Efficient Triplet Exciton Fusion in Molecularly Doped Polymer Light-Emitting Diodes

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.

Efficient Spin Interconversion by Molecular Conformation Dynamics of a Triplet Pair for Photon Up-Conversion in an Amorphous Solid

Solid-state materials with improved light-to-energy conversions in organic photovoltaics and in optoelectronics are expected to be developed by realizing efficient triplet-triplet annihilation (TTA) by manipulating the spin conversion processes to the singlet state. In this study, we elucidate the spin conversion mechanism for delayed fluorescence by TTA from a microscopic view of the molecular conformations. We examine the time evolution of the electron spin polarization of the triplet-pair state (TT state) in an amorphous solid-state system exhibiting highly efficient up-conversion emission by using time-resolved electron paramagnetic resonance. We clarified that the spin-state population of the singlet TT increased through the spin interconversion from triplet and quintet TT states during exciton diffusion with random orientation dynamics between the two triplets for the modulation of the exchange interaction, achieving a high quantum yield of up-conversion emission. This understanding provides us with a guide for the development of efficient light-to-energy conversion devices utilizing TTA.

Luminescent Radicals

Organic radicals are attracting increasing interest as a new class of molecular emitters. They demonstrate electronic excitation and relaxation dynamics based on their doublet or higher multiplet spin states, which are different from those based on singlet-triplet manifolds of conventional closed-shell molecules. Recent studies have disclosed luminescence properties and excited state dynamics unique to radicals, such as highly efficient electron-photon conversion in OLEDs, NIR emission, magnetoluminescence, an absence of heavy atom effect, and spin-dependent and spin-selective dynamics. These are difficult or sometimes impossible to achieve with closed-shell luminophores. This review focuses on luminescent organic radicals as an emerging photofunctional molecular system, and introduces the material developments, fundamental properties including luminescence, and photofunctions. Materials covered in this review range from monoradicals, radical oligomers, and radical polymers to metal complexes with radical ligands demonstrating radical-involved emission. In addition to stable radicals, transiently formed radicals generated in situ by external stimuli are introduced. This review shows that luminescent organic radicals have great potential to expand the chemical and spin spaces of luminescent molecular materials and thus broaden their applicability to photofunctional systems.

Bulk Heterojunction Upconversion Thin Films Fabricated via One-Step Solution Deposition

Upconversion of near-infrared light into the visible has achieved limited success in applications due to the difficulty of creating solid-state films with high external quantum efficiency (EQE). Recent developments have expanded the range of relevant materials for solid-state triplet-triplet annihilation upconversion through the use of a charge-transfer state sensitization process. Here, we report the single-step solution-processed deposition of a bulk heterojunction upconversion film using organic semiconductors. The use of a bulk heterojunction thin film enables a high contact area between sensitizer and annihilator materials in this interface-triplet-generation mechanism and allows for a facile single-step deposition process. Demonstrations of multiple deposition and patterning methods on glass and flexible substrates show the promise of this materials system for solid-state upconversion applications.

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.

Highly Efficient OLEDs by Using a Brominated Thermally Activated Delayed Fluorescent Host due to Balanced Carrier Transport and Enhanced Exciton Upconversion

Thermally activated delayed fluorescent (TADF) materials are naturally bipolar and can potentially serve as hosts. However, triplet excitons in TADF materials are long-lived and prone to unfavorable bimolecular processes. Implementing an efficient reverse system intersection (RISC) process is an effective solution. Moreover, although the general TADF host is bipolar, polarity differences still cause a mobility imbalance. In this work, we designed and synthesized a novel TADF host material, 11-(3-(4-(3-bromophenyl)-6-phenyl-1,3,5-triazin-2-yl)phenyl)-12,12-dimethyl-11,12-dihydroindeno[2,1-a]carbazole (Br-DMIC-TRZ). The upconversion of the TADF host and its doped films is facilitated due to enhanced spin-orbit coupling (SOC) induced by bromine, which exhibits a higher rate of RISC. This progress facilitates the involvement of more triplet excitons in luminescence. Meanwhile, the attachment of bromine to the acceptor fragment of TADF enhances the electron mobility, where hole mobility and electron mobility are more comparable. Enhanced exciton upconversion and balanced carrier transport allow devices formed based on brominated TADF hosts to outperform other hosts. The Br-TADF-based devices with three dopants sensitized achieved improvements of 29.8, 21.4, and 24.4% compared to the DMIC-TRZ-based device. This work provides a feasible molecular design strategy for further developing efficient hosts.

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.

