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Working mechanism of CT-assisted OLEDs. a Energy transfer schematic and mechanism of previously reported OLEDs with EL at sub-bandgap voltages. b Energy level diagram of rubrene and C60 based exciplex OLED
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It is commonly accepted that a full bandgap voltage is required to achieving efficient electroluminescence (EL) in organic light-emitting diodes. In this work, we demonstrated organic molecules with a large singlet-triplet splitting can achieve efficient EL at voltages below the bandgap voltage. The EL originates from delayed fluorescence due to tr...
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... The DSB derivative described in the literature contain conjugated units that are linked together via different bridges which are in most cases non-conjugated spacers forming therefore insulated blocks [16]. These insulated sequences can act as a barrier for charge injection and mobility and thus reduce the performance of the diodes [17,18]. However, few papers describe the incorporation of diphenylsulfone as electron acceptors to tune the OLEDs characteristics [19,20]. ...
New semi-conducting molecular materiel (M-DSB) based on the distyrylbenzene π-conjugated system was synthesized via the Wittig reaction. The structural properties of M-DSB, displaying three different stereoisomers E-E, Z-Z and Z-E, were investigated through density functional theory (DFT) optimization. The detailed TD-DFT analysis provided insights into the structure-property relationships and revealed good agreement with experimental data. This photoactive material exhibits nearly the same UV–visible spectra in dilute solution and as solid thin film, with a maximum absorption at 363 nm and an optical gap of 2.99 eV. A broad and red-shifted photoluminescence spectrum was obtained in the solid state with blue emission compared to the solution which shows purplish blue emission with relatively narrow emission. The HOMO/LUMO energy levels were calculated by cyclic voltammetry measurements and indicate a p-type semiconductor material. The temperature and frequency dependence of the conductivity shows an activation energy value of about 0.89 eV for the M-DSB thin layer. Impedance spectroscopy of the ITO/M-DSB/Al device indicates a typical behavior of a single relaxation process and confirms that the electrical response becomes faster and the mobility of the charge carriers is enhanced by increasing the temperature.
... Recently, several papers have been published reporting low-voltage operation of blue OLEDs 34,35 . However, the turn-on voltage was approximately 2.5 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.
... Since the first developments of organic light-emitting diodes (OLED) using fluorescent emitters in 1987 1 , a lot of efforts have been devoted to upgrade the efficiency and lifetime by exploiting a mechanism fully utilizing excitons for radiative transition [2][3][4][5][6][7][8] . Among the approaches, pure organic based thermally activated delayed fluorescence (TADF) emitters have been spotlighted as an alternative to highly efficient phosphorescent emitters utilizing expensive noble metals. ...
Multiple resonance (MR) thermally activated delayed fluorescence emitters have been actively studied as pure blue dopants for organic light-emitting diodes (OLEDs) because of excellent color purity and high efficiency. However, the reported MR emitter, 2,5,13,16-tetra-tert-butylindolo[3,2,1-jk]indolo[1′,2′,3′:1,7]indolo[2,3-b]carbazole (tDIDCz) based on bis-fused indolocarbazole framework could not demonstrate efficient triplet-to-singlet spin crossover. In this work, we report two isomeric MR emitters designed to promote triplet exciton harvesting by reconstructing the electronic structure of tDIDCz. To manage excited states, strong electron donors were introduced at the 2,5-/1,6-position of tDIDCz. As a result, 2,5-positions managed tDIDCz shows long-range charge transfer characteristics while preserving the MR nature. Quantum chemical calculation demonstrates direct spin-orbit coupling by long-range charge transfer and spin-vibronic coupling assisted reverse intersystem crossing by short-range charge transfer simultaneously contribute to triplet-to-singlet spin crossover. Consequently, high performance blue OLED recorded a high external quantum efficiency of 30.8% at a color coordinate of (0.13, 0.13).
... 17 −22 One mechanism that enables a process to ensure a practical operational device lifetime with a relatively high exciton production yield is triplet−triplet upconversion (TTU). 23 −27 In TTU processes for blue OLEDs, two triplet excitons (with energies around 1.8 eV such as anthracene derivatives) can collide to form one bright excited singlet exciton (with energies around 3.0 eV). Thus, this strategy allows low-triplet-energy design while still achieving better exciton utilization efficiency. ...
