Low Trap State Research Articles

High-power and efficiency W-band InAlGaN/AlN/GaN high-electron-mobility transistors for future high-capacity wireless communications

This study describes high-power and high-efficiency W-band InAlGaN/AlN/GaN high-electron-mobility transistors (HEMTs) for future sub-terahertz wireless communications. A low-thermal-budget selective-area growth (SAG) process was developed to obtain low contact resistance with low trap states. Transmission lines and substrate structures were optimized to obtain high-thermal conductivity and low substrate resonance. Consequently, a high output power of 28.7 dBm (742 mW), output power density of 4.6 W mm−1, and power-added efficiency (PAE) of 28.0% were achieved with pre-matched InAlGaN/AlN/GaN HEMTs at 90 GHz, which were superior combination of output power and PAE compared to the conventional high-temperature SAG process.

Reduced trap state density in AlGaN/GaN HEMTs with low-temperature CVD-grown BN gate dielectric

In this Letter, low-temperature (400 °C) chemical vapor deposition-grown boron nitride (BN) was investigated as the gate dielectric for AlGaN/GaN metal–insulator–semiconductor high electron mobility transistors (MISHEMTs) on a Si substrate. Comprehensive characterizations using x-ray photoelectron spectroscopy, reflection electron energy loss spectroscopy, atomic force microscope, high-resolution transmission electron microscopy, and time-of-flight secondary ion mass spectrometry were conducted to analyze the deposited BN dielectric. Compared with conventional Schottky-gate HEMTs, the MISHEMTs exhibited significantly enhanced performance with 3 orders of magnitude lower reverse gate leakage current, a lower off-state current of 1 × 10−7 mA/mm, a higher on/off current ratio of 108, and lower on-resistance of 5.40 Ω mm. The frequency-dependent conductance measurement was performed to analyze the BN/HEMT interface, unveiling a low interface trap state density (Dit) on the order of 5 × 1011–6 × 1011 cm−2 eV−1. This work shows the effectiveness of low-temperature BN dielectrics and their potential for advancing GaN MISHEMTs toward high-performance power and RF electronics applications.

Energy Transfer-Dominated Quasi-2D Blue Perovskite Light-Emitting Diodes.

The bromide-chloride mixed quasi-two-dimensional (2D) perovskite, with a natural quantum well structure and tunable exciton binding energy, has gained significant attention for high-performance blue perovskite light-emitting diodes (PeLEDs). However, the relative importance of having a low trap state density or efficient exciton transfer for high-efficiency electroluminescence (EL) performance remains elusive. Here, two molecules with the benzoic acid group, sodium 4-fluorobenzoate (SFB) and 3,5-dibromobenzoic acid (DBA), are used to modulate the phase distribution and trap state to explore the effect between energy transfer and defect passivation. As a result, when the n = 1 phase is inhibited in both films, the DBA@SFB-modified perovskite films achieve a higher photoluminescence quantum yield (PLQY) than the SFB-modified perovskite films due to effective defect passivation. However, DBA@SFB-modified PeLEDs exhibit lower external quantum efficiency (EQE) compared to SFB-modified PeLEDs due to the poor exciton transfer between the low-dimensional phase. This demonstrates that passivation strategies may enhance photoluminescence through reducing nonradiative recombination, but the effect of phase distribution is pivotal for EL performance by efficient energy transfer in quasi-2D perovskites. Femtosecond time-resolved transient absorption measurements confirm the fastest carrier dynamics in SFB-modified perovskite films, further corroborating the above result. This work provides useful information about phase modulation and defect passivation for high-efficiency blue quasi-2D PeLEDs.

Interfacial diffused reaction at two-phase solution induced crystallization for high-quality perovskite single crystals

A novel and universal two-phase solution method is developed to grow high-quality halide perovskite single crystals in this work. It can be expected that this method will help to advance the application of perovskite optoelectronic devices.

