Direct measurement of the singlet generation yield in polymer light-emitting diodes

Structurally Tunable Donor-Bridge-Fluorophore Architecture Enables Highly Efficient and Concentration-Independent Narrowband Electroluminescence

Luminescent materials with narrowband emission have extraordinary significance for developing ultrahigh-definition display. B-N-containing multiple resonance thermally activated delayed fluorescence (MR-TADF) materials are strong contenders. However, their device performances pervasively encounter detrimental aggregation-caused quenching effect that is highly vulnerable to doping concentration, complicating device fabrication. Therefore, constructing highly efficient and concentration-independent MR-TADF emitters is of pragmatic importance for improving device controllability and reproducibility, simplifying manufacturing procedures, and conserving production costs. Here, by systematic arrangement of donor triphenylamine and fluorophore BNCz on distinct bridges, a spatial confinement strategy has been developed with a donor-bridge-fluorophore architecture. Structurally fine modulation and progressive evolution to construct molecular entities with congested steric hindrance effect that can suppress intermolecular interactions without substantially affecting the luminescence tone of fluorophore BNCz, resulting in highly efficient and concentration-independent narrowband emitters; through isomer engineering, two isomers BN-PCz-TPA and TPA-PCz-BN with different crystal stacking patterns are synthesized by altering the connection mode between triphenylamine and BNCz. As a result, BN-PCz-TPA-based device showcases maximum external quantum efficiency (EQE) of 36.3% with narrow full-width at half-maximum of 27 nm at 10 wt% doping concentration. Even at 20 wt% doping concentration, the maximum EQE remains at 32.5% and the emission spectrum is almost unchanged.

Impact of Photoluminescence Imaging Methodology on Transport Parameters in Semiconductors

Triplet-triplet annihilation-induced delayed emission provides a pathway for investigating triplets via emission spectroscopy. This bimolecular annihilation depends directly on the transport properties of triplet excitons in disordered organic semiconductors. Photoluminescence (PL) imaging is a direct method for studying exciton and charge-carrier diffusivity. However, most of these studies neglect dispersive transport. Early time scale measurements using this technique can lead to an overestimation of the diffusion coefficient (DT) or diffusion length (Ld). In this study, we investigated the time-dependent triplet DT using PL imaging. We observed an overestimation of Ld in classical delayed PL imaging, often 1 order of magnitude higher than the actual Ld value. We compared various thicknesses of polymeric thin films to study the dispersive nature of triplet excitons. Transient analysis of delayed PL imaging and steady state imaging reveals the importance of considering the time-dependent nature of DT for the triplet excitons in disordered electronic materials.

Constructing Highly Efficient Circularly Polarized Multiple-Resonance Thermally Activated Delayed Fluorescence Materials with Intrinsically Helical Chirality

Advanced circularly polarized multiple-resonance thermally activated delayed fluorescence (CP-MR-TADF) materials synergize the advantages of circularly polarized luminescence (CPL), narrowband emission, and the TADF characteristic, which can be fabricated into highly efficient circularly polarized organic light-emitting diodes (CP-OLEDs) with high color purity, directly facing the urgent market strategic demand of ultrahigh-definition and 3D displays. In this work, based on an edge-topology molecular-engineering (ETME) strategy, a pair of high-performance CP-MR-TADF enantiomers, (P and M)-BN-Py, is developed, which merges the intrinsically helical chirality into the MR framework. The optimized CP-OLEDs with (P and M)-BN-Py emitters and the newly developed ambipolar transport host PhCbBCz exhibit pure green emission with sharp peaks of 532 nm, full-width at half-maximum (FWHM) of 37 nm, and Commission Internationale de L'Eclairage (CIE) coordinates of (0.29, 0.68). Importantly, they achieve remarkable maximum external quantum efficiencies (EQEs) of 30.6% and 29.2%, and clear circularly polarized electroluminescence (CPEL) signals with electroluminescence dissymmetry factors (gEL s) of -4.37 × 10-4 and +4.35 × 10-4 for (P)-BN-Py and (M)-BN-Py, respectively.

