Accelerated Redox Reactions Enable Stable Tin-Lead Mixed Perovskite Solar Cells.

The facile oxidation of Sn2+ to Sn4+ poses an inherent challenge that limits the efficiency and stability of tin-lead mixed (Sn-Pb) perovskite solar cells (PSCs) and all-perovskite tandem devices. In this work, we discover the sustainable redox reactions enabling self-healing Sn-Pb perovskites, where their intractable oxidation degradation can be recovered to their original state under light soaking. Quantitative and operando spectroscopies are used to investigate the redox chemistry, revealing that metallic Pb0 from the photolysis of perovskite reacts with Sn4+ to regenerate Pb2+ and Sn2+ spontaneously. Given the sluggish redox reaction kinetics, V3+ /V2+ ionic pair is designed as an effective redox shuttle to accelerate the recovery of Sn-Pb perovskites from oxidation. The target Sn-Pb PSCs enabled by V3+ /V2+ ionic pair deliver an improved power conversion efficiency (PCE) of 21.22 % and excellent device lifespan, retaining nearly 90 % of its initial PCE after maximum power point tracking under light for 1,000 hours.

Blue emitting exciplex for yellow and white organic light-emitting diodes.

White organic light-emitting diodes (WOLEDs) have several desirable features, but their commercialization is hindered by the poor stability of blue light emitters and high production costs due to complicated device structures. Herein, we investigate a standard blue emitting hole transporting material (HTM) N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)benzidine (NPB) and its exciplex emission upon combining with a suitable electron transporting material (ETM), 3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole (TAZ). Blue and yellow OLEDs with simple device structures are developed by using a blend layer, NPB:TAZ, as a blue emitter as well as a host for yellow phosphorescent dopant iridium (III) bis(4-phenylthieno[3,2-c]pyridinato-N,C2')acetylacetonate (PO-01). Strategic device design then exploits the ambipolar charge transport properties of tetracene as a spacer layer to connect these blue and yellow emitting units. The tetracene-linked device demonstrates more promising results compared to those using a conventional charge generation layer (CGL). Judicious choice of the spacer prevents exciton diffusion from the blue emitter unit, yet facilitates charge carrier transport to the yellow emitter unit to enable additional exciplex formation. This complementary behavior of the spacer improves the blue emission properties concomitantly yielding reasonable yellow emission. The overall white light emission properties are enhanced, achieving CIE coordinates (0.36, 0.39) and color temperature (4643 K) similar to daylight. Employing intermolecular exciplex emission in OLEDs simplifies the device architecture via its dual functionality as a host and as an emitter.

Large area inkjet-printed OLED fabrication with solution-processed TADF ink.

This work demonstrates successful large area inkjet printing of a thermally activated delayed fluorescence (TADF) material as the emitting layer of organic light-emitting diodes (OLEDs). TADF materials enable efficient light emission without relying on heavy metals such as platinum or iridium. However, low-cost manufacturing of large-scale TADF OLEDs has been restricted due to their incompatibility with solution processing techniques. In this study, we develop ink formulation for a TADF material and show successful ink jet printing of intricate patterns over a large area (6400 mm2) without the use of any lithography. The stable ink is successfully achieved using a non-chlorinated binary solvent mixture for a solution processable TADF material, 3-(9,9-dimethylacridin-10(9H)-yl)-9H-xanthen-9-one dispersed in 4,4'-bis-(N-carbazolyl)-1,1'-biphenyl host. Using this ink, large area ink jet printed OLEDs with performance comparable to the control spin coated OLEDs are successfully achieved. In this work, we also show the impact of ink viscosity, density, and surface tension on the droplet formation and film quality as well as its potential for large-area roll-to-roll printing on a flexible substrate. The results represent a major step towards the use of TADF materials for large-area OLEDs without employing any lithography.

Harvesting Triplet Excitons in High Mobility Emissive Organic Semiconductor for Efficiency Enhancement of Light-Emitting Transistors.