Room-temperature growth of perovskite single crystals via antisolvent-assisted confinement for high-performance electroluminescent devices

Organic-inorganic halide perovskite single crystals (OIHP-SCs) are promising light-emitting materials for bright electroluminescence (EL) devices due to their superior optoelectronic properties and excellent stability. However, the conventional high-temperature growth methods will inevitably disturb the ordered growth and introduce massive defects in OIHP-SCs, leading to the degradation of device performance. The preparation of thin OIHP-SCs with low trap-state densities for high-performance EL devices remains a formidable challenge. Here, we develop an efficient antisolvent-assisted confined growth (AACG) method for the room-temperature growth of formamidinium lead bromide (FAPbBr3) perovskite SCs. An antisolvent vapour environment is intentionally introduced in the spatially confined growth process to lower the free energy barrier for nucleation, enabling the growth of high-quality FAPbBr3 SCs at room temperature. Consequently, FAPbBr3 SCs with ultralow trap-state density of 2.15 × 109 cm−3 and amplified spontaneous emission (ASE) threshold of 7.1 µJ cm−2 could be achieved, representing the best results ever reported for FAPbBr3 SCs. By leveraging the high crystal quality of FAPbBr3 SCs, SC-based perovskite light-emitting diodes (PeLEDs) with a high luminance of over 26,000 cd m−2 and a peak current efficiency (CE) of 13.02 cd A−1 are demonstrated. Our work opens up new opportunities for the development of high-performance EL devices based on OIHP-SCs.

Anomalous p-type characteristic and recrystallization upon aging of hot-cast CH3NH3PbI3 perovskite thin films grown under atmospheric air

Hot-cast CH3NH3PbI3 perovskite thin films are fabricated in ambient air using heated substrates. All the perovskite films prepared are stored in a low-humidity environment. Interestingly, it has been observed that the perovskite films fabricated with high-temperature substrates degrade at a slower rate compared to those prepared at lower temperatures. Even after one week, the morphology of the films remains unchanged when produced with high-temperature substrates under 35–40 % relative humidity (RH). The hot-cast films exhibit p-type characteristics, as evidenced by their surface photovoltage signals. During the aging process, the iodine ions seem to vaporize as evidenced by the reduction of the I/Pb ratios in the energy-dispersive X-ray spectroscopy (EDX) spectra. The hot-cast perovskite films show relatively low surface trap states, ranging from 1015–1016 cm−3, along with low energy band tails of approximately 20 meV.

A bithiophene-cored interfacial passivating layer for Sb2S3 solar cells with 7.1% efficiency

Sb2S3, which is an excellent performance of the third generation of solar cells with superior photovoltaic properties, high stability and environmental friendliness. However, because of the low formation energies of the cation-replaced-anion antisites and anion vacancies, the presence of these trap states result in increased unpaired Sb atoms, which could serve as charge recombination centers on the surface and grain boundaries. Here, we introduce a bithiophene-cored compound (BTR-TPA, 4,4'-([2,2′-bithiophene]-5,5′-diyl)bis(N,N-bis(4-methoxyphenyl)aniline)) as an interlayer between the Sb2S3 and hole transport layers to passivate the trap states. Suitable energy levels, inhibited antimony oxide impurity phase, low trap states together with remarkable photoelectric properties are obtained via introducing such an interface layer. As a result, the optimal power conversion efficiency (PCE) of Sb2S3 solar cells is raised from 6.25% (control device) to 7.12%. This research demonstrates that interface passivation is an effective means of enhancing the PCE of Sb2S3 solar cells.

Centimeter-Sized Single Crystals of Dion-Jacobson Phase Lead-Free Double Perovskite for Efficient X-ray Detection.

