Triplets contribute to both an increase and loss in fluorescent yield in organic light emitting diodes

A perspective on next-generation hyperfluorescent organic light-emitting diodes

Hyperfluorescence, also known as thermally activated delayed fluorescence (TADF) sensitized fluorescence, is known as a next-generation efficient and innovative process for high-performance organic light-emitting diodes (OLEDs). High external quantum efficiency (EQE) and good color purity are crucial parameters for display applications. Hyperfluorescent OLEDs (HF-OLEDs) take the lead in this respect as they utilize the advantages of both TADF emitters and fluorescent dopants, realizing high EQE with color saturation and long-term stability. Hyperfluorescence is mediated through Förster resonance energy transfer (FRET) from a TADF sensitizer to the final fluorescent emitter. However, competing loss mechanisms such as Dexter energy transfer (DET) of triplet excitons and direct charge trapping on the final emitter need to be mitigated in order to achieve fluorescence emission with high efficiency. Despite tremendous progress, appropriate guidelines and fine optimization are still required to address these loss channels and to improve the device operational lifetime. This perspective aims to provide an overview of the evolution of HF-OLEDs by reviewing both molecular and device design pathways for highly efficient narrowband devices covering all colors of the visible spectrum. Existing challenges and potential solutions, such as molecules with peripheral inert substitution, multi-resonant (MR) TADF emitters as final dopants, and exciplex-sensitized HF-OLEDs, are discussed. Furthermore, the operational device lifetime is reviewed in detail before concluding with suggestions for future device development.

Reverse Intersystem Crossing Dynamics in Vibronically Modulated Inverted Singlet-Triplet Gap System: A Wigner Phase Space Study

We inspect the origin of the inverted singlet-triplet gap (INVEST) and slow change in the reverse intersystem crossing (rISC) rate with temperature, as recently observed. A Wigner phase space study reveals that, though INVEST is found at equilibrium geometry, variation in the exchange interaction and the doubles-excitation for other geometries in the harmonic region leads to non-INVEST behavior. This highlights the importance of nuclear degrees of freedom for the INVEST phenomenon, and in this case, geometric puckering of the studied molecule determines INVEST and the associated rISC dynamics.

Demystifying Triplet-Triplet Annihilation Mechanism in the CsPbI3-Rubrene-DBP Upconversion System

A triplet-triplet annihilation-based upconversion (TTA-UC) system, employing a multichromophore assembly, is convenient to harvest low-energy photons for light energy conversion and optoelectronic applications. The primary donor in the TTA-UC system, typically a low-bandgap semiconductor, captures the low-energy photons and transfers triplet energy to an annihilator dye molecule, which in turn generates a high-energy singlet excited state via T-T annihilation. We have now succeeded in revealing kinetic and mechanistic details of multistep energy transfer processes in the CsPbI3-rubrene-perylene derivative (DBP) films by analyzing time-resolved emission and absorption measurements. The initial triplet energy transfer between CsPbI3 and rubrene occurs with an efficiency of 70% and a rate constant of 9 × 108 s-1. The rubrene triplets undergo T-T annihilation via simple second-order kinetics to form an excited singlet state exhibiting a delayed emission up to 10 μs, which is significantly greater than the intrinsic lifetime of 15 ns. The emitter DBP (a perylene derivative) captures the singlet energy quite effectively and delivers the upconverted emission in sync with the delayed emission of rubrene. The quadratic dependence of DBP emission on the excitation light intensity shows the importance of the T-T annihilation process in dictating the overall upconversion process. The kinetic parameters evaluated in this study, which divulge the critical steps dictating energy transfer in a TTA-UC system, should aid in the design of new light harvesting assemblies.

Deep Learning Prediction of Triplet-Triplet Annihilation Parameters in Blue Fluorescent Organic Light-Emitting Diodes

The triplet-triplet annihilation (TTA) ratio and the rate coefficient (kTT) of TTA are key factors in estimating the contribution of triplet excitons to radiative singlet excitons in fluorescent TTA organic light-emitting diodes. In this study, deep learning models are implemented to predict key factors from transient electroluminescence (trEL) data using new numerical equations. A new TTA model is developed that considers both polaron and exciton dynamics, enabling the distinction between prompt and delayed singlet decays with a fundamental understanding of the mechanism. In addition, deep learning models for predicting the kinetic coefficients and TTA ratio are established. After comprehensive optimization inspired by photophysics, determination coefficient values of 0.992 and 0.999 are achieved in the prediction of kTT and TTA ratio, respectively, indicating a nearly perfect prediction. The contribution of each kinetic parameter of polaron and exciton dynamics to the trEL curve is discussed using various deep-learning models.

Impact of Spontaneous Orientational Polarization on Triplet-Triplet Upconversion-Based Blue Organic Light-Emitting Diodes

The spontaneous orientation polarization (SOP) of a permanent dipole moment of the molecule induces a giant surface potential (GSP) in an organic semiconductor film, and GSP is expected to be a crucial parameter for understanding the operational mechanism of organic light-emitting diodes (OLEDs). This study demonstrates that the voltage-dependent migration of a carrier recombination zone induced by a polar electron transporting layer (ETL) having a positive SOP causes a decline in the overall performance of the OLED in triplet-triplet upconversion (TTU) based on OLEDs. Specifically, the TTU efficiency in an OLED with 2,2',2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) as the ETL decreased by 20% due to the reduction of electrically generated triplet exciton density. This decrease resulted in a lower external electroluminescence (EL) quantum efficiency (EQE) of 5.4% at 1 mA cm-2, while the OLED with a nonpolar ETL resulted in an EQE of around 8.1% at 1 mA cm-2. We confirmed a shift in the recombination zone from the current density dependence of the EL spectra in the OLEDs. Our results indicate that the fixed carrier recombination zone near a hole transport layer and an emitting layer (HTL/EML) strongly enhanced the TTU process, while the polar EML/ETL interface induced the migration of the recombination zone depending on voltage, resulting in the decrease of triplet exciton density.

