication strategy for metal halide perovskite (MHP) FETs is shown, by which transparent perovskite FETs are successfully obtained using low-temperature (<150 °C) solution process. This strategy involves the employment of ferroelectric copolymer poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) as the dielectric, which conquers the challenging issue of gate-electric-field screening effect in MHP FETs. Additionally, an ultra-thin SnO₂ is inserted between the source/drain electrodes and MHPs to facilitate electron injection. Consequently, n-type semi-transparent MAPbBr₃ FETs and fully transparent MAPbCl₃ FETs which can operate well at room temperature with mobility over 10^-3  cm²  V^-1  s^-1 and on/off ratio >10³ are achieved for the first time. The low-temperature solution processability of these FETs makes them particularly attractive for applications in low-cost, large-area transparent electronics.

High-Entropy Perovskites for Energy Conversion and Storage: Design, Synthesis, and Potential Applications

Perovskites have shown tremendous promise as functional materials for several energy conversion and storage technologies, including rechargeable batteries, (electro)catalysts, fuel cells, and solar cells. Due to their excellent operational stability and performance, high-entropy perovskites (HEPs) have emerged as a new type of perovskite framework. Herein, this work reviews the recent progress in the development of HEPs, including synthesis methods and applications. Effective strategies for the design of HEPs through atomistic computations are also surveyed. Finally, an outlook of this field provides guidance for the development of new and improved HEPs.

In situgrowth strategy to construct perovskite quantum dot@covalent organic framework composites with enhanced water stability

Metal halide perovskite quantum dots (QDs) have excellent optoelectronic properties; however, their poor stability under water or thermal conditions remains an obstacle to commercialization. Here, we used a carboxyl functional group (-COOH) to enhance the ability of a covalent organic framework (COF) to adsorb lead ions and grow CH₃NH₃PbBr₃(MAPbBr₃) QDsin situinto a mesoporous carboxyl-functionalized COF to construct MAPbBr₃QDs@COF core-shell-like composites to improve the stability of perovskites. Owing to the protection of the COF, the as-prepared composites exhibited enhanced water stability, and the characteristic fluorescence was maintained for more than 15 d. These MAPbBr₃QDs@COF composites can be used to fabricate white light-emitting diodes with a color comparable to natural white emission. This work demonstrates the importance of functional groups for thein situgrowth of perovskite QDs, and coating with a porous structure is an effective way to improve the stability of metal halide perovskites.

Recent progress in layered metal halide perovskites for solar cells, photodetectors, and field-effect transistors

Metal halide perovskite materials demonstrate immense potential for photovoltaic and electronic applications. In particular, two-dimensional (2D) layered metal halide perovskites have advantages over their 3D counterparts in optoelectronic applications due to their outstanding stability, structural flexibility with a tunable bandgap, and electronic confinement effect. This review article first analyzes the crystallography of different 2D perovskite phases [the Ruddlesden-Popper (RP) phase, the Dion-Jacobson (DJ) phase, and the alternating cations in the interlayer space (ACI) phase] at the molecular level and compares their common electronic properties, such as out-of-plane conductivity, crucial to vertical devices. This paper then critically reviews the recent development of optoelectronic devices, namely solar cells, photodetectors and field effect transistors, based on layered 2D perovskite materials and points out their limitations and potential compared to their 3D counterparts. It also identifies the important application-specific future research directions for different optoelectronic devices providing a comprehensive view guiding new research directions in this field.

Low-Operating-Voltage Two-Dimensional Tin Perovskite Field-Effect Transistors with Multilayer Gate Dielectrics Based on a Fluorinated Copolymer

