Sub-5 nm Ultrathin In2O3 Transistors for High-Performance and Low-Power Electronic Applications

Ultrathin oxide semiconductors are promising candidates for back-end-of-line (BEOL) compatible transistors and monolithic three-dimensional integration. Experimentally, ultrathin indium oxide (In2O3) field-effect transistors (FETs) with thicknesses down to 0.4 nm exhibit an extremely high drain current (104 μA/μm) and transconductance (4000 μS/μm). Here, we employ ab initio quantum transport simulation to investigate the performance limit of sub-5 nm gate length (Lg) ultrathin In2O3 FETs. Based on the International Technology Roadmap for Semiconductors (ITRS) criteria for high-performance (HP) devices, the scaling limit of ultrathin In2O3 FETs can reach 2 nm in terms of on-state current, delay time, and power dissipation. The wide bandgap nature of ultrathin In2O3 (3.0 eV) renders it a suitable candidate for ITRS low-power (LP) electronics with Lg down to 3 nm. Notably, both the HP and LP ultrathin In2O3 FETs exhibit superior energy-delay products as compared to those of other common 2D semiconductors such as monolayer MoS2 and MoTe2. These findings unveil the potential of ultrathin In2O3 in HP and LP nanoelectronic device applications.

Nanopatterning Induced Si Doping in Amorphous Ga2O3 for Enhanced Electrical Properties and Ultra-Fast Photodetection

Ga2O3 has emerged as a promising material for the wide-bandgap industry aiming at devices beyond the limits of conventional silicon. Amorphous Ga2O3 is widely being used for flexible electronics, but suffers from very high resistivity. Conventional methods of doping like ion implantation require high temperatures post-processing, thereby limiting their use. Herein, an unconventional method of doping Ga2O3 films with Si, thereby enhancing its electrical properties, is reported. Ion-beam sputtering (500 eV Ar+) is utilized to nanopattern SiO2-coated Si substrate leaving the topmost part rich in elemental Si. This helps in enhancing the carrier conduction by increasing n-type doping of the subsequently coated 5 nm amorphous Ga2O3 films, corroborated by room-temperature resistivity measurement and valence band spectra, respectively, while the nanopatterns formed help in better light management. Finally, as proof of concept, metal-semiconductor-metal (MSM) photoconductor devices fabricated on doped, rippled films show superior properties with responsivity increasing from 6 to 433 mA W-1 while having fast detection speeds of 861 µs/710 µs (rise/fall time) as opposed to non-rippled devices (377 ms/392 ms). The results demonstrate a facile, cost-effective, and large-area method to dope amorphous Ga2O3 films in a bottom-up approach which may be employed for increasing the electrical conductivity of other amorphous oxide semiconductors as well.

Enhancing Ultraviolet Stability and Performance of Wide Bandgap Perovskite Solar Cells Through Ultraviolet Light-Absorbing Passivator

Ultraviolet light (UV) has caused tremendous damage to perovskite solar cells (PSCs), degrading the perovskite and shortening their lifetime. Defects act as non-radiative recombination sites, accelerate the degradation process, reduce the efficiency of the device and weaken the stability of solar cell. In this work, to realize efficient and stable p-i-n wide bandgap solar cells under UV, a synergetic strategy utilizing UV light-absorbing passivator, (Trifluoroacetyl) benzotriazole (TFABI), enhance UV photostability and regulate the defect passivation is proposed. By using TFABI, the degradation of the perovskite absorption layer under UV light is suppressed, spectral response is enhanced and the Pb vacancy defects are passivated. As a result, the target device achieves an efficiency of 21.54%, exhibiting excellent long-term stability under 365 nm UV irradiation. After 60 h of irradiation, it retains 85% of its initial value (60 mW cm-2 , RH 25-30%, 25 °C).

Recent Advances in Wide Bandgap Perovskite Solar Cells: Focus on Lead-Free Materials for Tandem Structures

A tandem solar cell, which is composed of a wide bandgap (WBG) top sub-cell and a narrow bandgap (NBG) bottom subcell, harnesses maximum photons in the wide spectral range, resulting in higher efficiency than single-junction solar cells. WBG (>1.6 eV) perovskites are currently being studied a lot based on lead mixed-halide perovskites, and the power conversion efficiency of lead mixed-halide WBG perovskite solar cells (PSCs) reaches 21.1%. Despite the excellent device performance of lead WBG PSCs, their commercialization is hampered by their Pb toxicity and low stability. Hence, lead-free, less toxic WBG perovskite absorbers are needed for constructing lead-free perovskite tandem solar cells. In this review, various strategies for achieving high-efficiency WBG lead-free PSCs are discussed, drawing inspiration from prior research on WBG lead-based PSCs. The existing issues of WBG perovskites such as VOC loss are discussed, and toxicity issues associated with lead-based perovskites are also addressed. Subsequently, the natures of lead-free WBG perovskites are reviewed, and recently emerged strategies to enhance device performance are proposed. Finally, their applications in lead-free all perovskite tandem solar cells are introduced. This review presents helpful guidelines for eco-friendly and high-efficiency lead-free all perovskite tandem solar cells.