In the process of triplet-triplet upconversion (TTU), a bright excited singlet can be generated because of the collision of two dark excited triplets. In particular, the efficiency of TTU is crucial for achieving a high exciton production yield in blue fluorescence organic light-emitting diodes (OLEDs) beyond the theoretical limit. While the theoretical upper limit of TTU contribution yield is expected to be 60%, blue OLEDs with the maximum TTU contribution are still scarce. Herein, we present a proof of concept for realizing the maximum TTU contribution yield in blue OLEDs, achieved through the doping of thermally activated delayed fluorescence (TADF) molecules in the carrier recombination zone. The bipolar carrier transport ability of TADF materials enables direct carrier recombination on the molecules, resulting in the expansion of the recombination zone. Although the external electroluminescence quantum efficiency of OLEDs is slightly lower than that of conventional TTU-OLEDs due to the low photoluminescence quantum yield of the doped layer, the TTU efficiency approaches the upper limit. Furthermore, the operational device lifetime of OLEDs employing TADF molecules increased by five times compared to the conventional ones, highlighting the expansion of the recombination zone as a crucial factor for enhancing overall OLED performance in TTU-OLEDs.
... Recently, several papers have been published reporting low-voltage operation of blue OLEDs. 31,32 However, the turn-on voltage was approximately 2.5 V. The ultralow voltage operation at approximately 1.5 V for blue emission has not been achieved even with inorganic LEDs. ...
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. 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 the triplet-triplet annihilation, which forms one emissive singlet from two triplet excitons. We found that the essential component for efficient blue emission is a smaller energy difference between the CT state and triplet exciton, accelerating the energy transfer between the two states and achieving the optimal performance by avoiding direct decay from the CT state to the ground state. Our study demonstrates that the developed OLED allows for a much longer operation lifetime than that from a typical blue phosphorescent OLED because the blue emission originates from a stable low-energy triplet exciton that avoids degrading the constituent materials.
... In fluorescent OLEDs, triplet-triplet annihilation methods have obtained EQEs of 11.5% and 8.6% for red and yellow light respectively [4], by placing an additional assisting layer between the EEL and ETL, the assisting layer contains a blue host material and a green dopant. However, as green component is present in the device, it will be hard to apply the assisting layer technique to enhance the EQE of blue OLEDs. ...
... To be able to restrict the triplet excitons, it is necessary to have an HTL material and an EEL material, both of which have a greater triplet level than the emitting materials. EEL-2, a newly synthesised EEL material, exhibits an electron mobility of around 104 cm 2 V -1 s and an electron affinity level about identical to the blue host material [4]. Molecular orientation is another strategy for promoting OLED efficiency. ...
Organic light emitting diode (OLED) is one of the main lighting devices and can be used as mobile phone screens, so its performance enhancement is worthy of an in-depth study. This paper discusses three types of devices with different light emitting principles in the history of OLED development in chronological order and their performances, as well as ways to enhance their external quantum efficiency (EQE). For fluorescent, phosphorescent and thermally activated delayed fluorescent (TADF) OLEDs, adding additional layers and doping are effective means of improving the performance. For fluorescent OLED, the EQE can be increased up to 11.5% by adding an efficiency enhancement layer and doping the emitting layer with a new blue dopant with a higher orientation coefficient. For phosphorescent OLED, a hole transport layer is utilized to block excitons within the FIrpic-doped emissive layer leading to an EQE of 16.7%. For TADF OLED, the soluble doped TADF OLEDs is helpful at improving the quantum efficiency up to 18.3%. This paper looks forward to the maturation of these strategies and their practical application, and the identification of more technologies that can enhance the performance of OLED devices to help make it more usable.
... Organic luminescent molecules are widely used as functional materials in various optoelectronic devices, such as organic light-emitting diodes (OLEDs) [1][2][3][4], sensors [5,6], scintillators [7][8][9], lasers [10,11], luminescent solar concentrators [12], bioimaging systems [13,14] and many other applications upon growing demands of science and technology. Nevertheless, the improvement of spectral and photophysical characteristics of such materials remains an important task. ...
Organic luminescent materials are widely used in various electronic and optoelectronic devices upon growing demands of science and technology. Enhancement of spectral-luminescence characteristics for such materials and in depth understanding of “structure-property” relationships remain challenging tasks. Herein, we report on synthesis and comprehensive investigation of the series of novel luminescent push-pull molecules with triphenylamine unit as an electron donor block and thiophene as a π-spacer, which end-capped with various types of electron-withdrawing groups (EWGs), which are commonly used for the molecular design of various functional materials in organic electronics. The results allowed us to evaluate the impact of EWG type used on the target materials characteristics. Phenyl-substituted EWGs were found to be more suitable for the design of highly thermally and electrochemically stable materials with relatively high melting temperatures and melting enthalpies. Depending on the EWG nature luminescence maxima of the luminophores demonstrated significant variability, e.g. from 509 nm to 750 nm, while the photoluminescence quantum yield (PLQY) values laid in the range of 1–89%. All luminophores showed good compatibility with a polystyrene (PS) matrix, in which PLQYs were generally higher (up to 25-fold enhancement) compared to the corresponding solutions or polycrystalline films. The changes of spectral characteristics observed for these luminophores were well described using basic relations of the semi-empirical theory of solvatochromism. Based on lifetime of excited states measurements, it was shown that the excited state non-radiative deactivation constants values the major contributors to PLQY values in THF solutions, while increase of the PLQY values in PS films can be associated with decrease of the probability of non-radiative deactivation of the excited states.