2D Dion-Jacobson (DJ) phase hybrid perovskites have shown great promise in the photoelectronic field owing to their outstanding optoelectronic performance and superior structural rigidity. However, DJ phase lead-free double perovskites are still a virgin land with direct X-ray detection. Herein,wehave designed and synthesized a new DJ phase lead-free layered double perovskite of (HIS)2 AgSbBr8 (1, HIS2+ = histammonium). Centimeter-sized (18 × 10 × 5 mm3 ) single crystals of 1 are successfully grown via the temperature cooling technique, exhibiting remarkable semiconductive characteristics such as a high resistivity (2.2 × 1011 Ω cm), a low trap state density (3.56 × 1010 cm-3 ), and a large mobility-lifetime product (1.72 × 10-3 cm2 V-1 ). Strikingly, its single-crystal-based X-ray detector shows a high sensitivity of 223 µC Gy-1 air cm-2 under 33.3V mm-1 , a low detection limit (84.2 nGyair s-1 ) and superior anti-fatigue. As far asweknow,wefirstly demonstrates the potential of 2D DJ phase lead-free hybrid double perovskite in X-ray detection, showing excellent photoelectric response and operational stability. This work will pave a promising pathway to the innovative application of hybrid perovskites for eco-friendly and efficient X-ray detection.

High‐Quality CsAg2I3Microwires Grown by Spatial Confinement Method for Self‐Powered UV Photodetectors

AbstractRecently, lead halide perovskites arouse intensive attentions in photoelectric fields owing to their low trap state density, high carrier mobility, large extinction coefficient, and low‐temperature solution processing technique. However, their future commercial development is severely hampered by the lead toxicity and instability. Although lead‐free metal‐halide perovskites recently have been extensively studied in optoelectronics, their low‐dimensional counterparts can further expand their novel optical and electronic properties for potential applications. In this work, the planar growth of one‐dimensional (1D) CsAg2I3 single‐crystal microwires (MWs) on diverse substrates by space‐confined method is reported. The growth kinetic process of CsAg2I3 single‐crystal MWs is elucidated by in situ observation. Furthermore, the CsAg2I3 single‐crystal MWs exhibit potential applications in ultraviolet light photodetectors as a photoactive layer. The fabricated heterojunction photodetector exhibits pronounced photoelectric performances working in self‐powered mode, including a specific detectivity of 2.09 × 108 Jones, a high responsivity of 26.5 mA W−1, and fast response speed of 258.43/289.32 ms under light illumination of 265 nm, due to the high crystallinity and effective photogenerated carrier transfer. This study provides a vital insight into the rational design of low‐dimensional perovskites for versatile optoelectronic applications.

Aprotic Solvent Effect in Preparation of Organo Lead Iodide Perovskite Nanowires by Two-Step Spin-Coating Procedure

Lead-iodide perovskite (CH3NH3PbI3 ≡ MAPbI3) nanowires (NWs) were prepared by a two-step spin-coating technique by modifying one of the perovskite precursors with a small amount of aprotic solvent. In the two-step spin-coating technique, the perovskite precursors are MAI and PbI2. The first MAI powder is dissolved in isopropanol (IPA) to produce an isopropanol-MAI solution, while the second PbI2 is used in N, N-dimethylformamide (DMF) as an aprotic solvent. Here, a small amount of DMF was used with an IPA solution of MAI to grow 1D NWs based on MAPbI3 perovskite. Then, the film was formed directly from the MAPbI3 perovskite nanowires (PNWs) by coating the PbI2 layer with an IPA solution of MAI modified by DMF. The amount of DMF in the MAI/IPA solution was adjusted in the range between 0 and 50 μl. One-dimensional (1D) PNWs (∼100 nm diameter) and three-dimensional (3D) perovskite nanocrystals are compared. The structural and optical properties of the PNWs films are systematically investigated using X-ray diffraction patterns (XRD), scanning electron microscopy images (SEM), UV–vis absorption, and photoluminescence (PL). The result is that the presence of an additional solvent and its optimized amount in the MAI/IPA solution can increase the length and efficiency of charge transfer by facilitating perovskite transformation, as evidenced by the intensity of PL and the formation of a high-quality perovskite film. Compact, hole-free films with low trap states (crystal defects) were obtained. Further investigation of the lead iodide perovskite showed that reducing the dimensionality from 3D to 1D resulted in shorter wavelengths of the absorption edge and the PL peak in both the absorption and PL spectra. The shorter wavelengths indicate more localized exciton states in NWs. Finally, the amplified spontaneous emission (ASE) properties were obtained under picosecond laser excitation and a low ASE threshold was found at 10 and 53 μJ/cm2 for 1D and 3D, respectively, at about 300 nm film thickness.