Organic light-emitting transistors (OLETs), a kind of highly integrated and minimized optoelectronic device, demonstrate great potential applications in various fields. The construction of high-performance OLETs requires the integration of high charge carrier mobility, strong emission, and high triplet exciton utilization efficiency in the active layer. However, it remains a significant long-term challenge, especially for single component active layer OLETs. Herein, the successful harvesting of triplet excitons in a high mobility emissive molecule, 2,6-diphenylanthracene (DPA), through the triplet-triplet annihilation process is demonstrated. By incorporating a highly emissive guest into the DPA host system, an obvious increase in photoluminescence efficiency along with exciton utilization efficiency results in an obvious enhancement of external quantum efficiency of 7.2 times for OLETs compared to the non-doped devices. Moreover, well-tunable multi-color electroluminescence, especially white emission with Commission Internationale del'Eclairage  of (0.31, 0.35), from OLETs is also achieved by modulating the doping concentration with a controlled energy transfer process. This work opens a new avenue for integrating strong emission and efficient exciton utilization in high-mobility organic semiconductors for high-performance OLETs and advancing their related functional device applications.

Amplified Spontaneous Emission from Zwitterionic Excited-State Intramolecular Proton Transfer.

The unique four-level photocycle characteristics of excited-state intramolecular proton transfer (ESIPT) materials enable population inversion and large spectral separation between absorption and emission through their respective enol and keto forms. This leads to minimal or no self-absorption losses, a favorable feature in acting as an optical gain medium. While conventional ESIPT materials with an enol-keto tautomerism process are widely known, zwitterionic ESIPT materials, particularly those with high photoluminescence, are scarce. Facilitated by the synthesis and characterization of a new family of 2-hydroxyphenyl benzothiazole (HBT) with fluorene substituents, HBT-Fl1 and HBT-Fl2, we herein report the first efficient zwitterionic ESIPT lasing material (HBT-Fl2). The zwitterionic ESIPT HBT-Fl2 not only shows a remarkably low solid-state amplified spontaneous emission (ASE) threshold of 5.3 µJ/cm2 with an ASE peak at 609 nm but also exhibits high ASE photostability. Coupled with its substantially large Stokes shift (≈236 nm ≈10,390 cm-1) and an extremely small overlap of excited-state absorption with ASE emission, comprehensive density functional theory (DFT) and time-dependent DFT studies reveal the zwitterionic characteristics of HBT-Fl2. In opposition to conventional ESIPT with π-delocalized tautomerism as observed in analogue HBT-Fl1 and parent HBT, HBT-Fl2 instead shows charge redistribution in the proton transfer through the fluorene conjugation. This structural motif provides a design tactic in the innovation of new zwitterionic ESIPT materials for efficient light amplification in red and longer-wavelength emission.

Cibalackrot Dendrimers for Hyperfluorescent Organic Light-Emitting Diodes.

Hyperfluorescent organic light-emitting diodes (HF-OLEDs) enable a cascading Förster resonance energy transfer (FRET) from a suitable thermally activated delayed fluorescent (TADF) assistant host to a fluorescent end-emitter to give efficient OLEDs with relatively narrowed electroluminescence compared to TADF-OLEDs. Efficient HF-OLEDs require optimal FRET with minimum triplet diffusion via Dexter-type energy transfer (DET) from the TADF assistant host to the fluorescent end-emitter. To hinder DET, steric protection of the end-emitters has been proposed to disrupt triplet energy transfer. In this work, the first HF-OLEDs based on structurally well-defined macromolecules, dendrimers is reported. The dendrimers contain new highly twisted dendrons attached to a Cibalackrot core, resulting in high solubility in organic solvents. HF-OLEDs based on dendrimer blend films are fabricated to show external quantum efficiencies of >10% at 100 cd m-2 . Importantly, dendronization with the bulky dendrons is found to have no negative impact to the FRET efficiency, indicating the excellent potential of the dendritic macromolecular motifs for HF-OLEDs. To fully prevent the undesired triplet diffusion, Cibalackrot dendrimers HF-OLEDs are expected to be further improved by adding additional dendrons to the Cibalackrot core and/or increasing dendrimer generations.

Low Light Amplification Threshold and Reduced Efficiency Roll-Off in Thick Emissive Layer OLEDs from a Diketopyrrolopyrrole Derivative.