... The ever-increasing demand for high-efficiency light-emitting diodes (LEDs) fuels multidisciplinary efforts in material discoveries, device structural optimization, and mechanism elucidation. Substantial progress has been made in materials engineering, evidenced by booming organic emitters [1][2][3][4][5][6][7][8][9] and recently the most sought-after perovskite family [10][11][12][13][14]. They share similarities of high photoluminescence quantum yields (PLQYs), readily tunable bandgaps, low-cost solution processability, etc. ...
Perovskite light-emitting diodes (PeLEDs) have attracted much attention due to their superior performance. When a bottleneck of energy conversion efficiency is achieved with materials engineering, nanostructure incorporation proves to be a feasible approach to further improve device efficiencies via light extraction enhancement. The finite-difference time-domain simulation is widely used for optical analysis of nanostructured optoelectronic devices, but reliable modeling of PeLEDs with nanostructured emissive layers remains unmet due to the difficulty of locating dipole light sources. Herein we established a hybrid process for modeling light emission behaviors of such nanostructured PeLEDs by calibrating light source distribution through electrical simulations. This hybrid modeling method serves as a universal tool for structure optimization of light-emitting diodes with nanostructured emissive layers.
... The electrical transported inside the OLED can be modeled by the one-dimensional time independent drift-diffusion model [28][29][30]: ...
... The mobility of organic material is calculated by the Pool-Frenkel equation [28,30] as follow: ...
... The recombination of holes and electrons to exctions occurs due to attractive Coulombic interaction. The bulk recombination rate of free holes and electrons is calculated in accordance with Langevin theory [30]. ...
In this paper, a comparative study between four OLEDs devices is carried out. The bi- layers device (A) (consists of) Hole Injection Layer (HIL)/Electron Transport Layer (ETL), the multilayer device (B) (consists of) HIL Layer/Hole Transport Layer (HTL)/ETL Layer. The influence of the hole transporting material on the performance of the three layers OLEDs was investigated. Three different HTL materials were used: α- NPD, TAPC and p-TTA with the same electron transporting material as Alq3; (these holes transport material consists the devices (B), (C) and (D) respectively). The carrier injection, Langevin recombination rate, singlet exciton density and the power of luminescent are demonstrated. The simulation results shows that the insertion of a thin HTL layer between HIL and ETL layers increases the characteristics of the device (B)as: 6.19.1025 cm-3s-1 of the Langevin recombination rate, 1.16.1015cm-3 of the singlet exciton density and 0.04232 W/μm2 of the luminescence power. Moreover, the insertion of TAPC as HTL material gives rise to 1.36.1026 cm-3s-1 of the Langevin recombination rate, 2.1015cm-3 of the singlet exciton density and 0.075 w/μm2 of the luminescence power.
... Since the molecular orbital theory was proposed in the 20th century, the concepts of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) have been used in various research areas [1][2][3][4][5][6][7][8] . For example, Fukui et al. found that π-electrons in the HOMO played decisive roles in the reaction of aromatic hydrocarbons and extended this result to the concept of frontier molecular orbitals (i.e., the HOMO and LUMO) 9,10 . ...
The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies, which are key factors in optoelectronic devices, must be accurately estimated for newly designed materials. Here, we developed a deep learning (DL) model that was trained with an experimental database containing the HOMO and LUMO energies of 3026 organic molecules in solvents or solids and was capable of predicting the HOMO and LUMO energies of molecules with the mean absolute errors of 0.058 eV. Additionally, we demonstrated that our DL model was efficiently used to virtually screen optimal host and emitter molecules for organic light-emitting diodes (OLEDs). Deep-blue fluorescent OLEDs, which were fabricated with emitter and host molecules selected via DL prediction, exhibited narrow emission (bandwidth = 36 nm) at 412 nm and an external quantum efficiency of 6.58%. Our DL-assisted virtual screening method can be further applied to the development of component materials in optoelectronics.
