Electroluminescence Quantum Efficiency Research Articles

Alternating Donor–Acceptor π‐Conjugated Macrocycle Exhibiting Efficient Thermally Activated Delayed Fluorescence and Spontaneous Horizontal Molecular Orientation

A versatile design of thermally activated delayed fluorescence (TADF) π‐conjugated macrocycles incorporating electron‐donor (D) and acceptor (A) units into a cyclo‐meta‐phenylene motif with an alternating pattern is presented. The new π‐conjugated macrocycle, which combines three 5‐(N‐carbazolyl)‐phenylen‐1,3‐diyl as D units and three 6‐phenyl‐1,3,5‐triazin‐2,4‐diyl as A units, possesses a small singlet–triplet energy gap and hence can emit efficient green TADF both in solution and doped thin films. Comparative experimental and computational investigations of the electronic and photophysical properties of the macrocycle with its analogous noncyclic compound reveal key advantages of the cyclic molecular configuration for actual emitters. Organic light‐emitting diodes incorporating the TADF π‐conjugated macrocycle as an emitter demonstrate high external electroluminescence quantum efficiencies of up to 15.7%, outperforming the devices based on the noncyclic emitter. Herein, the importance of geometric design for producing novel organic emitters with fascinating optoelectronic and morphological characteristics is highlighted.

New trifluoromethyl modified iridium(III) complex for high-efficiency sky-blue phosphorescent organic light-emitting diode

A new and efficient trifluoromethyl modified sky-blue iridium(III) coordination compound (Ir-CF3) is well designed and prepared by using bis(trifluoromethyl) modified 4-methyl-2-phenylpyridine as the cyclometalating ligand and 2-(1H-imidazol-2-yl)pyridine as the auxiliary ligand. Ir-CF3 shows intensive sky-blue phosphorescence (465 nm with 494 nm as shoulder), high solution photoluminescence efficiency (0.22). The incorporation of trifluoromethyl moiety into the complex not only enhances the robust chemical stability but also provides the steric hindrance beneficial to organic light-emitting diodes. The sky-blue OLEDs based on this complex show narrow emission band with full width at half maxima of 54 nm. The peak power efficiency, current efficiency and external quantum efficiency of electroluminescence are as high as 36.05 lm/W, 51.45 cd/A and 21.74%, respectively.

Efficient light-emitting diodes from organic radicals with doublet emission

Organic light-emitting diodes (OLEDs) with doublet-spin radical emitters have emerged as a new route to efficient display technologies. In contrast to standard organic semiconductors, radical materials have unpaired electrons. This feature results in the most well-known examples of organic radicals being where they are reactive species in chemical reactions. Stabilized radicals can be used in optoelectronic applications, which exploit their optical and spin properties, allowing up to 100% internal quantum efficiency (IQE) for electroluminescence. Highly efficient OLEDs have been demonstrated, which operate in the doublet-spin electronic state manifold with doublet emission. The radical-based devices present a departure from the singlet- and triplet-level considerations that impose efficiency limits in OLEDs for typical organic semiconductors (25% IQE). This Perspective focuses on radical doublet emitters for optoelectronics, outlining how the photo- and spin-physics of unpaired electron systems present new avenues for research in light-emitting applications.

A New Conjugated Polymer that Enables the Integration of Photovoltaic and Light-Emitting Functions in One Device.

Exploring the intriguing bifunctional nature of organic semiconductors and investigating the feasibility of fabricating bifunctional devices are of great significance in realizing various applications with one device. Here, the design of a new wide-bandgap polymer named PBQx-TCl (optical bandgap of 2.05eV) is reported, and its applications in photovoltaic and light-emitting devices are studied. By fabricating devices with nonfullerene acceptors BTA3 and BTP-eC9, it is shown that the devices exhibit a high power conversion efficiency (PCE) of 18.0% under air mass 1.5G illumination conditions and an outstanding PCE of 28.5% for a 1 cm2 device and 26.0% for a 10 cm2 device under illumination from a 1000 lux light-emitting diode. In addition, the PBQx-TCl:BTA3-based device also demonstrates a moderate organic light-emitting diode performance with an electroluminescence external quantum efficiency approaching 0.2% and a broad emission range of 630-1000nm. These results suggest that the polymer PBQx-TCl-based devices exhibit outstanding photovoltaic performance and potential light-emitting functions.

