Hole Mobility Enhancement Research Articles

Simultaneous Suppression of Multilayer Ion Migration via Molecular Complexation Strategy toward High-Performance Regular Perovskite Solar Cells.

The migration and diffusion of Li+, I- and Ag impedes the realization of long-term operationally stable perovskite solar cells (PSCs). Herein, we report a multifunctional and universal molecular complexation strategy to simultaneously stabilize hole transport layer (HTL), perovskite layer and Ag electrode by the suppression of Li+, I- and Ag migration via directly incorporating bis(2,4,6-trichlorophenyl) oxalate (TCPO) into HTL. Meanwhile, TCPO co-doping results in enhanced hole mobility of HTL, advantageous energy band alignment and mitigated interfacial defects, thereby leading to facilitated hole extraction and minimized nonradiative recombination losses. TCPO-doped regular device achieves a peak power conversion efficiency (PCE) of 25.68% (certified 25.59%). The unencapsulated TCPO doped devices maintain over 90% of their initial efficiencies after 730 h of continuous operation under one sun illumination, 2800 h of storage at 30% relative humidity, and 1200 h of exposure to 65 °C, which represents one of the best stabilities reported for regular PSCs. This work provides a new approach to enhance the PCE and long-term stability of PSCs by host-guest complexation strategy via rational design of multifunctional ligand molecules.

Simultaneous Suppression of Multilayer Ion Migration via Molecular Complexation Strategy toward High‐Performance Regular Perovskite Solar Cells

The migration and diffusion of Li+, I‐ and Ag impedes the realization of long‐term operationally stable perovskite solar cells (PSCs). Herein, we report a multifunctional and universal molecular complexation strategy to simultaneously stabilize hole transport layer (HTL), perovskite layer and Ag electrode by the suppression of Li+, I‐ and Ag migration via directly incorporating bis(2,4,6‐trichlorophenyl) oxalate (TCPO) into HTL. Meanwhile, TCPO co‐doping results in enhanced hole mobility of HTL, advantageous energy band alignment and mitigated interfacial defects, thereby leading to facilitated hole extraction and minimized nonradiative recombination losses. TCPO‐doped regular device achieves a peak power conversion efficiency (PCE) of 25.68% (certified 25.59%). The unencapsulated TCPO doped devices maintain over 90% of their initial efficiencies after 730 h of continuous operation under one sun illumination, 2800 h of storage at 30% relative humidity, and 1200 h of exposure to 65 °C, which represents one of the best stabilities reported for regular PSCs. This work provides a new approach to enhance the PCE and long‐term stability of PSCs by host‐guest complexation strategy via rational design of multifunctional ligand molecules.

Computational Screening Two-Dimensional Conjugated Microporous Polymer Applied in Perovskite Solar Cells.

A two-dimensional (2D) conjugated microporous polymer with a structure of 2D nanosheets has been synthesized. Theoretical calculations and experimental results reveal that the Fermi level of this 2D polymer aligns well with perovskite absorber, and its conduction band is high enough to block electron transport into the anode. This 2D polymer is used to modify the hole transport layer, significantly improving its photoelectric properties, including enhanced hole mobility, matched energy level, and reduced recombination. Furthermore, the 2D polymer exhibits a mesoporous structure, allowing perovskite to fill into its loose framework, increasing the hole export area and providing a large hole transport flux. As a result, the efficiency of inverted perovskite solar cells enhances to 24.64 % from 21.17 % of control device without 2D conjugated microporous polymer. Given that this material can be synthesized on a large scale, this work has significant implications for the future development of 2D polymers in perovskite solar cells, potentially accelerating industrialization.

Piezo Electric Properties of Epitaxial SiGe

Piezo resistivity of B- and P-doped epitaxial Si1-xGex (x=10-30%) is investigated to assess its application potential for thin film strain sensors. The gauge factor (GF) is calculated based on the change in resistivity to the externally induced compressive uniaxial strain along the current flow direction. In the case of B-doped SiGe, the resistivity decreases under the induced compressive strain which may be related to hole mobility enhancement, while no influence on the resistivity of the P-doped SiGe. A significant increase in the GF is observed by lowering B concentration. At the same B concentration, slightly higher GF is observed for higher Ge content. Moreover, the GF is slightly improved by lowering the SiGe growth temperature. The slight improvement may be related to improved crystallinity. These results suggest that the low-doped p-type SiGe deposited at low temperature has reasonable GF and can potentially be applied in strain sensors.

