Efficient up-conversion of triplet excitons into a singlet state and its application for organic light emitting diodes

Open AccessCCS ChemistryMINI REVIEW1 Aug 2020The Leap from Organic Light-Emitting Diodes to Organic Semiconductor Laser Diodes Chihaya Adachi and Atula S. D. Sandanayaka Chihaya Adachi *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Center for Organic Photonics and Electronics Research, Kyushu University, Fukuoka 819-0395 Google Scholar More articles by this author and Atula S. D. Sandanayaka *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Center for Organic Photonics and Electronics Research, Kyushu University, Fukuoka 819-0395 Department of Physical Sciences and Technologies, Faculty of Applied Sciences, Sabaragamuwa University of Sri Lanka, Belihuloya 70140 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000327 SectionsAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail In recent years, organic light-emitting device technology has expanded from organic light-emitting diodes (OLEDs) to organic semiconductor laser diodes (OSLDs) with the progress of sophisticated molecular and device architectural designs. In OLEDs, the development of thermally activated delayed fluorescence molecules has been intensively investigated recently. As a result, the internal quantum efficiency of OLEDs containing relatively simple aromatic compounds without precious metals has reached almost 100%. Furthermore, incorporating a distributed feedback resonator structure into the OLED architecture has yielded OSLDs that exhibit the features of current-pumped lasing. In this short review, the authors describe the recent paradigm shift from OLEDs to OSLDs, mainly from the perspective of materials innovation. Download figure Download PowerPoint Progress of Emitter Materials in Organic Light-Emitting Diodes In an organic light-emitting diode (OLED), electrons and holes are injected from the cathode and anode, respectively, into multiple organic layers with thicknesses of ∼100 nm and transported in these layers. The recombination of electrons and holes in the light-emitting layer generates excitons, which might deactivate radiatively, leading to light emission from the OLED. At the time of exciton generation, four eigenstates are formed from a combination of electrons and holes according to spin statistics (Figure 1).1 In this event, the excited singlet state with spin s = 0 is generated with a probability of 25% and the excited triplet states with s = 1 are generated with a probability of 75%. Singlet excitons are usually generated with a probability of almost 100% in the photoexcitation process, whereas triplet excitons are generated with a 75% probability in the electrical excitation process (Figure 2). Thus, achieving radiative deactivation of triplet excitons generated by electrical excitation is the key to realizing highly efficient OLEDs. However, most organic molecules are fluorescent materials that emit light from singlet excitons; emission from their triplet excited states is not usually observed at room temperature because of the competition of nonradiative deactivation. Therefore, triplet-state emission from fluorescent materials is typically limited to low temperatures, such as that of liquid nitrogen. OLED research started in the 1950s with single crystals of anthracene, which is a typical fluorescent molecule (first generation).2,3 Until around 1997, only fluorescent materials were used as light-emitting materials (Figure 3). It was then discovered that the excited triplet energy level of anthracene derivatives could be controlled systematically by introducing a wide variety of substituents, which led to the use of triplet–triplet upconversion (TTU) to raise their electroluminescence (EL) efficiency to higher than that of typical fluorescent molecules. At present, the external quantum efficiency (EQE) of TTU-based OLEDs is >10%, which exceeds the theoretical limit of fluorescence-based OLEDs, that is, the EQE = 5%. In addition, TTU-based emitters with durable molecular structures have been developed, resulted in their practical application in blue OLEDs. Figure 1 | Four eigenstates generated under current excitation. Statistically, the recombination of electrons and holes produces 25% excited singlets and 75% excited triplets. (a) Conceptual diagram of the four spin states. (b) Spin function. Download figure Download PowerPoint Since the early 1950s, it has been widely recognized from theoretical considerations that high EL efficiency can be obtained in OLEDs by using phosphorescence, which is direct luminescence from a triplet excited state. In the early 1990s, some phosphorescent materials such as keto-coumarin derivatives,4,5 Eu derivatives,6,7 and Tb derivatives7 were examined. However, the EQE of OLEDs containing these phosphorescent materials was much lower than that of fluorescent OLEDs. Then in the latter half of the 1990s, some organometallic complexes containing heavy metals such as Os, Au, Pt, and Ir were examined, aimed for OLED application. In fact, Ma and Che first