Sulfur atom manipulates geometric isomerism of diphosphinine oxides for efficient delayed fluorescence diodes

Geometric isomerism has significant influences on intermolecular interactions. Therefore, despite the challenge, geometry manipulation is desired for developing high-performance optoelectronic materials. Herein, we demonstrate that heteroatom incorporation is a feasible approach to control cis- and trans- configurations of diphosphinine oxide compounds. Molecular geometries are highly dependent on heteroatom position, which is manifested by a cis- (cDPDPbDTO) and a trans- dithiophene-fused diphosphinine dioxide (tDPDPcDTO). It is showed that effects between S and P atoms bonded with the same C atoms bend the fused ring of cDPDPbDTO, giving rise to its cis-configuration. In contrast to trans-featured tDPDPcDTO containing a planar fused ring, cDPDPbDTO reveals more compact packing mode, accompanied by limited π-π stacking, and remarkably condensed excited states. Consequently, in addition to higher carrier mobility, cDPDPbDTO is much more effective in quenching suppression. Doped with a bluish green thermally activated delayed fluorescence (TADF) dye 2CzPN, cDPDPbDTO hosted film achieves fourfold increased radiative rate constant, and tenfold and sixfold decreased singlet and triplet nonradiative rate constants, respectively, as well as a 40%-improved reverse intersystem crossing (RISC) efficiency (97%). As the result, cDPDPbDTO endows its TADF systems with the state-of-the-art photoluminescence and electroluminescence quantum efficiencies of 95% and 26%, respectively.

Highly Efficient Near‐Infrared Electrofluorescence from a Thermally Activated Delayed Fluorescence Molecule

AbstractNear‐IR organic light‐emitting diodes (NIR‐OLEDs) are potential light‐sources for various sensing applications as OLEDs have unique features such as ultra‐flexibility and low‐cost fabrication. However, the low external electroluminescence (EL) quantum efficiency (EQE) of NIR‐OLEDs is a critical obstacle for potential applications. Here, we demonstrate a highly efficient NIR emitter with thermally activated delayed fluorescence (TADF) and its application to NIR‐OLEDs. The NIR‐TADF emitter, TPA‐PZTCN, has a high photoluminescence quantum yield of over 40 % with a peak wavelength at 729 nm even in a highly doped co‐deposited film. The EL peak wavelength of the NIR‐OLED is 734 nm with an EQE of 13.4 %, unprecedented among rare‐metal‐free NIR‐OLEDs in this spectral range. TPA‐PZTCN can sensitize a deeper NIR fluorophore to achieve a peak wavelength of approximately 900 nm, resulting in an EQE of over 1 % in a TADF‐sensitized NIR‐OLED with high operational device durability (LT95>600 h.).

Highly Efficient Near-Infrared Electrofluorescence from a Thermally Activated Delayed Fluorescence Molecule.

Near-IR organic light-emitting diodes (NIR-OLEDs) are potential light-sources for various sensing applications as OLEDs have unique features such as ultra-flexibility and low-cost fabrication. However, the low external electroluminescence (EL) quantum efficiency (EQE) of NIR-OLEDs is a critical obstacle for potential applications. Here, we demonstrate a highly efficient NIR emitter with thermally activated delayed fluorescence (TADF) and its application to NIR-OLEDs. The NIR-TADF emitter, TPA-PZTCN, has a high photoluminescence quantum yield of over 40 % with a peak wavelength at 729 nm even in a highly doped co-deposited film. The EL peak wavelength of the NIR-OLED is 734 nm with an EQE of 13.4 %, unprecedented among rare-metal-free NIR-OLEDs in this spectral range. TPA-PZTCN can sensitize a deeper NIR fluorophore to achieve a peak wavelength of approximately 900 nm, resulting in an EQE of over 1 % in a TADF-sensitized NIR-OLED with high operational device durability (LT95 >600 h.).

Ligand-engineered bandgap stability in mixed-halide perovskite LEDs.