Magnetic Nanocomposite Modified Hybrid Hole-Transport Layer for Constructing Organic Solar Cells with High Efficiencies.

An interface modification layer holds paramount significance in reducing interface carrier recombination and improving the ohmic contact between the active layer and the electrode in organic solar cells (OSCs). Modifying or doping the widely used hole-transport layer (HTL) PEDOT:PSS to adjust the work function, conductivity, and acidity has become a common strategy for achieving high-performance OSCs. Metal oxides and two-dimensional materials as secondary dopants into PEDOT:PSS, respectively, as well as a replacement of PEDOT:PSS both exhibit immense potential for achieving high-performance OSCs due to their excellent electrical properties. Herein, we report a method utilizing a Fe3O4/GO magnetic nanocomposite as a secondary dopant for PEDOT:PSS to modulate its inherent properties for constructing high-efficiency OSCs. The magnetic nanocomposite hybrid HTL exhibits a suitable optical transmittance and higher work function. Meanwhile, it is found that the addition of Fe3O4/GO magnetic nanoparticles expands the domain of PEDOT and enhances the phase separation between PEDOT and PSS segments, thereby improving the conductivity of PEDOT:PSS. By fine-tuning the doping ratio of a Fe3O4/GO magnetic nanocomposite in PEDOT:PSS, the best power conversion efficiency of OSCs based on PM6:L8-BO was up to 18.91%. The notable enhancement of the device's performance was due to the enhanced hole mobility and the improved charge extraction, further complemented by the decreased likelihood of interface recombination brought about by the hybrid HTL. Compared with PEDOT:PSS-based OSCs, an enhanced stability of the hybrid HTL-based device was also obtained. In addition, the diverse adaptability of the hybrid HTL was demonstrated in enhancing the performance of OSCs that are based on PM6:Y6 and PBDB-T:ITIC. The effectiveness and versatility of a magnetic nanocomposite hybrid HTL present opportunities for achieving high-performance OSCs.

Solid‐State NMR Exploration of Factors for Enhancement of Hole Mobility by Introduction of Poly(styrene) Into Poly(3‐hexylthiophene)

AbstractSolid‐state cross‐polarization magic angle spinning (CPMAS) 13C NMR is employed to examine the morphological factors that contribute to the enhanced hole mobility observed in poly(3‐hexylthiophene) (P3HT) by the introduction of electrically inert poly(styrene) (PSt). Chain mobilities of crystalline and amorphous phases in the P3HT domain are evaluated utilizing T1C (13C spin‐lattice relaxation time in the laboratory frame). The crystallinity of P3HT component is estimated based on the spectral editing method through T1ρH (1H spin‐lattice relaxation time in the rotating frame) filtered CPMAS. Moreover, the miscibility of P3HT crystalline and P3HT amorphous domains is estimated. These results suggest the formation of the rigid amorphous (short‐range ordered amorphous) in a block copolymer (P3HT‐block‐PSt). An increase in the proportion of the crystallite and proximate presence of each crystallite in a blend sample of P3HT with PSt (P3HT‐blend‐PSt) are also indicated. Enhanced mobility is attributed to the larger portion of rigid amorphous domain for P3HT‐block‐PSt, and to higher crystalline content for P3HT‐blend‐PSt.

Enhanced Carrier Transport Performance of Monolayer Hafnium Disulphide by Strain Engineering.

For semiconducting two-dimensional transition metal dichalcogenides (TMDs), the carrier transport properties of the material are affected by strain engineering. In this study, we investigate the carrier mobility of monolayer hafnium disulphide (HfS2) under different biaxial strains by first-principles calculations combined with the Kubo-Greenwood mobility approach and the compact band model. The decrease/increase in the effective mass of the conduction band (CB) of monolayer HfS2 caused by biaxial tensile/compressive strain is the major reason for the enhancement/degradation of its electron mobility. The lower hole effective mass of the valence bands (VB) in monolayer HfS2 under biaxial compressive strain improves its hole transport performance compared to that under biaxial tensile strain. In summary, biaxial compressive strain causes a decrease in both the effective mass and phonon scattering rate of monolayer HfS2, resulting in an increase in its carrier mobility. Under the biaxial compressive strain reaches 4%, the electron mobility enhancement ratio of the CB of monolayer HfS2 is ~90%. For the VB of monolayer HfS2, the maximum hole mobility enhancement ratio appears to be ~13% at a biaxial compressive strain of 4%. Our results indicate that the carrier transport performance of monolayer HfS2 can be greatly improved by strain engineering.

