Design of Intramolecular Dihedral Angle between Electronic Donor and Acceptor in Thermally Activated Delayed Fluorescence Molecules.

In order to improve the exciton utilization efficiency (ηexc) of organic light-emitting materials, we addressed the ideal donor-acceptor dihedral angle (θD-A) in the TADF molecule by striking a balance between two photophysical processes. One is the conversion of triplet excitons into singlet excitons, and the other is the radiative process from a low-lying excited state to the ground state. Using a combination of first-principles calculations and molecular dynamics simulations, we investigated the impact of θD-A on the splitting energy and spin-orbit coupling between singlet and triplet excitons as well as the transition dipole moment for carbazole benzonitrile (CzBN) derivatives. By comparison with the reverse intersystem crossing rate (krISC), fluorescence emission rate (kr), and ηexc, we proposed a potential highest ηexc (of 94.4%) with the ideal θD-A of 77° for blue light CzBN derivatives; the calculated results have a good agreement with experimental measurement. The structure-efficiency physical connection between the molecular structure (θD-A) and efficiency provided an ideal parameter for a potential candidate for blue TADF-OLED materials.

An all-two-dimensional Fe-FET retinomorphic sensor based on the novel gate dielectric In2Se3-xOx.

Two-dimensional (2D) ferroelectric field-effect transistors (Fe-FETs) have attracted extensive interest as a competitive platform for implementing future-generation functional electronics, including digital memory and brain-inspired computing circuits. In 2D Fe-FETs, the 2D ferroelectric materials are more suitable as gate dielectric materials compared to 3D ferroelectric materials. However, the current 2D ferroelectric materials (represented by α-In2Se3) need to be integrated with other 3D gate dielectric layers because of their high conductivity as a ferroelectric semiconductor. This 2D/3D hybrid structure can lead to compatibility problems in practical devices. In this study, a new 2D gate dielectric material that is compatible with the complementary metal-oxide semiconductor process was found by using oxygen plasma treatment. The 2D gate dielectric material obtained shows excellent performance, with an equivalent oxide thickness of less than 0.15 nm, and excellent insulation, with a leakage current of less than 2 × 10-5 A cm-2 (under a 1 V gate voltage). Based on this dielectric layer and the α-In2Se3 ferroelectric gate material, we fabricated an all-2D Fe-FET high-performance photodetector with a high on/off ratio (∼105) and detectivity (>1013 Jones). Moreover, the photoelectric device integrates perception, memory and computing characteristics, indicating that it can be applied to an artificial neural network for visual recognition.

Prediction of sub-pyramid texturing as the next step towards high efficiency silicon heterojunction solar cells.

The interfacial morphology of crystalline silicon/hydrogenated amorphous silicon (c-Si/a-Si:H) is a key success factor to approach the theoretical efficiency of Si-based solar cells, especially Si heterojunction technology. The unexpected crystalline silicon epitaxial growth and interfacial nanotwins formation remain a challenging issue for silicon heterojunction technology. Here, we design a hybrid interface by tuning pyramid apex-angle to improve c-Si/a-Si:H interfacial morphology in silicon solar cells. The pyramid apex-angle (slightly smaller than 70.53°) consists of hybrid (111)0.9/(011)0.1 c-Si planes, rather than pure (111) planes in conventional texture pyramid. Employing microsecond-long low-temperature (500 K) molecular dynamic simulations, the hybrid (111)/(011) plane prevents from both c-Si epitaxial growth and nanotwin formation. More importantly, given there is not any additional industrial preparation process, the hybrid c-Si plane could improve c-Si/a-Si:H interfacial morphology for a-Si passivated contacts technique, and wide-applied for all silicon-based solar cells as well.

Strain-Tunable Electronic and Transport Properties of One-Dimensional Fibrous Phosphorus Nanotubes.

The one-dimensional van der Waals (1D vdW) material fibrous red phosphorus (FRP) nanotubes are a promising direct-bandgap semiconductor with high carrier mobility and anisotropic optical responses because of low deformation potential and dangling-bond-free anisotropic interface. Employing first-principles calculations, we captured the potential of 1D FRP nanotubes. The thermal stability of 1D FRP nanotubes was confirmed by phonon calculation. Meanwhile, Raman spectroscopy indicated the strong vibration mode (366 cm-1) is along the phosphorus nanotube. Interestingly, spatial anisotropy bandgaps were found along with various stacking orientations. The charge transport calculations showed that the 1D FRP nanotube has a high hole mobility (499.2 cm2 V-1 s-1), considering the weak acoustic phonon scattering. More importantly, we found that the hole mobility changes dramatically (down to 7.1 cm2 V-1 s-1) under the strain, and the strain-dependent charge transport property of 1D FRP nanotubes could be considered to have many potential applications for electronics, optoelectronics, and switching devices.