Cation exchange strategy to construct nanopatterned Zn:NiOx electrode with highly conductive interface for efficient inverted perovskite solar cells

Inorganic hole-transporting materials (HTMs) with excellent hole extraction and transfer properties are highly desirable for inverted perovskite solar cells (PSCs). However, the widely employed oxide HTMs (e.g., NiOx nanocrystal) often suffer from the low p-type conductivity and severe trap-assisted recombination at HTMs/perovskite interface, which significantly limits the final performance of PSCs. Herein, the facile template-assisted cation exchange strategy is developed to fabricate nanopatterned Zn:NiOx as efficient HTMs for PSCs, for the first time. Promisingly, by virtue of the advantageous 1D nanoscale architecture and synergistic substitutional Zn doping, high conductivity, high hole mobility and low interfacial charge trap states density have been achieved by such nanopatterned Zn:NiOx HTMs. These features endow the highly conductive interface for PSCs to accelerate hole extraction and minimize recombination loss. As a consequence, PSCs with nanopatterned Zn:NiOx deliver a champion efficiency of ≈20 % with open-circuit voltage of as high as 1.14 V. This work demonstrates and paves a new insight and avenue for the rational design of efficient oxide HTMs.

Heterojunction In Situ Constructed by a Novel Amino Acid-Based Organic Spacer for Efficient and Stable Perovskite Solar Cells.

The optical properties and stability of metal halide perovskites can be improved by reducing their dimensionality. Because defects at the perovskite film grain body and boundaries cause significant energetic losses by nonradiative recombination, perovskite films with manageable crystal size and macroscopic grains are essential to improve the photovoltaic properties. Through theoretical calculation models and experiments, we show that the carboxyl group of 4-ammonium butyric acid-based cation (4-ABA+) can interact with the three-dimensional (3D) perovskite to produce in situ a secondary grain growth by post-treatment. It passivates the trap defects and broadens the light absorption. 4-ABA+ could induce a 2D capping layer on top of 3D mixed cation-based perovskite to construct a 2D/3D heterojunction. The 4-ABA+-modified perovskite film consists of large-sized grains with extremely low trap state densities and possesses a longer charge carrier lifetime and good stability, resulting in efficient perovskite solar cells with a champion efficiency of 23.16% and a VOC of 1.20 V. We show that the 4-ABA+-treated devices outperform the 3-ammonium propionic acid (3-APA+)- and 5-ammonium valeric acid (5-AVA+)-treated ones. Moreover, the devices exhibit high stability under high humidity and continuous light soaking conditions. This work gives a hint that our approach based on 4-ABA+ treatment is key to achieving better electrical properties, a controlled crystal growth, and highly stable perovskite solar cells.

Study of intrinsic point defects in γ-In2Se3 based on first principles calculations for the realization of an efficient UV photodetector

