Huanhuan Li

Thermally Activated Delayed Fluorescence Materials Towards the Breakthrough of Organoelectronics

The design and characterization of thermally activated delayed fluorescence (TADF) materials for optoelectronic applications represents an active area of recent research in organoelectronics. Noble metal-free TADF molecules offer unique optical and electronic properties arising from the efficient transition and interconversion between the lowest singlet (S1 ) and triplet (T1 ) excited states. Their ability to harvest triplet excitons for fluorescence through facilitated reverse intersystem crossing (T1 →S1 ) could directly impact their properties and performances, which is attractive for a wide variety of low-cost optoelectronic devices. TADF-based organic light-emitting diodes, oxygen, and temperature sensors show significantly upgraded device performances that are comparable to the ones of traditional rare-metal complexes. Here we present an overview of the quick development in TADF mechanisms, materials, and applications. Fundamental principles on design strategies of TADF materials and the common relationship between the molecular structures and optoelectronic properties for diverse research topics and a survey of recent progress in the development of TADF materials, with a particular emphasis on their different types of metal-organic complexes, D-A molecules, and fullerenes, are highlighted. The success in the breakthrough of the theoretical and technical challenges that arise in developing high-performance TADF materials may pave the way to shape the future of organoelectronics.

Dynamically Adaptive Characteristics of Resonance Variation for Selectively Enhancing Electrical Performance of Organic Semiconductors

Increased resonance: The selective tuning of the optoelectronic properties of organic semiconductors is possible by enantiotropic resonance variation. Using resonance forms of N(+)=P-O(-) in a series of arylamine-phosphine oxide hybrids afforded low-voltage-driven phosphorescent OLEDs with outstanding performances.

Understanding the Control of Singlet-Triplet Splitting for Organic Exciton Manipulating: A Combined Theoretical and Experimental Approach

Developing organic optoelectronic materials with desired photophysical properties has always been at the forefront of organic electronics. The variation of singlet-triplet splitting (ΔEST) can provide useful means in modulating organic excitons for diversified photophysical phenomena, but controlling ΔEST in a desired manner within a large tuning scope remains a daunting challenge. Here, we demonstrate a convenient and quantitative approach to relate ΔEST to the frontier orbital overlap and separation distance via a set of newly developed parameters using natural transition orbital analysis to consider whole pictures of electron transitions for both the lowest singlet (S1) and triplet (T1) excited states. These critical parameters revealed that both separated S1 and T1 states leads to ultralow ΔEST; separated S1 and overlapped T1 states results in small ΔEST; and both overlapped S1 and T1 states induces large ΔEST. Importantly, we realized a widely-tuned ΔEST in a range from ultralow (0.0003 eV) to extra-large (1.47 eV) via a subtle symmetric control of triazine molecules, based on time-dependent density functional theory calculations combined with experimental explorations. These findings provide keen insights into ΔEST control for feasible excited state tuning, offering valuable guidelines for the construction of molecules with desired optoelectronic properties.

A Solution-Processed Resonance Host for Highly Efficient Electrophosphorescent Devices with Extremely Low Efficiency Roll-off.

Solution-processible N-P=O resonance-host molecules are applied successfully in spin-coated phosphorescent light-emitting diodes (PHOLEDs) to selectively and self-adaptively tune the electrical properties for balanced charge transportation. The resonance-molecule-hosted PHOLEDs exhibit a high maximum quantum efficiency of 16.5% and a low efficiency roll-off down to 0.7%, highlighting the significant progress of these solution-processed PHOLEDs with high efficiency and flat efficiency roll-off achieved simultaneously.

Achieving Optimal Self-Adaptivity for Dynamic Tuning of Organic Semiconductors through Resonance Engineering

Current static-state explorations of organic semiconductors for optimal material properties and device performance are hindered by limited insights into the dynamically changed molecular states and charge transport and energy transfer processes upon device operation. Here, we propose a simple yet successful strategy, resonance variation-based dynamic adaptation (RVDA), to realize optimized self-adaptive properties in donor-resonance-acceptor molecules by engineering the resonance variation for dynamic tuning of organic semiconductors. Organic light-emitting diodes hosted by these RVDA materials exhibit remarkably high performance, with external quantum efficiencies up to 21.7% and favorable device stability. Our approach, which supports simultaneous realization of dynamically adapted and selectively enhanced properties via resonance engineering, illustrates a feasible design map for the preparation of smart organic semiconductors capable of dynamic structure and property modulations, promoting the studies of organic electronics from static to dynamic.

Heteroatom-bridged benzothiazolyls for organic solar cells: a theoretical study.