demonstrated the feasibility of metal complexes to obtain high-efficiency OLEDs using Os(CN)2(PPh)3X,8 although their very first device showed a rather low EQE of <0.1%. This study initiated the examination of various luminescent materials, which revealed that PtOEP9 and Ir(ppy)310,11,12 showed great promise for use in OLEDs. Indeed, an internal quantum efficiency (IQE) of almost 100% was realized for OLEDs with Ir(ppy)3 derivatives and sophisticated device architectures,12 giving rise to second-generation luminescent materials. Then the molecular structure of Ir complexes was optimized considering device durability, resulting in current practical devices that operate in the green and red regions. However, Ir is inherently scarce and expensive. Furthermore, even after 15 years of research and development, it is still difficult to achieve highly stable blue phosphorescent OLEDs.13 Figure 3 | Progress of OLED light-emitting molecules: first generation (fluorescent molecules), second generation (phosphorescent molecules), and third generation (TADF). TTA is an extension of first-generation technology. Download figure Download PowerPoint In 2012, our research group reported a current-to-photon conversion efficiency of nearly 100% using advanced thermally activated delayed fluorescence (TADF) materials as third-generation luminescent materials,14 following our lead studies.15–17 To achieve efficient TADF, a small energy difference between the lowest singlet and triplet excited states (ΔEST) is needed to facilitate reverse intersystem crossing (RISC). In TADF, the RISC process is used as an emission light path (Figure 2). Moreover, the phenomenon of TADF itself was first confirmed in the 1930s,18 but the efficiency of upconversion was rather low, masking it as a possible OLED mechanism.19–21 However, focusing on precise molecular design with the aim of minimizing ΔEST has led to pure aromatic compounds with ΔEST as small as several hundreds of millielectronvolts with almost 100% upconversion efficiency. As a result, OLEDs with an IQE of 100% were realized.14 Figure 2 | Mechanisms of exciton generation under current excitation. (a) Conventional fluorescence and phosphorescence emission mechanisms under optical and electrical excitations. In case of fluorescence molecules, only 25% of electrically generated excitons contributes for light emission, while phosphorescence molecules can harvest 100% excitons for light emission via direct triplet exciton formation and indirect triplet formation through ISC. (b) TADF mechanism. In case of thermally activated delayed fluorescence (TADF) mechanism, both electrically generated singlet and triplet excitons contribute for prompt and delayed emissions, leading to 100% emission from the singlet state. ISC, intersystem crossing; RISC, reverse intersystem crossing; TADF, thermally activated delayed fluorescence; NRD, nonradiative decay process. Download figure Download PowerPoint So far, many reported TADF molecules comprise donor–acceptor (D–A) structures in which the electronic configurations of the ground and excited states are orthogonal to each other, like the n–π* transition but not π–π*. Thus, it is vital to understand the mechanism of effective spin upconversion in the TADF system. In the case of D–A-type TADF molecules, it has been well recognized that there are two major electronic states, such as locally excited (LE) and charge-transfer (CT) states, that form multiple energy levels depending on the molecular structures.14 A recent study clarified that LE and CT states could mix partially to form ψ(LE + CT) states. Upconversion from an excited triplet to an excited singlet state is a transition between different spin states, and according to the El-Sayed rule,22 a transition between triplet CT and singlet CT states or triplet LE and singlet LE states is a forbidden process when the wavefunctions of these states are composed of pure components. Thus, the transition between the same types of pure electronic states does not occur, but instead, as a mechanism to promote the triplet-to-singlet RISC transition, and a model was proposed in which the transition between the singlet CT and triplet CT states goes through an intermediate triplet LE transition state (Figure 4). Quantum chemical calculations have also revealed that in actual molecules pure CT and LE states do not exist, and in many cases, the electronic level is a mixture of CT and LE states.23–25 Furthermore, it has been pointed out that the presence of different CT levels, such as through-space and through-bond levels, plays an essential role in upconversion.26,27 Figure 4 | A possible mechanism of the electronic transition from the lowest triplet excited state to the lowest singlet excited state. Spin conversion from 3CT to 1CT occurs via 3LE. The CT state is based on the electronic transition from the donor site to the acceptor site in a molecule, and the LE state is the electronic state localized at the donor site. In practical devices, the mixing of CT and LE states occurs, promoting the RISC process. Download