Hot-Exciton Mechanism and AIE Effect Boost the Performance of Deep-Red Emitters in Non-Doped OLEDs

Luminescent materials possessing a "hot-exciton" mechanism and aggregation-induced emission (AIE) qualities are well-suited for use as emitting materials in nondoped organic light-emitting diodes (OLEDs), particularly in deep-red regions where their ground state and singlet excited state surfaces are in proximity, leading to the formation of multiple nonradiative channels. However, designing molecules that artificially combine the hot-exciton mechanism and AIE attributes remains a formidable task. In this study, a versatile strategy is presented to achieve hot-exciton fluorescence with AIE property by increasing the first singlet excited (S1 ) state through modulation of the conjugation length of the newly created acceptor unit, matching the energy level of high-lying triplet (Tn ) states, and enhancing exciton utilization efficiency by employing suitable donor moieties. This approach reduces the aggregation-caused quenching (ACQ) in the aggregate state, resulting in the proof-of-concept emitter DT-IPD, which produces an unprecedented external quantum efficiency (EQE) of 12.2% and Commission Internationale de I'Eclairage (CIE) coordinates of (0.69, 0.30) in a deep-red non-doped OLED at 685 nm, representing the highest performance among all deep-red OLEDs based on materials with hot-exciton mechanisms. This work provides novel insights into the design of more efficient hot-exciton emitters with AIE properties.

Mechanism of Enhanced Triplet-Triplet Upconversion in Organic Molecules

We use time-dependent density functional theory (TDDFT) to investigate the mechanism of efficient triplet-triplet upconversion (TTU) in certain organic materials. In particular, we focus on materials where some singlets are generated in a two-step spin-nonconserving process (T1 + T1 → T2 → S1). For this mechanism to contribute significantly, the intersystem crossing (ISC) from the high-lying triplet to the singlet (T2 → S1) must outcompete the internal conversion (IC) to the low-lying triplet (T2 → T1). By considering multiple families of materials, we show that the T2 → S1 ISC can be enhanced in a number of ways: the substitution of electron-donating (ED) and electron-withdrawing (EW) groups at appropriate positions; the substitution of bulky groups that distort the molecular geometry; and the substitution of heavy atoms that enhance the spin-orbit coupling (SOC). In the first two cases, the enhancements are consistent with El-Sayed's rule in that rapid T2 → S1 ISC requires significant differences in the characters of the S1 and the T2 wavefunctions. Together, these effects enable a wide tunability of T2 → S1 ISC rates over at least 5 orders of magnitude. Meanwhile, the T2 → T1 IC is inhibited in these systems due to the large T2 - T1 energy gap >0.5 eV, which entails a high energy barrier to the T2 → T1 IC and the prediction of a slow rate regardless of the substituents or the presence of heavy atoms. In this way, tuning the T2 → S1 ISC appears to provide an effective strategy to achieve systematic improvement of TTU materials.

Overcoming the Limitation of Spin Statistics in Organic Light Emitting Diodes (OLEDs): Hot Exciton Mechanism and Its Characterization

Triplet harvesting processes are essential for enhancing efficiencies of fluorescent organic light-emitting diodes. Besides more conventional thermally activated delayed fluorescence and triplet-triplet annihilation, the hot exciton mechanism has been recently noticed because it helps reduce the efficiency roll-off and improve device stability. Hot exciton materials enable the conversion of triplet excitons to singlet ones via reverse inter-system crossing from high-lying triplet states and thereby the depopulation of long-lived triplet excitons that are prone to chemical and/or efficiency degradation. Although their anti-Kasha characteristics have not been clearly explained, numerous molecules with behaviors assigned to the hot exciton mechanism have been reported. Indeed, the related developments appear to have just passed the stage of infancy now, and there will likely be more roles that computational elucidations can play. With this perspective in mind, we review some selected experimental studies on the mechanism and the related designs and then on computational studies. On the computational side, we examine what has been found and what is still missing with regard to properly understanding this interesting mechanism. We further discuss potential future points of computational interests toward aiming for eventually presenting in silico design guides.

High-Level Reverse Intersystem Crossing and Molecular Rigidity Improve Spin Statistics for Triplet-Triplet Annihilation Upconversion

The structural factors affecting triplet-triplet annihilation (TTA) at the molecular level are not well-understood. Here, our steady-state photoluminescence and transient absorption results demonstrate that the spin statistical factor, η, decreases from 0.60 to 0.46 and 0.14 going from 9,10-diphenylanthracene (DPA) to the 1,5-DPA and 2,6-DPA isomers, respectively, during photon upconversion with a platinum octaethylporphyrin sensitizer. Density functional theory (DFT) shows that η depends on the energetics of hot triplet states and molecular rigidity. The significantly high conical intersection energy between the S₀ and T₁ states for 9,10-DPA gives its longer triplet lifetime. Time-dependent DFT calculations show that 9,10-DPA and 1,5-DPA can undergo high-level reverse intersystem crossing from their T₂ and T₃ states, respectively, to the bright S₁ state, increasing the limit of the spin statistical factor. Both factors ultimately serve to enhance the TTA efficiency. This work provides insight into designing molecules for efficient light-emitting and photon upconversion applications.