High-Performance Semiconducting Carbon Nanotube Transistors Using Naphthalene Diimide-Based Polymers with Biaxially Extended Conjugated Side Chains.

Polymer-wrapped single-walled carbon nanotubes (SWNTs) are a potential method for obtaining high-purity semiconducting (sc) SWNT solutions. Conjugated polymers (CPs) can selectively sort sc-SWNTs with different chiralities, and the structure of the polymer side chains influences this sorting capability. While extensive research has been conducted on modifying the physical, optical, and electrical properties of CPs through side-chain modifications, the impact of these modifications on the sorting efficiency of sc-SWNTs remains underexplored. This study investigates the introduction of various conjugated side chains into naphthalene diimide-based CPs to create a biaxially extended conjugation pattern. The CP with a branched conjugated side chain (P3) exhibits reduced aggregation, resulting in improved wrapping ability and the formation of larger bundles of high-purity sc-SWNTs. Grazing incidence X-ray diffraction analysis confirms that the potential interaction between sc-SWNTs and CPs occurs through π-π stacking. The field-effect transistor device fabricated with P3/sc-SWNTs demonstrates exceptional performance, with a significantly enhanced hole mobility of 4.72 cm2 V-1 s-1 and high endurance/bias stability. These findings suggest that biaxially extended side-chain modification is a promising strategy for improving the sorting efficiency and performance of sc-SWNTs by using CPs. This achievement can facilitate the development of more efficient and stable electronic devices.

Giant Enhancement of Hole Mobility for 4H-Silicon Carbide through Suppressing Interband Electron-Phonon Scattering.

4H-silicon carbide (4H-SiC) possesses a high Baliga figure of merit, making it a promising material for power electronics. However, its applications are limited by low hole mobility. Herein, we found that the hole mobility of 4H-SiC is mainly limited by the strong interband electron-phonon scattering using mode-level first-principles calculations. Our research indicates that applying compressive strain can reverse the sign of crystal-field splitting and change the ordering of electron bands close to the valence band maximum. Therefore, the interband electron-phonon scattering is severely suppressed and the electron group velocity is significantly increased. The out-of-plane hole mobility of 4H-SiC can be greatly enhanced by ∼200% with 2% uniaxial compressive strain applied. This work provides new insights into the electron transport mechanisms in semiconductors and suggests a strategy to improve hole mobility that could be applied to other semiconductors with hexagonal crystalline geometries.

Chitosan Mediated FAPbBr3 Nanocrystal Thin‐Films with High Color Purity and Enhanced Hole Mobility

Composite materials which combine the advantages of both embedded constituents have been a successful strategy for developing novel multi‐functional materials. Here, we demonstrate the synthesis of nanoscale materials of organic‐inorganic hybrid lead halide perovskites (OIH‐LHP) with enhanced optoelectronic properties and photostability by utilizing biomimetic ligand, chitosan. Notably, nanocrystals fabricated with chitosan mediated ligands exhibit an increase in photoluminescence quantum yield by 300% and fluorescence lifetime by 1.5 times. Consequently, the lifetime distribution of the photoluminescence emission decreases to a value as low as 6 ns. More interestingly, even though chitosan is a wide band gap insulating material, its incorporation onto the perovskite NCs enhances the bulk hole mobility by at least an order of magnitude in comparison to the pristine material. With detailed spectroscopic and structural studies, we correlate these enhancements in optoelectronic parameters to the improvement in the crystal quality, phase purity, and minimized poly‐dispersity of the nanocrystals. Development of such composite materials with unprecedented optoelectronic properties and outstanding stability will contribute immensely towards high performance nanocrystal devices.