figure Download PowerPoint D–A compounds are considered the fundamental TADF structure for designing high-performance TADF molecules, and many such molecules have now been developed. It has also been clarified that high-performance TADF properties could be achieved using other novel molecular skeletons. In 2014, it was reported that an n–π*-type heptazine derivative without a D–A skeleton exhibited TADF properties.28 Although the photoluminescence quantum yield of guest–host thin films with the heptazine derivative was about 30%, its TADF lifetime was extremely short (about 250 ns). Furthermore, Hatakeyama et al.29–33 proposed a separation mechanism of the highest occupied and lowest unoccupied molecular orbitals using the charge-resonance effect, which yielded a high-performance TADF molecule. Since this molecule had a rigid molecular skeleton, it showed a very narrow emission spectrum with a full width at half maximum (FWHM) of 27 nm, making it an excellent candidate for display applications. Currently, the molecular skeletons of TADF materials include D–A type, charge-resonance type, multiple heterocycles utilizing the n–π* excited state, and proton transfer molecules.32 Therefore, a wide variety of molecular skeletons could be used to realize TADF, and it is expected that further molecular designs would be developed in the future. In this way, OLED research started with fluorescent molecules, progressed to the development of room-temperature phosphorescent molecules, and then rapidly evolved to focus on TADF molecules. Besides, very recent studies have demonstrated some novel conceptual light-emitting materials based on organic radical and organic–inorganic perovskite materials, which use triplet-to-triplet,33 doublet-to-doublet,34 and band-to-band transitions,35,36 respectively. Indeed, various developments are being made because of the high degree of freedom in the molecular design of organic molecules. Active Molecules for Organic Lasers Another attractive feature of organic light-emitting molecules is their ability to amplify light; that is, laser action. Since the first reports of lasing from organic materials using Eu complexes by Sorokin, Lankard, and Schafer more than 50 years ago,37–41 various molecular skeletons have been developed for this purpose. Research has centered on styrylamine-, coumarin-, and cyanine-based materials, keeping their application to liquid dye lasers in mind, and the number of such lasing materials exceeds tens of thousands.42 Especially since 1995, the development of materials for solid-state waveguide thin-film lasers has progressed along with that of OLED light-emitting molecules, and various molecular skeletons exhibiting low lasing thresholds have been reported.43–58 Figure 5 summarizes the lasing/amplified spontaneous emission (ASE) threshold of representative laser materials in solid films. It has been recognized that stilbene and fluorene units in both small molecules and polymers provide excellent lasing behaviors, indicating all possessing rigid backbones with high photoluminescent quantum yield (PLQY) and radiative decay rates. Actually, some reports have aimed to develop current injection lasers using organic materials.59,60 Because organic molecules exhibit strong concentration quenching, a thin solid film consisting of a few mol % of the laser molecules dispersed in a host matrix, that is, guest–host system, is used in such current injection lasers. Figure 5 | Correlation between the molecular structures of organic laser molecules and thresholds of ASE and lasing. Download figure Download PowerPoint Of these various molecular skeletons, it has been reported that laser molecules with a stilbene skeleton exhibit a low threshold value for ASE and lasing.61,62 In particular, 4,4′-bis[(N-carbazole)styryl]biphenyl (BSB-Cz) showed an ASE oscillation wavelength (λASE) of 461 nm in a thin-film waveguide structure with 6 wt % BSB-Cz: 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl CBP as the active layer and an ASE threshold (Eth) of 0.32 ± 0.1 μJ/cm2, which is extremely low (Figure 6).61 The fluorescence lifetime (τf) of this thin film was short (∼1.0 ns), its fluorescence quantum yield (Φf) reached almost 100%, and its radiative deactivation rate constant (kr) was large (1 × 109 s−1). Because Φf and τf of this film did not show temperature dependence from 5 to 300 K, nonradiative deactivation was suppressed entirely even at room temperature. λASE of BSB-Cz occurs near the 0–1 transition in its fluorescence emission spectrum, which suggests the slight self-absorption of the 0–0 transition. Here, λASE is discussed based on kr, the stimulated emission cross section (σem), and the absorption cross section (σABS). kr (kr = ΦPL/τf) is calculated from τf and the emission quantum efficiency (ΦPL) of each codeposited thin film. σem is calculated using the following formula,63,64 σ em ( λ ) = λ 4 E f ( λ ) 8 π n 2 ( λ ) c τ f (1) n f = ∫ E f ( λ ) d λ (2) Figure 6 | Laser oscillation characteristics and optical properties of a 6 wt % BSB-Cz:CBP thin film as an active