Wide-Bandgap Rare-Earth Iodate Single Crystals for Superior X-Ray Detection and Imaging

Semiconductor-based X-ray detectors with low detectable thresholds become critical in medical radiography applications. However, their performance is generally limited by intrinsic defects or unresolved issues of materials, and developing a novel scintillation semiconductor for low-dose X-ray detection is a highly urgent objective. Herein, a high-quality rare-earth iodate Tm(IO₃ )₃ single crystal grown through low-cost solution processing is reported with a wide bandgap of 4.1 eV and a large atomic number of 53.2. The roles of IO and TmO groups for charge transport in the Tm(IO₃ )₃ are revealed with the structural difference between the [101] and [ 1 ¯ 01 ] $[{\bar{1}}01]$ crystal orientations. Based on anisotropic responses of material properties and detection performances, it is found that the [ 1 ¯ 01 ${\bar{1}}01$ ] orientation, the path with fewer IO groups, achieves a high resistivity of 1.02 × 10¹¹ Ω cm. Consequently, a single-crystal detector exhibits a low dark current and small baseline drifting due to the wide bandgap, high resistivity and less ion migration of Tm(IO₃ )₃ , resulting in a low detection limit of 85.2 nGy_air s^-1 . An excellent X-ray imaging performance with a high sensitivity of 4406.6 µC Gy_air ^-1 cm^-2 is also shown in the Tm(IO₃ )₃ device. These findings provide a new material design perspective for high-performance X-ray imaging applications.

Recent Strategies for High-Performing Indoor Perovskite Photovoltaics

The development of digital technology has made our lives more advanced as a society familiar with the Internet of Things (IoT). Solar cells are among the most promising candidates for power supply in IoT sensors. Perovskite photovoltaics (PPVs), which have already attained 25% and 40% power conversion efficiencies for outdoor and indoor light, respectively, are the best candidates for self-powered IoT system integration. In this review, we discuss recent research progress on PPVs under indoor light conditions, with a focus on device engineering to achieve high-performance indoor PPVs (Id-PPVs), including bandgap optimization and defect management. Finally, we discuss the challenges of Id-PPVs development and its interpretation as a potential research direction in the field.

Wide Bandgap Conjugated Polymers Based on Difluorobenzoxadiazole for Efficient Non-Fullerene Organic Solar Cells

Wide bandgap polymers with a donor-acceptor alternating structure play a key role in constructing high-efficiency organic solar cells (OSCs). However, only a handful of high-performance polymers are available owing to the limited choices of acceptor units. 5,6-Difluorobenzo[c][1,2,5]oxadiazole (ffBX) is a promising acceptor unit with high ionization potential, and can afford high charge carrier mobility and strong aggregation for the resulting polymers. Historically, ffBX was successfully used in constructing high-performance polymer donors for fullerene-based OSCs. However, this unit is far less been explored in non-fullerene OSCs. In this work, we report three ffBX-based wide bandgap polymers (Oc00, Oc25, and Oc50) with varied solubilizing side chain content for application in non-fullerene OSCs. The polymers show matched energy levels and complementary optical absorption with the state-of-the-art non-fullerene acceptor Y6. Moreover, the polymer solubility, solid state packing, and bulk-heterojunction morphology were finely tuned via side chain engineering. Encouragingly, a decent efficiency of 14.25% was realized by the polymer Oc25 when blended with Y6 due to the efficient charge transport and favorable active layer morphology. These results suggest the promising prospect of ffBX in constructing high-performance polymer donors for non-fullerene OSCs. This article is protected by copyright. All rights reserved.