Plasmonic Metal Nanostructures Meet Triplet-Triplet Annihilation-Based Photon Upconversion Systems: Performance Improvements and Application Trends

Improving the performance of upconversion systems based on triplet-triplet annihilation (TTA-UC) can have far-reaching implications for various fields, including solar devices, nano-bioimaging, and nanotherapy. This review focuses on the use of localized surface plasmon (LSP) resonance of metal nanostructures to enhance the performance of TTA-UC systems and explores their potential applications. After introducing the basic driving mechanism of TTA-UC and typical sensitizers used in these systems, we discuss recent studies that have utilized new sensitizers with distinct characteristics. Furthermore, we confirm that the enhancement in upconverted emission can be explained, at least in part, by the mechanism of "metal-enhanced fluorescence", which is attributed to LSP resonance-induced fluorescence enhancement. Next, we describe selected experiments that demonstrate the enhancement in upconverted emission in plasmonic TTA-UC systems, as well as the emerging trends in their application. We present specific examples of studies in which the enhancement in upconverted emission has significantly improved the performance of photocatalysts under both sunlight and indoor lighting. Additionally, we discuss the potential for future developments in plasmonic TTA-UC systems.

Nanoengineering Triplet-Triplet Annihilation Upconversion: From Materials to Real-World Applications

Using light to control matter has captured the imagination of scientists for generations, as there is an abundance of photons at our disposal. Yet delivering photons beyond the surface to many photoresponsive systems has proven challenging, particularly at scale, due to light attenuation via absorption and scattering losses. Triplet-triplet annihilation upconversion (TTA-UC), a process which allows for low energy photons to be converted to high energy photons, is poised to overcome these challenges by allowing for precise spatial generation of high energy photons due to its nonlinear nature. With a wide range of sensitizer and annihilator motifs available for TTA-UC, many researchers seek to integrate these materials in solution or solid-state applications. In this Review, we discuss nanoengineering deployment strategies and highlight their uses in recent state-of-the-art examples of TTA-UC integrated in both solution and solid-state applications. Considering both implementation tactics and application-specific requirements, we identify critical needs to push TTA-UC-based applications from an academic curiosity to a scalable technology.

Water-Stable Upconverting Coordination Polymer Nanoparticles for Transparent Films and Anticounterfeiting Patterns with Air-Stable Upconversion

Photon upconversion (UC) based on triplet-triplet annihilation is a very promising phenomenon with potential application in several areas, though, due to the intrinsic mechanism, the achievement of diffusion-limited solid materials with air-stable UC is still a challenge. Herein, we report UC coordination polymer nanoparticles (CPNs) combining sensitizer and emitter molecules especially designed with alkyl spacers that promote the amorphous character. Beyond the characteristic constraints of crystalline MOFs, amorphous CPNs facilitate high dye density and flexible ratio tunability. To show the universality of the approach, two types of UC-CPNs are reported, exhibiting highly photostable UC in two different visible spectral regions. Given their nanoscale, narrow size distribution, and good chemical/colloidal stability in water, the CPNs were also successfully printed as anticounterfeiting patterns and used to make highly transparent and photostable films for luminescent solar concentrators, both showing air-stable UC.

Suppressing singlet-triplet annihilation processes to achieve highly efficient deep-blue AIE-based OLEDs

Aggregation-induced emission (AIE) materials are attractive for the fabrication of high efficiency organic light-emitting diodes (OLEDs) by harnessing "hot excitons" from the high-lying triplet exciton states (T_n, n ≥ 2) and high photoluminescence (PL) quantum efficiency in solid films. However, the electroluminescence (EL) efficiency of most AIE-based OLEDs does not meet our expectation due to some unrevealed exciton loss processes. Herein, we further enhance the efficiency of blue AIE-based OLEDs, and find experimentally and theoretically that the serious exciton loss is caused by the quenching of radiative singlet excitons and long-lived triplet excitons [singlet-triplet annihilation (STA)]. In order to suppress the STA process, 1-(2,5-dimethyl-4-(1-pyrenyl)phenyl)pyrene (DMPPP) with triplet-triplet annihilation up-conversion was doped in two AIE emitters to reduce the triplet excitons on the lowest triplet excited state (T₁) of AIE molecules. It can be seen that the external quantum efficiency (EQE) of the resulting blue OLEDs was enhanced to 11.8% with CIE coordinates of (0.15, 0.07) and a negligible efficiency roll-off, realizing the efficiency breakthrough of deep-blue AIE-based OLEDs. This work establishes a physical insight in revealing the exciton loss processes and the fabrication of high-performance AIE-based OLEDs.