The fabrication of organic-inorganic perovskite field-effect transistors (FETs) with polymer gate dielectrics is challenging because of the solvent corrosion and wettability issues at interfaces. A few polymers have been integrated into perovskite transistors; however, these devices have high operating voltages due to low dielectric constants. Herein, poly(vinylidenefluoride-co-trifluoroethylene) (PVDF-TrFE) with a high dielectric constant is introduced into bottom-gate phenylethylammonium tin iodide perovskite [(PEA)₂SnI₄] FETs. Polytetrafluoroethylene (PTFE) and cross-linked poly(4-vinylphenol) (PVP) (CL-PVP) are used to address the issues of solvent corrosion and wettability. We design the PVDF-TrFE/PTFE and PVDF-TrFE/PTFE/CL-PVP dielectric layers, where the ferroelectric properties of PVDF-TrFE are reduced by PTFE. The (PEA)₂SnI₄ FETs operate at relatively low gate voltages, exhibiting good overall performance with average hole mobilities of 0.42 and 0.36 cm² V^-1 s^-1. Our findings provide a feasible strategy for constructing low-operating-voltage perovskite FETs with large-dielectric-constant ferroelectric polymers as gate dielectrics by a solution processing technique.

Charge transport in mixed metal halide perovskite semiconductors

Investigation of the inherent field-driven charge transport behaviour of three-dimensional lead halide perovskites has largely remained challenging, owing to undesirable ionic migration effects near room temperature and dipolar disorder instabilities prevalent specifically in methylammonium-and-lead-based high-performing three-dimensional perovskite compositions. Here, we address both these challenges and demonstrate that field-effect transistors based on methylammonium-free, mixed metal (Pb/Sn) perovskite compositions do not suffer from ion migration effects as notably as their pure-Pb counterparts and reliably exhibit hysteresis-free p-type transport with a mobility reaching 5.4 cm² V^-1 s^-1. The reduced ion migration is visualized through photoluminescence microscopy under bias and is manifested as an activated temperature dependence of the field-effect mobility with a low activation energy (~48 meV) consistent with the presence of the shallow defects present in these materials. An understanding of the long-range electronic charge transport in these inherently doped mixed metal halide perovskites will contribute immensely towards high-performance optoelectronic devices.

Stable and efficient soft perovskite crystalline film based solar cells prepared with a facile encapsulation method

The quasi Fermi level for electrons in a soft perovskite crystalline thin film and the contact qualities at the PCBM/perovskite and perovskite/P3CT-Na interfaces can be increased using a facile encapsulation method, which improves the device performance and stability of the resultant perovskite solar cells. In the encapsulated perovskite solar cells, the averaged open-circuit voltage (V_OC) largely increases from 0.981 V to 1.090 V after 9 days mainly due to the increased quasi Fermi levels. Besides, the reflectance and photoluminescence (PL) spectra show improved contact qualities at the PCBM/perovskite and perovskite/P3CT-Na interfaces, which can be used to explain the increase in the short-circuit current density (J_SC) from 21.68 mA cm^-2 to 23.48 mA cm^-2 after the encapsulation process. Besides, nanosecond time-resolved PL and temperature-dependent PL spectra can be used to explain the increased V_OC, which is mainly due to the increased shallow defect density and thereby increasing the exciton binding energy of the encapsulated perovskite sample. It is noted that the averaged power conversion efficiency (PCE) slowly decreases from 18.24% to 16.52% within 45 days.

Metal Halide Perovskite/Electrode Contacts in Charge-Transporting-Layer-Free Devices

Metal halide perovskites have drawn substantial interest in optoelectronic devices in the past decade. Perovskite/electrode contacts are crucial for constructing high-performance charge-transporting-layer-free perovskite devices, such as solar cells, field-effect transistors, artificial synapses, memories, etc. Many studies have evidenced that the perovskite layer can directly contact the electrodes, showing abundant physicochemical, electronic, and photoelectric properties in charge-transporting-layer-free perovskite devices. Meanwhile, for perovskite/metal contacts, some critical interfacial physical and chemical processes are reported, including band bending, interface dipoles, metal halogenation, and perovskite decomposition induced by metal electrodes. Thus, a systematic summary of the role of metal halide perovskite/electrode contacts on device performance is essential. This review summarizes and discusses charge carrier dynamics, electronic band engineering, electrode corrosion, electrochemical metallization and dissolution, perovskite decomposition, and interface engineering in perovskite/electrode contacts-based electronic devices for a comprehensive understanding of the contacts. The physicochemical, electronic, and morphological properties of various perovskite/electrode contacts, as well as relevant engineering techniques, are presented. Finally, the current challenges are analyzed, and appropriate recommendations are put forward. It can be expected that further research will lead to significant breakthroughs in their application and promote reforms and innovations in future solid-state physics and materials science.