layer. (a) Chemical structures of BSB-Cz and CBP as an active emitter and host, respectively. (b) Temperature dependence of the emission quantum efficiency and emission lifetime of the thin film. (c) Lasing oscillation spectrum. (d) Excitation power dependence of emission intensity. The threshold is around 0.32 μJ/cm2. Download figure Download PowerPoint In Eq. (1), Ef(λ) is the quantum yield distribution, and n is the refractive index. σABS54 was calculated using Eq. (3), in which n = 1.8. σ ABS , Sol ( λ ) = 1000 ɛ ( λ ) ln 10 N A (3)where ɛ(λ) is the molar extinction coefficient, and NA is Avogadro’s number. In the 6 wt % BSB-Cz:CBP thin film, a high value of σem = 2.7 × 10−16 cm2 was obtained. Furthermore, the effective stimulated emission cross section (σemeff) is the difference between σem and the cross section related to a loss (σABS and the singlet and triplet excited-state absorption cross sections, σSS and σTT, respectively), and is given by Eq. (4). σ emeff = σ em − ( σ ABS + σ SS + σ TT ) (4) Figure 7 shows the spectra of σeff and σABS and the excited-state absorption spectrum of a 6 wt % BSB-Cz:CBP coevaporated thin film. In BSB-Cz, the singlet excited-state absorption, triplet excited-state absorption, and ground-state absorption spectra do not have a large overlap with λASE. Thus, the 6 wt % BSB-Cz:CBP codeposited thin film has a high kr (i.e., a large σem), σABS as small as <10−19 cm2 at λASE, and an excited-state absorption. The absence of these absorptions provides a very large σemeff, leading to a very low Eth. Figure 7 | (a) Ground-state absorption spectrum (solid red line), fluorescence spectrum, laser oscillation spectrum (solid blue line), S–S absorption spectrum (blue circles), and T–T absorption spectrum (orange circles) of BSB-Cz. (b) Energy-level diagram of BSB-Cz. Download figure Download PowerPoint Laser Oscillation Characteristics Under Optical Excitation As described earlier, BSB-Cz is suitable for optical amplification because of its high Φf, low probability of intersystem crossing, and the absence of overlapping excited-state absorption in the λASE region.66 For laser oscillation, it is necessary to introduce an optical resonator structure; however, in an amorphous organic thin film with a thickness of several hundred nanometers, it is difficult to form an end face like in the case of an inorganic semiconductor crystal with a distributed Bragg reflector structure. Thus, for organic thin-film lasers, it is ideal for forming a distributed feedback (DFB) resonator structure, which could outcouple the emission perpendicular to the longitudinal direction of the device. Among DFB resonator structures, the mixed-order DFB structure, which has a primary feedback region that produces strong optical feedback and a secondary Bragg scattering region that allows light extraction, is suitable for organic thin-film lasers. In a DFB resonator structure, the Bragg condition is given by Eq. (5), m λ Bragg = 2 n eff Λ (5)where m is the diffraction order, λBragg is the Bragg wavelength, neff is the effective refractive index of the gain medium, and Λ is the grating period. Laser oscillation occurs when this condition is satisfied.25 Using the reported values of neff and λBragg for BSB-Cz, the optimum Λ for m = 1 and 2 in DFB laser devices are 140 and 280 nm, respectively. Figure 8 shows a DFB grating observed by scanning electron microscopy (SEM). The DFB grating was designed to possess a depth of 65 ± 5 nm and Λ of 140 ± 5 and 280 ± 5 nm. The primary and secondary DFB grating lengths were approximately 15.12 and 10.08 µm, respectively. By forming a 200 nm-thick BSB-Cz film on the grating by vacuum deposition, the surface morphology of the organic layer possessed a lattice structure with a surface modulation depth of 20–30 nm. Figure 9 shows the oscillation characteristics of a mixed-order DFB device under optical excitation. With increasing excitation intensity, the FWHM decreased remarkably, and at Eth = ∼0.2 μJ/cm2, laser oscillation occurred from the vicinity of the stopband at the central oscillation wavelength of 481 nm. In this mixed-order-type DFB structure, compared with those of devices with ASE and second-order DFB structures, Eth was decreased by about 1/3 and 1/2, respectively, demonstrating the superior performance of the mixed-order-type DFB structure. These results confirmed the light confinement effect of the mixed-order DFB structure with BSB-Cz. Figure 8 | (a) Schematic of a mixed-order DFB structure with first- and second-order gratings. (b, c) SEM images of the DFB structure with a 140-nm primary structure and 280-nm secondary structure. Download figure Download PowerPoint The limited overlap of the excited-state absorption, ground-state absorption, and emission spectra of BSB-Cz suggest the possibility to realize quasi-continuous-wave (qCW) laser oscillation. Figure 10 shows the qCW laser oscillation characteristics of a device with BSB-Cz. Continuous laser action was obtained even at a high frequency of 80 MHz. Besides, continuous laser action was observed even with a long