A "Flexible" Purely Organic Molecule Exhibiting Strong Spin-Orbital Coupling: Toward Nondoped Room-Temperature Phosphorescence OLEDs

Purely organic materials usually exhibit weak spin-orbital coupling (SOC) effect because of the lack of noble heavy metals, and the generation and direct emission from the triplet state is spin-forbidden. This would lead to slow intersystem crossing, long triplet lifetime, and low phosphorescence quantum yield. Herein, strong spin-orbital coupling between singlet and triplet was observed in a "flexible" and twist thianthrene-pyrimidine-based purely organic compound in an amorphous film state, which shows a fast intersystem crossing process and a high phosphorescence rate of 1.1 × 10³ s^-1. The heavy atom sulfur and nitrogen atoms in the molecule can provide n-π* transition character for efficient spin-orbital coupling. Moreover, the flexible molecule skeleton enables conformational change and molecular vibration in excited states, which was proved to be vital for efficient vibrational spin-orbital coupling. Benefitting from the strong SOC effect, a nondoped purely organic phosphorescence light-emitting diode was fabricated, which achieves a maximum external quantum efficiency of 7.98%, corresponding to an exciton utilization ratio exceeding 87.6%.

The Rise and Future of Discrete Organic-Inorganic Hybrid Nanomaterials

Hybrid nanomaterials (HNs), the combination of organic semiconductor ligands attached to nanocrystal semiconductor quantum dots, have applications that span a range of practical fields, including biology, chemistry, medical imaging, and optoelectronics. Specifically, HNs operate as discrete, tunable systems that can perform prompt fluorescence, energy transfer, singlet fission, upconversion, and/or thermally activated delayed fluorescence. Interest in HNs has naturally grown over the years due to their tunability and broad spectrum of applications. This Review presents a brief introduction to the components of HNs, before expanding on the characterization and applications of HNs. Finally, the future of HN applications is discussed.

Singlet and triplet to doublet energy transfer: improving organic light-emitting diodes with radicals

Organic light-emitting diodes (OLEDs) must be engineered to circumvent the efficiency limit imposed by the 3:1 ratio of triplet to singlet exciton formation following electron-hole capture. Here we show the spin nature of luminescent radicals such as TTM-3PCz allows direct energy harvesting from both singlet and triplet excitons through energy transfer, with subsequent rapid and efficient light emission from the doublet excitons. This is demonstrated with a model Thermally-Activated Delayed Fluorescence (TADF) organic semiconductor, 4CzIPN, where reverse intersystem crossing from triplets is characteristically slow (50% emission by 1 µs). The radical:TADF combination shows much faster emission via the doublet channel (80% emission by 100 ns) than the comparable TADF-only system, and sustains higher electroluminescent efficiency with increasing current density than a radical-only device. By unlocking energy transfer channels between singlet, triplet and doublet excitons, further technology opportunities are enabled for optoelectronics using organic radicals.

Organic light-emitting diodes must be engineered to circumvent efficiency limits imposed by the ratio of triplet to singlet exciton formation, following electron-hole capture. Here, authors unlock energy transfer channels between singlet, triplet and doublet excitons using thermally activated delayed fluorescence and radical emitters towards more efficient light-emitting devices.

Efficiency of Polymer Light-Emitting Diodes: A Perspective

The various contributions to the external quantum efficiency (EQE) of polymer light-emitting diodes (PLEDs) are discussed. The EQE of an organic light-emitting diode is governed by a number of parameters, such as the electrical efficiency, the photoluminescence quantum yield (PLQY), the optical outcoupling efficiency and the spin statistics for singlet exciton generation. In the last decade, the electrical efficiency has been determined from a numerical PLED device model. More recently, an optical model to simulate the fraction of photons outcoupled to air for PLEDs with a broad recombination zone has been developed. Together with the directly measured PLQY, the EQE of a PLED can then be estimated. However, it has been observed that the measured EQEs of fluorescent PLEDs, including the model system super-yellow poly(p-phenylene vinylene) (SY-PPV) often exceed the expected values. To solve this discrepancy, it is demonstrate that the electrical PLED model has to be expanded by the inclusion of triplet-triplet annihilation (TTA), which is shown to be responsible for a substantial EQE enhancement. Experimentally, it is obtained that TTA contributes to a singlet-exciton generation efficiency of ≈40% in SY-PPV PLEDs, giving rise to an EQE of ≈4% instead of the expected value of 2.5%.