Role of solution concentration in formation kinetics of bromide perovskite thin films during spin-coating monitored by optical in situ metrology

Optoelectronic devices based on metal halide perovskites continue to show a improved performance, and solution-based coating techniques pave the way for large-area applications. However, not all parameters influencing the thin film formation process of metal halide perovskites are identified and entirely rationalised over their full compositional range, thus hampering optimised thin film fabrication. Furthermore, while the perovskite deposition via spin-coating and annealing is an easily accessible technique, more profound insights into the chemical formation process are still lacking. Varying the precursor solution concentration is commonly used to vary the resulting thin film thickness. This study shows that varying the precursor solution concentration also affects the thin film morphology and optoelectronic quality. Hence, we herein investigate the influence of the precursor solution concentration on the formation process of a pure bromide-based triple cation perovskite (Cs0.05MA0.10FA0.85PbBr3) by fiber-based optical in situ measurement. During the spin-coating process, in situ UV-vis and PL measurements reveal formation kinetics are strongly dependent on the concentration. Furthermore, we identify delayed nucleation and retarded growth kinetics for more concentrated precursor solutions. In addition, we quantify the shifting chemical equilibrium of colloidal pre-coordination in the precursor solution depending on concentration. Namely, colloids are pre-organised to a higher degree and higher-coordination lead-bromide complexes tend to form in more concentrated precursor solutions. Thus, the modified solution chemistry rationalises retarded perovskite formation kinetics and highlights the precursor concentration as an influential and optimisable parameter for solution-based thin film deposition.

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.

Enhanced Optoelectronic Performance Induced by Ion Migration in Lead-Free CsCu2I3 Single-Crystal Microrods

Lead-free perovskite has attracted great attention in realizing high-performance optoelectronic devices due to their excellent atmospheric stability and nontoxic characteristics. Although a pronounced ion migration effect has been observed in this new class of materials, its potential in enhancing the overall device performance is yet to be fully explored. In this work, we studied the effect of ion migrations on the carrier transport behavior and found that the recoverable migration process can contribute to enhancing the on/off ratio in a lead-free CsCu2I3 single-crystal microrod-based photodetector. In detail, we synthesized CsCu2I3 single-crystal microrods via an in-plane self-assembly supersaturated crystallization approach. These microrods with well-defined morphologies were then used to construct ultraviolet (UV)-band photodetectors, which outperform most reported lead-free perovskite photodetectors based on individual single crystals. Simultaneously, ion migration can result in asymmetric band bending in the two-terminal device, as confirmed by surface potential profiling with Kelvin probe force microscopy (KPFM). Such an effect can be harnessed to increase the on/off ratio by almost an order of magnitude. Furthermore, the lead-free CsCu2I3 single crystal exhibits excellent thermal and air stabilities. These findings demonstrate that the CsCu2I3 single-crystal microrods can be used in stable and efficient photodetection, and the ion migration effect can potentially be utilized for improving the optoelectronic performance of lead-free devices.

Grain engineering for improved charge carrier transport in two-dimensional lead-free perovskite field-effect transistors

Controlling crystal growth and reducing the number of grain boundaries are crucial to maximize the charge carrier transport in organic-inorganic perovskite field-effect transistors (FETs). Herein, the crystallization and growth kinetics of a Sn(II)-based 2D perovskite, using 2-thiopheneethylammonium (TEA) as the organic cation spacer, were effectively regulated by the hot-casting method. With increasing crystalline grain size, the local charge carrier mobility is found to increase moderately from 13 cm² V^-1 s^-1 to 16 cm² V^-1 s^-1, as inferred from terahertz (THz) spectroscopy. In contrast, the FET operation parameters, including mobility, threshold voltage, hysteresis, and subthreshold swing, improve substantially with larger grain size. The optimized 2D (TEA)₂SnI₄ transistor exhibits hole mobility of up to 0.34 cm² V^-1 s^-1 at 295 K and a higher value of 1.8 cm² V^-1 s^-1 at 100 K. Our work provides an important insight into the grain engineering of 2D perovskites for high-performance FETs.