Engineering the Non-Radiative Recombination of Mixed-Halide Perovskites with Optimal Bandgap for Indoor Photovoltaics

Indoor photovoltaics have attracted increasing attention, since they can provide sustainable energy through the recycling of photon energy from household dim lighting. However, solar cells exhibiting high performance under sunlight may not perform well under indoor light conditions, mainly due to the mismatch of the irradiance spectrum. In particular, most of the indoor light sources emit visible photons with negligible near-infrared irradiance. According to the detailed balance theory, the optimal bandgap for indoor photovoltaics should be relatively larger, considering the trade-off between photocurrent and photovoltage losses. In this work, a systematic comparison of the theoretical limits of the conventional and indoor photovoltaics is presented. Then the non-radiative recombination losses are reduced by a synergetic treatment with Pb(SCN)₂ and PEABr, resulting relatively high open circuit voltage of 1.29 V and power conversion efficiency of 17.32% under 1 sun illumination. Furthermore, the devices are fully characterized under weak indoor light (1000 lux, 4000 K LED) achieving a high efficiency of 37.18%, which is promising for real applications.

A High-Performance Nonfused Wide-Bandgap Acceptor for Versatile Photovoltaic Applications

Wide-bandgap (WBG) nonfullerene acceptors (NFAs) with nonfused conjugated structures play a critical role in organic photovoltaic (OPV) cells. Here, NFAs named GS-OEH, GS-OC6, and GS-ISO, with optical bandgaps larger than 1.70 eV, are synthesized without using the fused ring structures. Compared with GS-OEH and GS-OC6, GS-ISO exhibits much stronger crystallinity, leading to a smaller energetic disorder and a larger exciton diffusion coefficient. GS-ISO also possesses a higher electroluminescence external quantum efficiency of 1.0 × 10^-2 . The OPV cell based on PBDB-TF:GS-ISO demonstrates a power conversion efficiency (PCE) of 11.62% under the standard one sun illumination. Besides, the PBDB-TF:GS-ISO-based cell with effective area of 1.0 cm²  exhibits a PCE of 28.37% under 2700 K illumination of 500 lux. A tandem OPV cell using PBDB-TF:GS-ISO as the front subcell shows an outstanding efficiency of 19.10%. Importantly, the GS-ISO-based OPV cell exhibits promising stability under the continuous illumination of simulated sunlight. This study indicates that the molecular design strategy demonstrated in this work has great superiority in developing nonfused NFAs and also that GS-ISO is a promising WBG acceptor for versatile photovoltaic applications.

First demonstration of robust tri-gateβ-Ga2O3nano-membrane field-effect transistors

Nano-membrane tri-gateβ-gallium oxide (β-Ga₂O₃) field-effect transistors (FETs) on SiO₂/Si substrate fabricated via exfoliation have been demonstrated for the first time. By employing electron beam lithography, the minimum-sized features can be defined with the footprint channel width of 50 nm. For high-quality interface betweenβ-Ga₂O₃and gate dielectric, atomic layer-deposited 15 nm thick aluminum oxide (Al₂O₃) was utilized with tri-methyl-aluminum (TMA) self-cleaning surface treatment. The fabricated devices demonstrate extremely low subthreshold slope (SS) of 61 mV dec^-1, high drain current (I_DS) ON/OFF ratio of 1.5 × 10⁹, and negligible transfer characteristic hysteresis. We also experimentally demonstrated robustness of these devices with current-voltage (I-V) characteristics measured at temperatures up to 400 °C.

The Effect of Annealing Ambience on the Material and Photodetector Characteristics of Sputtered ZnGa2O4 Films

Spinel ZnGa₂O₄ films were grown on c-plane sapphire substrates at the substrate temperature of 400 °C by radio-frequency magnetron sputtering. Post thermal annealing was employed at the annealing temperature of 700 °C in order to enhance their crystal quality. The effect of thermal annealing on the microstructural and optoelectronic properties of ZnGa₂O₄ films was systematically investigated in various ambiences, such as air, nitrogen, and oxygen. The X-ray diffraction patterns of annealed ZnGa₂O₄ films showed the crystalline structure to have (111) crystallographic planes. Transmission electron micrographs verified that ZnGa₂O₄ film annealed under air ambience possesses a quasi-single-crystalline structure. This ZnGa₂O₄ film annealed under air ambience exhibited a smooth surface, an excellent average transmittance above 82% in the visible region, and a wide bandgap of 5.05 eV. The oxygen vacancies under different annealing ambiences were revealed a substantial impact on the material and photodetector characteristics by X-ray photoelectron spectrum investigations. ZnGa₂O₄ film exhibits optimal performance as a metal-semiconductor-metal photodetector when annealed under air ambience. Under these conditions, ZnGa₂O₄ film exhibits a higher photo/dark current ratio of ~10⁴ order, as well as a high responsivity of 2.53 A/W at the bias of 5 V under an incident optical light of 240 nm. These results demonstrate that quasi-single-crystalline ZnGa₂O₄ films have significant potential in deep-ultraviolet applications.