Altering singlet fission pathways in perylene-dimers; perylene-diimide versus perylene-monoimide

We used a systematic approach to shed light on the inherent differences in perylenes, namely monoimides versus diimides, including coplanarity and dipole moment, and their impact on singlet fission (SF) by designing, synthesizing, and probing a full fledged series of phenylene- and naphthalene-linked dimers. Next to changing the functionality of the perylene core, we probed the effect of the spacers and their varying degrees of rotational freedom, molecular electrostatic potentials, and intramolecular interactions on the SF-mechanism and -efficiencies. An arsenal of spectroscopic techniques revealed that for perylene-monoimides, a strong charge-transfer mixing with the singlet and triplet excited states restricts SF and yields low triplet quantum yields. This is accompanied by an up-conversion channel that includes geminate triplet-triplet recombination. Using perylene-diimides alters the SF-mechanism by populating a charge-separated-state intermediate, which either favors or shuts-down SF. Napthylene-spacers bring about higher triplet quantum yields and overall better SF-performance for all perylene-monoimides and perylene-diimides. The key to better SF-performance is rotational freedom because it facilitates the overall excited-state polarization and amplifies intramolecular interactions between chromophores.

Photon-upconverters for blue organic light-emitting diodes: a low-cost, sky-blue example

In the research ecosystem's quest towards having deployable organic light-emitting diodes with higher-energy emission (e.g., blue light), we advocate focusing on fluorescent emitters, due to their relative stability and colour purity, and developing design strategies to significantly improve their efficiencies. We propose that all triplet-triplet annihilation upconversion (TTA-UC) emitters would make good candidates for triplet fusion-enhanced OLEDs ("FuLEDs"), due to the energetically uphill nature of the photophysical process, and their common requirements. We demonstrate this with the low-cost sky-blue 1,3-diphenylisobenzofuran (DPBF). Having satisfied the criteria for TTA-UC, we show DPBF as a photon upconverter (I th 92 mW cm-2), and henceforth demonstrate it as a bright emitter for FuLEDs. Notably, the devices achieved 6.5% external quantum efficiency (above the ∼5% threshold without triplet contribution), and triplet-exciton-fusion-generated fluorescence contributes up to 44% of the electroluminescence, as shown by transient measurements. Here, triplet fusion translates to a quantum yield (Φ TTA-UC) of 19%, at an electrical excitation of ∼0.01 mW cm-2. The enhancement is meaningful for commercial blue OLED displays. We also found DPBF to have decent hole mobilities of ∼0.08 cm2 V-1 s-1. This additional finding can lead to DPBF being used in other capacities in various printable electronics.

Aggregation-induced emission luminogen with excellent triplet-triplet upconversion efficiency for highly efficient non-doped blue organic light-emitting diodes

By combining aggregation-induced emission (AIE) effect and a triplet-triplet upconversion (TTU) process, a blue emitter with excellent photoluminescence quantum efficiency and high upconversion efficiency in the film state is developed, from which a highly efficient non-doped blue TTU organic light-emitting diode (TTU-OLED) was realized.

Versatile, stable, and air-tolerant triplet–triplet annihilation upconversion block copolymer micelles

A versatile, stable, and highly air-tolerant triplet–triplet annihilation up-conversion system based on block copolymer micelles was designed and fabricated.

Lifting Triplet Energy and Bipolar Characteristics by Limiting the Rotation of the Peripheral Groups in Host Materials to Achieve High-Efficiency Blue OLED

Bipolar host materials with high triplet energy are of great significance for highly efficient blue organic light-emitting diodes (OLEDs). In this work, three donor-acceptor-donor (D-A-D) type host materials with identical non-rigid diphenylsulfone center but differing in rotation degree of peripheral amino substituted derivatives from rotating freely diphenylamine (SODP) to rotating partially iminodibenzyl (SOId) and rotating restricted carbazole (SOCz) were designed and synthesized. It was demonstrated that the triplet energy (E_T ) level of the materials promoted by limiting the rotation degree of the peripheral groups, which was 2.72 eV for SODP, 2.73 eV for SOId and 2.78 eV for SOCz, respectively. Besides, the results of the single-carrier devices indicate SOCz possess better bipolar characteristic. Using FIrpic as guest emitter, the blue OLED with SOCz as host material exhibited superior device performance with a low turn-on voltage of 3.3 V, a maximum current efficiency (CE) of 30.1 cd A^-1 , a maximum power efficiency (PE) of 32.2 lm W^-1 , and a maximum external quantum efficiency (EQE) of 14.0%. This work provides a beneficial guideline for realizing promising host materials in efficient blue OLEDs.

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.