Mesomorphism, high-contrast negative fluorosolvatochromism and photoinduced fluorescence conversion of thienylcyanostyrene-pyridinium salts

Novel thienylcyanostyrene and pyridinium-based luminescent ionic liquid crystals (ILCs) characterized with different length of N-alkyl chains (N–C3H7 and N–C8H17) and different size of counter anions (Br−, BF4−, PF6− and Tf2N−) on pyridinium moiety have been synthesized. The chemical modifications of N-alkyl chains and counter anions are easily achieved via pyridine alkylation followed with further ion exchanges, which played a key role in tuning supramolecular self-assembly of these ILCs. The compounds with shorter N–C3 alkyl chain (E-CTPC3X, X = Br−, BF4−, PF6− and Tf2N−) can self-assemble into 3D cubic phase or 2D hexagonal columnar phase depending on the size of counter anion, while the analogues with longer N–C8 alkyl chain (E-CTPC8X, X = BF4− and Tf2N−) only result smectic A phases. Moreover, the prolongation of the N-alkyl chain or enlargement of counter anion could significantly decrease the clear points of LC phases avoiding the thermal decomposition at high temperature, whereas introducing counter anions such as BF4−, PF6− that can form hydrogen bonds have stabilized the LC phases. The notable negative fluorosolvatochromism and the photoinduced fluorescence conversion with high-contrast of these ILCs were observed using UV–vis absorption and photoluminescence (PL) spectra, and was further analyzed by dynamic 1H NMR. These ILCs had been used as dopant to modify the n-Si/PEDOT:PSS heterojunction solar cells (HSCs) with 2 fold hole mobility (μhole) enhancement of the PEDOT:PSS layer.

Oxygen Stoichiometry Engineering in P‐Type NiOx for High‐Performance NiO/Ga2O3 Heterostructure p–n Diode

P‐type NiOx is employed for the fabrication of NiO/Ga2O3 p–n diode. Addressing the challenge of low hole mobility in NiOx, an extensive investigation into the impact of oxygen stoichiometry engineering in NiOx is conducted. The meticulous optimization of the O2/Ar ratio to 30% during the sputtering process results in significant improvements, notably achieving enhanced hole mobility of 1.61 cm2 V−1 s. It leads to a low specific on‐resistance of 2.79 mΩ cm2 and a high rectification ratio of ≈1011, underscoring the efficacy of recombination transport mechanism driven by enhanced hole mobility. Detailed band alignment analysis between NiOx and Ga2O3 reveals a small band offset, with a valence band offset of 2.47 eV and a conduction band offset of 1.70 eV. It suggests a tailored modification of band alignment through the engineering the oxygen stoichiometry in NiOx, facilitating enhanced recombination conduction. The device exhibits a superior breakdown voltage (Vb) of 2780 V and a notable Baliga's figure of merit (BFOM) of 2.77 GW cm−2, surpassing the SiC unipolar figure of merit. The insights gained from this work are expected to inform future designs and optimizations of high‐performance Ga2O3 electronic devices.

Hole mobility enhancement in monolayer WSe2 p-type transistors through molecular doping

Hole mobility enhancement in monolayer WSe2 p-type transistors through molecular doping

Enhancement of hole mobility in high-rate reactively sputtered Cu2O thin films induced by laser thermal annealing

In presented work, a reactive high-power impulse magnetron sputtering (r-HiPIMS) was used for high-rate deposition (≈ 170 nm/min) of Cu2O films. Films were deposited on a standard soda-lime glass (SLG) substrate at a temperature of 190 °C. As-deposited films exhibit poor hole mobility in the orders of ≈ 1 cm2/Vs. We have systematically studied the effect of laser thermal annealing (LTA) procedure performed using high-power infrared laser under different laser parameters (number of pulses, length of the pulse). We have found, LTA procedure could significantly enhance the hole mobility (up to 24 cm2/Vs in our case). We have also fitted the results of a temperature-dependent Hall measurement to clarify the mechanism of the reported increase in hole mobility. Moreover, we have discussed the effect of the LTA procedure on microstructure (crystallinity, surface morphology) and on the value of optical band gap.

Improvement of Thermal Stability and Photoelectric Performance of Cs2PbI2Cl2/CsPbI2.5Br0.5 Perovskite Solar Cells by Triple-Layer Inorganic Hole Transport Materials.