A Wide Bandgap Polymer Donor Composed of Benzodithiophene and Oxime-Substituted Thiophene for High-Performance Organic Solar Cells

Oxime-substituted thiophene (TO) is used as an acceptor (A) unit to copolymerize with the benzodithiophene (BDT) donor (D) unit to form a novel D-A polymer donor, PBDTTO, which has a low-lying highest occupied molecular orbital energy level (E_HOMO) of -5.60 eV and a wide bandgap of 2.03 eV, forming complementary absorption and matching energy levels with the narrow bandgap nonfullerene acceptors. Organic solar cells using PBDTTO and Y6 as the donor and acceptor, respectively, exhibited a J_SC of 27.03 mA cm^-2, a V_OC of 0.83 V, and a fill factor of 0.59, reaching a high power conversion efficiency of 13.29%. The unencapsulated devices show good long-term stability in ambient air. Compared with the acceptor monomers used in other high-performance BDT-based D-A polymer donors, which are synthesized tediously in low yields, the TO acceptor monomer can be conveniently synthesized in only two steps with a high overall yield of 70%. These results demonstrate that TO unit can be used as a promising acceptor unit for developing BDT-based D-A polymer donors at low cost while maintaining high photovoltaic performance.

Towards P-Type Conduction in Hexagonal Boron Nitride: Doping Study and Electrical Measurements Analysis of hBN/AlGaN Heterojunctions

Reliable p-doped hexagonal boron nitride (h-BN) could enable wide bandgap optoelectronic devices such as deep ultra-violet light emitting diodes (UV LEDs), solar blind photodiodes and neutron detectors. We report the study of Mg in h-BN layers as well as Mg h-BN/AlGaN heterostructures. Mg incorporation in h-BN was studied under different biscyclopentadienyl-magnesium (Cp2Mg) molar flow rates. 2θ-ω x-ray diffraction scans clearly evidence a single peak, corresponding to the (002) reflection plane of h-BN with a full-width half maximum increasing with Mg incorporation in h-BN. For a large range of Cp2Mg molar flow rates, the surface of Mg doped h-BN layers exhibited characteristic pleats, confirming that Mg doped h-BN remains layered. Secondary ion mass spectrometry analysis showed Mg incorporation, up to 4 × 10¹⁸ /cm³ in h-BN. Electrical conductivity of Mg h-BN increased with increased Mg-doping. Heterostructures of Mg h-BN grown on n-type Al rich AlGaN (58% Al content) were made with the intent of forming a p-n heterojunction. The I-V characteristics revealed rectifying behavior for temperatures from 123 to 423 K. Under ultraviolet illumination, photocurrent was generated, as is typical for p-n diodes. C-V measurements evidence a built-in potential of 3.89 V. These encouraging results can indicate p-type behavior, opening a pathway for a new class of wide bandgap p-type layers.

Bistriazoles with a Biphenyl Core Derivative as an Electron-Favorable Bipolar Host of Efficient Blue Phosphorescent Organic Light-Emitting Diodes

High-quality host materials are indispensable for the construction in the emitting layer of efficient organic light-emitting diodes (OLEDs), especially in a guest and host system. The good carrier transport and energy transfer between the host and emitters are out of necessity. In this work, a wide bandgap and bipolar organic compound, 2,2'-bis(4,5-diphenyl-(1,2,4)-triazol-3-yl)biphenyl (BTBP), conjugating two electron-transporting triazole moieties on a hole-transporting biphenyl core, was synthesized and characterized. The wide bandgap of 4.0 eV makes the promise in efficient energy transfer between the host and various color emitters to apply as the universal host, especially for blue emitters. The close electron and hole mobilities perform the same order of 10-5 cm2·V-1·s-1, identified as bipolar behavior and benefited for carrier balance at low bias. Although carrier transportation belongs to bipolar behavior at a low electrical field, the electron mobility is much faster than the hole one at a high electrical field and belongs to electron-transporting behavior. Employing the BTBP as the host matrix mixed with a phosphor dopant, iridium(III)bis[4,6-di-fluorophenyl-pyridinato-N,C2]picolinate, a high-efficiency sky-blue phosphorescent organic light-emitting diode (OLED) was achieved with a maximum current efficiency of 65.9 cd/A, maximum power efficiency of 62.8 lm/W, and maximum external quantum efficiency of 30.2%.