CdSe nanocrystal sensitized photon upconverting film

Here, films using CdSe nanocrystal (NC) triplet photosensitizers in conjunction with diphenylanthracene (DPA) emitters were assembled to address several challenges to practical applications for solution-based photon upconversion. By using poly(9-vinylcarbazole) as a phosphorescent host in this film, volatile organic solvents are eliminated, the spontaneous crystallization of the emitter is significantly retarded, and ∼1.5% photon upconversion quantum yield (out of a maximum of 50%) is obtained. Transient absorption spectroscopy on nanosecond-to-microsecond time scales reveals this efficiency is enabled by an exceptionally long triplet lifetime of 3.4 ± 0.3 ms. Ultimately, we find the upconversion efficiency is limited by incomplete triplet–triplet annihilation, which occurs with a rate 3–4 orders of magnitude slower than in solution-phase upconversion systems.

Here, films using CdSe nanocrystal (NC) triplet photosensitizers in conjunction with diphenylanthracene (DPA) emitters doe for the conversion of green to blue light.

(DiMeIHeptCl)Pd: A Low-Load Catalyst for Solvent-Free (Melt) Amination

(DiMeIHept^Cl)Pd, a hyper-branched N-aryl Pd NHC catalyst, has been shown to be efficient at performing amine arylation reactions in solvent-free ("melt") conditions. The highly lipophilic environment of the alkyl chains flanking the Pd center serves as lubricant to allow the complex to navigate through the paste-like environment of these mixtures. The protocol can be used on a multi-gram scale to make a variety of aniline derivatives, including substrates containing alcohol moieties.

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.

Singlet exciton fission in a modified acene with improved stability and high photoluminescence yield

We report a fully efficient singlet exciton fission material with high ambient chemical stability. 10,21-Bis(triisopropylsilylethynyl)tetrabenzo[a,c,l,n]pentacene (TTBP) combines an acene core with triphenylene wings that protect the formal pentacene from chemical degradation. The electronic energy levels position singlet exciton fission to be endothermic, similar to tetracene despite the triphenylenes. TTBP exhibits rapid early time singlet fission with quantitative yield of triplet pairs within 100 ps followed by thermally activated separation to free triplet excitons over 65 ns. TTBP exhibits high photoluminescence quantum efficiency, close to 100% when dilute and 20% for solid films, arising from triplet-triplet annihilation. In using such a system for exciton multiplication in a solar cell, maximum thermodynamic performance requires radiative decay of the triplet population, observed here as emission from the singlet formed by recombination of triplet pairs. Combining chemical stabilisation with efficient endothermic fission provides a promising avenue towards singlet fission materials for use in photovoltaics.

Designing optimised molecules for singlet fission is crucial to improve the efficiency of solar cells beyond its theoretical limit. Here, the authors investigate pentacene derivative TTBP, which exhibits high stability and luminescence yield, and find it highly suitable for exciton multiplication purposes.

Unconventional singlet fission materials

Singlet fission (SF) is a photophysical downconversion pathway, in which a singlet excitation transforms into two triplet excited states. As such, it constitutes an exciton multiplication generation process, which is currently at the focal point for future integration into solar energy conversion devices. Beyond this, various other exciting applications were proposed, including quantum cryptography or organic light emitting diodes. Also, the mechanistic understanding evolved rapidly during the last year. Unfortunately, the number of suitable SF-chromophores is still limited. This is per se problematic, considering the wide range of envisaged applicability. With that in mind, we emphasize uncommon SF-scaffolds and outline requirements as well as strategies to expand the chromophore pool of SF-materials.

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.

Understanding the luminescent nature of organic radicals for efficient doublet emitters and pure-red light-emitting diodes

The doublet-spin nature of radical emitters is advantageous for applications in organic light-emitting diodes, as it avoids the formation of triplet excitons that limit the electroluminescence efficiency of non-radical emitters. However, radicals generally show low optical absorption and photoluminescence yields. Here we explain the poor optical properties of radicals based on alternant hydrocarbons, and establish design rules to increase the absorption and luminescence yields for donor-acceptor-type radicals. We show that non-alternant systems are necessary to lift the degeneracy of the lowest energy orbital excitations; moreover, intensity borrowing from an intense high-lying transition by the low-energy charge-transfer excitation enhances the oscillator strength of the emitter. We apply these rules to design tris(2,4,6-trichlorophenyl)methyl-pyridoindolyl derivatives with a high photoluminescence quantum yield (>90%). Organic light-emitting diodes based on these molecules showed a pure-red emission with an over 12% external quantum efficiency. These insights may be beneficial for the rational design and discovery of highly luminescent doublet emitters.

Polymer Light Emitting Diodes with Doublet Emission

Organic light-emitting radicals have developed rapidly due to their unique doublet emission and great potential in display technology. Although some organic light-emitting diodes (OLEDs) exploiting small-molecular radicals as the emitters have been reported, there is no report about the polymer-radical-based OLEDs until now. Herein, a kind of polymer radical, PS-CzTTM, is adopted as the emitter to fabricate solution-processed OLEDs. A maximum external quantum efficiency of 3.0% is achieved for a deep-red device with an emissive layer of PS-CzTTM lightly doped in 2,2',2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-benzimidazole) (TPBi). Temperature-dependent time-resolved photoluminescent spectra and transient electroluminescence of radical emitters and devices are first measured. The results demonstrate that the emission channels for both thin films and devices are from the transition of doublet excitons, indicating that the unique doublet emission mechanism of radicals is maintained in PS-CzTTM films and PS-CzTTM-based OLEDs.