Lead halide perovskites are promising semiconductors for light-emitting applications because they exhibit bright, bandgap-tunable luminescence with high colour purity1,2. Photoluminescence quantum yields close to unity have been achieved for perovskite nanocrystals across a broad range of emission colours, and light-emitting diodes with external quantum efficiencies exceeding 20 per cent-approaching those of commercial organic light-emitting diodes-have been demonstrated in both the infrared and the green emission channels1,3,4. However, owing to the formation of lower-bandgap iodide-rich domains, efficient and colour-stable red electroluminescence from mixed-halide perovskites has not yet been realized5,6. Here we report the treatment of mixed-halide perovskite nanocrystals with multidentate ligands to suppress halide segregation under electroluminescent operation. We demonstrate colour-stable, red emission centred at 620 nanometres, with an electroluminescence external quantum efficiency of 20.3 per cent. We show that a key function of the ligand treatment is to 'clean' the nanocrystal surface through the removal of lead atoms. Density functional theory calculations reveal that the binding between the ligands and the nanocrystal surface suppresses the formation of iodine Frenkel defects, which in turn inhibits halide segregation. Our work exemplifies how the functionality of metal halide perovskites is extremely sensitive to the nature of the (nano)crystalline surface and presents a route through which to control the formation and migration of surface defects. This is critical to achieve bandgap stability for light emission and could also have a broader impact on other optoelectronic applications-such as photovoltaics-for which bandgap stability is required.

Morphology controlled nanocrystalline CsPbBr3 thin-film for metal halide perovskite light emitting diodes

Morphology and crystal grain size of inorganic CsPbBr3 metal halide perovskite (MHP) films are controllable through a diffusion-controlled anti-solvent dripping crystallization method using an anti-solvent mixture of toluene (Tol):isopropanol (IPA). The 8:2 (v:v) Tol:IPA anti-solvent enables the production of a pinhole-free CsPbBr3 MHP film with the smallest crystal grain size and the brightest photoluminescence (PL). Accordingly, the 8:2 CsPbBr3 MHP film reveals a very uniform and strong PL mapping image. As a result, the 8:2 CsPbBr3 MHP light emitting diode (LED) exhibits ∼12.8, ∼7.4, and ∼7.9 fold enhanced maximum electroluminescence (ELmax), current efficiency (CEmax), and external quantum efficiency (EQEmax) compared to the 10:0 pristine device. The optimized 8:2 MHP LED shows an ELmax of 22,298cd/m2, CEmax of 36.65cd/A and EQEmax of 9.64%, respectively. In addition, the 8:2 CsPbBr3 MHP LED with controlled morphology and grain size has better operating stability than the pristine device.

Cis-Quinacridone-Based Delayed Fluorescence Emitters: Seemingly Old but Renewed Functional Luminogens.

Herein, we report a material design of linear cis-quinacridone (cis-QA) derivatives as delayed fluorescence luminogens. In contrast to the widely studied traditional trans-isomers, the functionality of cis-QA and its derivatives remains unexplored and unclarified. Through combined computational and experimental investigations, we revealed that cis-QA derivatives can function as fascinating narrowband deep-blue delayed fluorescence emitters for organic light-emitting diodes (OLEDs). The best-performing deep-blue OLEDs incorporating these cis-QA luminogens achieved high external electroluminescence quantum efficiencies of up to 19.0 % and high color purity with chromaticity coordinates of (0.13, 0.14).

Cis‐Quinacridone‐Based Delayed Fluorescence Emitters: Seemingly Old but Renewed Functional Luminogens

AbstractHerein, we report a material design of linear cis‐quinacridone (cis‐QA) derivatives as delayed fluorescence luminogens. In contrast to the widely studied traditional trans‐isomers, the functionality of cis‐QA and its derivatives remains unexplored and unclarified. Through combined computational and experimental investigations, we revealed that cis‐QA derivatives can function as fascinating narrowband deep‐blue delayed fluorescence emitters for organic light‐emitting diodes (OLEDs). The best‐performing deep‐blue OLEDs incorporating these cis‐QA luminogens achieved high external electroluminescence quantum efficiencies of up to 19.0 % and high color purity with chromaticity coordinates of (0.13, 0.14).

Efficient perovskite solar cells via improved carrier management.

Metal halide perovskite solar cells (PSCs) are an emerging photovoltaic technology with the potential to disrupt the mature silicon solar cell market. Great improvements in device performance over the past few years, thanks to the development of fabrication protocols1-3, chemical compositions4,5 and phase stabilization methods6-10, have made PSCs one of the most efficient and low-cost solution-processable photovoltaic technologies. However, the light-harvesting performance of these devices is still limited by excessive charge carrier recombination. Despite much effort, the performance of the best-performing PSCs is capped by relatively low fill factors and high open-circuit voltage deficits (the radiative open-circuit voltage limit minus the high open-circuit voltage)11. Improvements in charge carrier management, which is closely tied to the fill factor and the open-circuit voltage, thus provide a path towards increasing the device performance of PSCs, and reaching their theoretical efficiency limit12. Here we report a holistic approach to improving the performance of PSCs through enhanced charge carrier management. First, we develop an electron transport layer with an ideal film coverage, thickness and composition by tuning the chemical bath deposition of tin dioxide (SnO2). Second, we decouple the passivation strategy between the bulk and the interface, leading to improved properties, while minimizing the bandgap penalty. In forward bias, our devices exhibit an electroluminescence external quantum efficiency of up to 17.2 per cent and an electroluminescence energy conversion efficiency of up to 21.6 per cent. As solar cells, they achieve a certified power conversion efficiency of 25.2 per cent, corresponding to 80.5 per cent of the thermodynamic limit of its bandgap.