pulse excitation of 800 µs to 30 ms.55 The optical gain and loss coefficient estimated from the ASE characteristics of the doped film (optical waveguide structure with a thickness of 200 nm) using the variable stripe method were 40 and 3 cm−1, respectively. These results confirmed that BSB-Cz is an attractive candidate for qCW lasers able to operate even under long-pulsed light excitation. Figure 10 | Quasi-CW lasing characteristics of mixed-order DFB structures. Streak images of the oscillation state when (a) the excitation frequency was changed from 0.01 to 80 MHz, and (b) the pulse width was 30 ms and 800 (c) Excitation dependence of the lasing threshold The doped film exhibited a lower Eth than that of the film. continuous Download figure Download PowerPoint of Organic Semiconductor Laser With the of fluorescent molecules, phosphorescent molecules, and TADF molecules, OLEDs, of conversion with the IQE of 100%, are now At the same the of an organic semiconductor laser diode has long been a in organic semiconductor proposed an using an Eu in the active layer in but 30 years have since organic semiconductor laser that by current excitation is expected to have great because of its low and wavelength from the to the Furthermore, such organic semiconductor lasers are attractive for use at the of organic such as in optical on Figure 9 | (a) Laser oscillation characteristics of the mixed-order DFB structure. (b) of oscillation and FWHM on excitation intensity. (c) of emission around the with the calculated Download figure Download PowerPoint In our research group reported of lasing by current The device structure was based on that of a OLED. To electrical the primary and secondary DFB structures in the optical resonator and a fluorescent thin film of BSB-Cz as the organic semiconductor active layer were between an thin and cathode (Figure In this organic amorphous thin-film a thin-film structure is to a high of for effective current and the thickness of the organic active layer was limited to nm. Furthermore, to form with the the cathode of the organic active layer was with and a layer was on the of the organic active layer to achieve Figure | Schematic diagram of the structure of a was obtained using BSB-Cz on the cathode and layer on the cathode Download figure Download PowerPoint Conventional OLEDs are based on a to and electrons and holes and the generated excitons in the light-emitting layer. This structure to achieve high and the EL emission efficiency of OLEDs did not to current of about 1 However, an injection of 1 is needed for laser oscillation. At such a high current and various exciton deactivation by the Therefore, it is necessary to use a structure containing to both and deactivate excitons in the of the light-emitting layer. current revealed that the of both electrons and holes in the BSB-Cz layer were about Thus, that there are at the between the and the organic the recombination site be near the of the BSB-Cz emission layer. In fact, the characteristics of the showed a constant EQE to a high current of without A organic molecules under current excitation is the presence of and radical absorption. Because many organic molecules have a absorption spectrum in the radical state, the absorption by overlap with the oscillation wavelength laser oscillation. BSB-Cz shows strong absorption around to 1000 nm, which does not overlap with the emission spectrum near nm. Therefore, BSB-Cz the overlap of the absorption from the ground state, excited singlet absorption, excited triplet absorption, and absorption, the to achieve excellent performance as a laser molecule for current excitation. A laser was using a 6 wt % BSB-Cz:CBP codeposited thin film as an active and as an optical resonator structure suitable for an first- and second-order DFB structures were into an OLED device to Figure shows the laser oscillation characteristics of the device under the current excitation. a current of about a narrow and emission were obtained. A in FWHM was observed with a and a width of nm or was obtained. The current threshold was almost to the threshold value estimated under The efficiency under current excitation was which was the same as that under optical excitation In the efficiency of the device without a metal was which suggests that the loss by the metal was Figure | Lasing characteristics of a current-pumped (a) of the oscillation spectrum near the threshold on current (b) of oscillation and FWHM on current Download figure Download PowerPoint the of laser oscillation, the current a very short device of OLEDs has been by by and and chemical of light-emitting molecules using device However, a very high current compared with that used for OLEDs. the yield of OSLDs was because of the of device DFB In the along with the of the laser the mechanism of by the of the excited triplet states be BSB-Cz is to because of its which is relatively to it has been confirmed that the of triplet into organic semiconductor laser structures the of lasing In the in to the triplet exciton deactivation mechanism, it is necessary to the