In-situ observation of nucleation and property evolution in films grown with an atmospheric pressure spatial atomic layer deposition system

Atmospheric pressure—spatial atomic layer deposition (AP-SALD) is a promising open-air deposition technique for high-throughput manufacturing of nanoscale films, yet the nucleation and property evolution in these films has not been studied in detail. In this work, in situ reflectance spectroscopy was implemented in an AP-SALD system to measure the properties of Zinc oxide (ZnO) and Aluminum oxide (Al2O3) films during their deposition. For the first time, this revealed a substrate nucleation period for this technique, where the length of the nucleation time was sensitive to the deposition parameters. The in situ characterization of thickness showed that varying the deposition parameters can achieve a wide range of growth rates (0.1–3 nm/cycle), and the evolution of optical properties throughout film growth was observed. For ZnO, the initial bandgap increased when deposited at lower temperatures and subsequently decreased as the film thickness increased. Similarly, for Al2O3 the refractive index was lower for films deposited at a lower temperature and subsequently increased as the film thickness increased. Notably, where other implementations of reflectance spectroscopy require previous knowledge of the film’s optical properties to fit the spectra to optical dispersion models, the approach developed here utilizes a large range of initial guesses that are inputted into a Levenberg-Marquardt fitting algorithm in parallel to accurately determine both the film thickness and complex refractive index.

Design Strategy of Anthracene-Based Fluorophores toward High-Efficiency Deep Blue Organic Light-Emitting Diodes Utilizing Triplet-Triplet Fusion

In contrast to the red and green regions, conventional fluorescent emitters continue to serve as blue emitters in commercialized organic light-emitting diodes. Many researchers have studied anthracene moieties as blue emitters, given their appropriate energy levels and good emission properties. We herein report two new deep blue-emitting anthracene derivatives that include p-xylene as moieties connecting the anthracene cores to side groups. We enhanced the efficiency by maximizing triplet-triplet fusion (TTF) without sacrificing emission color. The large steric hindrance imposed by the methyl groups of p-xylene creates a perpendicular geometry between p-xylene and the neighboring aromatic rings. Any extension of π-conjugation is thus disrupted, and the isolated core anthracene moiety emits a deep blue color with a high photoluminescence quantum yield. Moreover, the extensive steric hindrance suppresses vibration and rotation because the molecules are rigid. The high horizontal dipole ratio attributable to the large aspect ratio increases the outcoupling efficiency of the emitted light. Furthermore, the charge mobility and triplet harvesting ability are enhanced by decreasing the bulkiness of the side groups. Molecular dynamics simulation revealed that the bulkiness of the side group significantly impacted molecular density, which in turn affected the charge transport and TTF. We used two molecules, 2PPIAn (containing a phenyl side group) and 4PPIAn (containing a terphenyl side group), to form nondoped emission layers that exhibited maximum external quantum efficiencies of 8.9 and 7.1% with Commission Internationale de L'Eclairage coordinates of (0.150, 0.060) and (0.152, 0.085), respectively.

Rational Design of Carbazole- and Carboline-Based Polymeric Host Materials for Realizing High-Efficiency Solution-Processed Thermally Activated Delayed Fluorescence Organic Light-Emitting Diode

Recently, various host materials have been developed for solution-processed thermally activated delayed fluorescent organic light-emitting diodes (TADF-OLEDs). Compared with small-molecule hosts, polymeric hosts are advantageous for inducing a uniform distribution and segregation of dopant molecules in the emissive layer without undesired large-scale phase separation. In this study, new polymer hosts were demonstrated, in which a bipolar conjugative moiety consisting of a carbazole (Cz) donor and an α-carboline (α-Cb) acceptor was bound to the polystyrene backbone through a non-conjugated linker. They exhibited high triplet energies of >2.8 eV, and their emission spectra overlapped with the absorption spectrum of a green TADF emitter, which allowed facile energy transfer from the polymeric host to the small-molecule dopants. High device performance was observed, with external quantum efficiencies (EQEs) of 13.65, 17.09, and 17.48% for solution-processed green TADF-OLEDs using PSCzCz, PSCzCb, and PSCbCz, respectively, as hosts for the EML. The EQEs of bipolar host (PSCzCb and PSCbCz)-based devices were higher than those of unipolar host (poly(N-vinylcarbazole) and PSCzCz)-based devices owing to the well-balanced charge-carrier transport. According to these results, the polymeric host bearing a bipolar Cz and α-Cb coupled moiety is a promising material for solution-processable TADF-OLEDs.