Low Thermal Budget Heteroepitaxial Gallium Oxide Thin Films Enabled by Atomic Layer Deposition

This work explores the applicability of atomic layer deposition (ALD) in producing highly oriented crystalline gallium oxide films on foreign substrates at low thermal budgets. The effects of substrate, deposition temperature, and annealing process on formation of crystalline gallium oxide are discussed. The Ga₂O₃ films exhibited a strong preferred orientation on the c-plane sapphire substrate. The onset of formation of crystalline gallium oxide is determined, at which only two sets of planes, i.e., α-Ga₂O₃ (006) and β-Ga₂O₃ (4̅02), are present parallel to the surface. More specifically, this work reports, for the first time, that epitaxial gallium oxide films on sapphire start to form at deposition temperatures ≥ 190 °C by using an optimized plasma-enhanced ALD process such that α-Ga₂O₃ (006)∥α-Al₂O₃ (006) and β-Ga₂O₃ (2̅01)∥α-Al₂O₃ (006). Both α-Ga₂O₃ (006) and β-Ga₂O₃ (2̅01) planes are polar planes (i.e., consisting of only one type of atom, either Ga or O) and, therefore, favorable to form by ALD at such low deposition temperatures. Ellipsometry and van der Pauw measurements confirmed that the crystalline films have optical and electrical properties close to bulk gallium oxide. The film grown at 277 °C was determined to have superior properties among as-deposited films. Using TEM to locate α-Ga₂O₃ and β-Ga₂O₃ domains in the as-deposited crystalline films, we proposed a short annealing scheme to limit the development of α-Ga₂O₃ domains in the film and produce pure β-Ga₂O₃ films via an energy-efficient process. A pure β-Ga₂O₃ phase on sapphire with β-Ga₂O₃ (2̅01)∥α-Al₂O₃ (006) was successfully achieved by using the proposed process at the low annealing temperature of 550 °C preceded by the low deposition temperature of 190 °C. The results of this work enable epitaxial growth of gallium oxide thin films, with superior material properties offered by ALD, not only with potential applications as a high-performance material in reducing global energy consumption but also with an energy-efficient fabrication process.

Indium-Tin-Oxide Transistors with One Nanometer Thick Channel and Ferroelectric Gating

In this work, we demonstrate high-performance indium-tin-oxide (ITO) transistors with a channel thickness down to 1 nm and ferroelectric Hf_0.5Zr_0.5O₂ as gate dielectric. An on-current of 0.243 A/mm is achieved on submicron gate-length ITO transistors with a channel thickness of 1 nm, while it increases to as high as 1.06 A/mm when the channel thickness increases to 2 nm. A raised source/drain structure with a thickness of 10 nm is employed, contributing to a low contact resistance of 0.15 Ω·mm and a low contact resistivity of 1.1 × 10^-7 Ω·cm². The ITO transistor with a recessed channel and ferroelectric gating demonstrates several advantages over 2D semiconductor transistors and other thin-film transistors, including large-area wafer-size nanometer thin-film formation, low contact resistance and contact resistivity, an atomic thin channel being immune to short channel effects, large gate modulation of high carrier density by ferroelectric gating, high-quality gate dielectric and passivation formation, and a large bandgap for the low-power back-end-of-line complementary metal-oxide-semiconductor application.

Surfactant-assisted doctor-blading-printed FAPbBr3 films for efficient semitransparent perovskite solar cells

Organic-inorganic hybrid perovskite solar cells have generated wide interest due to the rapid development of their photovoltaic conversion efficiencies. However, the majority of the reported devices have been fabricated via spin coating with a device area of < 1 cm². In this study, we fabricated a wide-bandgap formamidinium lead bromide (FAPbBr₃) film using a cost-effective, high-yielding doctor-blade-coating process. The effects of different surfactants, such as 1-α-phosphatidylcholine, polyoxyethylene sorbitan monooleate, sodium lauryl sulfonate, and hexadecyl trimethyl ammonium bromide, were studied during the printing process. Accompanying the optimization of the blading temperature, crystal sizes of over 10 µm and large-area perovskite films of 5 cm × 5 cm were obtained using this method. The printed FAPbBr₃ solar cells exhibited a short-circuit current density of 8.22 mA/cm², an open-circuit voltage of 1.175 V, and an efficiency of 7.29%. Subsequently, we replaced the gold with silver nanowires as the top electrode to prepare a semitransparent perovskite solar cell with an average transmittance (400-800 nm) of 25.42%, achieving a high-power efficiency of 5.11%. This study demonstrates efficient doctor-blading printing for preparing large-area FAPbBr₃ films that possess high potential for applications in building integrated photovoltaics.