Achieving pure room temperature phosphorescence (RTP) in phenoselenazine-based organic emitters through synergism among heavy atom effect, enhanced n → π* transitions and magnified electron coupling by the A-D-A molecular configuration

The weak spin-orbit coupling (SOC) in metal-free organic molecules poses a challenge in achieving phosphorescence emission. To attain pure phosphorescence in RTP organic emitters, a promising molecular design concept has been proposed. This involves incorporating n → π* transitions and leveraging the heavy atomic effect within the spin-orbit charge transfer-induced intersystem crossing (SOCT-ISC) mechanism of bipolar molecules. Following this design concept, two bipolar metal-free organic molecules (PhSeB and PhSeDB) with donor-acceptor (D-A) and acceptor-donor-acceptor (A-D-A) configurations have been synthesized. When the molecular configuration changes from D-A to A-D-A, PhSeDB exhibits stronger electron coupling and n → π* transitions, which can further enhance the spin-orbit coupling (SOC) together with the heave atom effect from the selenium atom. By the advanced synergism among enhanced n → π* transitions, heavy atom effect and magnified electron coupling to efficiently promote phosphorescence emission, PhSeDB can achieve pure RTP emission in both the solution and doped solid film. Thanks to the higher spin-orbit coupling matrix elements (SOCMEs) for T1 ↔ S0, PhSeDB attains the highest phosphorescence quantum yield (ca. 0.78) among all the RTP organic emitters reported. Consequently, the purely organic phosphorescent light-emitting diodes (POPLEDs) based on PhSeDB achieve the highest external quantum efficiencies of 18.2% and luminance of 3000 cd m-2. These encouraging results underscore the significant potential of this innovative molecular design concept for highly efficient POPLEDs.

Robust luminogens as cutting-edge tools for efficient light emission in recent decades

Blue luminogens play a vital role in white lighting and potential metal-free fluorescent materials and their high-lying excited states contribute to harvesting triplet excitons in devices. However, in TADF-OLEDs (ΔEST < 0.1 eV), although T1 excitons transfer to S1via RISC with 100% IQE, the longer lifetime of blue TADF suffers from efficiency roll-off (RO). In this case, hybridized local and charge transfer (HLCT) materials have attracted significant interest in lighting owing to their 100% hot exciton harvesting and enhanced efficiency. Both academics and industrialists widely use the HLCT strategy to improve the efficiency of fluorescent organic light-emitting diodes (FOLEDs) by harvesting dark triplet excitons through the RISC process. Aggregation-induced emissive materials (AIEgens) possess tight packing in the aggregation state, and twisted AIEgens with HLCT behaviour have a shortened conjugation length, inducing blue emission and making them suitable candidates for OLED applications. TTA-OLEDs are used in commercial BOLEDs because of their moderate efficiency and reasonable operation lifetime. In this review, we discuss the devices based on TTA fluorophores, TADF fluorophores, HLCT fluorophores, AIEgens and HLCT-sensitized fluorophores (HLCT-SF), which break through the statistical limitations.

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.

Anthracene derivatives with strong spin-orbit coupling and efficient high-lying reverse intersystem crossing beyond the El-Sayed rule

The attention to materials with hot exciton channel and triplet-triplet fusion (TTF) mediated high-lying reverse intersystem crossing (hRISC) has been raised for their ability to convert non-emissive 'dark' triplets into radiative singlet excitons. This spin conversion process results in high exciton utilization efficiency (EUE) that exceeds the theoretical limits. Notably, it is known that such spin conversion processes from the high-lying excited triplet to the singlet state are facilitated by the orthogonal orbital transition effect governed by the El-Sayed's rule. In this study, an anthracene derivative with indenoquinoline substituent 7,7-dimethyl-9-(10-(4-(naphthalen-1-yl)phenyl)anthracen-9-yl)-7H-indeno[1,2-f]quinoline (2MIQ-NPA) was synthesized and analyzed to investigate whether the hRISC process occurs in these molecules, even when the El-Sayed's rule is not followed. The hRISC channels of the emitter were fully unraveled through DFT calculations and experiments, which were quantitatively subdivided using transient electroluminescence measurements. The results showed that 2MIQ-NPA, which does not follow the El-Sayed's rule and has a relatively strong spin-orbit coupling matrix element of 0.116 cm-1 between the high-lying triplet state of T4 and the lowest singlet state of S1, effectively converted triplet excitons into singlet excitons with an EUE of 64.3%, contributed by a direct hot exciton channel of 19.2% and a TTF-mediated hot exciton channel of 15.1%. Despite the low outcoupling efficiency, the non-doped device with 2MIQ-NPA achieved an excellent device performance with an external quantum efficiency of 7.0%.

Understanding the degradation mechanism of TTA-based blue fluorescent OLEDs by exciton dynamics and transient electroluminescence measurements

The lifetime of blue organic light-emitting diodes (OLEDs) has always been a big challenge in practical applications. Blue OLEDs based on triplet-triplet annihilation (TTA) up-conversion materials have potential to achieve long lifetimes due to fusing two triplet excitons to one radiative singlet exciton, but there is a lack of an in-depth understanding of exciton dynamics on degradation mechanisms. In this work, we established a numerical model of exciton dynamics to study the impact factors in the stability of doped blue OLEDs based on TTA up-conversion hosts. By performing transient electroluminescence experiments, the intrinsic parameters related to the TTA up-conversion process of aging devices were determined. By combining the change of excess charge density in the emitting layer (EML) with aging time, it is concluded that the TTA materials are damaged by the excess electrons in the EML during ageing, which is the main degradation mechanism of OLEDs. This work provides a theoretical basis for preparing long-lifetime blue fluorescent OLEDs.