Precise Functionalization of a Multiple-Resonance Framework: Constructing Narrowband Organic Electroluminescent Materials with External Quantum Efficiency over 40

It is of important strategic significance to develop high-efficiency narrowband organic electroluminescent materials that can be employed to fabricate ultrahigh-definition displays with wide color gamut. This topic implies a great challenge to molecular design and synthesis, especially for the development of universality, diversity, scalability, and robustness of molecular architectonics. In this work, a synthetic methodology is demonstrated for functionalizing brominated BN-containing multiple-resonance (MR) frameworks with multifarious functional groups, such as donors, acceptors, and moieties without obvious push-pull electron properties. The m-DPAcP-BNCz-based organic light-emitting diode (OLED) exhibits green emission with a full-width at half-maximum (FWHM) of 28 nm and a maximum external quantum efficiency (EQE) of 40.6%. The outstanding performance of m-DPAcP-BNCz is attributed to the perfect integration of the inherent advantages of the MR framework and the donor-acceptor configuration, which can not only achieve bathochromic shift and narrowband emission, but also obtain high photoluminescence (PL) quantum yield (Φ_PL ) and horizontal emitting dipole orientation ratio (Θ_// ). This straightforward and efficient approach provides insightful guidance for the construction and enrichment of more high-efficiency narrowband emitters.

Highly Efficient Electroluminescence from Narrowband Green Circularly Polarized Multiple Resonance Thermally Activated Delayed Fluorescence Enantiomers

Purely organic fluorescent materials that concurrently exhibit high efficiency, narrowband emission, and circularly polarized luminescence (CPL) remain an unaddressed issue despite their promising applications in wide color gamut- and 3D-display. Herein, the CPL optical property and multiple resonance (MR) effect induced thermally activated delayed fluorescence (MR-TADF) emission are integrated with high color purity and luminous efficiency together. Two pairs of highly efficient green CP-MR-TADF enantiomers, namely, (R/S)-OBN-2CN-BN and (R/S)-OBN-4CN-BN, are developed. The enantiomer-based organic light-emitting diodes (OLEDs) exhibit pure green emission with narrow full-width at half-maximums (FWHMs) of 30 and 33 nm, high maximum external quantum efficiencies (EQEs) of 29.4% and 24.5%, and clear circularly polarized electroluminescence (CPEL) signals with electroluminescence dissymmetry factors (g_EL ) of +1.43 × 10^-3 /-1.27 × 10^-3 and +4.60 × 10^-4 /-4.76 × 10^-4 , respectively. This is the first example of a highly efficient OLED that exhibits CPEL signal, narrowband emission, and TADF concurrently.

Role of Bimolecular Exciton Kinetics in Controlling the Efficiency of Organic Light-Emitting Diodes

Here, we have carried out a spectroscopic investigation on the operational organic light-emitting diodes (OLEDs) to determine the role of emission layer thickness on the optoelectronic performance of OLEDs based on a poly(9,9-dioctylfluorene- alt-benzothiadiazole) (F8BT) copolymer system. Our study shows that delayed fluorescence (DF) via triplet-triplet annihilation (TTA) contributes significantly to boost the OLED efficiency through its fractional contribution. Interestingly, we note that DF contribution varies as a function of the emissive layer thickness. From the time-resolved electroluminescence (TREL) and triplet absorption (under electrical excitation) studies, we have seen that the emissive layer thickness controls triplet exciton generation and decay processes. From TREL, we have also shown that singlet-triplet annihilation (STA) is the dominant fluorescence quenching mechanism in bulk of the emissive layer, whereas thinner devices have significant exciton quenching at the interface of the injection layer/F8BT. The strength of STA differs in thin versus thick samples, which has been correlated with the spectral & spatial overlap integral of singlet and triplet states. Hence, STA strength and triplet population density are critical parameters for an explanation of high efficiency in unusually thick F8BT OLEDs.

Singlet Exciton Fraction in Electroluminescence from Conjugated Polymer

The efficiency of electrofluorescent polymer light-emitting diodes is determined by singlet exciton fraction (χ_S) formation and its value still remains controversial. In this work, χ_S in spiropolyfluorene (SPF) is determined by analyzing transient emission of phosphor-dopant probe. The χ_S is found to range from 50% to 76%, depending on applied voltage. Higher applied voltage gives larger χ_S. Besides, more rapid increment in χ_S with applied voltage is observed in the higher-molecular-weight polymer. The voltage or molecular weight dependence of χ_S suggests the probability of singlet exciton (SE) generation through triplet-triplet annihilation (TTA) is enhanced due to higher triplet exciton (TE) concentration at higher applied voltage or accommodation of more TEs in a polymer chain with high molecular weight, thereby increasing probability of TTA. At lower applied voltage, χ_S is contributed by charge recombination. Its value (χ_S ~50%) higher than the statistical limit 25% is in agreement with efficient interconversion between triplet and singlet polaron pairs (PP) and with larger formation rate of SE relative to that of TE.

