Frontier Molecular Orbital Engineering of Aromatic Donor Fusion: Modularly Constructing Highly Efficient Narrowband Yellow Electroluminescence

The development of high-performance multiple resonance thermally activated delayed fluorescence (MR-TADF) materials with narrowband yellow emission is highly critical for various applications in industries, such as the automotive, aerospace, and microelectronic industries. However, the modular construction approaches to expeditiously access narrowband yellow-emitting materials is relatively rare. Here, a unique molecular design concept based on frontier molecular orbital engineering (FMOE) of aromatic donor fusion is proposed to strategically address this issue. Donor fusion is a modular approach with a “leveraging effect”; through direct polycyclization of donor attached to the MR parent core, it is facile to achieve red-shifted emission by a large margin. As a result, two representative model molecules, namely BN-Cz and BN-Cb, have been constructed successfully. The BN-Cz- and BN-Cb-based sensitized organic light-emitting diodes (OLEDs) exhibit bright yellow emission with peaks of 560 and 556 nm, full-width at half-maxima (fwhm’s) of 49 and 45 nm, Commission Internationale de L’Eclairage coordinates of (0.44, 0.55) and (0.43, 0.56), and maximum external quantum efficiencies (EQEs) of 32.9% and 29.7%, respectively. The excellent optoelectronic performances render BN-Cz and BN-Cb one of the most outstanding yellow-emitting MR-TADF materials

Two-Dimensional Organic Single Crystals with Scale Regulated, Phase-Switchable, Polymorphism-Dependent, and Amplified Spontaneous Emission Properties

The successful preparation of two-dimensional (2D) single crystals can promote the development of organic optoelectronic devices with excellent performance. A Schiff base compound salicylidene­(4-dimethylamino)­aniline with aggregation induced emission (AIE) property was employed as the building block to fabricate 2D thin single crystal plates with scales from around 50 μm to 1.5 cm. Yellow and red emissive polymorphs were concomitantly obtained during crystallization. The single-crystal-to-single-crystal (SC-to-SC) transformation from yellow polymorph to red one was demonstrated. Furthermore, both polymorphs exhibited amplified spontaneous emission (ASE) properties. Interestingly, the red polymorph displayed size-dependent ASE characteristics. The larger red polymorph showed near-infrared ASE with maximum at 706 nm, whereas the smaller one presented red ASE with maximum at 610 nm. These results suggest that the different scale single crystalline thin films with perfect optoelectronic properties may be fabricated by using the organic molecules with 2D assembly feature

High Performance Small-Molecule Cathode Interlayer Materials with D‑A‑D Conjugated Central Skeletons and Side Flexible Alcohol/Water-Soluble Groups for Polymer Solar Cells

A new class of organic cathode interfacial layer (CIL) materials based on isoindigo derivatives (IID) substituted with pyridinium or sulfonate zwitterion groups were designed, synthesized, and applied in polymer solar cells (PSCs) with PTB7:PC₇₁BM (PTB7: polythieno­[3,4-<i>b</i>]-thiophene-<i>co</i>-benzodithiophene and PC₇₁BM: [6,6]-phenyl C71-butyric acidmethyl ester) as an active layer. Compared with the control device, PSCs with an IID-based CIL show simultaneous enhancement of open-circuit voltage (<i>V</i>_oc), short-circuit current (<i>J</i>_sc), and fill factor (FF). Systematic optimizations of the central conjugated core and side flexible alcohol-soluble groups demonstrated that isoindigo-based CIL material with thiophene and sulfonate zwitterion substituent groups can efficiently enhance the PSC performance. The highest power conversion efficiency (PCE) of 9.12%, which is 1.75 times that of the control device without CIL, was achieved for the PSC having an isoindigo-based CIL. For the PSCs with an isoindigo-based CIL, the molecule-dependent performance property studies revealed that the central conjugated core with D-A-D characteristics and the side chains with sulfonate zwitterions groups represents an efficient strategy for constructing high performance CILs. Our study results may open a new avenue toward high performance PSCs

Large π‑Conjugated Quinacridone Derivatives: Syntheses, Characterizations, Emission, and Charge Transport Properties

Two 11-ring-fused quinacridone derivatives, TTQA and DCNTTQA, have been synthesized by ferric chloride mediated cyclization and Knoevenagel reaction. Replacement of the carbonyl groups (in TTQA) with dicyanoethylene groups (in DCNTTQA) not only red-shifted the emission to the near-infrared region but also led to a nonplanar skeleton that significantly improved the solubility of DCNTTQA. Moreover, dicyanoethylene groups rendered DCNTTQA low-lying HOMO and LUMO levels. DCNTTQA-based solution-processed field-effect transistors showed a hole mobility up to 0.217 cm² V^–1 s^–1

Single-Molecule-based White-Light Emissive Organic Solids with Molecular-Packing-Dependent Thermally Activated Delayed Fluorescence

White-light-emitting single molecules have attracted broad attention because of their great potential for use in flat-panel displays and future light sources. We report a unique molecule of 3-(diphenylamino)-9<i>H</i>-xanthen-9-one (3-DPH-XO), which was found to exhibit bright white-light emission in the solid state caused by the spontaneous formation of a mixture with different polymorphs. Single-crystal analyses demonstrate that noncovalent interactions (such as π···π stacking, hydrogen bonding, and C–H···π interactions) induce different stacking arrangements (polymorphs <b>A</b>, <b>B</b>, and <b>C</b>) with different photophysical properties in a molecular solid. In addition, crystals <b>B</b> and <b>C</b> with the acceptor···acceptor stacking feature show the thermally activated delayed fluorescence (TADF) characteristics, indicating that appropriate noncovalent interactions could enhance the reverse intersystem crossing process and consequently lead to delayed fluorescence. This discovery provides an effective strategy for the design of new white-light-emitting single molecules as well as TADF materials