A High-Performance In-Memory Photodetector Realized by Charge Storage in a van der Waals MISFET.

The emerging data-intensive applications in optoelectronics are driving innovation toward the fused integration of sensing, memory, and computing to break through the restrictions of the von Neumann architecture. However, the present photodetectors with only optoelectronic conversion functions cannot satisfy the growing demands of the multifunctions required in single devices. Here, a novel route for the integration of non-volatile memory into a photodetector is proposed, with a WSe2 /h-BN van der Waals heterostructure on a Si/SiO2 substrate to realize in-memory photodetection. This photodetector exhibits an ultrahigh readout photocurrent of 3.4 µA and photoresponsivity of 337.8 A W-1 in the solar-blind wavelength region, together with an extended retention time of more than 10 years. Furthermore, the charge-storage-based non-volatile mechanism of h-BN/SiO2 is successfully proven through a novel analysis of in situ optoelectronic electron energy-loss spectroscopy. These results represent a leap forward to future applications and insightful mechanisms of in-memory photodetection.

High performance sub-bandgap photodetection via internal photoemission based on ideal metal/2D-material van der Waals Schottky interface.

Two-dimensional (2D) materials have been demonstrated to be promising candidates to design high performance photodetectors owing to their strong light-matter interaction. However, the performance of 2D material photodetectors is still unsatisfactory, such as slow response speed due to defects and vulnerable contact interface, which impede their rapid development in the field of optoelectronics. In this paper, we obtained the ideal and large photosensitive van der Waals Schottky interface by the laminating-flipping method. Hence, a fast response speed (<1 ms) and high detectivity (>1012 Jones) are observed on the van der Waals Schottky junction photodiode. More importantly, benefiting from the flat Schottky interface (the roughness ∼0.6 nm), a sub-bandgap light response modulated by the Schottky barrier height (cut-off edge at 1050 nm) has been detected based on the large Au/MoSe2 sensitive Schottky interface internal photoemission. As a result, a universal strategy for the sub-bandgap near-infrared van der Waals Schottky junction detector of 2D materials was obtained.

Strain Effect Enhanced Ultrasensitive MoS2 Nanoscroll Avalanche Photodetector.

Two-dimensional (2D) materials and their derived quasi one-dimensional structure provide incredible possibilities for the field of photoelectric detection due to their intrinsic optical and electrical properties. However, the photogenerated carriers in atomically thin media are poor due to the low optical absorption, which greatly limits their performance. Here, in the MoS2 nanoscroll photodetector, we meticulously investigated the avalanche multiplication effect. The results show that by employing the nanoscroll structure, the required threshold electrical field for triggering avalanche multiplication is significantly lower than that of MoS2 flake due to the modulation of the energy band and intervalley scattering through the strain effect. Consequently, avalanche multiplication could efficiently enhance the photoresponsivity to >104 A/W. Furthermore, enhanced avalanche multiplication could be generalized to other TMDCs through theoretical prediction. The results not only are significant for the understanding of the intrinsic nature of 2D materials but also reveal meaningful advances in high-performance and low-power consumption photodetection.

Enhanced Performance of a CVD MoS2 Photodetector by Chemical in Situ n-Type Doping.

Transition metal dichalcogenides (TMDs) are a category of promising two-dimensional (2D) materials for the optoelectronic devices, and their unique characteristics include tunable band gap, nondangling bonds as well as compatibility to large-scale fabrication, for instance, chemical vapor deposition (CVD). MoS2 is one of the first TMDs that is well studied in the photodetection area widely. However, the low photoresponse restricts its applications in photodetectors unless the device is applied with ultrahigh source-drain voltage ( VDS) and gate voltage ( VGS). In this work, the photoresponse of a MoS2 photodetector was improved by a chemical in situ doping method using gold chloride hydrate. The responsivity and specific detectivity were increased to 99.9 A/W and 9.4 × 1012 Jones under low VDS (0.1 V) and VGS (0 V), which are 14.6 times and 4.8 times higher than those of a pristine photodetector, respectively. The photoresponse enhancement results from chlorine n-type doping in CVD MoS2 which reduces the trapping of photoinduced electrons and promotes the photogating effect. This novel doping strategy leads to great applications of high-performance MoS2 photodetectors potentially and opens a new avenue to enhance photoresponse for other 2D materials.