Charge Generation Unit Research Articles

Achieving slow efficiency roll-off and tunable spectrum in high-performance tandem WOLEDs with mixed hosts

Highly efficient blue and warm white tandem OLEDs were designed and realized by doping blue thermally activated delayed fluorescence (TADF) emitter and red phosphorescent emitter into mixed hosts system together. Based on the optimization of charge generation unit (CGU), blue tandem device with external quantum efficiency of 30.93 % and color coordinates of (0.165, 0.365) was firstly realized. To achieve efficient and high-quality white OLEDs (WOLEDs), red phosphorescent emitter PQ2Ir (dpm) was doped into TCTA:26DCzPPy (1:1) to construct another electroluminescent (EL) unit near the anode. By optimizing the doping concentration of PQ2Ir (dpm), maximizing excitons utilization probability and controlling energy transfer processes, the optimal tandem WOLED with turn-on voltage, maximum brightness, current efficiency (CE), power efficiency (PE) and external quantum efficiency (EQE) of 5.2 V, 29,595 cd m−2, 65.19 cd A−1, 38.94 lm W−1 and 40.46 %, respectively, was demonstrated. Excitingly, this device can still maintain an EQE up to 25.59 % at the certain brightness of 1000 cd m−2, which is important for lighting application. The correlated color temperature (CCT) of this device tunes from 4401 K at 1000 cd m−2 to 2642 K at 20,000 cd m−2, thereby accommodating various scene requirements.

Lateral current suppression in tandem organic light-emitting diodes by adopting a buffer layer

For the tandem organic light emitting diodes (TOLEDs) with the charge generation unit (CGU) of LiF/Al/MoO 3 , there is a significant current lateral spreading causing light emission over an extremely large area outside the OLEDs pixel, due to the conductive interfacial layer caused by the oxidation-reduction reaction between Al and MoO 3 layers. To crack this nut, a buffer layer of 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) is inserted between Al and MoO 3 layers. The result shows the HAT-CN buffer layer eliminates the spread light emission in the TOLEDs. What's more, the device characteristics show the new CGU of LiF/Al/HAT-CN/MoO 3 has stronger charge generation and injection capabilities. The white TOLEDs with the new CGU exhibit a high external quantum efficiency (EQE) of 28.4%. Compared with the single OLEDs and the TOLEDs with the CGU of LiF/Al/MoO 3 , the efficiency is increased by 143% and 44%, respectively. • The efficient charge generation unit (CGU) of LiF/Al/HAT-CN/MoO 3 was developed. • The current lateral spreading was completely suppressed by adopting the CGU of LiF/Al/HAT-CN/MoO 3 . • The white tandem OLEDs exhibited an external quantum efficiency of 28.4%.

Enhanced Efficiency of Tandem Organic Light‐Emitting Diodes via Manipulating Heterojunction Composition of Charge Generation Unit

Tandem organic light‐emitting diodes (OLEDs) with an organic photovoltaic heterojunction (OPV‐HJ) as a charge generation unit (CGU), consisting of fullerence (C60) and copper phthalocyanine (CuPc), are fabricated. The device structures are ITO/EL unit/C60–CuPc HJ/MoO3/EL unit/Al, where EL unit is composed of NPB/Alq3/LiF. The efficiency improvement of tandem OLEDs is achieved by manipulating the structure and composition of C60–CuPc HJs. The charge generation and transport characteristics of CGUs are strongly dependent on the detailed structures of C60–CuPc HJs and the optimized CGU is 0.5:1 C60:CuPc bulk‐HJ. The improved efficiency of the tandem device with 0.5:1 C60:CuPc bulk‐HJ as CGU demonstrates to be the result of better charge carrier balance induced by the optimal charge transport and charge generation of C60:CuPc bulk‐HJs according to their surface morphology and absorption spectra. These results shed light on that C60:CuPc OPV‐HJ‐based CGU is a promising connection unit for the fabrication of high‐efficiency tandem OLEDs.