Realizing External Quantum Efficiency over 25% with Low Efficiency Roll-Off in Polymer-Based Light-Emitting Diodes Synergistically Utilizing Intramolecular Sensitization and Bipolar Thermally Activated Delayed Fluorescence Monomer

A pure-blue light-emitting material is one of the key components in the preparation of organic light-emitting diode (OLED) displays. Although high-efficiency blue OLEDs have been realized in therma...

Dual-Responsive Thermally Activated Delayed Fluorescence of Spiropyran Derivatives

Thermally activated delayed fluorescence (TADF)‐type compounds have great potential as emitter molecules in organic light‐emitting diodes, allowing for electrofluorescence with 100% internal quantum efficiency. In small molecules, TADF is achieved through the formation of intramolecular charge‐transfer states. The only design limitation is the requirement that donor and acceptor entities spatially decouple the highest occupied and lowest unoccupied molecular orbitals, respectively, to minimize exchange splitting. The development of polymeric TADF emitters, on the contrary, has seen comparably small progress and those are typically built up from monomeric units that show promising TADF properties in small molecule studies beforehand. By contrast, herein, a way to achieve TADF properties in cyclic oligomers and polymers composed of non‐TADF building blocks is shown. Due to a strongly decreased energy splitting of the polymer with respect to the individual repeating unit between the lowest singlet and triplet excited state (ΔEST) and a sufficiently high radiative decay rate kSr, a highly efficient TADF polymer with up to 71% photoluminescence quantum yield is obtained. For the first time, an encouraging method is provided for producing highly efficient TADF oligomers and polymers from solely non‐TADF units via induced conjugation, opening a new design strategy exclusive for polymers.

Organic light emitting diodes (OLEDs) have been the focus of intense study since the late 1980s, when the low voltage organic electroluminescence in small organic molecules such as Alq3, and large organic molecules such as polymers (PPV), was reported. Since that time, research has continued to demonstrate the potential of OLEDs as viable systems for displays and eco-friendly lighting applications. OLEDs offer full colour display, reduced manufacturing cost, larger viewing angle, more flexible, lower power consumption, better contrast, slimmer, etc. which help in replacing the other technologies such as LCD. The operation of OLEDs involves injection of charge carriers into organic semiconducting layers, recombination of charge carriers, formation of singlet and triplet excitons, and emission of light during decay of excitons. The maximum internal quantum efficiency of fluorescent OLEDs consisting of the emissive layer of fluorescent organic material is 25% because in this case only the 25% singlet excitons can emit light. The maximum internal quantum efficiency of phosphorescent OLEDs consisting of the emissive layer of fluorescent organic material mixed with phosphorescent material of heavy metal complexes such as platinum complexes, iridium complexes, etc. is nearly 100% because in this case both the 25% singlet excitons and 75% triplet excitons emit light. Recently, a new class of OLEDs based on thermally activated delayed fluorescence (TADF) has been reported, in which the energy gap between the singlet and triplet excited states is minimized by design, thereby promoting highly efficient spin up-conversion from non-radiative triplet states to radiative singlet states while maintaining high radiative decay rates of more than 106decays per second. These molecules harness both singlet and triplet excitons for light emission through fluorescence decay channels and provides an intrinsic fluorescence efficiency in excess of 90 per cent and a very high external electroluminescence efficiency of more than 19 per cent, which is comparable to that achieved in high-efficiency phosphorescence-based OLEDs.The OLED technology can be used to make screens large enough for laptop, cell phones, desktop computers, televisions, etc. OLED materials could someday be applied to plastic and other materials to create wall-size video panels, roll-up screens for laptops, automotive displays, and even head wearable displays. Presently, the OLEDs are opening up completely new design possibilities for lighting in the world of tomorrow whereby the offices and living rooms could be illuminated by lighting panels on the ceiling. The present paper describes the salient features of OLEDs and discusses the applications of OLEDs in displays and solid state lighting devices. Finally, the challenges in the field of OLEDs are explored. Contents of Paper

Thermally activated delayed fluorescence (TADF) relies on the presence of a very small energy gap, ΔEST, between the lowest singlet and triplet excited states. ΔEST is thus a key factor in the molecular design of more efficient materials. However, its accurate theoretical estimation remains challenging, especially in the solid state due to the influence of polarization effects. We have quantitatively studied ΔEST as a function of dielectric constant, ε, for four representative organic molecules using the methodology we recently proposed at the Tamm-Dancoff approximation ωB97X level of theory, where the range-separation parameter ω is optimized with the polarizable continuum model. The results are found to be in very good agreement with experimental data. Importantly, the polarization effects can lead to a marked reduction in the ΔEST value, which is favorable for TADF applications. This ΔEST decrease in the solid state is related to the hybrid characters of the lowest singlet and triplet excited states, whose dominant contribution switches to charge-transfer-like with increasing ε. The present work provides a theoretical understanding on the influence of polarization effect on the singlet-triplet gap and confirms our methodology to be a reliable tool for the prediction and development of novel TADF materials.