Highly Efficient Long-Wavelength Thermally Activated Delayed Fluorescence OLEDs Based on Dicyanopyrazino Phenanthrene Derivatives

Highly efficient long-wavelength thermally activated delayed fluorescence (TADF) materials are developed using 2,3-dicyanopyrazino phenanthrene (DCPP) as the electron acceptor (A), and carbazole (Cz), diphenylamine (DPA), or 9,9-dimethyl-9,10-dihydroacridine (DMAC) as the electron donor (D). Because of the large, rigid π-conjugated structure and strong electron-withdrawing capability of DCPP, TADF molecules with emitting colors ranging from yellow to deep-red are realized with different electron-donating groups and π-conjugation length. The connecting modes between donor and acceptor, that is, with or without the phenyl ring as π-bridge, are also investigated to study the π-bridge effect on the thermal, photophysical, electrochemical, and electroluminescent properties. Yellow, orange, red, and deep-red organic light-emitting diodes (OLEDs) based on DCPP derivatives exhibit high efficiencies of 47.6 cd A^–1 (14.8%), 34.5 cd A^–1 (16.9%), 12.8 cd A^–1 (10.1%), and 13.2 cd A^–1 (15.1%), with Commission Internationale de L’Eclairage (CIE) coordinates of (0.44, 0.54), (0.53, 0.46), (0.60, 0.40), and (0.64, 0.36), respectively, which are among the best values for long-wavelength TADF OLEDs

Multiheterojunction Phototransistors Based on Graphene–PbSe Quantum Dot Hybrids

Graphene-semiconductor quantum dot (QD) hybrid field effect phototransistors (FEpTs) have attracted much interest due to their ultrahigh gain and responsivity in photo detection. However, most reported results are based on single-layer heterojunction, and the multiheterojunction FEpTs are often ignored. Here, we design two typical multiheterojunction FEpTs based on graphene–PbSe quantum dot (QD) hybrids, including QD at the bottom layer (QD-bottom) and graphene at the bottom layer (G-bottom) FEpTs. Through a comparative study, G-bottom FEpTs showed a multisaturation behavior due to the multigraphene layer effect, which was absent in the QD-bottom FEpTs. The mobilities for electrons and holes were μ_E = 147 cm² V^–1 s^–1 and μ_E = 137 cm² V^–1 s^–1 in the G-bottom FEpTs and μ_E = 14 cm² V^–1 s^–1 and μ_E = 59 cm² V^–1 s^–1 in the QD-bottom FEpTs. Higher responsivity (∼10⁶ A W^–1) and faster response rate were both achieved by the G-bottom FEpTs. All of the advantages in G-bottom FEpTs were attributed to the back-gate effect. Therefore, high performance is expected in those FEpTs whose heterojunctions are designed to be close to the back-gate

Ambipolar Quantum-Dot-Based Low-Voltage Nonvolatile Memory with Double Floating Gates

Considerable research efforts have been devoted to promoting memory performance, especially the memory window and retention time. In this work, we develop an innovative field-effect-transistor memory with graphene oxide (GO)/gold nanoparticles (Au NPs) as double floating gates (DFG) and PbS quantum dots (QDs) as the semiconductor layer. QDs can provide both electrons and holes in the channel, which offers a chance for the floating gates to trap both of them to achieve bidirectional threshold voltage shifts after programming and erasing operations. Due to the DFG structure covering the GO sheets on the Au NP monolayer, the enhanced memory window (∼2.72 V at a programming/erasing voltage of ±10 V) can be attributed to more charge carriers being trapped in the floating gates. More importantly, because of the different energy levels between GO and Au NPs, the DFG construction brings about an energy barrier that prevents the trapped charges from leaking back to the channel, so that the retention capability is significantly improved. The outstanding memory performance will give the QD-based DFG memory great potential to have its own place in the flash memory market

PbS-Decorated WS₂ Phototransistors with Fast Response

Tungsten disulfide (WS₂), as a typical metal dichalcogenides (TMDs), has aroused keen research interests in photodetection. Here, field effect phototransistors (FE_pTs) based on heterojunction between monolayer WS₂ and PbS colloidal quantum dots are demonstrated to show high photoresponsivity (up to ∼14 A/W), wide electric bandwidth (∼396 Hz), and excellent stability. Meanwhile, the devices exhibit fast photoresponse times of ∼153 μs (rise time) and ∼226 μs (fall time) due to the assistance of heterojunction on the transfer of photoexcitons. Therefore, excellent device performances strongly underscore monolayer WS₂–PbS quantum dot as a promising material for future photoelectronic applications

Broadband Phototransistor Based on CH₃NH₃PbI₃ Perovskite and PbSe Quantum Dot Heterojunction

Organic lead halide perovskites have received a huge amount of interest since emergence, because of tremendous potential applications in optoelectronic devices. Here field effect phototransistors (FE_pTs) based on CH₃NH₃PbI₃ perovskite/PbSe colloidal quantum dot heterostructure are demonstrated. The high light absorption and optoelectric conversion efficiency, due to the combination of perovskite and quantum dots, maintain the responsivities in a high level, especially at 460 nm up to 1.2 A/W. The phototransistor exhibits bipolar behaviors, and the carrier mobilities are determined to be 0.147 cm²V^–1s^–1 for holes and 0.16 cm²V^–1s^–1 for electrons. The device has a wide spectral response spectrum ranging from 300 to 1500 nm. A short photoresponse time is less than 3 ms due to the assistance of heterojunction on the transfer of photoexcitons. The excellent performances presented in the device especially emphasize the CH₃NH₃PbI₃ perovskite–PbSe quantum dot as a promising material for future photoelectronic applications