Conventional hole transport layer (HTL) Spiro-OMeTAD requires the addition of hygroscopic dopants due to its low conductivity and hole mobility, resulting in a high preparation cost and poor device stability. Cuprous thiocyanate (CuSCN) is a cost-effective alternative with a suitable energy structure and high hole mobility. However, CuSCN-based perovskite solar cells (PSCs) are affected by environmental factors, and the solvents of an HTL can potentially corrode the perovskite layer. In this study, a Co3O4/CuSCN/Co3O4 sandwich structure was proposed as an HTL for inorganic Cs2PbI2Cl2/CsPbI2.5Br0.5 PSCs to address these issues. The Co3O4 layers can serve as buffer and encapsulation layers, protecting the perovskite layer from solvent-induced corrosion and enhancing hole mobility at the interface. Based on this sandwich structure, the photovoltaic performances of the Cs2PbI2Cl2/CsPbI2.5Br0.5 PSCs are significantly improved, with the power conversion efficiency (PCE) increasing from 9.87% (without Co3O4) to 11.06%. Furthermore, the thermal stability of the devices is also significantly enhanced, retaining 80% of its initial PCE after 40 h of continuous aging at 60 °C. These results indicate that the Co3O4/CuSCN/Co3O4 sandwich structure can effectively mitigate the corrosion of the perovskite layer by solvents of an HTL and significantly improves the photovoltaic performance and thermal stability of devices.

Durable Perovskite Solar Cells with 24.5% Average Efficiency: The Role of Rigid Conjugated Core in Molecular Semiconductors.

Efficient and robust n-i-p perovskite solar cells necessitate superior organic hole-transport materials with both mechanical and electronic prowess. Deciphering the structure-property relationship of these materials is crucial for practical perovskite solar cell applications. Through direct arylation, two high glass transition temperature molecular semiconductors, DBC-ETPA (202°C) and TPE-ETPA (180°C) are synthesized, using dibenzo[g,p]chrysene (DBC) and 1,1,2,2-tetraphenylethene (TPE) tetrabromides with triphenylene-ethylenedioxythiophene-dimethoxytriphenylamine (ETPA). In comparison to spiro-OMeTAD, both semiconductors exhibit shallower HOMO energy levels, resulting in increased hole densities (generated by air oxidation doping) and accelerated hole extraction from photoexcited perovskite. Experimental and theoretical studies highlight the more rigid DBC core, enhancing hole mobility due to reduced reorganization energy and lower energy disorder. Importantly, DBC-ETPA possesses a higher cohesive energy density, leading to lower ion diffusion coefficients and higher Young's moduli. Leveraging these attributes, DBC-ETPA is employed as the primary hole-transport layer component, yielding perovskite solar cells with an average efficiency of 24.5%, surpassing spiro-OMeTAD reference cells (24.0%). Furthermore, DBC-ETPA-based cells exhibit superior operational stability and 85°C thermal storage stability.

Charge Carrier Regulation for Efficient Blue Quantum-Dot Light-Emitting Diodes Via a High-Mobility Coplanar Cyclopentane[b]thiopyran Derivative.

The performance of blue quantum dot light-emitting diodes (QLEDs) is limited by unbalanced charge injection, resulting from insufficient holes caused by low mobility or significant energy barriers. Here, we introduce an angular-shaped heteroarene based on cyclopentane[b]thiopyran (C8-SS) to modify the hole transport layer poly-N-vinylcarbazole (PVK), in blue QLEDs. C8-SS exhibits high hole mobility and conductivity due to the π···π and S···π interactions. Introducing C8-SS to PVK significantly enhanced hole mobility, increasing it by 2 orders of magnitude from 2.44 × 10-6 to 1.73 × 10-4 cm2 V-1 s-1. Benefiting from high mobility and conductivity, PVK:C8-SS-based QLEDs exhibit a low turn-on voltage (Von) of 3.2 V. More importantly, the optimized QLEDs achieve a high peak power efficiency (PE) of 7.13 lm/W, which is 2.65 times that of the control QLEDs. The as-proposed interface engineering provides a novel and effective strategy for achieving high-performance blue QLEDs in low-energy consumption lighting applications.

Facilely Modified Nickel-Based Hole Transporting Layers for Organic Solar Cells with 19.12% Efficiency and Enhanced Stability.