The role of chemical design in the performance of organic semiconductors

Organic semiconductors are solution-processable, lightweight and flexible and are increasingly being used as the active layer in a wide range of new technologies. The versatility of synthetic organic chemistry enables the materials to be tuned such that they can be incorporated into biological sensors, wearable electronics, photovoltaics and flexible displays. These devices can be improved by improving their material components, not only by developing the synthetic chemistry but also by improving the analytical and computational techniques that enable us to understand the factors that govern material properties. Judicious molecular design provides control of the semiconductor frontier molecular orbital energy distribution and guides the hierarchical assembly of organic semiconductors into functional films where we can manipulate the properties and motion of charges and excited states. This Review describes how molecular design plays an integral role in developing organic semiconductors for electronic devices in present and emerging technologies.

Visible-to-UV Photon Upconversion in Nanostructured Chromophoric Ionic Liquids

Visible-to-ultraviolet (vis-to-UV) triplet-triplet annihilation based photon upconversion (TTA-UC) is achieved in a non-volatile chromophoric ionic liquid (IL) for the first time. A novel IL is synthesized by combining UV-emitting anion 4-(2-phenyloxazol-5-yl)benzenesulfonate (PPOS) and trihexyltetradecylphosphonium cation (P66614). The nanostructured organization of chromophoric anions is demonstrated by synchrotron X-ray and optical measurements. When the IL is doped with a triplet sensitizer tris(2-phenylpyridinato)iridium(III) (Ir(ppy)3), the visible-to-UV TTA-UC with a relatively low threshold excitation intensity of 61 mW cm-2 is achieved. This is due to a large triplet diffusion coefficient in the IL (1.4×10-7 cm2 s-1) as well as a high absorption coefficient 15 cm-1 and a long PPOS triplet lifetime of 1.55 ms, all implemented in the condensed IL system. This work demonstrates the unique potential of ILs to control chromophore arrangements for desired functions.

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.

The Physics of Light Emission in Halide Perovskite Devices

Light emission is a critical property that must be maximized and controlled to reach the performance limits in optoelectronic devices such as photovoltaic solar cells and light-emitting diodes. Halide perovskites are an exciting family of materials for these applications owing to uniquely promising attributes that favor strong luminescence in device structures. Herein, the current understanding of the physics of light emission in state-of-the-art metal-halide perovskite devices is presented. Photon generation and management, and how these can be further exploited in device structures, are discussed. Key processes involved in photoluminescence and electroluminescence in devices as well as recent efforts to reduce nonradiative losses in neat films and interfaces are discussed. Finally, pathways toward reaching device efficiency limits and how the unique properties of perovskites provide a tremendous opportunity to significantly disrupt both the power generation and lighting industries are outlined.

Design of efficient non-doped blue emitters: toward the improvement of charge transport

Charge transport and electronic transition properties of a series of newly designed anthracene-based non-doped blue emitters were investigated by density functional theory calculations. For a highly efficient non-doped device, Cz3PhAn-based emitters were designed to suppress the hole and electron reorganization energies required for structural relaxation with respect to the changes of charged states. As a result, the hole hopping rates of triphenylamine (TPA) and phenylbenzimidazole (PBI) substituted Cz3PhAn derivatives (1, 4, and 5-7) were tremendously enhanced as compared to that of Cz3PhAn due to the suppression of the reorganization energy of holes, λ h. Moreover, 1 and 4 emitters showed almost identical hopping rates of holes and electrons, which can possibly lead to a perfect charge balance and high efficiency. The photo-physical properties showed that the emission energy of all 1-10 emitters is in 439-473 nm range. It is expected that our rational design strategy can help develop non-doped blue fluorescent emitters for high efficiency.

Diffraction imaging of nanocrystalline structures in organic semiconductor molecular thin films

The properties of organic solids depend on their structure and morphology, yet direct imaging using conventional electron microscopy methods is hampered by the complex internal structure of these materials and their sensitivity to electron beams. Here, we manage to observe the nanocrystalline structure of two organic molecular thin-film systems using transmission electron microscopy by employing a scanning nanodiffraction method that allows for full access to reciprocal space over the size of a spatially localized probe (~2 nm). The morphologies revealed by this technique vary from grains with pronounced segmentation of the structure-characterized by sharp grain boundaries and overlapping domains-to liquid-crystal structures with crystalline orientations varying smoothly over all possible rotations that contain disclinations representing singularities in the director field. The results show how structure-property relationships can be visualized in organic systems using techniques previously only available for hard materials such as metals and ceramics.