Host-Free Deep-Blue Organic Light-Emitting Transistors Based on a Novel Fluorescent Emitter

Halogen-bonding boosting the high performance X-ray imaging of organic scintillators

Organic scintillators with efficient X-ray excited luminescence are essential for medical diagnostics and security screening. However, achieving excellent organic scintillation materials is challenging due to low X-ray absorption coefficients and inferior radioluminescence (RL) intensity. Herein, supramolecular interactions are incorporated, particularly halogen bonding, into organic scintillators to enhance their radioluminescence properties. By introducing heavy atoms (X = Cl, Br, I) into 9,10-bis(4-pyridyl)anthracene (BPA), the formation of halogen bonding (BPA-X) enhances their X-ray absorption coefficient and restricts the molecular vibration and rotation, which boosts their RL intensity. The RL intensity of BPA-Cl and BPA-Br fluorochromes increased by over 2 and 6.3 times compared to BPA, respectively. Especially, BPA-Br exhibits an ultrafast decay time of 8.25 ns and low detection limits of 25.95 ± 2.49 nGy s-1. The flexible film constructed with BPA-Br exhibited excellent X-ray imaging capabilities. Furthermore, this approach is also applicable to organic phosphors. The formation of halogen bonding in bromophenyl-methylpyridinium iodide (PYI) led to a fourfold increase in RL intensity compared to bromophenyl-methyl-pyridinium (PY). It suggests that halogen bonding serves as a promising and effective molecular design strategy for the development of high-performance organic scintillator materials, presenting new opportunities for their applications in radiology and security screening.

Organic Polarized Light-Emitting Transistors

Electrically driven polarized light-emitting sources are central to various applications including quantum computers, optical communication, and 3D displays, but serious challenges remain due to the inevitable incorporation of complex optical elements in conventional devices. Here, organic polarized light-emitting transistors (OPLETs), a kind of novel device that integrates the functions of organic field-effect transistors, organic light-emitting diodes, and polarizers into one unique device, are demonstrated with a degree of polarization (DOP) as high as 0.97, which is comparable to completely linearly polarized light (DOP = 1). Under the modulation of gate voltage, robust and efficient polarization emission is proven, ascribed to the intrinsic in-plane anisotropy of the molecular transition dipole moment in organic semiconductors and the open-ended feature of OPLETs instead of other factors. As a result, high-contrast optical imaging and anti-counterfeiting security are successfully demonstrated based on OPLETs, establishing a new direction for photonic and electronic integration toward on-chip miniaturized optoelectronic applications.

Improving the Performance of Red Organic Light-Emitting Transistors by Utilizing a High-k Organic/Inorganic Bilayer Dielectric

Integration of electrical switching and light emission in a single unit makes organic light-emitting transistors (OLETs) highly promising multifunctional devices for next-generation active-matrix flat-panel displays and related applications. Here, high-performance red OLETs are fabricated in a multilayer configuration that incorporates a zirconia (ZrO_x)/cross-linked poly(vinyl alcohol) (C-PVA) bilayer as a dielectric. The developed organic/inorganic bilayer dielectric renders high dielectric constant as well as improved dielectric/semiconductor interface quality, contributing to enhanced carrier mobility and high current density. In addition, an efficient red phosphorescent organic emitter doped in a bihost system is employed as the emitting layer for an effective exciton formation and light generation. Consequently, our optimized red OLETs displayed a high brightness of 16 470 cd m^-2 and a peak external quantum efficiency of 11.9% under a low gate and source-drain voltage of -24 V. To further boost the device performance, an electron-blocking layer is introduced for ameliorated charge-carrier balance and hence suppressed exciton-charge quenching, which resulted in an improved maximum brightness of 20 030 cd m^-2. We anticipate that the new device optimization approaches proposed in this work would spur further development of efficient OLETs with high brightness and curtailed efficiency roll-off.