Intrinsic (Trap-Free) Transistors Based on Epitaxial Single-Crystal Perovskites

The first experimental realization of the intrinsic (not dominated by defects) charge conduction regime in lead-halide perovskite field-effect transistors (FETs) is reported. The advance is enabled by: i) a new vapor-phase epitaxy technique that results in large-area single-crystalline cesium lead bromide (CsPbBr₃ ) films with excellent structural and surface properties, including atomically flat surface morphology, essentially free from defects and traps at the level relevant to device operation; ii) an extensive materials analysis of these films using a variety of thin-film and surface probes certifying the chemical and structural quality of the material; and iii) the fabrication of nearly ideal (trap-free) FETs with characteristics superior to any reported to date. These devices allow the investigation of the intrinsic FET and (gated) Hall-effect carrier mobilities as functions of temperature. The intrinsic mobility is found to increase on cooling from ≈30 cm² V^-1 s^-1 at room temperature to ≈250 cm² V^-1 s^-1 at 50 K, revealing a band transport limited by phonon scattering. Establishing the intrinsic (phonon-limited) mobility provides a solid test for theoretical descriptions of carrier transport in perovskites, reveals basic limits to the technology, and points to a path for future high-performance perovskite electronic devices.

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.

Designing Two-Dimensional Halide Perovskites Based on High-Throughput Calculations and Machine Learning

The interactions between ions and the low-dimensional halide perovskites are critical to realizing the next-generation energy storage devices such as photorechargeable ion batteries and ion capacitors. In this study, we performed high-throughput calculations and machine-learning analysis for ion adsorption on two-dimensional A₂BX₄ halide perovskites. The first-principles calculations obtained an initial data set containing adsorption energies of 640 compositionally engineered ion/perovskite systems with diverse ions including Li⁺, Zn²⁺, K⁺, Na⁺, Al³⁺, Ca²⁺, Mg²⁺, and F^-. The machine learning algorithms including k-nearest neighbors (KNN), Kriging, Random Forest, Rpart, SVM, and Xgboost algorithms were compared, and the Xgboost algorithm achieved the best accuracy (r = 0.97, R² = 0.93) and was selected to predict the virtual design space consisting of 11 976 ion/perovskite systems. The features were then analyzed and ranked according to their Pearson correlations to the output values. In particular, to better understand the features, diverse feature selection methods were employed to comprehensively evaluate the features. The machine-learning-predicted virtual design space was subsequently screened to select stable lead-free ion/perovskite systems with suitable band gaps and halogen mixing features. The present study provides a theoretical foundation to design halide perovskite materials for ion-based energy storage applications such as secondary ion batteries, ion capacitors, and solar-rechargeable batteries.

Electrode Engineering in Halide Perovskite Electronics: Plenty of Room at the Interfaces

Contact engineering is a prerequisite for achieving desirable functionality and performance of semiconductor electronics, which is particularly critical for organic-inorganic hybrid halide perovskites due to their ionic nature and highly reactive interfaces. Although the interfaces between perovskites and charge-transporting layers have attracted lots of attention due to the photovoltaic and light-emitting diode applications, achieving reliable perovskite/electrode contacts for electronic devices, such as transistors and memories, remains as a bottleneck. Herein, a critical review on the elusive nature of perovskite/electrode interfaces with a focus on the interfacial electrochemistry effects is presented. The basic guidelines of electrode selection are given for establishing non-polarized interfaces and optimal energy level alignment for perovskite materials. Furthermore, state-of-the-art strategies on interface-related electrode engineering are reviewed and discussed, which aim at achieving ohmic transport and eliminating hysteresis in perovskite devices. The role and multiple functionalities of self-assembled monolayers that offer a unique approach toward improving perovskite/electrode contacts are also discussed. The insights on electrode engineering pave the way to advancing stable and reliable perovskite devices in diverse electronic applications.