Enhancing operational stability of OLEDs based on subatomic modified thermally activated delayed fluorescence compounds

The realization of operationally stable blue organic light-emitting diodes is a challenging issue across the field. While device optimization has been a focus to effectively prolong device lifetime, strategies based on molecular engineering of chemical structures, particularly at the subatomic level, remains little. Herein, we explore the effect of targeted deuteration on donor and/or acceptor units of thermally activated delayed fluorescence emitters and investigate the structure-property relationship between intrinsic molecular stability, based on isotopic effect, and device operational stability. We show that the deuteration of the acceptor unit is critical to enhance the photostability of thermally activated delayed fluorescence compounds and hence device lifetime in addition to that of the donor units, which is commonly neglected due to the limited availability and synthetic complexity of deuterated acceptors. Based on these isotopic analogues, we observe a gradual increase in the device operational stability and achieve the long-lifetime time to 90% of the initial luminance of 23.4 h at the luminance of 1000 cd m-2 for thermally activated delayed fluorescence-sensitized organic light-emitting diodes. We anticipate our strategic deuteration approach provides insights and demonstrates the importance on structural modification materials at a subatomic level towards prolonging the device operational stability.

Exciton dynamics of an aggregation-induced delayed fluorescence emitter in non-doped OLEDs and its application as host for high-efficiency red phosphorescent OLEDs

Aggregation-induced delayed fluorescence (AIDF) materials have great potential in non-doped OLEDs due to their high photoluminescence (PL) quantum efficiency in film, high exciton utilization in the aggregated state and negligible efficiency roll-off at high luminance. However, their efficient mechanism in OLEDs is not yet well understood. Here, the exciton dynamics are used to investigate the electroluminescence (EL) mechanism of an AIDF emitter (4-(10H-phenoxazin-10-yl)phenyl)-(9-phenyl-9H-carbazol-3-yl)methanone (CP-BP-PXZ) in detail. It can be seen that the high efficiency and negligible efficiency roll-off in non-doped OLEDs based on CP-BP-PXZ as the emitter are ascribed to the effective reverse intersystem crossing (RISC) from high level triplet T2 to singlet S1 in the aggregated state. Furthermore, CP-BP-PXZ also exhibits excellent properties as a phosphor host due to its good AIDF properties. Thus, high-efficiency red phosphorescent OLEDs with low roll-off efficiency are successfully fabricated based on CP-BP-PXZ as the host. The maximum external quantum efficiency (EQEmax) reaches 23% and is maintained at 21% at a luminance of 1000 cd m-2.

Integrating the atomically separated frontier molecular orbital distribution of two multiple resonance frameworks through a single bond for high-efficiency narrowband emission

Atomically separated frontier molecular orbital (FMO) distribution plays a crucial role in achieving narrowband emissions for multiple resonance (MR)-type thermally activated delayed fluorescence emitters. Directly peripherally decorating a MR framework with donor or acceptor groups is a common strategy for developing MR emitters. However, this approach always induces bonding features and thus spectral broadening as a side effect. How direct donor/acceptor decoration enhances atomic FMO separation while avoiding bonding features has not been explored. For this aim, two MR derivatives are synthesized by integrating two MR frameworks at different sites. Following resonance alignment, DOBNA-m-CzBN avoids breaking nonbonding FMO features at the single connecting bond and shows enhanced MR characteristics, with a sharp emission at 491 nm and a full width at half maximum (FWHM) of 24 nm/118 meV. Conversely, DOBNA-p-CzBN emerges as a bonding feature due to its continuous π-conjugation extension, with a broadened FWHM of 26 nm/132 meV peaking at 497 nm. Impressively, both emitters exhibit outstanding external quantum efficiencies of 37.8-38.6% in organic light-emitting diodes (OLEDs), demonstrating improved performance with rigid acceptor decoration. Distinctly, the electroluminescence of DOBNA-m-CzBN shows a narrower FWHM than that of DOBNA-p-CzBN. This work for the first time reports the enhancement of atomic FMO separation for MR emitters via peripheral decoration through a single bond and provides a more comprehensive illustration for further development of MR emitters.

Blue organic light-emitting diode with a turn-on voltage of 1.47 V

Among the three primary colors, blue emission in organic light-emitting diodes (OLEDs) are highly important but very difficult to develop. OLEDs have already been commercialized; however, blue OLEDs have the problem of requiring a high applied voltage due to the high-energy of blue emission. Herein, an ultralow voltage turn-on at 1.47 V for blue emission with a peak wavelength at 462 nm (2.68 eV) is demonstrated in an OLED device with a typical blue-fluorescent emitter that is widely utilized in a commercial display. This OLED reaches 100 cd/m2, which is equivalent to the luminance of a typical commercial display, at 1.97 V. Blue emission from the OLED is achieved by the selective excitation of the low-energy triplet states at a low applied voltage by using the charge transfer (CT) state as a precursor and triplet-triplet annihilation, which forms one emissive singlet from two triplet excitons.