Thermal equilibration between singlet and triplet excited states in organic fluorophore for submicrosecond delayed fluorescence.

In any complex molecular system, electronic excited states with different spin multiplicities can be described via a simple statistical thermodynamic formalism if the states are in thermal equilibrium. However, this ideal situation has hitherto been infeasible for efficient fluorescent organic molecules. Here, we report a highly emissive metal-free purely organic fluorophore that enables thermal equilibration between singlet and triplet excited states. The key to this unconventional excitonic behavior is the exceptionally fast spin-flipping reverse intersystem crossing from the triplet to singlet excited states with a rate constant exceeding 108 per second, which is considerably higher than that of radiative decay (fluorescence) from the singlet excited state. The present fluorophoric system exhibits an emission lifetime as short as 750 nanoseconds and, therefore, allows organic light-emitting diodes to demonstrate external electroluminescence quantum efficiency exceeding 20% even at a practical high luminance of more than 10,000 candelas per square meter.

Relationship between charge transfer state electroluminescence and the degradation of organic photovoltaics

The degradation of archetype organic photovoltaics comprising both vacuum and solution-deposited bulk heterojunction active regions is investigated and quantified using a theory based on detailed balance, which relates the open-circuit voltage to the efficiency of charge transfer state emission. To describe this relationship, we account for the difference between electroluminescent external quantum efficiency and the charge transfer emission efficiency. An empirical factor, m, is introduced to distinguish between nonradiative defect sites both within, m = 1, and outside, m >1, of the photoactive heterojunction. The m-factor is used to determine the primary sources of degradation for archetype solution- and vacuum-processed material systems. We conclude that degradation occurs primarily within the donor–acceptor heterojunction for the vacuum-processed devices (where m = 1.020 ± 0.002) and outside of the photoactive heterojunction for the solution-processed devices studied, both with and without an anode buffer layer (where m = 2.93 ± 0.09 and m = 1.90 ± 0.01, respectively).

Phenyl‐triggered photophysical switching between normal fluorescence and delayed fluorescence in phthalonitrile‐based luminophores

AbstractHerein, a facile strategy for switching luminescent properties between normal fluorescence and thermally activated delayed fluorescence (TADF) is presented. Two luminophoric molecules, VPN‐Ph and VPN‐H, combining phthalonitrile as an electron‐accepting core and triphenylamines as electron‐donating peripheries with and without two phenyl groups, are newly developed. A comparative study on their structural and photophysical properties is conducted. While non‐phenyl VPN‐H does not exhibit TADF but normal fluorescence, phenyl‐introduced VPN‐Ph exhibits TADF with a high photoluminescence quantum yield as a consequence of the phenyl‐triggered steric congestion. By virtue of the TADF feature, an organic light‐emitting diode (OLED) incorporating VPN‐Ph as an emitter achieves a maximum external electroluminescence (EL) quantum efficiency as high as 28.0%, which is five times higher than that of the VPN‐H‐based OLED. Thus, phenyl‐triggered geometric modulation has a drastic impact on the resulting photophysical and EL properties, leading to TADF on/off switching.

Sky-blue delayed fluorescence molecules based on pyridine-substituted acridone for efficient organic light-emitting diodes

Novel sky-blue thermally activated delayed fluorescence molecules with aggregation-induced emission are developed, which provide sky-blue electroluminescence (482–492 nm) and high external quantum efficiencies (21.8–26.8%).