Efficient inverted organic light-emitting devices using a charge-generation unit as electron-injection layers

Efficient electron injection from cathode to electron transport layer is generally required to realize high-performance inverted organic light-emitting devices (IOLEDs). In this work, highly efficient IOLEDs are developed by employing a non-doped charge-generation unit of 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN)/Al/LiF as electron-injection layers (EILs). The ultraviolet photoelectron spectroscopy (UPS) shows that the insertion of HAT-CN layer reduces the work function of EILs by 0.21 eV. Combined with the current-voltage characteristics of electron-only devices, the role of the HAT-CN layer in promoting electron injection is confirmed. What's more, the double EILs device with Rubrene as a probe verifies the strong hole blocking ability of HAT-CN/Al/LiF. Based on the EILs, the inverted blue phosphorescent OLEDs with a maximum current efficiency of 29.4 cd/A are realized. The excellent performance proves the superiority of HAT-CN/Al/LiF as EILs. Working diagrams of CGU as EILs.In this work, highly efficient IOLEDs are developed by employing a non-doped CGU of HAT-CN/Al/LiF as EILs. The EILs show strong electron injection and hole blocking capabilities. •The charge generation unit as the electron injection layers greatly improves the injection of electrons. •The non-doped electron injection layers improves the stability and repeatability of the devices. •Based on the EILs, the inverted blue OLEDs with the maximum current efficiency of 29.4 cd/A and a maximum EQE of 14.6% are realized.

P‐99: Low Driving Voltage and High Color‐Stability White OLEDs on Silicon via Thin‐Charge Generation Unit for Microdisplays

In this research, we studied white organic light emitting diodes on silicon (OLEDoS) devices with low driving voltage and high color stability for microdisplay in augmented reality (AR) applications. In this device architecture, a thin charge generation unit (CGU) with 10 nm thickness was introduced into blue fluorescent and yellowish‐green phosphorescent emitting layers. It showed 5.8 V driving voltage at 1,000 cd/m2 with (0.276, 0.382) color coordinates which is sufficiently cool white emission. It also presented maximum luminance of 50,015 cd/m2 at 10 V driving voltage. It also showed high color stability of (± 0.007, ± 0.006) according to the luminance change from 500 to 2,000 cd/m2.

Highly efficient ultra-flexible tandem organic light-emitting diodes adopting a non-doped charge generation unit

An efficient CGU is applied to the high efficiency super-flexible white TOLEDs, which show a higher PE than the single OLEDs. After being bent for 3000 cycles, the devices retain ∼90% of the original luminance.

Efficient tandem organic light-emitting diodes with non-doped structures.

Highly efficient tandem organic light-emitting diodes (TOLEDs) were achieved based on a non-doped charge generation unit (CGU) consisting of LiF/Al/C60/4,4',4"-tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine (m-MTDATA) and ultrathin emitting layers. The current-voltage characteristics of the CGU devices and electron-only devices and the capacitance-voltage characteristics of the CGU-based capacitance devices were characterized to explore the charge generation and injection mechanisms. The charge generation process occurs at the interface of C60/m-MTDATA through electron transferring from the highest occupied molecular orbital of m-MTDATA to the lowest unoccupied molecular orbital of C60. It is found that the thinner C60 layer contributes to efficient electron injection. Under the optimal structure, the blue TOLEDs exhibit a maximum current efficiency (CEmax) of 43.3 cd/A. The CEmax and maximum external quantum efficiency (EQEmax) of the white TOLEDs reach 84.6 cd/A and 26.7%, respectively.

Highly efficient tandem blue phosphorescent organic LEDs with external quantum efficiency exceeding 42%

High-performance tandem blue phosphorescent organic LEDs (T-BPhOLEDs) are important for solid-state lighting and flat-panel display. Herein, we have realized a highly efficient T-BPhOLED by using the photovoltaic-type charge generation unit (CGU) of LiF/Al/fullerene/zinc-phthalocyanine/MoO3, blue phosphorescent emitter of bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) and electron transporter of 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene. The charge generation ability of CGU could be enhanced by the photovoltaic function of fullerene/zinc-phthalocyanine in T-BPhOLEDs. Under high luminance, a cycle composed of the CGU and emission layers in T-BPhOLEDs could convert some of the internal radiation into external radiation for improving performance. Simultaneously, the optimal T-BPhOLED exhibits a high external quantum efficiency of 42.9% at 100 cd·m−2.