Thermally activated delayed fluorescence (TADF), a unique molecular fluorescence mechanism, plays a key role in designing emitters of high efficiency. Carbon fullerenes such as C&lt;sub&gt;60&lt;/sub&gt; and C&lt;sub&gt;70&lt;/sub&gt; exhibit strong TADF with intensity even higher than that of the prompt fluorescence, owing to their long lifetimes of triplet state and modest singlet-triplet energy gaps. Thus, there arises the intriguing question whether other fullerene-like clusters can also have fluorescence and host the TADF effect. In this work, by time-dependent density functional theory (TD-DFT) calculations, we explore the excited-states of the experimentally reported boron nitride cage clusters B&lt;sub&gt;12&lt;/sub&gt;N&lt;sub&gt;12&lt;/sub&gt;, B&lt;sub&gt;24&lt;/sub&gt;N&lt;sub&gt;24&lt;/sub&gt; and B&lt;sub&gt;36&lt;/sub&gt;N&lt;sub&gt;36&lt;/sub&gt;, as well as compound clusters B&lt;sub&gt;12&lt;/sub&gt;P&lt;sub&gt;12&lt;/sub&gt;, Al&lt;sub&gt;12&lt;/sub&gt;N&lt;sub&gt;12&lt;/sub&gt; and Ga&lt;sub&gt;12&lt;/sub&gt;N&lt;sub&gt;12&lt;/sub&gt; with the same geometry as B&lt;sub&gt;12&lt;/sub&gt;N&lt;sub&gt;12&lt;/sub&gt;. Using the HSE06 hybrid functional, the predicted energy gaps of these fullerene-like clusters are obtained to range from 2.83 eV to 6.54 eV. They mainly absorb ultraviolet light, and their fluorescence spectra are all in the visible range from 405.36 nm to 706.93 nm, including red, orange, blue, and violet emission colors. For the boron nitride cages, the energy gap of excited states increases with the cluster size increasing, accompanied by a blue shift of emission wavelength. For the clusters with B&lt;sub&gt;12&lt;/sub&gt;N&lt;sub&gt;12&lt;/sub&gt; geometry and different elemental compositions, the excited energy gap decreases as the atomic radius increases, resulting in a red shift of emission wavelength. In addition, the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) of these compound cage clusters are distributed separately on different elements, resulting in small overlap between HOMO and LUMO wavefunctions. Consequently, these fullerene-like clusters exhibit small singlet-triplet energy differences below 0.29 eV, which is beneficial for the intersystem crossing between the excited singlet state and triplet state, and hence promoting the TADF process. Our theoretical results unveil the fluorescence characteristics of cage clusters other than carbon fullerenes, and provide important guidance for precisely modulating their emission colors by controlling the cluster sizes and elemental compositions. These experimentally feasible fullerene-like compound clusters possess many merits as fluorophors such as outstanding stabilities, non-toxicity, large energy gap, visible-light fluorescence, and small singlet-triplet energy gap. Therefore, they are promising luminescent materials for applications in display, sensors, biological detection and labelling, therapy, and medicine.

Thermally activated delayed fluorescence (TADF) molecules have attracted great attention as high efficient luminescent materials. Most of TADF molecules possess small energy gap between the first singlet excited state (S1) and the first triplet excited state (T1) to favor the up-conversion from T1 to S1. In this paper, a new TADF generation mechanism is revealed based on theoretical simulation. By systematic study of the light-emitting properties of SOBF-OMe in both toluene and in aggregation state, we find that the single SOBF-OMe could not realize TADF emission due to large energy gap as well as small up-conversion rates between S1 and T1. Through analysis of dimers, we find that dimers with intermolecular hydrogen bond (H-bond) are responsible for the generation of TADF, since smaller energy gap between S1 and T1 is found and the emission wavelength is in good agreement with experimental counterpart. The emission properties of SOBF-H are also studied for comparison, which reflect the important role of H-bond. Our theoretical results agree ith experimental results well and confirm the mechanism of H-bond induced TADF.