A New Wide Bandgap Donor Polymer for Efficient Nonfullerene Organic Solar Cells with a Large Open-Circuit Voltage

Significant progress has been made in nonfullerene small molecule acceptors (NF-SMAs) that leads to a consistent increase of power conversion efficiency (PCE) of nonfullerene organic solar cells (NF-OSCs). To achieve better compatibility with high-performance NF-SMAs, the direction of molecular design for donor polymers is toward wide bandgap (WBG), tailored properties, and preferentially ecofriendly processability for device fabrication. Here, a weak acceptor unit, methyl 2,5-dibromo-4-fluorothiophene-3-carboxylate (FE-T), is synthesized and copolymerized with benzo[1,2-b:4,5-b']dithiophene (BDT) to afford a series of nonhalogenated solvent processable WBG polymers P1-P3 with a distinct side chain on FE-T. The incorporation of FE-T leads to polymers with a deep highest occupied molecular orbital (HOMO) level of -5.60-5.70 eV, a complementary absorption to NF-SMAs, and a planar molecular conformation. When combined with the narrow bandgap acceptor ITIC-Th, the solar cell based on P1 with the shortest methyl chain on FE-T achieves a PCE of 11.39% with a large V oc of 1.01 V and a J sc of 17.89 mA cm-2. Moreover, a PCE of 12.11% is attained for ternary cells based on WBG P1, narrow bandgap PTB7-Th, and acceptor IEICO-4F. These results demonstrate that the new FE-T is a highly promising acceptor unit to construct WBG polymers for efficient NF-OSCs.

Comparison Study of Wide Bandgap Polymer (PBDB-T) and Narrow Bandgap Polymer (PBDTTT-EFT) as Donor for Perylene Diimide Based Polymer Solar Cells

Perylene diimide (PDI) derivatives as a kind of promising non-fullerene-based acceptor (NFA) have got rapid development. However, most of the relevant developmental work has focused on synthesizing novel PDI-based structures, and few paid attentions to the selection of the polymer donor in PDI-based solar cells. Wide bandgap polymer (PBDB-T) and narrow bandgap polymer (PBDTTT-EFT) are known as the most efficient polymer donors in polymer solar cells (PSCs). While PBDB-T is in favor with non-fullerene acceptors achieving power conversion efficiency (PCE) more than 12%, PBDTTT-EFT is one of the best electron donors with fullerene acceptors with PCE up to 10%. Despite the different absorption profiles, the working principle of these benchmark polymer donors with a same electron acceptor, specially PDI-based acceptors, was rarely compared. To this end, we used PBDB-T and PBDTTT-EFT as the electron donors, and 1,1'-bis(2-methoxyethoxyl)-7,7'-(2,5-thienyl) bis-PDI (Bis-PDI-T-EG) as the electron acceptor to fabricate PSCs, and systematically compared their differences in device performance, carrier mobility, recombination mechanism, and film morphology.

Solution-Processed Air-Stable Copper Bismuth Iodide for Photovoltaics

Bismuth-based solar cells have been under intensive interest as an efficient non-toxic absorber in photovoltaics. Within this new family of semiconductors, we herein report a new, long-term stable copper bismuth iodide (CuBiI₄ ). A solutionprocessed method under air atmosphere is used to prepare the material. The adopted HI-assisted dimethylacetamide (DMA) co-solvent can completely dissolve CuI and BiI₃ powders with high concentration compared with other organic solvents. Moreover, the high vapor pressure of tributyl phosphate, selected for the solvent vapor annealing (SVA), enables complete low-temperature (≤70 °C) film preparation, resulting in a stable, uniform, dense CuBiI₄ film. The average grain size increases with the precursor concentration, greatly improving the photoluminescence lifetime and hall mobility; a carrier lifetime of 3.03 ns as well as an appreciable hall mobility of 110 cm²  V^-1 s^-1 were obtained. XRD illustrates that the crystal structure is cubic (space group Fd3m) and favored in the [1 1 1] direction. Moreover, the photovoltaic performance of CuBiI₄ was also investigated. A wide bandgap (2.67 eV) solar cell with 0.82 % power conversion efficiency is presented, which exhibits excellent long-term stability over 1008 h under ambient conditions. This air-stable material may give an application in future tandem solar cells as a stable short-wavelength light absorber.

High Efficiency Nonfullerene Polymer Solar Cells with Thick Active Layer and Large Area

In this work, high-efficiency nonfullerene polymer solar cells (PSCs) are developed based on a thiazolothiazole-containing wide bandgap polymer PTZ1 as donor and a planar IDT-based narrow bandgap small molecule with four side chains (IDIC) as acceptor. Through thermal annealing treatment, a power conversion efficiency (PCE) of up to 11.5% with an open circuit voltage (V_oc ) of 0.92 V, a short-circuit current density (J_sc ) of 16.4 mA cm^-2 , and a fill factor of 76.2% is achieved. Furthermore, the PSCs based on PTZ1:IDIC still exhibit a relatively high PCE of 9.6% with the active layer thickness of 210 nm and a superior PCE of 10.5% with the device area of up to 0.81 cm² . These results indicate that PTZ1 is a promising polymer donor material for highly efficient fullerene-free PSCs and large-scale devices fabrication.