External quantum efficiency (EQE) roll-off under high current injection has been one of the major limiting factors toward the development of organic semiconductor laser diodes (OSLDs). While significant progress in this regard has been made on organic semiconductors (OSCs) emitting in the blue-green region of the visible spectrum, OSCs with longer wavelength emission (>600 nm) have fallen behind in both material development and the advancement in device architectures suitable for the realization of OSLDs. Therefore, to make simultaneous incremental advancements, a host-guest system comprising of a high performing poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) polymer and an efficient small molecule laser dye, dithiophenyl diketopyrrolopyrrole (DT-DPP), is used. This combination provides an extremely low amplified spontaneous emission threshold of 4.2 µJ cm-2 at an emission wavelength of 620 nm. The solution-processed organic light-emitting diodes (OLEDs) fabricated using this system exhibit a high external quantum efficiency (EQE) of 2.6% with low efficiency roll-off and high current injection up to 90 A cm-2 to yield ultrahigh luminance of over 1.5 million cd m-2 .

Probing polaron-induced exciton quenching in TADF based organic light-emitting diodes.

Polaron-induced exciton quenching in thermally activated delayed fluorescence (TADF)-based organic light-emitting diodes (OLEDs) can lead to external quantum efficiency (EQE) roll-off and device degradation. In this study, singlet-polaron annihilation (SPA) and triplet-polaron annihilation (TPA) were investigated under steady-state conditions and their relative contributions to EQE roll-off were quantified, using experimentally obtained parameters. It is observed that both TPA and SPA can lead to efficiency roll-off in 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) doped OLEDs. Charge imbalance and singlet-triplet annihilation (STA) were found to be the main contributing factors, whereas the device degradation process is mainly dominated by TPA. It is also shown that the impact of electric field-induced exciton dissociation is negligible under the DC operation regime (electric field < 0.5 MV cm-1). Through theoretical simulation, it is demonstrated that improvement to the charge recombination rate may reduce the effect of polaron-induced quenching, and thus significantly decrease the EQE roll-off.

Application of Triplet-Triplet Annihilation Upconversion in Organic Optoelectronic Devices: Advances and Perspectives.

Organic semiconductor materials have been widely used in various optoelectronic devices due to their rich optical and/or electrical properties, which are highly related to their excited states. Therefore, how to manage and utilize the excited states in organic semiconductors is essential for the realization of high-performance optoelectronic devices. Triplet-triplet annihilation (TTA) upconversion is a unique process of converting two non-emissive triplet excitons to one singlet exciton with higher energy. Efficient optical-to-electrical devices can be realized by harvesting sub-bandgap photons through TTA-based upconversion. In electrical-to-optical devices, triplets generated after the combination of electrons and holes also can be efficiently utilized via TTA, which resulted in a high internal conversion efficiency of 62.5%. Currently, many interesting explorations and significant advances have been demonstrated in these fields. In this review, a comprehensive summary of these intriguing advances on developing efficient TTA upconversion materials and their application in optoelectronic devices is systematically given along with some discussions. Finally, the key challenges and perspectives of TTA upconversion systems for further improvement for optoelectronic devices and other related research directions are provided. This review hopes to provide valuable guidelines for future related research and advancement in organic optoelectronics.

Liquid-phase sintering of lead halide perovskites and metal-organic framework glasses.

Lead halide perovskite (LHP) semiconductors show exceptional optoelectronic properties. Barriers for their applications, however, lie in their polymorphism, instability to polar solvents, phase segregation, and susceptibility to the leaching of lead ions. We report a family of scalable composites fabricated through liquid-phase sintering of LHPs and metal-organic framework glasses. The glass acts as a matrix for LHPs, effectively stabilizing nonequilibrium perovskite phases through interfacial interactions. These interactions also passivate LHP surface defects and impart bright, narrow-band photoluminescence with a wide gamut for creating white light-emitting diodes (LEDs). The processable composites show high stability against immersion in water and organic solvents as well as exposure to heat, light, air, and ambient humidity. These properties, together with their lead self-sequestration capability, can enable breakthrough applications for LHPs.