Great progress has been made in developing highly efficient thermally activated delayed fluorescent (TADF) materials. However, developing highly efficient long-wavelength TADF emitters is still a challenge because a small energy gap (ΔEST) between the lowest singlet (S1) and triplet excited states (T1) and a relatively high fluorescence rate are difficult to achieve simultaneously in one molecule. Here, eleven donor-acceptor (D-A) type molecules using N3,N3,N6,N6-tetraphenyl-9H-carbazole-3,6-diamine (named DAC-II) as the electron donor and the 2-phenyl-quinoxaline-based electron acceptor are designed via introducing different electron-donating and electron-withdrawing groups into the acceptor and changing the connection position between the donor (D) and the acceptor (A). Quantum chemical calculations indicated that introducing the electron-donating groups (-OCH3, -CH3) into the phenyl ring of the acceptor, molecules 2 and 3, cannot change the emission property of molecule 1, thus molecules 2 and 3 could also be used as green TADF emitters like molecule 1. Introducing an electron-withdrawing unit (-CF3) into molecule 1, molecule 4, reduces the ΔEST value to 0.10 eV, while the radiative decay rate (kVE) is also reduced correspondingly. Changing the connection position between D and A, molecules 5 to 8, cannot reduce the ΔEST value and lowers the kVE value compared with molecules 1 to 4. However, introducing electron-withdrawing groups (-2F, -4F and -CN) into the quinoxaline moiety, molecules 9 to 11, contributes to both small ΔEST and large kVE for the emission process. The values of ΔEST of molecules 9 to 11 are in the range of 0.21 to 0.30 eV, and the maximum emission wavelengths of molecules 10 and 11 are 576 and 590 nm, respectively, which are promising to be used as efficient yellow and orange TADF emitters in organic light-emitting diodes.

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 (3LEID), 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 (ΔEST) and mediation of the 3LEID 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.

Thermally activated delayed fluorescent (TADF) materials are those that have a small energy gap between the lowest singlet and triplet excited states, enabling singlet excitons through thermal upconversion of triplet excitons even at room temperature. Thus, they can achieve external electroluminescence (EL) quantum efficiency (EQE) equal to that of phosphorescence based organic light-emitting diodes (OLEDs) [1]. Considering fluorescence and phosphorescence based OLEDs as the first and second generations, respectively, TADF materials can be positioned as the third generation realizing highly efficient OLEDs. Recently, the best EQE of 31.2% in green TADF based OLEDs was demonstrated [2]. Further, EQE can be greatly improved by transition dipoles horizontally oriented to a substrate surface; such a preferential dipole orientation is achievable by controlling molecular orientation.

1,8-Naphthalimide-based hybrids for efficient red thermally activated delayed fluorescence organic light-emitting diodes

Thermally activated delayed fluorescence (TADF) is a photophysical phenomenon that involves electronically coupled singlet and triplet excited states. Materials exhibiting TADF have most prominently been employed in organic light-emitting diodes (OLEDs). Electroluminescent devices with TADF emitters are capable of converting up to 100% of the excitons generated to light. The microsecond long delayed lifetimes and the sensitivity of the emission to the environment have been exploited in sensing, imaging, and photocatalysis applications. TADF relies on there being energetically similar singlet and triplet excited states, which enables not only intersystem crossing (ISC) but also the endothermic conversion of triplet excitons to singlet excitons via reverse intersystem crossing (RISC). The coupling of the singlet and triplet excited states leads to a biexponential decay of the emission that is observed in the transient photoluminescence (PL) of the material. It means that although emission is from the singlet, at long time its dynamics are controlled by the triplet population via the RISC process. This review provides an overview of the methods used in the literature to analyze the PL decay of TADF compounds and to infer the rate constants that govern all facets of the TADF process. While the photophysics of TADF is often analyzed using transient PL, most applications of TADF emitters occur in a steady-state regime facilitated by constant exciton generation and recombination. Thus, this review also discusses the link between parameters of the kinetics and the performance of TADF OLEDs.

Recently all-organic thermally activated delayed fluorescent (TADF) emitters have attracted great attention. In TADF emitters, nonemissive triplet states can be also harvested via population of emissive singlet states through reverse intersystem crossing (RISC). The RISC can be induced by the small energy gap between the lowest singlet (S1) and triplet (T1) states. Due to this ability of TADF, 100% internal quantum efficiency and high maximum external quantum efficiency (EQEmax) comparable to those of phosphorescent organic light emitting diodes (OLEDs) have been already reported in TADF OLEDs. In this study, we demonstrate high efficiency TADF OLEDs which are attributed to employing triazine acceptor type TADF compounds having high RISC rates(kRISC).