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Tandem organic light-emitting devices (OLEDs) have attracted considerable attention for solid-state lighting and flat panel displays because their tandem architecture enables high efficiency and long operational lifetime simultaneously. In the tandem OLED structure, plural light-emitting units (LEUs) are stacked in series through a charge generatio...
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... This charge generation process can be described by the 'temperatureindependent tunneling' model, where electrons tunnel from the highest occupied molecular orbital (HOMO) or valence band of p-type materials to the lowest unoccupied molecular orbital (LUMO) or conduction band of n-type materials, induced by an electric field, resulting in the formation of electron-hole pairs at the p-n interface [70][71][72][73][74]. Common n-type materials used in CGLs include transition metal oxides such as molybdenum oxide (MoO 3 ), tungsten oxide (WO x ), and zinc oxide (ZnO), and organic compounds such as tetrafluoro-tetracyano-quinodimethane (F4TCNQ) and 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) [56,60,66,67,[75][76][77][78][79][80][81][82][83][84]. P-type materials include organic compounds such as 1,1-bis-(4-bis(4-methyl-phenyl)-aminophenyl)-cyclohexane (TAPC), N,N ′ -di(naphth-1-yl)-N,N ′diphenyl-benzidine (NPD), 4,4 ′ ,4 ′′ -tris(N-3-methylphenyl-Nphenylamino) triphenylamine (m-MTDATA), and polymers like poly (3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) [28,29,52,78,85,86]. ...
... For instance, the relatively high deposition temperatures of some metal oxides and organic compounds can cause degradation if less thermally stable materials are present at the bottom [87][88][89][90]. Furthermore, some unstable materials, such as F4TCNQ, can lead to increased driving V during long-term operation [85,91]. ...
Due to their unique properties, charge-generation layers (CGLs) have been used as interconnect layers for organic and quantum-dot light-emitting devices (LEDs) consisting of multiple emission units. Furthermore, CGLs have also been integrated into single-emission-unit LEDs and alternating-current LEDs. The charge-generation structures provide charge carriers (electrons and holes) to the devices under an external electric field, instead of charge injection from the electrodes. Therefore, there is no strict requirement for precise matching of energy levels between the electrodes and charge-injection layers. This affords greater flexibility for device design and enhances the efficiency and operational lifespan of devices. In this review, we summarize the development of charge-generation structures and discuss the existing challenges and opportunities. A particular focus is placed on the working mechanism of CGLs and their applications in various LEDs. Additionally, issues such as voltage drop in CGLs, charge generation efficiency, increased operating voltage for the devices, and optimizations of existing CGLs are discussed.
... So, the commonly used connecting electrodes are low work function metals, such as Al [10], Ag [11], and Ca [12]. To ensure a high transmittance, the thickness of the connecting electrode is usually optimizing to 1 nm [3,[13][14][15][16]. In addition to the above two requirements, however, other optical properties of the connecting electrode are also important for the performance of tandem OLEDs, especially in the strong microcavity environments. ...
Connecting electrodes play a crucial role to assist charge injection into the adjacent electroluminescent units in tandem organic light-emitting diodes (OLEDs). In this study, we demonstrate that Mg:Ag alloy is an effective connecting electrode for bottom- and top-emitting tandem OLEDs. Optical cavity design and simulation are also conducted to predict the luminance of tandem OLEDs. It is found that the theoretical luminance of tandem OLEDs is close to but not higher than twofold enhancement over the luminance of a single OLED optimized to the first resonance mode, which is theoretically higher than high-order resonance modes. It is also found that the optical properties of Mg:Ag connecting electrodes, while having relatively small influence on weak microcavity bottom-emitting tandem OLEDs, have large influence on strong microcavity top-emitting tandem OLEDs.
... 22 In tandem LEDs, two or more light-emitting units (LEUs) are connected through an interconnecting layer (ICL). When bias is applied, multiple electron−hole pairs can be injected through the electrodes and charge generation layer into the different LEUs; therefore, it is possible to simultaneously emit multiple photons, which results in an increase in CE. 23,24 The generation and recombination of excitons also occur on each emission layer, which prevents the close range charge transfer between different QDs; therefore, the variation in EL spectra recorded at different driving voltages can be reduced effectively. 25,26 The design of proper ICLs is critical to achieving highperformance tandem LEDs, as they play an important role in tuning the electron and hole carriers' generation and injection properties in the devices. ...