Hole transporting layers (HTLs), strategically positioned between electrode and light absorber, play a pivotal role in shaping charge extraction and transport in organic solar cells (OSCs). However, the commonly used poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL, with its hygroscopic and acidic nature, undermines the operational durability of OSC devices. Herein, an environmentally friendly approach is developed utilizing nickel acetate tetrahydrate (NiAc·4H2O) and [2-(9H-carbazol-9-yl)ethyl] phosphonic acid (2PACz) as the NiAc·4H2O/2PACz HTL, aiming at overcoming the limitations posed by the conventional PEDOT:PSS one. Encouragingly, a remarkable power conversion efficiency (PCE) of 19.12% is obtained for the OSCs employing NiAc·4H2O/2PACz as the HTL, surpassing that of devices with the PEDOT:PSS HTL (17.59%), which is ranked among the highest ones of OSCs. This improvement is attributed to the appropriate work function, enhanced hole mobility, facilitated exciton dissociation efficiency, and lower recombination loss of NiAc·4H2O/2PACz-based devices. Furthermore, the NiAc·4H2O/2PACz-based OSCs exhibit superior operational stability compared to their PEDOT:PSS-based counterparts. Of significant note, the NiAc·4H2O/2PACz HTL demonstrates a broad generality, boosting the PCE of the PM6:PY-IT and PM6:Y6-based OSCs from 16.47% and 16.79% (with PEDOT:PSS-based analogs as HTLs) to 17.36% and 17.57%, respectively. These findings underscore the substantial potential of the NiAc·4H2O/2PACz HTL in advancing OSCs, offering improved performance and stability, thereby opening avenue for highly efficient and reliable solar energy harvesting technologies.

Multifunctional Iron–Porphyrin Additive for Hole‐Transporting Layer Toward Efficient and Stable Perovskite Solar Cells

Spiro‐OMeTAD, as a crucial component of hole‐transporting layer (HTL), exhibits limited mobility and conductivity, and the lithium bis‐trifluoromethanesulfonimide dopant is sensitive to water vapor, which imposes restrictions on the photovoltaic properties of perovskite solar cells (PSCs). Herein, the iron–porphyrin (FePP) is introduced into Spiro‐OMeTAD solution as additive, which facilitates the oxidation process of Spiro‐OMeTAD, leading to the enhancement of hole mobility and hole extraction and transport. Besides, the surface Pb2+ defects of perovskite film are cured by the presence of carboxylic acids (COOH) in FePP. As a result, the photovoltaic properties of PSCs with FePP additive have been improved with a power conversion efficiency (PCE) of 21.58%. Moreover, FePP can further anchor Li+ ions in HTL to prevent it from being invaded by water vapor. Dramatically, the degradation of unencapsulated devices with FePP is suppressed significantly, which retains 82.0% of its original PCE under 10–20% relative humidity (RH) after 7100 h and maintains about 79.6% of its original PCE under 50–60% RH after 1000 h. Thus, this study shows that the design and development of multifunctional HTL additives holds great potential for achieving highly efficient and durable PSCs.

Charge transport and ion migration in perovskite-incorporated conjugated polymer semiconductor

The migration of intrinsic ions in metal halide perovskites and their interfaces has been shown to contribute to hysteresis and performance degradation in perovskite-based electronic devices, particularly in photovoltaics and transistors. Accordingly, controlling the film morphology, microstructure, and ionic defects in perovskite semiconductors is essential for advancing and achieving high-performance perovskite field-effect transistors (FETs). In this study, we demonstrate a well-controlled method to systematically probe the structure-property relationships, origin of hysteresis, and intrinsic ion migration effects in formamidinium iodide and lead iodide (FAI + PbI2)-based perovskite by incorporating it in a semicrystalline conjugated poly(3-hexylthiophene) (P3HT) polymer. Optimized FETs exhibited over 100% hole mobility enhancement owing to unperturbed edge-on crystalline orientation of the P3HT chains caused by the incorporated perovskite, P3HT-(FAI + PbI2) interactions, and better charge injection properties. However, the optimized devices exhibited improved current modulation with dual-sweep hysteresis, which was attributed to the ion migration effect contributed by the polarization of the lead/iodine-related ions and defects. Furthermore, operational stability investigation of the P3HT-(FAI + PbI2) FETs in the air revealed gradual current decay owing to charge trapping in contrast to the control P3HT FETs. This work provides a fundamental understanding of the origin of hysteresis and instabilities in metal perovskite materials and their transistor-based devices.