Redistributed Current Density in Lateral Organic Light-Emitting Transistors Enabling Uniform Area Emission with Good Stability and Arbitrary Tunability

Organic light-emitting transistors (OLETs), integrating the functions of an organic field-effect transistor (OFET) and organic light-emitting diode (OLED) in a single device, are promising for the next-generation display technology. However, the great challenge of achieving uniform area emission in OLETs with good stability and arbitrary tunability hinders their development in this field. Herein, an effective solution to obtain well-defined area emission in lateral OLETs by incorporating a charge-transport buffer (CTB) layer between the conducting channel and emitting layer is proposed. Comprehensive theoretical simulation and experimental results demonstrate redistributed potential beneath the drain electrode under the shielding effect of the CBT layer, resulting in a highly uniform current density. In this case, uniform recombination of balanced holes and electrons can be guaranteed, which is essential for the formation of area emission in the following OLETs. RGB OLETs with uniform area emission are constructed, which show good gate tunable ability (ON/OFF ratio 10⁶ ), high loop stability (over 200 cycles) and high aperture ratio (over 80%) due to the arbitrary tunability of the device geometry. This work provides a new avenue for constructing area-emission lateral OLETs, which have great potential for display technology because of their good compatibility with conventional fabrication techniques.

Understanding and Tailoring Excited State Properties in Solution-Processable Oligo(p-phenyleneethynylene)s: Highly Fluorescent Hybridized Local and Charge Transfer Character via Experiment and Theory

Rod-shaped oligo(p-phenyleneethynylene) (OPE) offers an attractive π-framework for the development of solution-processable highly fluorescent molecules having tunable hybridized local and charge transfer (HLCT) excited states and (reverse) intersystem crossing ((R)ISC) channels. Herein, an HLCT oligo(p-phenyleneethynylene) library was studied for the first time in the literature in detail systematically via experiment and theory. The design, synthesis, and full characterization of a new highly fluorescent (Φ_PL-solution ∼ 1) sky blue emissive 4',4‴-((2,5-bis((2-ethylhexyl)oxy)-1,4-phenylene)bis(ethyne-2,1-diyl))bis(N,N-diphenyl-[1,1'-biphenyl]-4-amine) (2EHO-TPA-PE) was also reported. The new molecule consists of a D'-Ar-π-D-π-Ar-D' molecular architecture with an extended π-spacer and no acceptor unit, and detailed structural, physicochemical, single-crystal, and optoelectronic characterizations were performed. A high solid-state quantum efficiency (Φ_{PL-solid state} ∼ 0.8) was achieved as a result of suppressed exciton-phonon/vibronic couplings (no π-π interactions and multiple (14 per dimeric form) strong C-H···π interactions). Strong solution-phase/solid-state dipole-dependent tunable excited state behavior (local excited (LE) → HLCT → charge transfer (CT)) and decay dynamics covering a wide spectral region were demonstrated, and the CT state was observed to be highly fluorescent despite extremely large Stokes shift (∼130 nm)/fwhm (∼125 nm) and significant charge separation (0.75 charge·nm). Employing the Lippert-Mataga model, along with detailed photophysical studies and TDDFT calculations, key relationships between molecular design-electronic structure-exciton characteristics were elucidated with regards to HLCT and hot exciton channel formations. The interstate coupling between CT and LE states and the interplay of this coupling with respect to medium polarity were explored. A key relationship between excited-state symmetry breaking process and the formation of HLCT state was discussed for TPA-ended rod-shaped OPE π-systems. (R)ISC-related delayed fluorescence (τ ∼ 2-6 ns) processes were evident following the prompt decays (∼0.4-0.9 ns) both in the solution and in the solid-state. As a unique observation, the delayed fluorescence could be tuned and facilitated via small dielectric changes in the medium. Our results and the molecular engineering perspectives presented in this study may provide unique insights into the structural and electronic factors governing tunable excited state and hot-exciton channel formations in OPEs for (un)conventional solution-processed luminescence applications.

Recent Advances in Multi-Layer Light-Emitting Heterostructure Transistors

Light-emitting transistors (LETs) have attracted tremendous academic and industrial interest due to their dual functions of electrical switching and light emission in a single device, which can considerably reduce system complexity and manufacturing costs, especially in the area of flat panel and flexible displays as well as lighting and lasers. In recent years, enhanced LET performance has been achieved by introducing multiple-layer heterostructures in the charge-carrying/light-emitting LET channel versus the best-reported performance in single active layer LETs, rendering multi-layer LETs promising candidates for next-generation display technologies. In this review, the fundamental structures and working principles of multi-layer heterostructure LETs are introduced. Next, developments in multi-layer LETs are discussed based on co-planar LETs, non-planar LETs, and vertical LETs including organic, quantum dot, and perovskite light emitters. Finally, this review concludes with a summary and a perspective on the future of this research field.