Tunable Light-Emission Properties of Solution-Processable N-Heterocyclic Carbene Cyclometalated Gold(III) Complexes for Organic Light-Emitting Diodes.

N-Heterocyclic carbene (NHC) cyclometalated gold(III) complexes remain very scarce and therefore their photophysical properties remain currently underexplored. Moreover, gold(III) complexes emitting in the blue region of the electromagnetic spectrum are rare. In this work, a series of four phosphorescent gold(III) complexes was investigated bearing four different NHC monocyclometalated (C^C*)-type ligands and a dianionic (N^N)-type ancillary ligand ((N^N)=5,5'-(propane-2,2-diyl)bis(3-(trifluoromethyl)-1 H-pyrazole) (mepzH2 )). The complexes exhibit strong phosphorescence when doped in poly(methyl methacrylate) (PMMA) at room temperature, which were systematically tuned from sky-blue [λPL =456 nm, CIE coordinates: (0.20, 034)] to green [λPL =516 nm, CIE coordinates: (0.31, 0.54)] by varying the monocyclometalated (C^C*) ligand framework. The complexes revealed high quantum efficiencies (ϕPL ) of up to 43 % and excited-state lifetimes (τ0 ) between 15-266 µs. The radiative rate constant values found for these complexes (kr =103 -104  s-1 ) are the highest found in comparison to previously known best-performing monocyclometalated gold(III) complexes. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations of these complexes further lend support to the excited-state nature of these complexes. The calculations showed a significant contribution of the gold(III) metal center in the lowest unoccupied molecular orbitals (LUMOs) of up to 18 %, which was found to be unique for this class of cyclometalated gold(III) complexes. Additionally, organic light-emitting diodes (OLEDs) were fabricated by using a solution process to provide the first insight into the electroluminescent (EL) properties of this new class of gold(III) complexes.

Solid cyclooctatetraene-based triplet quencher demonstrating excellent suppression of singlet-triplet annihilation in optical and electrical excitation.

Triplet excitons have been identified as the major obstacle to the realisation of organic laser diodes, as accumulation of triplet excitons leads to significant losses under continuous wave (CW) operation and/or electrical excitation. Here, we report the design and synthesis of a solid-state organic triplet quencher, as well as in-depth studies of its dispersion into a solution processable bis-stilbene-based laser dye. By blending the laser dye with 20 wt% of the quencher, negligible effects on the ASE thresholds, but a complete suppression of singlet-triplet annihilation (STA) and a 20-fold increase in excited-state photostability of the laser dye under CW excitation, were achieved. We used small-area OLEDs (0.2 mm2) to demonstrate efficient STA suppression by the quencher in the nanosecond range, supported by simulations to provide insights into the observed STA quenching under electrical excitation. The results demonstrate excellent triplet quenching ability under both optical and electrical excitations in the nanosecond range, coupled with excellent solution processability.

Charge and exciton dynamics of OLEDs under high voltage nanosecond pulse: towards injection lasing.

Electrical pumping of organic semiconductor devices involves charge injection, transport, device on/off dynamics, exciton formation and annihilation processes. A comprehensive model analysing those entwined processes together is most helpful in determining the dominating loss pathways. In this paper, we report experimental and theoretical results of Super Yellow (Poly(p-phenylene vinylene) co-polymer) organic light emitting diodes operating at high current density under high voltage nanosecond pulses. We demonstrate complete exciton and charge carrier dynamics of devices, starting from charge injection to light emission, in a time scale spanning from the sub-ns to microsecond region, and compare results with optical pumping. The experimental data is accurately replicated by simulation, which provides a robust test platform for any organic materials. The universality of our model is successfully demonstrated by its application to three other laser active materials. The findings provide a tool to narrow the search for material and device designs for injection lasing.

Mobility Evaluation of [1]Benzothieno[3,2- b][1]benzothiophene Derivatives: Limitation and Impact on Charge Transport.