Doping of Sn-based two-dimensional perovskite semiconductor for high-performance field-effect transistors and thermoelectric devices

Doping is an important technique for semiconductor materials and devices, yet effective and controllable doping of organic-inorganic halide perovskites is still a challenge. Here, we demonstrate a facile way to dope two-dimensional Sn-based perovskite (PEA)2SnI4 by incorporating SnI4 in the precursor solutions. It is observed that Sn4+ produces p-doping effect on the perovskite, which increases the electrical conductivity by 105 times. The dopant SnI4 is also found to improve the film morphology of (PEA)2SnI4, leading to reduced trap states. This doping technique allows us to improve the room temperature mobility of (PEA)2SnI4 field-effect transistors from 0.25 to 0.68 cm2 V-1 s-1 thanks to reduced trapping effects in the doped devices. Moreover, the doping technique enables the characterization and improvement of the thermoelectric performance of (PEA)2SnI4 films, which show a high power factor of 3.92 μW m-1 K-2 at doping ratio of 5 mol %.

High-performance hysteresis-free perovskite transistors through anion engineering

Despite the impressive development of metal halide perovskites in diverse optoelectronics, progress on high-performance transistors employing state-of-the-art perovskite channels has been limited due to ion migration and large organic spacer isolation. Herein, we report high-performance hysteresis-free p-channel perovskite thin-film transistors (TFTs) based on methylammonium tin iodide (MASnI₃) and rationalise the effects of halide (I/Br/Cl) anion engineering on film quality improvement and tin/iodine vacancy suppression, realising high hole mobilities of 20 cm² V^-1 s^-1, current on/off ratios exceeding 10⁷, and threshold voltages of 0 V along with high operational stabilities and reproducibilities. We reveal ion migration has a negligible contribution to the hysteresis of Sn-based perovskite TFTs; instead, minority carrier trapping is the primary cause. Finally, we integrate the perovskite TFTs with commercialised n-channel indium gallium zinc oxide TFTs on a single chip to construct high-gain complementary inverters, facilitating the development of halide perovskite semiconductors for printable electronics and circuits.

Halide Segregation in Mixed Halide Perovskites: Visualization and Mechanisms

Photoinduced halide segregation in mixed halide perovskites is an intriguing phenomenon and simultaneously a stability issue. In-depth probing this effect and unveiling the underpinning mechanisms are of great interest and significance. This article reviews the progress in visualized investigation of halide segregation, especially light-induced, by means of spatially-resolved imaging techniques. Furthermore, the current understanding of photoinduced phase separation based on several possible mechanisms is summarized and discussed. Finally, the remained open questions and future outlook in this field are outlined.

Morphological Control of 2D Hybrid Organic-Inorganic Semiconductor AgSePh

Silver phenylselenolate (AgSePh) is a hybrid organic-inorganic two-dimensional (2D) semiconductor exhibiting narrow blue emission, in-plane anisotropy, and large exciton binding energy. Here, we show that the addition of carefully chosen solvent vapors during the chemical transformation of metallic silver to AgSePh allows for control over the size and orientation of AgSePh crystals. By testing 28 solvent vapors (with different polarities, boiling points, and functional groups), we controlled the resulting crystal size from <200 nm up to a few μm. Furthermore, choice of solvent vapor can substantially improve the orientational homogeneity of 2D crystals with respect to the substrate. In particular, solvents known to form complexes with silver ions, such as dimethyl sulfoxide (DMSO), led to the largest lateral crystal dimensions and parallel crystal orientation. We perform systematic optical and electrical characterizations on DMSO vapor-grown AgSePh films demonstrating improved crystalline quality, lower defect densities, higher photoconductivity, lower dark conductivity, suppression of ionic migration, and reduced midgap photoluminescence at low temperature. Overall, this work provides a strategy for realizing AgSePh films with improved optical properties and reveals the roles of solvent vapors on the chemical transformation of metallic silver.