Electrically driven organic laser using integrated OLED pumping

Organic semiconductors are carbon-based materials that combine optoelectronic properties with simple fabrication and the scope for tuning by changing their chemical structure1-3. They have been successfully used to make organic light-emitting diodes2,4,5 (OLEDs, now widely found in mobile phone displays and televisions), solar cells1, transistors6 and sensors7. However, making electrically driven organic semiconductor lasers is very challenging8,9. It is difficult because organic semiconductors typically support only low current densities, suffer substantial absorption from injected charges and triplets, and have additional losses due to contacts10,11. In short, injecting charges into the gain medium leads to intolerable losses. Here we take an alternative approach in which charge injection and lasing are spatially separated, thereby greatly reducing losses. We achieve this by developing an integrated device structure that efficiently couples an OLED, with exceptionally high internal-light generation, with a polymer distributed feedback laser. Under the electrical driving of the integrated structure, we observe a threshold in light output versus drive current, with a narrow emission spectrum and the formation of a beam above the threshold. These observations confirm lasing. Our results provide an organic electronic device that has not been previously demonstrated, and show that indirect electrical pumping by an OLED is a very effective way of realizing an electrically driven organic semiconductor laser. This provides an approach to visible lasers that could see applications in spectroscopy, metrology and sensing.

High Efficiency over 15% by Breaking the Theoretical Efficiency Limit of Fluorescent Organic Light-Emitting Diodes with Localized Surface Plasmon Resonance Effects

The theoretical efficiency limit of fluorescence organic light-emitting diodes (OLEDs) was successfully surpassed by utilizing the localized surface plasmon resonance (LSPR) effect with conventional emissive materials. The interaction between polaritons and plexcitons generated during the LSPR process was also analyzed experimentally. As a result, the external quantum efficiency (EQE) increased dramatically from 6.01 to 15.43%, significantly exceeding the theoretical efficiency limit of fluorescent OLEDs. Additionally, we introduced a new concept of the LSPR effect, called "LSPR sensitizer", which allowed for simultaneous improvement in color conversion and efficiency through cascade transfer of the LSPR effect. To the best of our knowledge, the EQE and the current efficiency of our LSPR-OLED are the highest among LSPR-based fluorescent OLEDs to date.

Achieving Bright Organic Light-Emitting Field-Effect Transistors with Sustained Efficiency through Hybrid Contact Design

Organic light-emitting field-effect transistors (OLEFETs) with bilayer structures have been widely studied due to their potential to integrate high-mobility organic transistors and efficient organic light-emitting diodes. However, these devices face a major challenge of imbalance charge transport, leading to a severe efficiency roll-off at high brightness. Here, we propose a solution to this challenge by introducing a transparent organic/inorganic hybrid contact with specially designed electronic structures. Our design aims to steadily accumulate the electrons injected into the emissive polymer, allowing the light-emitting interface to effectively capture more holes even when the hole current increases. Our numerical simulations show that the capture efficiency of these steady electrons will dominate charge recombination and lead to a sustained external quantum efficiency of 0.23% over 3 orders of magnitude of brightness (4 to 7700 cd/m²) and current density (1.2 to 2700 mA/cm²) from -4 to -100 V. The same enhancement is retained even after increasing the external quantum efficiency (EQE) to 0.51%. The high and tunable brightness with stable efficiency offered by hybrid-contact OLEFETs makes them ideal light-emitting devices for various applications. These devices have the potential to revolutionize the field of organic electronics by overcoming the fundamental challenge of imbalance charge transport.

Investigation progresses of rare earth complexes as emitters or sensitizers in organic light-emitting diodes

Due to unique photo-physical characteristics, rare earth (RE) complexes play important roles in many fields, for example, telecommunications, life science, and organic light-emitting diodes (OLEDs). Especially, thanks to narrow emission bandwidth and 100% theoretical internal quantum efficiency (IQE), the study of RE complexes in the electroluminescence field has been a hot research topic in recent 30 years. As a leading technology in solid-state light source fields, OLEDs have attracted great interest from academic researchers and commercial endeavors. In the last decades, OLED-based products have trickled into the commercial market and developed quickly into portable display devices. Here, we briefly introduce the luminescent characteristics and electroluminescent (EL) study of RE complexes in material synthesis and device design. Moreover, we emphatically reveal the innovative application of RE complexes as sensitizers in OLEDs. Through experimental validation, the application of RE complexes as sensitizers can realize the complementary advantages of RE complexes and transition metal complexes, leading to significantly improved performances of OLEDs. The application of RE complexes as sensitizers provides a new strategy for designing and developing novel high performances OLEDs.

Achieving Ultimate Narrowband and Ultrapure Blue Organic Light-Emitting Diodes Based on Polycyclo-Heteraborin Multi-Resonance Delayed-Fluorescence Emitters

To achieve an ultimate wide color gamut for ultrahigh-definition displays, there is great demand for the development of organic light-emitting diodes (OLEDs) enabling monochromatic, ultrapure blue electroluminescence (EL). Herein, high-efficiency and ultrapure blue OLEDs based on polycyclo-heteraborin multi-resonance thermally activated delayed fluorescence (MR-TADF) materials, BOBO-Z, BOBS-Z, and BSBS-Z, are reported. The key to the design of the present luminophores is the exquisite combination and interplay of multiple boron, nitrogen, oxygen, and sulfur heteroatoms embedded in a fused polycyclic π-system. Comprehensive photophysical and computational investigations of this family of MR-TADF materials reveal that the systematic implementation of chalcogen (oxygen and sulfur) atoms can finely modulate the emission color while maintaining a narrow bandwidth, as well as the spin-flipping rates between the excited singlet and triplet states. Consequently, OLEDs based on BOBO-Z, BOBS-Z, and BSBS-Z demonstrate narrowband and ultrapure blue EL emission, with peaks at 445-463 nm and full width at half maxima of 18-23 nm, leading to Commission Internationale de l'Éclairage-y coordinates in the range of 0.04-0.08. Particularly, for OLEDs incorporating sulfur-doped BOBS-Z and BSBS-Z, notably high maximum external EL quantum efficiencies of 26.9% and 26.8%, respectively, and small efficiency roll-offs are achieved concurrently.