Solution-processed electroluminescent tandem white quantum-dot light-emitting diodes (TWQLEDs) have the advantages of being low-cost, high-efficiency, and having a wide color gamut combined with color filters, making this a promising backlight technology for high-resolution displays. However, TWQLEDs are rarely reported due to the challenge of designing device structures and the deterioration of film morphology with component layers that can be deposited from solutions. Here, we report an interconnecting layer with the optical, electrical, and mechanical properties required for fully solution processed TWQLED. The optimized TWQLEDs exhibit a state-of-the-art current efficiency as high as 60.4 cd/A and an extremely high external quantum efficiency of 27.3% at luminance of 100,000cd/m2. A high color gamut of 124% NTSC 1931 standard can be achieved when combined with commercial color filters. These results represent the highest performance for solution-processed WQLEDs, unlocking the great application potential of TWQLEDs as backlights for new-generation displays.
... Tandem organic light-emitting diode (OLED) devices are useful for display and lighting applications where high brightness is necessary. In a tandem OLED device, two or more individual electroluminescent (EL) cells are physically stacked in parallel and electrically connected in series by a common interconnect layer, also known as charge-generation layer (CGL) [1,2]. An ideal CGL should have minimal electrical impedance and highest optical transparency, in addition to providing ohmic contacts to the adjacent EL cells. ...
The 8-hydroxyquinolinato lithium (Liq)|Ag system has been evaluated as the electron-injection layer for the MoOx| N,N′-di (1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) charge-generation layer in tandem organic light-emitting diode (OLED) devices. From current-voltage studies of single-stack devices, it is determined that Liq is a relatively poor electron-transport material. However, Liq is found to be useful as an electron-injection layer with activation by Ag. Efficient tandem devices have been obtained using Liq|Ag as the electron-injection layer adjacent to the MoOx|NPB charge-generation layer, where the optimal Ag thickness is about 20–40 Å and the activation of Liq can be seen with as little as 1 Å of Ag.
... Furthermore, there have been innovative studies on environmentally friendly organic electronics and printing technologies [3][4][5]. Among these technologies, organic light-emitting diodes (OLEDs), organic thin-film transistors (OTFTs), and organic photovoltaics (OPVs), which use organic materials, have been investigated for flexible devices [6][7][8], and their development is underway. In particular, interest in flexible OLED displays is increasing [9][10][11]. ...
Interest in flexible organic light-emitting diode (OLED) displays is increasing. Fabrication methods based low-temperature, low-cost, and large-area printing processes have been studied to simplify the manufacturing process. A gate driver is required to drive the OLED display. The gate driver is integrated into the panel to reduce the manufacturing cost of the display panel and to simplify the module structure. In this study, a shift register is proposed using organic thin-film transistors (OTFTs) to integrate flexible an OLED display with the gate driver. OTFT modeling was performed using the experimental data with a three-terminal silicon-on-sapphire field effect transistor (SOSFET) model. Furthermore, a shift register for the gate driver was designed using the required process design kit with the OTFT modeling data. Operation of the shift register was confirmed by HSPICE simulation. The layout of the shift register was designed using the solution process rule for OTFTs. Those results show the possibility of gate diver integrated in flexible OLED display.
... The search for more efficient electroluminescent devices is one of the main topics currently addressed in the literature on Organic Light Emitting Diodes (OLEDs). The creation of new polymers and small molecules for OLED devices with high internal quantum efficiency (IQE) [1,2], the use of different device architectures [3], and the exploration of unusual properties, like the thermally activated delayed fluorescence (TADF) [1,4,5] appear as great options to make more efficient devices. However, as important as the study of the emission layers, the better understanding of the carriers transporting and/or blocking layers, such as TFB [6][7][8], ZnO [9,10], TPD [11], among others, are of great importance for the improvement of these devices. ...