In 2 Se 3 have attracted researcher’s attention due to its potential optoelectronic and memory device applications, having various polymorphs with few of them having layered structure. There exists a non-layered phase with vacancy ordered in screw form known as γ-In 2 Se 3 , whose properties are found to get affected majorly by the presence of defects. Here, the first principles calculations based on the density functional theory are performed to study the intrinsic point defects in defect wurtzite structure of γ-In 2 Se 3 using GGA-PBE approximations. Thermodynamic charge transitions based on the formation energy of the defects are determined for all native defects, V In , V Se , In i , Se i , Se In , In Se , under In-rich and Se-rich experimental growth conditions. The charge transition remains the same but transition energies differ for possible coordination of Se vacancies, whereas the charge transition as well as transition energy vary for different coordination sites of In vacancies. The In interstitial, In i is the most favourable defect site under In-rich conditions whereas Se In at the VBM and V In1 near CBM are dominating under Se-rich conditions. The origin of n-type conductivity of γ-In 2 Se 3 is found to be the presence of In i interstitials and In Se antisite in the defect wurtzite γ-In 2 Se 3 . Further, the films grown using PLD shows an n-type conductivity under In-rich conditions leading to a good performance with photo responsivity and specific detectivity of 5.22 × 10 2 A/W and 5.08 × 10 13 Jones. The understanding of these vacancies determines their thermodynamic behaviour which can help for the controlled growth of γ-In 2 Se 3 which leads to better device performance. • Thermodynamic transition states are predicted for γ-In 2 Se 3 under In-rich and Se-rich growth conditions using first principles study. • Origin of n-type conductivity of γ-In 2 Se 3 is found to be the presence of In i and In Se and V Se is observed to introduce the in-gap states. • Highly crystalline n-type γ-In 2 Se 3 thin films are optimized under the In-rich growth conditions using PLD technique. • High concentrations of In i and In Se are identified using PL studies. • High responsivity and detectivity is achieved for the prepared device indicating low trap states due to V Se .

Full-Dimensional Grain Boundary Stress Release for Flexible Perovskite Indoor Photovoltaics.

Perovskite photovoltaics are strong potential candidates to drive low-power off-grid electronics for indoor applications. Compared with rigid devices, flexible perovskite devices can provide a more suitable surface for indoor small electronic devices, enabling them have a broader indoor application prospect. However, the mechanical stability of flexible perovskite photovoltaics is an urgent issue solved. Herein, a kind of 3D crosslinking agent named borax is selected to carry out grain boundary penetration treatment on perovskite film to realize full-dimensional stress release. This strategy improves the mechanical and phase stabilities of perovskite films subjected to external forces or large temperature changes. The fabricated perovskite photovoltaics deliver a champion power conversion efficiency (PCE) of 21.63% under AM 1.5G illumination, which is the highest one to date. The merit of low trap states under weak light makes the devices present a superior indoor PCE of 31.85% under 1062 lux (LED, 2956 K), which is currently the best flexible perovskite indoor photovoltaic device. This work provides a full-dimensional grain boundary stress release strategy for highly stable flexible perovskite indoor photovoltaics.

Silk fibroin induced homeotropic alignment of perovskite crystals toward high efficiency and stability

The photoactive lead halide perovskites with a highly crystalline structure and low trap state density were regarded as the prerequisites for achieving high photovoltaic performance. The perovskite thin films were typically synthesized by simple wet chemistry routes, where the fluctuations in the fabrication readily generate lattice defects and induce stacking faults during the crystal growth. The growth of highly crystalline and low-defect perovskite usually needs complicated synthetical manipulation and well-designed additive engineering. Here in this work, we reported a naturally available protein, silk fibroin (SF), whereby the highly ordered β-sheets component can adequately interact with perovskite intermediates and work as the crystal growth template and trap passivation agent, which induced facile self-assembly of the perovskite crystals with homeotropic alignment structure and low defect concentrations. As a result, the SF incorporated perovskite layer exhibited a much-improved conversion efficiency with charge carrier mobility 1000 times higher than that of pristine film, as well as showed robust stability under various harsh conditions.

Dopant-compensated Cs2AgBiBr6−xClx single crystals for photo-imaging and X-ray detection

A facile Cl ion doping method is developed for growing low trap state density double perovskite single crystals. Benefitting from the excellent crystal quality, the detector shows high performance in both photodetection and X-ray detection.

Interfacial "Anchoring Effect" Enables Efficient Large-Area Sky-Blue Perovskite Light-Emitting Diodes.