Generating Light from Upper Excited Triplet States: A Contribution to the Indirect Singlet Yield of a Polymer OLED, Helping to Exceed the 25% Singlet Exciton Limit

The mechanisms by which light is generated in an organic light emitting diode have slowly been elucidated over the last ten years. The role of triplet annihilation has demonstrated how the "spin statistical limit" can be surpassed, but it cannot account for all light produced in the most efficient devices. Here, a further mechanism is demonstrated by which upper excited triplet states can also contribute to indirect singlet production and delayed fluorescence. Since in a device the population of these T_N states is large, this indirect radiative decay channel can contribute a sizeable fraction of the total emission measured from a device. The role of intra- and interchain charge transfer states is critical in underpinning this mechanism.

Direct monitoring of bias-dependent variations in the exciton formation ratio of working organic light emitting diodes

In typical operation of organic light emitting diodes (OLEDs), excitons are assumed to generate with a ratio of 1:3 for singlet and triplet excitons, respectively, based on a simple spin statistics model. This assumption has been used in designing efficient OLEDs. Despite the larger generation ratio of triplet excitons, physical properties of fluorescent OLEDs are usually evaluated only through the electroluminescence (EL) intensity from singlets and the behaviors of triplets during the LED operation are virtually black-boxed, because the triplets are mostly non-emissive. Here, we employ transient spectroscopy combined with LED-operation for directly monitoring the non-emissive triplets of working OLEDs. The spectroscopic techniques are performed simultaneously with EL- and current measurements under various operation biases. The simultaneous measurements reveal that the relative formation ratio of singlet-to-triplet excitons dramatically changes with the magnitude of bias. The measurements also show that the generation efficiency of singlets scales with the bias, whereas that of triplets is nearly bias-independent. These features of the formation ratio and efficiency are compatibly explained by considering the yield of intersystem crossing and the energy separation of excitons from electron-hole pairs. The obtained findings via the spectroscopic measurements enable prediction of the formation pathways in OLEDs.

Thermally activated delayed fluorescence sensitized phosphorescence: a strategy to break the trade-off between efficiency and efficiency roll-off

Materials with thermally activated delayed fluorescence (TADF) realized 100% internal quantum efficiency (IQE) but suffered significant efficiency roll-off. Here, an exciton dynamics study reveals that materials with TADF may play opposite roles in affecting the efficiency roll-off: decreasing the triplet density due to the fast reverse intersystem crossing, on the one hand, and increasing the triplet density due to the weakened singlet radiation. We show theoretically and experimentally that TADF-sensitized phosphorescence can break this trade-off by exploiting the efficient Förster energy transfer and simultaneously achieve 100% IQE and low efficiency roll-off (with a critical current density of 460 mA cm(-2)).

Triplet-triplet annihilation in highly efficient fluorescent organic light-emitting diodes: current state and future outlook

Studies of delayed electroluminescence in highly efficient fluorescent organic light-emitting diodes (OLEDs) of many dissimilar architectures indicate that the triplet-triplet annihilation (TTA) significantly increases yield of excited singlet states-emitting molecules in this type of device thereby contributes substantially to their efficiency. Towards the end of the 2000s, the essential role of TTA in realizing highly efficient fluorescent devices was widely recognized. Analysis of a diverse set of fluorescent OLEDs shows that high efficiencies are often cor-related to TTA extents. It is therefore likely that it is the long-term empirical optimization of OLED efficiencies that has resulted in fortuitous emergence of TTA as a large and ubiquitous contributor to efficiency. TTA contributions as high as 20-30% are common in the state-of-the-art OLEDs, and even become dominant in special cases, where TTA is shown to substantially exceed the spin-statistical limit. The fundamental features of OLED efficiency enhancement via TTA-molecular structure-dependent contributions, current density-dependent intensities in practical devices and frequently observed antagonistic relationships between TTA extent and OLED lifetime-came to be understood over the course of the next few years. More recently, however, there was much less reported progress with respect to all-important quantitative details of the TTA mechanism. It should be emphasized that, to this day and despite the decades of work on improving blue phosphorescent OLEDs as well as the recent advent of thermally activated delayed fluorescence OLEDs, the majority of practical blue OLEDs still rely on TTA. Considering such practical importance of fluorescent blue OLEDs, the design of blue OLED-compatible materials capable of substantially exceeding the spin-statistical limit in TTA, elimination of the antagonistic relationship between TTA-related efficiency gains and lifetime losses, and designing devices with an extended range of current densities producing near-maximum TTA electroluminescence are the areas where future improvements would be most beneficial.

Efficiency roll-off in organic light-emitting diodes

Organic light-emitting diodes (OLEDs) have attracted much attention in research and industry thanks to their capability to emit light with high efficiency and to deliver high-quality white light that provides good color rendering. OLEDs feature homogeneous large area emission and can be produced on flexible substrates. In terms of efficiency, OLEDs can compete with highly efficient conventional light sources but their efficiency typically decreases at high brightness levels, an effect known as efficiency roll-off. In recent years, much effort has been undertaken to understand the underlying processes and to develop methods that improve the high-brightness performance of OLEDs. In this review, we summarize the current knowledge and provide a detailed description of the relevant principles, both for phosphorescent and fluorescent emitter molecules. In particular, we focus on exciton-quenching mechanisms, such as triplet-triplet annihilation, quenching by polarons, or field-induced quenching, but also discuss mechanisms such as changes in charge carrier balance. We further review methods that may reduce the roll-off and thus enable OLEDs to be used in high-brightness applications.