Numerical Study of Triplet Dynamics in Organic Semiconductors Aimed for the Active Utilization of Triplets by TADF under Continuous-Wave Lasing

The limitation of lasing duration less than nanosecond order has been a major problem for realizing organic solid-state continues-wave (CW) lasers and organic semiconductor laser diodes. Triplets accumulation under CW excitation has been well recognized as a critical inhibiting factor. To overcome this issue, the utilization of thermally activated delayed fluorescence (TADF) emitters is a promising mechanism because of efficient reverse intersystem crossing. Herein, we model the triplet accumulation processes under lasing and propose the active utilization of TADF for lasing based on our simulation analysis. We used the rate constants experimentally determined from the optical properties of a boron difluoride curcuminoid fluorophore showing both TADF and lasing. We demonstrate that the intersystem crossing efficiency is gradually increased after the convergence of relaxation oscillation, i.e., terminating laser oscillation. In addition, we found that when the reverse intersystem crossing rate is close to the intersystem crossing rate, CW lasing becomes dominant.

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.

An unprecedented spike of the electroluminescence turn-on transience from guest-doped OLEDs with strong electron-donating abilities of host carbazole groups

An unreported unprecedented spike of ∼μs line-width, followed by an overshoot, was discovered at the rising edge of transient electroluminescence (TEL) from guest-doped organic light-emitting diodes with strong electron-donating abilities from the host carbazole groups. By changing the device structures and TEL measurement parameters, a series of experimental results demonstrate that this TEL spike is not related to exciton interactions such as singlet-triplet and triplet-triplet annihilations but originated from the radiative recombination of pre-stored electrons with injected holes. Surprisingly, these pre-stored guest electrons do not come from the energy-level traps in the host-guest systems; instead, the guest molecules receive the electrons transferred from the host carbazole groups due to their strong electron-donating abilities. Moreover, the observed spikes show rich and extraordinary temperature dependences. Based on the detailed understanding of the spike formation mechanism, we have proposed the requirements for the occurrence of spike and realized the artificial adjustments of the spike intensity. For instance, the instantaneous luminescent intensity of this spike can reach over 80 times the magnitude of the TEL plateau. Accordingly, this work deepens the physical understanding of this novel spike in TEL and paves the way for fabricating an electro-optic sensor to detect instantaneous weak current signals.

Direct Transition from Triplet Excitons to Hybrid Light-Matter States via Triplet-Triplet Annihilation

Strong light–matter coupling generates hybrid states that inherit properties of both light and matter, effectively allowing the modification of the molecular potential energy landscape. This phenomenon opens up a plethora of options for manipulating the properties of molecules, with a broad range of applications in photochemistry and photophysics. In this article, we use strong light–matter coupling to transform an endothermic triplet–triplet annihilation process into an exothermic one. The resulting gradual on–off photon upconversion experiment demonstrates a direct conversion between molecular states and hybrid light–matter states. Our study provides a direct evidence that energy can relax from nonresonant low energy molecular states directly into hybrid light–matter states and lays the groundwork for tunable photon upconversion systems that modify molecular properties in situ by optical cavities rather than with chemical modifications.

Acceptor-Donor-Acceptor-Type Orange-Red Thermally Activated Delayed Fluorescence Materials Realizing External Quantum Efficiency Over 30% with Low Efficiency Roll-Off

Two new orange-red thermally activated delayed fluorescence (TADF) materials, PzTDBA and PzDBA, are reported. These materials are designed based on the acceptor-donor-acceptor (A-D-A) configuration, containing rigid boron acceptors and dihydrophenazine donor moieties. These materials exhibit a small ΔE_ST of 0.05-0.06 eV, photoluminescence quantum yield (PLQY) as high as near unity, and short delayed exciton lifetime (τ_d ) of less than 2.63 µs in 5 wt% doped film. Further, these materials show a high reverse intersystem crossing rate (k_risc ) on the order of 10⁶ s^-1 . The TADF devices fabricated with 5 wt% PzTDBA and PzDBA as emitting dopants show maximum EQE of 30.3% and 21.8% with extremely low roll-off of 3.6% and 3.2% at 1000 cd m^-2 and electroluminescence (EL) maxima at 576 nm and 595 nm, respectively. The low roll-off character of these materials is analyzed by using a roll-off model and the exciton annihilation quenching rates are found to be suppressed by the fast k_risc and short delayed exciton lifetime. These devices show operating device lifetimes (LT₅₀ ) of 159 and 193 h at 1000 cd m^-2 for PzTDBA and PzDBA, respectively. The high efficiency and low roll-off of these materials are attributed to the good electronic properties originatng from the A-D-A molecular configuration.

Recent progress in hot exciton materials for organic light-emitting diodes

According to Kasha's rule, high-lying excited states usually have little effect on fluorescence. However, in some molecular systems, the high-lying excited states partly or even mainly contribute to the photophysical properties, especially in the process of harvesting triplet excitons in organic electroluminescent devices. In the current review, we focus on a type of organic light-emitting diode (OLED) materials called "hot exciton" materials, which can effectively harness the non-radiative triplet excitons via reverse intersystem crossing (RISC) from high-lying triplet states to singlet states (T_n→ S_m; n≥ 2, m≥ 1). Since Ma and Yang proposed the hot exciton mechanism for OLED material design in 2012, there have been many reports aiming at the design and synthesis of novel hot exciton luminogens. Herein, we present a comprehensive review of the recent progress in hot exciton materials. The developments of the hot exciton mechanism are reviewed, the fundamental principles regarding molecular design are discussed, and representative reported hot exciton luminogens are summarized and analyzed, along with their structure-property relationships and OLED applications.