Overcoming Nanoscale Inhomogeneities in Thin-Film Perovskites via Exceptional Post-annealing Grain Growth for Enhanced Photodetection

Antisolvent-assisted spin coating has been widely used for fabricating metal halide perovskite films with smooth and compact morphology. However, localized nanoscale inhomogeneities exist in these films owing to rapid crystallization, undermining their overall optoelectronic performance. Here, we show that by relaxing the requirement for film smoothness, outstanding film quality can be obtained simply through a post-annealing grain growth process without passivation agents. The morphological changes, driven by a vaporized methylammonium chloride (MACl)-dimethylformamide (DMF) solution, lead to comprehensive defect elimination. Our nanoscale characterization visualizes the local defective clusters in the as-deposited film and their elimination following treatment, which couples with the observation of emissive grain boundaries and excellent inter- and intragrain optoelectronic uniformity in the polycrystalline film. Overcoming these performance-limiting inhomogeneities results in the enhancement of the photoresponse to low-light (<0.1 mW cm^-2) illumination by up to 40-fold, yielding high-performance photodiodes with superior low-light detection.

All in One: A Versatile n-Perovskite/p-Spiro-MeOTAD p-n Heterojunction Diode as a Photovoltaic Cell, Photodetector, and Memristive Photosynapse

With their excellent optoelectronic properties, halide perovskite (HP) semiconductors have witnessed successful applications in many fields, such as solar cells, LEDs, photodetectors, transistors, and memristors. Exploiting their fascinating physical nature for the development of single nanodevices with multifunctionalities is significant yet remains challenging. We report a multifunctional device based on the n-perovskite/p-spiro-MeOTAD p-n heterojunction diode that enables the integration of photovoltaic, photodetection, and photosynaptic functions in a single device. The device exhibits a high photoelectronic conversion efficiency (PCE) of 17.64% under AM 1.5G illumination and excellent photodetection characteristics including a low drive voltage of 0.01 V, a short response time of 0.17 s, high switching repeatability, and stability. Coupled with the superior photomemristive effect of the device that can be used for the emulation of short- and long-term memory formation of visual synapses, these results suggest that the HP-based p-n heterojunction devices hold great potential in multifunctional integrated device applications.

Traversing Excitonic and Ionic Landscapes: Reduced-Dimensionality-Inspired Design of Organometal Halide Semiconductors for Energy Applications

At the very heart of the global semiconductor industry lies the omnipresent push for new materials discovery. New materials constantly rise and fall out of fashion in the scientific literature, with those passing an initial phase of research scrutiny becoming hotbeds of characterization and optimization efforts. Yet, innumerable hours of painstaking research have been devoted to materials that have ultimately fallen by the wayside after crossing over an indefinable threshold, whereupon historical optimism is met with newfound skepticism. Materials have to perform well, and they have to do it quickly. In the past decade, metal-halide perovskites (MHPs) have garnered widespread attention. The hegemonic view in both academic and industrial circles is that these materials could be engineered to meet the demands of the semiconductor industry. Their promise as inexpensive solar cell devices is highly attractive, and it has been nothing short of remarkable that efficiencies have risen from 3.8% in 2009 to more than 25.5% in 2021. Moreover, MHPs are poised to be revolutionary materials in more ways than one. The highest MHP LED efficiency was recently reported (23.4%), and MHPs have demonstrated promise in photodetectors, memristors, and transistors. However, the many excellent properties of MHPs are contrasted by longstanding stability and reproducibility limitations that have hindered their commercialization. Overcoming the limitations of MHPs is ultimately a materials engineering problem, which should be solved by mapping more precise relationships between structure, composition, and device performance. In 1958, Francis Crick famously developed the central dogma of molecular biology which describes the unidirectional flow of information in biological systems. In the words of Crick, "nature has devised a unique instrument in which an underlying simplicity is used to express great subtlety and versatility." In this Account, taking inspiration from the hierarchical organization of nature, we describe a hierarchical approach to materials engineering of organic metal-halide semiconductors. We demonstrate that organo-metal halide semiconductors' dimensionality, composition, and morphology dictate their optoelectronic properties and can be exploited in defining more explicit relationships between structure and function. Here, we traverse three-dimensional (3D), two-dimensional (2D), and one-dimensional (1D) organo-metal halide semiconductors, detailing the morphological and compositional differences in each and the implications that can be drawn within each domain on the engineering process. Control over ion migration pathways via morphology engineering as well as control over charge formation in organic-inorganic semiconductors is demonstrated. Fundamental insights into the amount of static and dynamic disorder in the MHP lattice are provided, which can be continuously tuned as a function of composition and morphology. Using electroabsorption spectroscopy on 2D MHPs, a disorder-induced dipole moment in the exciton proportional to the summed value of static and dynamic disorder is measured. Spectroscopic isolation of exciton features in 2D MHP electroabsorption spectra allows us to obtain precise, model-independent measurements of exciton binding energies to study the effect of chemical substitutions, such as Sn²⁺ → Pb²⁺, on the value of the exciton binding energy. Finally, we conclude that this multidimensional platform, with the aid of machine learning and robotics, will be foundational in accurately predicting structure-property-device relationships in organo-metal halide semiconductors in the future.