Singlet Generation from Triplet Excitons in Fluorescent Organic Light-Emitting Diodes

A potential major drawback with organic light-emitting devices, (OLEDs) is the limit of 25% singlet exciton production through spin-dependent charge recombination. Recent device results, however, show that this limit does not hold and far higher efficiencies can be achieved in purely fluorescent-based systems (Wohlgenannt et al. (2001), Dhoot et al. (2002), Lin et al. (2003), Wilson et al. (2001), Cao et al. (1999), Baldo et al. (1999), and Kim et al. (2000)). Thus, the question arises; is recombination spin dependent (Tandon et al. (2003)) or are singlet excitons generated in secondary processes? Direct measurement of the singlet generation rate in working devices of 44% has been shown (Rothe et al. (2006)), which have been verified as being part due to direct singlets formed on recombination and part from triplet fusion, singlets produced during triplet annihilation (Kondakov et al. (2009), King et al. (2011), and Zhang and Forrest (2012)). Here, the various routes by which triplet excitons can generate singlet states are discussed and their relative contributions to the overall electroluminescence yield are given. The materials requirements to obtain maximum singlet production from triplet states are discussed. These triplet contributions can give very high device yields for fluorescent emitters, which in the case of blue devices can be highly advantageous. Further, new devices architectures open up which are simple and have intrinsically low turn on voltages, ideal for large-area OLED lighting applications.

Magnetic-field dependence of the electroluminescence of organic light-emitting diodes: a competition between exciton formation and spin mixing

We explore the magnetoelectroluminescence (MEL) of organic light-emitting diodes by evaluating the magnetic-field dependent fraction of singlet excitons formed. We use two- and multisite polaron-hopping models with spin mixing by hyperfine fields and different singlet and triplet exciton formation rates k(S) and k(T). A huge MEL is predicted when exciton formation is in competition with spin mixing and when k(T) is significantly larger than k(S). This competition also leads to a low-field structure in the MEL that is in agreement with recent experiments.

Enhancing shielding of triplet energy transfer to poly(p-phenylene)s from phosphor dopant by addition of branched alcohol for highly efficient electrophosphorescence

To obtain an efficient electrophosphorescent device, one needs to consider quenching of phosphor phosphorescence brought by the low triplet energy of the host because the exothermic energy transfer can effectively quench phosphor phosphorescence and markedly lower the device efficiency. Here, a facile approach of adding a branched alcohol (3-tert-butyl-2,2,4,4-tetramethylpentan-3-ol, ROH) into green emission phosphor-doped dialkoxyl-substituted poly(para-phenylene)s (PPPs) is demonstrated to effectively enhance shielding of triplet energy transfer to PPPs from the phosphor, resulting from a formation of self-assembly structure that block direct contact between phosphor and the main chains. The green electrophosphorescent device performance can be improved from 7.1 and 32.2 cd/A to 25.1 and 42 cd/A for PPP with dioctoxyl substituents (dC(8)OPPP) and with carbozole (Cz)-capped dialkoxyl-substituents (CzPPP), respectively. The latter result 42 cd/A is the highest record for green emission in polymer light emitting diode. This finding suggests that promotion of specific electro-optical properties for small molecule and polymer can be obtained through a self-assembling interaction in addition to chemical structure modification.

Organic electronic devices and their functional interfaces

A most appealing feature of the development of (opto)electronic devices based on conjugated organic materials is the highly visible link between fundamental research and technological advances. Improved understanding of organic material properties can often instantly be implemented in novel device architectures, which results in rapid progress in the performance and functionality of devices. An essential ingredient for this success is the strong interdisciplinary nature of the field of organic electronics, which brings together experts in chemistry, physics, and engineering, thus softening or even removing traditional boundaries between the disciplines. Naturally, a thorough comprehension of all properties of organic insulators, semiconductors, and conductors is the goal of current efforts. Furthermore, interfaces between dissimilar materials-organic/organic and organic/inorganic-are inherent in organic electronic devices. It has been recognized that these interfaces are a key for device function and efficiency, and detailed investigations of interface physics and chemistry are at the focus of research. Ultimately, a comprehensive understanding of phenomena at interfaces with organic materials will improve the rational design of highly functional organic electronic devices.