Among contemporary semiconductors, many of the best performing materials are based on [1]benzothieno[3,2- b][1]benzothiophene (BTBT). Alkylated derivatives of these small molecules not only provide high hole mobilities but also can be easily processed by thermal vacuum or solution deposition methods. Over the last decade, numerous publications have investigated molecular structures and charge transport properties to elucidate what makes these molecules so special. However, the race toward ever higher mobilities resulted in significantly deviating values, which exacerbates linking molecular structure to electronic properties. Moreover, a recently arisen debate on overestimation of organic field-effect transistor mobilities calls for a revaluation of these numbers. We synthesized and characterized four BTBT derivatives with either one or two alkyl chains (themselves consisting of either 8 or 10 carbon atoms) and investigated their spectroscopic, structural, and electrical properties. By employing two-probe, gated four-point probe and gated van der Pauw measurements, we compare field-effect mobility values at room and low temperatures and discuss their feasibility and viability. We attribute mobility changes to different angles between molecule planes and core-to-core double-layer stacking of asymmetric BTBT derivatives and show higher mobilities in the presence of more and longer alkyl chains. A so-called "zipper effect" brings BTBT cores in closer proximity promoting stronger intermolecular orbital coupling and hence higher charge transport.

Hybrid Area-Emitting Transistors: Solution Processable and with High Aperture Ratios.

Area emission is realized in all-solution-processed hybrid light-emitting transistors (HLETs). A new HLET design is presented with increased aperture ratio, and optical and electrical characteristics are shown.

High-Performance, Fullerene-Free Organic Photodiodes Based on a Solution-Processable Indigo.

A solution-processable dibromoindigo with an alkyoxyphenyl solubilizing group is developed and used as a new electron acceptor in organic photodiodes. The solution-processed fullerene-free organic photodiodes show an almost spectrally flat response with a high responsivity (0.4 A W(-1)) and a high detectivity (1 × 10(12) Jones). These values are comparable to silicon-based photodiodes.

Defining the light emitting area for displays in the unipolar regime of highly efficient light emitting transistors.

Light-emitting field effect transistors (LEFETs) are an emerging class of multifunctional optoelectronic devices. It combines the light emitting function of an OLED with the switching function of a transistor in a single device architecture. The dual functionality of LEFETs has the potential applications in active matrix displays. However, the key problem of existing LEFETs thus far has been their low EQEs at high brightness, poor ON/OFF and poorly defined light emitting area - a thin emissive zone at the edge of the electrodes. Here we report heterostructure LEFETs based on solution processed unipolar charge transport and an emissive polymer that have an EQE of up to 1% at a brightness of 1350 cd/m(2), ON/OFF ratio > 10(4) and a well-defined light emitting zone suitable for display pixel design. We show that a non-planar hole-injecting electrode combined with a semi-transparent electron-injecting electrode enables to achieve high EQE at high brightness and high ON/OFF ratio. Furthermore, we demonstrate that heterostructure LEFETs have a better frequency response (f cut-off = 2.6 kHz) compared to single layer LEFETs. The results presented here therefore are a major step along the pathway towards the realization of LEFETs for display applications.

All solution-processed, hybrid light emitting field-effect transistors.

All solution-processed, high performance hybrid light emitting transistors (HLETs) are realized. Using a novel combination of device architecture and materials a bilayer device comprised of an inorganic and organic semiconducting layer is fabricated and the optoelectronic properties are presented.

Simultaneous enhancement of brightness, efficiency, and switching in RGB organic light emitting transistors.

An innovative design strategy for light emitting field effect transistors (LEFETs) to harvest higher luminance and switching is presented. The strategy uses a non-planar electrode geometry in tri-layer LEFETs for simultaneous enhancement of the key parameters of quantum efficiency, brightness, switching, and mobility across the RGB color gamut.

Ordered polymer nanofibers enhance output brightness in bilayer light-emitting field-effect transistors.

Polymer light emitting field effect transistors are a class of light emitting devices that reveal interesting device physics. Device performance can be directly correlated to the most fundamental polymer science. Control over surface properties of the transistor dielectric can dramatically change the polymer morphology, introducing ordered phase. Electronic properties such as carrier mobility and injection efficiency on the interface can be promoted by ordered nanofibers in the polymer. Moreover, by controlling space charge in the polymer interface, the recombination zone can be spatially extended and thereby enhance the optical output.