Matching Charge Extraction Contact for Wide-Bandgap Perovskite Solar Cells

Efficient wide-bandgap (WBG) perovskite solar cells are needed to boost the efficiency of silicon solar cells to beyond Schottky-Queisser limit, but they suffer from a larger open circuit voltage (V_OC ) deficit than narrower bandgap ones. Here, it is shown that one major limitation of V_OC in WBG perovskite solar cells comes from the nonmatched energy levels of charge transport layers. Indene-C60 bisadduct (ICBA) with higher-lying lowest-unoccupied-molecular-orbital is needed for WBG perovskite solar cells, while its energy-disorder needs to be minimized before a larger V_OC can be observed. A simple method is applied to reduce the energy disorder by isolating isomer ICBA-tran3 from the as-synthesized ICBA-mixture. WBG perovskite solar cells with ICBA-tran3 show enhanced V_OC by 60 mV, reduced V_OC deficit of 0.5 V, and then a record stabilized power conversion efficiency of 18.5%. This work points out the importance of matching the charge transport layers in perovskite solar cells when the perovskites have a different composition and energy levels.

Recent Advances in Wide-Bandgap Photovoltaic Polymers

The past decade has witnessed significant advances in the field of organic solar cells (OSCs). Ongoing improvements in the power conversion efficiency of OSCs have been achieved, which were mainly attributed to the design and synthesis of novel conjugated polymers with different architectures and functional moieties. Among various conjugated polymers, the development of wide-bandgap (WBG) polymers has received less attention than that of low-bandgap and medium-bandgap polymers. Here, we briefly summarize recent advances in WBG polymers and their applications in organic photovoltaic (PV) devices, such as tandem, ternary, and non-fullerene solar cells. Addtionally, we also dissuss the application of high open-circuit voltage tandem solar cells in PV-driven electrochemical water dissociation. We mainly focus on the molecular design strategies, the structure-property correlations, and the photovoltaic performance of these WBG polymers. Finally, we extract empirical regularities and provide invigorating perspectives on the future development of WBG photovoltaic materials.

Side-Chain Influence of Wide-Bandgap Copolymers Based on Naphtho[1,2-b:5,6-b]bispyrazine and Benzo[1,2-b:4,5-b']dithiophene for Efficient Photovoltaic Applications

In this work, we reported new types of wide-bandgap copolymers, PBDTA-NPz, PBDTT-NPz, PBDTF-NPz, and PBDTP-NPz, on the basis of the naphtho[1,2-b:5,6-b]bispyrazine (NPz) acceptor building block for efficient photovoltaic applications. The influencing factors of the introduced side chains, including alkoxyl, alkylthienyl, alkylfuryl, and alkoxylphenyl, were investigated in detail. These copolymers possessed wide bandgaps ranging from 1.79 to 1.88 eV with different nonconjugated or conjugated side chains. They also possessed deep highest-occupied molecular orbital levels of less than -5.25 eV, which allowed the achievement of high V_oc's from their polymer solar cells (PSCs). The X-ray diffraction results indicated their excellent crystallinity and molecular stacking features, especially for PBDTF-NPz containing alkylfuryl side chains. Their photovoltaic performances were measured using bulk-heterojunction single-junction PSCs with a configuration of ITO/PEDOT:PSS/copolymer:PC₇₁BM/Ca/Al under the same processing conditions. Different side chains of NPz-based copolymers induced largely different device performances. Without the additive, 1,8-diiodooctane (DIO), the primary PBDTA-NPz, PBDTT-NPz, PBDTF-NPz, and PBDTP-NPz devices showed power conversion efficiencies (PCEs) of 4.53, 6.09, 7.06, and 3.49%, respectively. On adding 3 vol % DIO, the device performances were elevated to a higher level. The PBDTF-NPz devices exhibited the highest PCE of 8.63%, which resulted in improved V_oc, J_sc, and FF values caused by their inherent properties. Our results indicated that NPz is a potential acceptor unit to construct high-powered wide-bandgap copolymers for efficient PSCs in the future.