Proton irradiation effects on mechanochemically synthesized and flash-evaporated hybrid organic-inorganic lead halide perovskites

A hybrid organic-inorganic halide perovskite is a promising material for developing efficient solar cell devices, with potential applications in space science. In this study, we synthesized methylammonium lead iodide (MAPbI₃) perovskites via two methods: mechanochemical synthesis and flash evaporation. We irradiated these perovskites with highly energetic 10 MeV proton-beam doses of 10¹¹, 10¹², 10¹³, and 4 × 10¹³protons cm^-2and examined the proton irradiation effects on the physical properties of MAPbI₃perovskites. The physical properties of the mechanochemically synthesized MAPbI₃perovskites were not considerably affected after proton irradiation. However, the flash-evaporated MAPbI₃perovskites showed a new peak in x-ray diffraction and an increased fluorescence lifetime in time-resolved photoluminescence under high-dose conditions, indicating considerable changes in their physical properties. This difference in behavior between MAPbI₃perovskites synthesized via the abovementioned two methods may be attributed to differences in radiation hardness associated with the bonding strength of the constituents, particularly Pb-I bonds. Our study will help to understand the radiation effect of proton beams on organometallic halide perovskite materials.

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.

Room-Temperature Halide Perovskite Field-Effect Transistors by Ion Transport Mitigation

Solution-processed halide perovskites have emerged as excellent optoelectronic materials for applications in photovoltaic solar cells and light-emitting diodes. However, the presence of mobile ions in the material hinders the development of perovskite field-effect transistors (FETs) due to screening of the gate potential in the nearby perovskite channel, and the resulting impediment to achieving gate modulation of an electronic current at room temperature. Here, room-temperature operation is demonstrated in cesium lead tribromide (CsPbBr₃ ) perovskite-based FETs using an auxiliary ferroelectric gate of poly(vinylidenefluoride-co-trifluoroethylene) [P(VDF-TrFE)], to electrostatically fixate the mobile ions. The large interfacial polarization of the ferroelectric gate attracts the mobile ions away from the main nonferroelectric gate interface, thereby enabling modulation of the electronic current through the channel by the main gate. This strategy allows for realization of the p-type CsPbBr₃ channel and revealing the thermally activated nature of the hole charge transport. The proposed strategy is generic and can be applied for regulating ions in a variety of ionic-electronic mixed semiconductors.

Solution Epitaxy of Halide Perovskite Thin Single Crystals for Stable Transistors

Halide perovskites hold promise for energy and optoelectronic applications due to their fascinating photophysical properties and facile processing. Among various forms, epitaxial thin single crystals (TSCs) are highly desirable due to their high crystallinity, reduced defects, and easy epitaxial integration with other materials. However, a cost-effective method for obtaining TSCs with perfect epitaxial features remains elusive. Here, we demonstrate a direct epitaxial growth of high-quality all-inorganic perovskite CsPbBr₃ TSCs on various substrates through a facile solution process under near-ambient conditions. Structural characterizations reveal a high-quality epitaxy between the obtained perovskite TSCs and substrates, thus leading to efficiently reduced defects. The resultant TSCs display a low trap density (∼10¹¹ cm^-3) and a long carrier lifetime (∼10.16 ns). Top-gate/top-contact transistors based on these TSCs exhibit high on/off ratios of over 10⁵, an optimal hole mobility of 3.9 cm² V^-1 s^-1, almost hysteresis-free operation, and high stability at room temperature. Such a facile approach for the high-yield production of perovskite epitaxial TSCs will enable a broad range of high-performance electronic applications.