Utilizing Electroplex Emission to Achieve External Quantum Efficiency up to 18.1% in Nondoped Blue OLED

For the first time, electroplex emission is utilized to enhance the performance of nondoped blue organic light-emitting diodes (OLEDs). By decorating the twisted blue-emitting platform and adjusting the electronic structure, three molecules of 3Cz-Ph-CN, 3Cz-mPh-CN, and 3Ph-Cz-CN with a donor-acceptor structure are synthesized and investigated. When external voltage is applied, electroplex emission, which contributes to the emission performance of OLED, can be realized at the interface between the emitting layer and the electron-transporting layer. Accordingly, high external quantum efficiency of 18.1% can be achieved, while the emission wavelength of the device can be controlled in the blue region. Our results provide the possibility to enhance the performance of OLED through electroplex emission, in addition to the generally investigated thermally activated delayed fluorescence (TADF). Excitedly, when 3Ph-Cz-CN is used as host material in orange-emitting phosphorous OLEDs (PO-01 as the dopant), unprecedented high external quantum efficiency of 27.4% can also be achieved.

Impact of t-butyl substitution in a rubrene emitter for solid state NIR-to-visible photon upconversion

Solid state NIR-to-visible photon upconversion (UC) mediated by triplet-triplet annihilation (TTA) is necessitated by numerous practical applications. Yet, efficient TTA-UC remains a highly challenging task. In this work palladium phthalocyanine-sensitized NIR-to-vis solid UC films based on a popular rubrene emitter are thoroughly studied with the primary focus on revealing the impact of t-butyl substitution in rubrene on the TTA-UC performance. The solution-processed UC films were additionally doped with a small amount of emissive singlet sink tetraphenyldibenzoperiflanthene (DBP) for collecting upconverted singlets from rubrene and in this way diminishing detrimental singlet fission. Irrespective of the excitation conditions used, t-butyl-substituted rubrene (TBR) was found to exhibit enhanced TTA-UC performance as compared to that of rubrene at an optimal emitter doping of 80 wt% in polystyrene films. Explicitly, in the TTA dominated regime attained at high excitation densities, 4-fold higher UC quantum yield (ΦUC) achieved in TBR-based films was caused by the reduced fluorescence concentration quenching mainly due to suppressed singlet fission. Under low light conditions, i.e. in the regime governed by spontaneous triplet decay, even though triplet exciton diffusion was obstructed in TBR films by t-butyl moieties, the subsequently reduced TTA rate was counterbalanced by both suppressed singlet fission and non-radiative triplet quenching, still ensuring higher ΦUC of these films as compared to those of unsubstituted rubrene films.

Triplet-triplet upconversion enhanced by spin-orbit coupling in organic light-emitting diodes

Triplet–triplet upconversion, in which two triplet excitons are converted to one singlet exciton, is a well-known approach to exceed the limit of electroluminescence quantum efficiency in conventional fluorescence-based organic light-emitting diodes. Considering the spin multiplicity of triplet pairs, upconversion efficiency is usually limited to 20%. Although this limit can be exceeded when the energy of a triplet pair is lower than that of a second triplet excited state, such as for rubrene, it is generally difficult to engineer the energy levels of higher triplet excited states. Here, we investigate the upconversion efficiency of a series of new anthracene derivatives with different substituents. Some of these derivatives show upconversion efficiencies close to 50% even though the calculated energy levels of the second triplet excited states are lower than twice the lowest triplet energy. A possible upconversion mechanism is proposed based on the molecular structures and quantum chemical calculations.

Though triplet-triplet upconversion is a promising strategy for designing new deep blue-emitting organic materials, maximizing the efficiency of this process remains difficult. Here, the authors report the upconversion efficiency in anthracene derivatives based on a spin-orbit coupling mechanism.

Organic Light-Emitting Diodes Based on Conjugation-Induced Thermally Activated Delayed Fluorescence Polymers: Interplay Between Intra- and Intermolecular Charge Transfer States

In this work, interactions between different host materials and a blue TADF polymer named P1 are systematically investigated. In photoluminescence, the host can have substantial impact on the photoluminescence quantum yield (PLQY) and the intensity of delayed fluorescence (Φ_DF), where more than three orders of magnitude difference of Φ_DF in various hosts is observed, resulting from a polarity effect of the host material and energy transfer. Additionally, an intermolecular charge-transfer (CT) emission with pronounced TADF characteristics is observed between P1 and 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T), with a singlet-triplet splitting of 7 meV. It is noted that the contribution of harvested triplets in monochrome organic light-emitting diodes (OLEDs) correlates with Φ_DF. For devices based on intermolecular CT-emission, the harvested triplets contribute ~90% to the internal quantum efficiency. The results demonstrate the vital importance of host materials on improving the PLQY and sensitizing Φ_DF of TADF polymers for efficient devices. Solution-processed polychrome OLEDs with a color close to a white emission are presented, with the emission of intramolecular (P1) and intermolecular TADF (PO-T2T:P1).