Effect of acceptor concentration in the FRET controlled photoluminescence of PMMA-ZnO nanocomposite for the application of PLED device

Luminescent PMMA-ZnO nanocomposites are synthesised through bulk polymerization of MMA in presence of ex-situ prepared ZnO nanoplates. Variation of ZnO concentration affects the optical properties of the composite. Introduction of ZnO in the PMMA matrix makes the composite as semiconducting in nature by creating a lower energy gap in the composite. Again Förster resonance energy transfer (FRET) between PMMA (donor) to ZnO (acceptor) enhances emission of ZnO significantly. But the enhanced emission decreases with increase in ZnO concentration, i.e. the emission intensity of PMMA-ZnO nanocomposite shows an increasing trend with decrease in the content of ZnO. The energy transfer efficiency varies from 97% to 52% when the concentration of ZnO varies from 10 wt% to 30 wt%. The Photoluminescence quantum efficiency of the composite showing highest emission is calculated and found to be 74%. Corresponding theoretically calculated electroluminescence quantum efficiency is 4% at an operating voltage of 3.1 V assuming all the injected charge carriers can generate singlet exciton if the composite showing highest emission is used as an emissive layer in PLED.

Suppression of Electric Field-Induced Segregation in Sky-Blue Perovskite Light-Emitting Electrochemical Cells.

Inexpensive perovskite light-emitting devices fabricated by a simple wet chemical approach have recently demonstrated very prospective characteristics such as narrowband emission, low turn-on bias, high brightness, and high external quantum efficiency of electroluminescence, and have presented a good alternative to well-established technology of epitaxially grown III-V semiconducting alloys. Engineering of highly efficient perovskite light-emitting devices emitting green, red, and near-infrared light has been demonstrated in numerous reports and has faced no major fundamental limitations. On the contrary, the devices emitting blue light, in particular, based on 3D mixed-halide perovskites, suffer from electric field-induced phase separation (segregation). This crystal lattice defect-mediated phenomenon results in an undesirable color change of electroluminescence. Here we report a novel approach towards the suppression of the segregation in single-layer perovskite light-emitting electrochemical cells. Co-crystallization of direct band gap CsPb(Cl,Br) and indirect band gap CsPb(Cl,Br) phases in the presence of poly(ethylene oxide) during a thin film deposition affords passivation of surface defect states and an increase in the density of photoexcited charge carriers in CsPb(Cl,Br) grains. Furthermore, the hexahalide phase prevents the dissociation of the emissive grains in the strong electric field during the device operation. Entirely resistant to 5.7 × 10 V·m electric field-driven segregation light-emitting electrochemical cell exhibits stable emission at wavelength 479 nm with maximum external quantum efficiency 0.7%, maximum brightness 47 cd·m, and turn-on bias of 2.5 V.

Large cation ethylammonium incorporated perovskite for efficient and spectra stable blue light-emitting diodes

Perovskite light-emitting diodes (PeLEDs) have showed significant progress in recent years; the external quantum efficiency (EQE) of electroluminescence in green and red regions has exceeded 20%, but the efficiency in blue lags far behind. Here, a large cation CH3CH2NH2+ is added in PEA2(CsPbBr3)2PbBr4 perovskite to decrease the Pb–Br orbit coupling and increase the bandgap for blue emission. X-ray diffraction and nuclear magnetic resonance results confirmed that the EA has successfully replaced Cs+ cations to form PEA2(Cs1-xEAxPbBr3)2PbBr4. This method modulates the photoluminescence from the green region (508 nm) into blue (466 nm), and over 70% photoluminescence quantum yield in blue is obtained. In addition, the emission spectra is stable under light and thermal stress. With configuration of PeLEDs with 60% EABr, as high as 12.1% EQE of sky-blue electroluminescence located at 488 nm has been demonstrated, which will pave the way for the full color display for the PeLEDs.

Origin of exciplex degradation in organic light emitting diodes: Thermal stress effects over glass transition temperature of emission layer

Exploiting exciplexes in organic light-emitting diodes (OLEDs) to enhance electroluminescence lifetime and quantum efficiency is of considerable interest to researchers. The presence of exciplexes has been demonstrated in a host material interface in a multiple stacked OLED; therefore, understanding the degradation mechanism of exciplexes and host materials is essential to develop highly reliable and uniform OLEDs. Herein, we report thermal stress-driven exciplex degradation in a blue OLED, which comprises 4,4′-bis(N-carbazolyl)-,1′-biphenyl (CBP) as a host material without dopants. The device structure dependent-electroluminescence clearly confirms the formation of exciplexes surrounding the CBP interfaces at 398 and 450 nm, respectively. During the thermal stress over the glass transition temperature (Tg) of the CBP, the spectral intensity of the exciplex decreased significantly, and the ideality factor and characteristic trap energy increased abruptly when the thermal stress temperature was higher than the Tg of CBP, signaling the origin of thermal degradation effects on the exciplex and host material in our OLED.