While tremendous progress has recently been made in perovskite light‐emitting diodes (PeLEDs), large‐area blue devices feature inferior performance due to uneven morphologies and vast defects in the solution‐processed perovskite films. To alleviate these issues, a facile and reliable interface engineering scheme is reported for manipulating the crystallization of perovskite films enabled by a multifunctional molecule 2‐amino‐1,3‐propanediol (APDO)‐triggered “anchoring effect” at the grain‐growth interface. Sky‐blue perovskite films with large‐area uniformity and low trap states are obtained, showing the distinctly improved radiative recombination and hole‐transport capability. Based on the APDO‐induced interface engineering, synergistical boost in device performance is achieved for large‐area sky‐blue PeLED (measuring at 100 mm2) with a peak external quantum efficiency (EQE) of 9.2% and a highly prolonged operational lifetime. A decent EQE up to 6.1% is demonstrated for the largest sky‐blue device emitting at 400 mm2.

Advances in metal halide perovskite lasers: synthetic strategies, morphology control, and lasing emission

In the past decade, lead halide perovskites have emerged as potential optoelectronic materials in the fields of light-emitting diode, solar cell, photodetector, and laser, due to their low-cost synthesis method, tunable bandgap, high quantum yield, large absorption, gain coefficient, and low trap-state densities. In this review, we present a comprehensive discussion of lead halide perovskite applications, with an emphasis on recent advances in synthetic strategies, morphology control, and lasing performance. In particular, the synthetic strategies of solution and vapor progress and the morphology control of perovskite nanocrystals are reviewed. Furthermore, we systematically discuss the latest development of perovskite laser with various fundamental performances, which are highly dependent on the dimension and size of nanocrystals. Finally, considering current challenges and perspectives on the development of lead halide perovskite nanocrystals, we provide an outlook on achieving high-quality lead perovskite lasers and expanding their practical applications.

Multidentate anchoring through additive engineering for highly efficient Sb2S3 planar thin film solar cells

Sb2S3 is a promising candidate for the flexible solar cells or the top subcells in tandem solar cells due to its wide-bandgap, less toxic, acceptable cost and progressive power conversion efficiency (PCE). However, the poor quality and high trap states of Sb2S3 films limit the device performance further enhancement. Herein, we adopt a multidentate ionic liquid, tetramethylammonium hexafluorophosphate ([TMA][PF6]) as a novel additive to address this issue. The octahedral [PF6]− contains six different oriented fluorine atoms with the lone pair electrons, which could coordinate with Sb atoms due to the multidentate anchoring. Thus, the high-quality Sb2S3 film with low trap states has been achieved. Moreover, the Fermi level of the Sb2S3 film has been upshifted, thereby showing an effective charge transfer. As a result, all photovoltaic parameters of the optimized Sb2S3 devices are obviously enhanced, boosting the final PCE from 4.43 (control device) to 6.83 %. Our study about the multidentate anchoring is manifested to be an effective method to enhance the Sb2S3 device performance.

Effective additive for enhancing the performance of Sb2S3 planar thin film solar cells

Sb2S3 is a promising photovoltaic absorber with appropriate bandgap, excellent light absorption coefficient and great stability. However, the power conversion efficiency (PCE) of Sb2S3 planar thin film solar cells is unsatisfactory for further commercial application due to low crystallinity and high resistivity of Sb2S3 film. Here, we introduce an additive of 4-Chloro-3-nitrobenzenesulfonyl Chloride (CSCl) to alleviate these problems. The CSCl molecular contains two terminal Cl with lone pair electrons, which have the interaction with Sb atoms. Thus, the Sb2S3 film with enhanced crystallization and low trap states has been obtained and the resistivity is also decreased. Furthermore, CSCl additive raises the Fermi level of the Sb2S3 film, thereby enhancing the transport of electron from Sb2S3 to TiO2. Consequently, the optimal PCE of Sb2S3 solar cells is raised from 4.20% (control device) to 5.84%. Our research demonstrates a novel additive to enhance the photoelectric performance of Sb2S3 solar cells.