N-Benzoimidazole/Oxadiazole Hybrid Universal Electron Acceptors for Highly Efficient Exciplex-Type Thermally Activated Delayed Fluorescence OLEDs

Recently, donor/acceptor type exciplex have attracted considerable interests due to the low driving voltages and small singlet-triplet bandgaps for efficient reverse intersystem crossing to achieve 100% excitons for high efficiency thermally activated delayed fluorescence (TADF) OLEDs. Herein, two N-linked benzoimidazole/oxadiazole hybrid electron acceptors were designed and synthesized through simple catalyst-free C-N coupling reaction. 24iPBIOXD and iTPBIOXD exhibited deep-blue emission with peak at 421 and 459 nm in solution, 397 and 419 nm at film state, respectively. The HOMO/LUMO energy levels were −6.14/−2.80 for 24iPBIOXD and −6.17/−2.95 eV for iTPBIOXD. Both compounds could form exciplex with conventional electron donors such as TAPC, TCTA, and mCP. It is found that the electroluminescent performance for exciplex-type OLEDs as well as the delayed lifetime was dependent with the driving force of both HOMO and LUMO energy offsets on exciplex formation. The delayed lifetime from 579 to 2,045 ns was achieved at driving forces close to or larger than 1 eV. Two TAPC based devices possessing large HOMO/LUMO offsets of 1.09–1.34 eV exhibited the best EL performance, with maximum external quantum efficiency (EQE) of 9.3% for 24iPBIOXD and 7.0% for iTPBIOXD acceptor. The TCTA containing exciplex demonstrated moderate energy offsets (0.88–1.03 eV) and EL efficiency (~4%), while mCP systems showed the poorest EL performance (EQE <1%) and shortest delayed lifetime of <100 ns due to inadequate driving force of 0.47–0.75 eV for efficient exciplex formation.

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.

Functional 1,8-Naphthalimide AIE/AIEEgens: Recent Advances and Prospects

This comprehensive review surveys the up-to-date development of aggregation-induced emission/aggregation-induced emission enhancement (AIE/AIEE) active naphthalimide (NI)-based smart materials with potential for wide and real-world applications and that serves as a highly versatile building block with tunable absorption and emission in the complete visible region. The review article commences with a precise description of the importance of NI moiety and its several restricted area of applications owing to its aggregation caused quenching (ACQ) properties, followed by the discovery and importance of AIE/AIEE-active NIs. The introduction section tracked an overview of the state of the art in NI luminogens in multiple applications. It also includes a few mechanistic studies on the structure-property correlation of NIs and provides more insights into the condensed-state photophysical properties of small aggregation-prone systems. The review aims to ultimately accomplish current and forthcoming views comprising the use of the NIs for the detection of biologically active molecules, such as amino acids and proteins, recognition of toxic analytes, fabrication of light emitting diodes, and their potential in therapeutics and diagnostics.

Intersystem Crossing and Triplet Fusion in Singlet-Fission-Dominated Rubrene-Based OLEDs Under High Bias Current

Singlet fission is usually the only reaction channel for excited states in rubrene-based organic light-emitting diodes (OLEDs) at ambient temperature. Intriguingly, we discover that triplet fusion (TF) and intersystem crossing (ISC) within rubrene-based devices begin at moderate and high current densities (j), respectively. Both processes enhance with decreasing temperature. This behavior is discovered by analyzing the magneto-electroluminescence curves of the devices. The j-dependent magneto-conductance, measured at ambient temperature indicates that spin mixing within polaron pairs that are generated by triplet-charge annihilation (TQA) causes the occurrence of ISC, while the high concentrations of triplets are responsible for generating TF. Additionally, the reduction in exciton formation and the elevated TQA with decreasing temperature may contribute to the enhanced ISC at low temperatures. This work provides considerable insight into the different mechanisms that occur when a high density of excited states exist in rubrene and reasonable reasons for the absence of EL efficiency roll-off in rubrene-based OLEDs.

Tailoring the molecular design of twisted dihydrobenzodioxin phenanthroimidazole derivatives for non-doped blue organic light-emitting devices

Three fused polycyclic aryl fragments, namely, naphthyl, methoxynaphthyl, and pyrenyl have been used to construct blue-emissive phenanthroimidazole-functionalized target molecules, i.e., 1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-2-(naphthalen-1-yl)-1H-phenanthro[9,10-d]imidazole (1), 1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-2-(1-methoxynaphthalen-4-yl)-1H-phenanthro[9,10-d]imidazole (2), and 1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-2-(pyren-10-yl)-1H-phenanthro[9,10-d]imidazole (3). The up-conversion of triplets to singlets via a triplet–triplet annihilation (TTA) process is dominant in these compounds due to 2E_T1 > E_S1. The pyrenyl dihydrobenzodioxin phenanthroimidazole (3)-based nondoped OLED exhibits blue emission (450 nm) with CIE (0.15, 0.14), a luminance of 53 890 cd m⁻², power efficiency of 5.86 lm W⁻¹, external quantum efficiency of 5.30%, and current efficiency of 6.90 cd A⁻¹. The efficient device performance of pyrenyl dihydrobenzodioxin phenanthroimidazole is due to the TTA contribution to the electroluminescent process.

Efficient blue emitters, 1-(2,3-dihydrobenzodioxinyl)-2-naphthylphenanthroimidazole, 1-(2,3-dihydrobenzodioxinyl)-2-methoxynaphthylphenanthroimidazole and 1-(2,3-dihydrobenzodioxinyl)-2-pyrenylphenanthroimidazole have been reported.