Efficient Nonfullerene Polymer Solar Cells Enabled by a Novel Wide Bandgap Small Molecular Acceptor

A wide bandgap small molecular acceptor, SFBRCN, containing a 3D spirobifluorene core flaked with a 2,1,3-benzothiadiazole (BT) and end-capped with highly electron-deficient (3-ethylhexyl-4-oxothiazolidine-2-yl)dimalononitrile (RCN) units, has been successfully synthesized as a small molecular acceptor (SMA) for nonfullerene polymer solar cells (PSCs). This SMA exhibits a relatively wide optical bandgap of 2.03 eV, which provides a complementary absorption to commonly used low bandgap donor polymers, such as PTB7-Th. The strong electron-deficient BT and RCN units afford SFBRCN with a low-lying LUMO (lowest unoccupied molecular orbital) level, while the 3D structured spirobifluorene core can effectively suppress the self-aggregation tendency of the SMA, thus yielding a polymer:SMA blend with reasonably small domain size. As the results of such molecular design, SFBRCN enables nonfullerene PSCs with a high efficiency of 10.26%, which is the highest performance reported to date for a large bandgap nonfullerene SMA.

Graphene-β-Ga2 O3 Heterojunction for Highly Sensitive Deep UV Photodetector Application

A deep UV light photodetector is assembled by coating multilayer graphene on beta-gallium oxide (β-Ga₂ O₃ ) wafer. Optoelectronic analysis reveals that the heterojunction device is virtually blind to light illumination with wavelength longer than 280 nm, but is highly sensitive to 254 nm light with very good stability and reproducibility.

Adjusting Nitrogen Atom Orientations of Pyridine Ring in Tetraphenylsilane-Based Hosts for Highly Efficient Blue Phosphorescent Organic Light-Emitting Devices

Four wide bandgap host materials, namely, 9-(4-diphenyl(4-(pyridin-3-yl)phenyl)silyl-phenyl)-9H-carbazole (CSmP), 9-(4-diphenyl(4-(pyridin-2-yl)phenyl)silylphenyl)-9H-carbazole (CSoP), 9-(4-diphenyl(4-(pyridin-3-yl)phenyl)silylphenyl)-9H-3,9'-bicarbazole (DCSmP), and 9-(4-(diphenyl(4-(pyridin-2-yl)phenyl)silyl)phenyl)-9H-3,9'-bicarbazole (DCSoP), have developed by incorporation of pyridine with varied N atom orientation and carbazole/dimer carbazole units into the tetraphenylsilane skeleton for blue phosphorescent light-emitting diodes. These host materials all possess wide bandgap (3.54-3.64 eV) and high triplet energies (2.77-2.95 eV). As revealed by the absorption and emission spectra, theoretical calculations, and CV measurements, the N atom orientation exerts a strong influence on the LUMO energy level and electron-transportation behaviors without deterioring the photophysical properties. Among them, DCSmP with 3-pyridyl substituent manifests the best electron-transporting capability. The FIrpic-doped blue phosphorescent device using DCSmP as host material exhibits excellent electroluminescence performance with a maximum current efficiency of 40.1 cd A(-1) and a maximum external quantum efficiency of 20.0%. The current efficiency and external quantum efficiency are improved 3-fold, higher than those fabricated from DCSpP with 4-pyridyl as substituent, demonstrating an effective strategy for large improvement in device performance by a subtle change in molecular structure.

Stabilized Wide Bandgap MAPbBr x I3-x Perovskite by Enhanced Grain Size and Improved Crystallinity

The light instability of CH₃NH₃PbI _x Br_3-x is one of the biggest challenges for its application in tandem solar cells. Here we show that an improved crystallinity and grain size of CH₃NH₃PbI _x Br_3-x films could stabilize these materials under one sun illumination, improving both the efficiency and stability of the wide-bandgap perovskite solar cells.

Strong microwave absorption of hydrogenated wide bandgap semiconductor nanoparticles

Electromagnetic interactions in the microelectronvolt (μeV) or microwave region have numerous important applications in both civil and military fields, such as electronic communications, signal protection, and antireflective coatings on airplanes against microwave detection. Traditionally, nonmagnetic wide-bandgap metal oxide semiconductors lack these μeV electronic transitions and applications. Here, we demonstrate that these metal oxides can be fabricated as good microwave absorbers using a 2D electron gas plasma resonance at the disorder/order interface generated by a hydrogenation process. Using ZnO and TiO2 nanoparticles as examples, we show that large absorption with reflection loss values as large as -49.0 dB (99.99999%) is obtained in the microwave region. The frequency of absorption can be tuned with the particle size and hydrogenation condition. These results may pave the way for new applications for wide bandgap semiconductors, especially in the μeV regime.