On the basis of a typical organic photovoltaic (OPV) building block of 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTBT), a series of novel DTBT derivatives were designed following a heteroatom-bridging strategy to take advantage of the diversified interactions between heteroatoms and π-conjugated systems. These heteroatom-bridged DTBTs, whose outer electron-rich thiophene moieties are covalently fastened to the central electron-deficient benzothiadiazole with additional heteroatom bridges, exhibit promising features for OPV applications with rigid molecular structures, properly lain frontier molecular orbitals (FMOs), broad and intense absorption spectra, and adequate charge transport properties, as revealed by systematic theoretical calculations on molecular geometries, FMOs, absorption spectra, and relaxation and reorganization energies. The structure-property relationship investigations show that the mono-/di-heteroatom bridging is effective not only in tuning the rigidity of the molecular geometries but also in adjusting the optoelectronic properties of the resulting materials. Among the studied heteroatoms, the C and Si were found to be the most efficient in designing novel molecules for OPV applications. These theoretical insights may provide a solid basis for experimental synthesis and device investigations of the proposed heteroatom-bridged DTBTs as potential high-performance building blocks for bulk heterojunction OPV molecules.

Selectively Modulating Triplet Exciton Formation in Host Materials for Highly Efficient Blue Electrophosphorescence

The concept of limiting the triplet exciton formation to fundamentally alleviate triplet-involved quenching effects is introduced to construct host materials for highly efficient and stable blue phosphorescent organic light-emitting diodes (PhOLEDs). The low triplet exciton formation is realized by small triplet exciton formation fraction and rate with high binding energy and high reorganization energy of triplet exciton. Demonstrated in two analogue molecules in conventional donor-acceptor molecule structure for bipolar charge injection and transport with nearly the same frontier orbital energy levels and triplet excited energies, the new concept host material shows significantly suppressed triplet exciton formation in the host to avoid quenching effects, leading to much improved device efficiencies and stabilities. The low-voltage-driving blue PhOLED devices exhibit maximum efficiencies of 43.7 cd A(-1) for current efficiency, 32.7 lm W(-1) for power efficiency, and 20.7% for external quantum efficiency with low roll-off and remarkable relative quenching effect reduction ratio up to 41%. Our fundamental solution for preventing quenching effects of long-lived triplet excitons provides exciting opportunities for fabricating high-performance devices using the advanced host materials with intrinsically small triplet exciton formation cross section.

Direct silicon–nitrogen bonded host materials with enhanced σ–π conjugation for blue phosphorescent organic light-emitting diodes

Silicon-containing ultrahigh-energy gap hosts (UGHs) have emerged as important candidates of high-performance host materials with high thermal stability and triplet energy for blue phosphorescent organic light-emitting diodes (PhOLEDs). However, the highest occupied molecular orbital (HOMO) of these UGHs are generally too deep to support balanced hole injection and transportation in devices. Here, we propose a new design strategy of UGHs by multiple introduction of strong electron-donating and high-triplet-energy units of carbazoles into the electron-accepting arylsilanes in the N–Si–N structure. The facilely synthesized carbazole-arylsilanes in one-step show high thermal stability, triplet energy and charge mobilities with high-lying HOMOs due to enhanced σ–π conjugation in the N–Si–N structure as revealed by combined experimental and theoretical investigations. Impressively, blue PhOLEDs hosted by these novel N–Si–N bonded UGHs exhibit an improved maximum current efficiency up to 39.5 cd A−1, a power efficiency of 27.4 lm W−1, and an external quantum efficiency of 24.2%, demonstrating significant advances in the design of UGHs by adjusting the d-orbital participation of π-conjugation to enhance the σ–π conjugation in donor (D)–acceptor (A) molecular architectures.

Efficient synthesis and photovoltaic properties of highly rigid perylene-embedded benzothiazolyls

Perylene-embedded benzothiazolyls, designed by 2D-fusing of dithienylbenzothiadiazoles (DTBT), have been synthesized and characterized for organic photovoltaic (OPV) applications. A new polymer based on the newly-developed electron-accepting building block exhibits excellent solubility, stability, and photoabsorbability with optimal bandgaps and frontier orbitals, achieving a power efficiency up to 3.22%.

Manipulating charge transport in a π-stacked polymer through silicon incorporation

Given the fundamental importance in charge transport engineering for device operation in molecular electronics, manipulating strategies and material design principles for desired applications are highly anticipated. In stark contrast to conventional organic electronic devices, designing organic semiconductors that perform effectively as molecular nanofuses remains a challenge. Based on a novel silicon-containing π-stacked polymer of silafluorene (PVMSiF), we have successfully fabricated a molecular nanofuse device with a high ON–OFF ratio of up to 4 × 106 for the first time. Using a combination of absorption and photoluminescence (PL) spectra, X-ray diffraction (XRD), micro-PL analysis supported by theoretical insights into unit and backbone geometries and wave function delocalizations provided by density functional theory (DFT) and molecular dynamics (MD) simulations, we have demonstrated the fundamental correlation between the polymer structures and the spectacular fuse-like resistive switching behaviors. It was shown that the manipulation of charge transport in π-stacked polymers is possible via silicon incorporation; molecular nanofuse devices based on π-stacked polymers can be realized following a silicon-stimulated filament mechanism with breakable π–π stacking at the charged states. These findings may have important consequences on future material studies and device applications.