The density functional theory (DFT) simulation method has been highlighted in last years, due to its ability to predict optical, electronic and molecular properties of different materials, presenting good agreement with experimental results. In this work, we conducted a study of the structural and optical properties of the copolymer poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-sec-butylphenyl)diphenylamine)] (TFB) which appears with great emphasis in the literature being applied mainly as hole transporting and electron blocking layer in devices. Raman and FT-IR measurements were taken, and the obtained results were compared with those calculated via DFT method (DFT/B3LYP and 6–31g*). The comparison between experimental and DFT results were performed, and a good agreement between both results was observed, showing discrepancy lower than 7% for the vibrational peak positions, in energy. Also, photoluminescence (PL) measurements were performed at room and liquid nitrogen temperature. The PL spectra were reconstructed via Franck Condon, through the Huang-Rhys parameters, by using the vibrational modes obtained both experimentally and via DFT. This procedure allowed to determine the contribution of the different vibrational modes to the photoluminescence vibronic bands.
... I n recent years, organic light-emitting devices (OLEDs) with a tandem structure have been extensively studied because of their unique advantages of high luminance and current efficiency at low current density and long lifetime. 1) In tandem OLEDs, two emitting units are electrically connected via a charge generation unit (CGU). Under an applied electric field, the holes and electrons generated within the CGU inject into the adjacent hole-transporting layer (HTL) and electron-transporting layer (ETL), respectively, and then recombine with electrons from the cathode side and holes from the anode side to generate photons in each emitting layer. ...
We have realized highly efficient tandem organic light-emitting devices (OLEDs) employing an easily fabricated charge generation unit (CGU) combining 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile with ultrathin bilayers of CsN3 and Al. The charge generation and separation processes of the CGU have been demonstrated by studying the differences in the current density-voltage characteristics of external-carrier-excluding devices. At high luminances of 1000 and 10000 cd/m², the current efficiencies of the phosphorescent tandem device are about 2.2- and 2.3-fold those of the corresponding single-unit device, respectively. Simultaneously, an efficient tandem white OLED exhibiting high color stability and warm white emission has also been fabricated.
... To make these devices more efficient, more than two EL units were introduced. For example, Kido [89,91,92], Tang [93,94] and Liao et al. [41,66,95], developed a TOLED with two and three units using a p-n junction CGL as the intermediate layer and showed excellent light outcoupling and carrier injection properties with significantly improved operational lifetime [43,82]. ...
... Therefore, the thickness of metal intermediate layer should be less than 5 nm. For example, the metals of Ag, Au, Mg:Ag, and Al were commonly used as the intermediate layer in TOLEDs [92,96,97]. In this TOLED, the ultrathin intermediate layer serves as an additional electrode, which could separately drive two EL units to generate red, blue and white light emission, as shown in Fig. 2a [98]. ...
... A monochromatic TOLED usually aims at increasing current efficiency, brightness and device lifetime for display panels [43,99,100,102,103]. In contrast, a multi-color TOLED usually focuses on generating stable white or multi-light emission for solidstate lighting sources [64,92]. In the point view of processing method, the p-n type intermediate layer of TOLEDs can be fabricated using thermal evaporation or solution processing. ...
In order to satisfy the special demand of AR products for ultra‐high luminescence and high resolution full‐color silicon‐based OLED micro displays, a silicon‐based OLED micro display with resolution of more than 4000 ppi (0.36 inch with 1280×720) and a luminescence of more than 5000 nit is investigated. First, the design scheme of high‐resolution silicon‐based OLED driver is introduced. Based on the scheme of strong microcavity effect OLED device, the structure of red, green and blue monochrome luminescence enhancement is verified, and the current efficiency reaches 60 cd A ‐1 , 95 cd A ‐1 and 6 cd A ‐1 , respectively. Finally, the development of HD036 chip is finished, and the high luminescence full‐color silicon‐based OLED micro display device with ppi more than 4000, luminescence more than 5000 nit, and color gamut more than 85% is achieved. The effect of virtual reality overlay display is achieved by the verification of AR machine.