Structure-Property Correlation in Luminescent Indolo[3,2-b]indole (IDID) Derivatives: Unraveling the Mechanism of High Efficiency Thermally Activated Delayed Fluorescence (TADF)

A series of indolo[3,2-b]indole (IDID) derivatives are designed as a novel structural platform for thermally activated delayed fluorescence (TADF) emitters. Intramolecular charge transfer (ICT)-type molecules consisting of IDID donor (D) and various acceptor (A) moieties are synthesized and characterized in the protocol of the systematical structure-property correlation. IDID derivatives exhibit high efficiency, prompt fluorescence as well as TADF with emission ranges tuned by the chemical structure of the acceptor units. Interestingly, almost all of the IDID derivatives show an identical energy level of the lowest triplet excited state (T₁) attributed to the locally excited triplet state of the IDID backbone (³LE_ID), while that of their lowest singlet excited state (S₁) is largely tuned by varying the acceptor units. Thus, we demonstrate the underlying mechanism in terms of the molecular engineering. Among the compounds, Tria-phIDID and BP-phIDID generate efficient delayed fluorescence based on the small energy gap between the lowest singlet and triplet excited states (ΔE_ST) and mediation of the ³LE_ID state. Organic light-emitting diodes with these Tria-phIDID and BP-phIDID as a dopant in the emitting layer show highly efficient electroluminescence with maximum external quantum efficiencies of 20.8% and 13.9%, respectively.

Donor-σ-Acceptor Motifs: Thermally Activated Delayed Fluorescence Emitters with Dual Upconversion

A family of organic emitters with a donor-σ-acceptor (D-σ-A) motif is presented. Owing to the weakly coupled D-σ-A intramolecular charge-transfer state, a transition from the localized excited triplet state (³ LE) and charge-transfer triplet state (³ CT) to the charge-transfer singlet state (¹ CT) occurred with a small activation energy and high photoluminescence quantum efficiency. Two thermally activated delayed fluorescence (TADF) components were identified, one of which has a very short lifetime of 200-400 ns and the other a longer TADF lifetime of the order of microseconds. In particular, the two D-σ-A materials presented strong blue emission with TADF properties in toluene. These results will shed light on the molecular design of new TADF emitters with short delayed lifetimes.

Electronic energy pooling in organic systems: a cross-disciplinary tutorial review

Electronic energy pooling via excited state (exciton) annihilation, primarily in organic systems, is reviewed in tutorial form. Cross-disciplinary terminologies and references are used and reference is made to the historical origins of the phenomena. Applications in organic photovoltaic and electroluminescent devices are addressed. Particular attention is paid to the kinetics of the processes involved; a standard format for all systems is developed. Within the organic materials framework, all triplet–triplet, triplet–singlet, and singlet–singlet annihilation processes are discussed. Examples from gas, liquid, and solid phase systems, including both homo- and hetero-species interactions, are employed. Particular attention is given to triplet–triplet annihilation processes in which product states other than the lowest excited singlet state are formed.

Role of Triplet-State Shelving in Organic Photovoltaics: Single-Chain Aggregates of Poly(3-hexylthiophene) versus Mesoscopic Multichain Aggregates

Triplet excitons have been the focus of considerable attention with regards to the functioning of polymer solar cells because these species are long-lived and quench subsequently generated singlet excitons in their vicinity. The role of triplets in poly(3-hexylthiophene) (P3HT) has been investigated extensively with contrary conclusions regarding their importance. We probe the various roles triplets can play in P3HT by analyzing the photoluminescence (PL) from isolated single-chain aggregates and multichain mesoscopic aggregates. Solvent vapor annealing allows deterministic growth of P3HT aggregates consisting of ∼20 chains, which exhibit red-shifted and broadened PL compared to single-chain aggregates. The multichain aggregates exhibit a decrease of photon antibunching contrast compared to single-chain aggregates, implying rather weak interchain excitonic coupling and energy transfer. Nevertheless, the influence of triplet-quenching oxygen on PL and a photon correlation analysis of aggregate PL reveal that triplets are quenched by intermolecular interactions in the bulk state.

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.

Unusual Nonemissive Behavior of Rubrene J-Aggregates: A Rare Violation

Structure-property correlations in rubrene (RB) colloidal J-aggregates were unravelled by steady state and time-resolved spectroscopy in conjunction with excited state density functional calculations. The RB J-aggregate with a slippage angle θ = 30.4°, estimated from the monomeric transition dipole moment directions, exhibited a broad fwhm of 1073 cm^-1 and a 5 nm red-shifted absorption band carrying a transition dipole moment (M⃗_λagg = 1.80 D) almost equivalent to the monomeric dye (M⃗_λmon = 1.89 D). A significantly low magnitude of exciton coupling energy, ΔE_exc = -358 cm^-1 for the rhombic-RB colloidal J-aggregates resulted owing to the weaker electronic communication between the largely separated RB subunits (r = 7.2 Å) and a restricted exciton delocalization over the RB J-dimer (N = 2). The RB J-dimer exhibited a perfect balance between the computed singlet (2.53 eV) and the triplet (1.29 eV) exciton energies for singlet fission (SF). Supporting this, the PL decay profile of the J-aggregates revealed a delayed fluorescence, substantiating triplet pair formation via SF. The experimental evidence for the long-lived triplet formation was furthermore confirmed by its transient absorption (T₁ → T_N) at 530 nm. Consequently, a high probability for SF and a low probability for triplet-triplet recombination, leading to a dramatic lowering in photoluminescence quantum yield from 0.172 down to 0.035 was noted. The electronic structure calculations for the RB J-dimer followed TD-DFT-M062X/6-31G+(d,p) level of theory following integral equation formalism polarizable continuum model (IEFPCM) in water. S₁ excited state for RB J-dimer was carefully analyzed using integral overlap of electron and hole density distribution (ϕ) and the defined t-indexes along all three spatial directions, and was found to be of locally excited in character.