Inverted organic light-emitting devices using a charge-generation unit as an electron injector

Abstract Efficient green fluorescent and phosphorescent inverted organic light emitting devices (IOLEDs) with a non-doped charge generation unit (CGU) of NPB/MoO3/Al/LiF as an electron injector have been developed. It is found that the electrons are generated effectively at the NPB/MoO3 interface and injected from MoO3 to the electron transporting layer via assistant electron injection layers of Al/LiF. More importantly, the performance of the devices shows insensitivity to the work function of the used cathode. Under the optimal structure of NPB(10 nm)/MoO3(5 nm)/Al(3 nm)/LiF(1 nm), the green fluorescent IOLEDs present a maximum current efficiency (CEmax) of 3.5 cd/A. Furthermore, the green phosphorescent IOLEDs based on this CGU show a turn-on voltage of 3.8 V, a CEmax of 54.6 cd/A and a maximum external quantum efficiency of 15.8%. The results suggest that the introduction of non-doped CGU in IOLEDs provides a promising strategy to integrate IOLEDs with the n-channel thin film transistors for applications in active-matrix OLEDs.

Remarkable enhancement of efficiencies for red, green, and blue tandem phosphorescent organic light-emitting diodes by utilizing a non-doped photovoltaic-type charge generation unit

We have achieved a remarkable enhancement in current efficiencies and power efficiencies for red, green, and blue tandem phosphorescent organic LEDs (T-PhOLEDs) by employing LiF/Al/fullerene/zinc-phthalocyanine/MoO3 as a charge generation unit (CGU). Fullerene/zinc-phthalocyanine with a proper absorption property in the visible region can absorb a part of photons from emission layers within T-PhOLEDs, which facilitates the charge generation and separation within CGU. At the identical luminance of 1,000 cd m−2, the current efficiencies for red, green, and blue T-PhOLEDs are 3.3-, 2.1-, and 2.9-fold those of the corresponding single-unit devices, and power efficiencies are increased by 72.2%, 13.6%, and 37.2%, respectively.

Improving the color-rendering index of a tandem warm white organic light-emitting device by employing a simple fabrication process.

We realized an efficient spectra-stable tandem white organic light-emitting device (WOLED) with pleasurable warm white emission and an extremely high color-rendering index (CRI) simultaneously by connecting a fluorescent blue unit and a phosphorescent dual-color unit via an easy-fabrication charge generation unit (CGU). Over a wide range of driving current density, the tandem warm white OLED exhibited general CRI (Ra) value exceeding 85 and special CRI (R9) value close to 57. In addition, Duvs are within the tolerance for CRI measurements, ensuring validity and logicality of the calculated results concerning Ra and R9. The obtained tandem OLED showed the maximum efficiencies of 38cd/A and 22.9lm/W, and still maintained high efficiencies of 35.5cd/A and 17.2lm/W at the luminance of 1000 cd/m2. Moreover, we studied the origins of both extreme color stability in dual-color OLED and extremely efficient electron injection within the CGU. To the best of our knowledge, this is the first report regarding quantitative values of both R9 and the Duv for a high-Ra tandem WOLED.

Hybrid warm-white organic light-emitting device based on tandem structure.

We realized an efficient hybrid tandem warm white organic light-emitting device with extremely stable color purity in forward direction by employing an easy-fabrication charge generation unit (CGU) combining 1, 4, 5, 8, 9, 11-Hexaazatriphenylene-hexacarbonitrile with ultrathin bilayers of LiF and Al. The tandem white device exhibits maximum efficiencies of 37.3 cd/A, 23.3 lm/W and still remains at high efficiencies of 34.4 cd/A, 17 lm/W at the luminance of 1000 cd/m2. When the operating current density increases from 5 mA/cm2 to 30 mA/cm2, the variations in Commission Internationale de l'Eclairage (CIE) coordinates are only (0.007, 0.003). Moreover, we also studied the cause of color variation in tandem white device by performing systematic optical simulation and discovered the origin of alleviation in efficiency roll-off for tandem white device by applying triplet-polaron quenching (TPQ) model.

Highly efficient tandem organic light-emitting devices employing an easily fabricated charge generation unit

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.

Highly-efficient tandem organic light-emitting device employing bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimi-dine doped with cesium azide in charge generation unit

Abstract In this research, we have investigated the charge generation and electron injection processes in charge generation unit (CGU) consisting of CsN3-doped Bis-4, 6-(3, 5-di-3-pyridylphenyl)-2-methylpyrimi-dine (CsN3:B3PYMPM)/Al/1, 4, 5, 8, 9, 11-Hexaazatriphenylene-hexacarbonitrile (HAT-CN). By employing this CGU, a high-efficiency phosphorescent tandem device achieving best efficiencies of 164.3 cd/A, 46.51% and 109.8 lm/W has been realized. Particularly, in addition to the improvement in current efficiency by more than two times, the tandem device has better power efficiency compared to the corresponding single-unit device at the same current density. Simultaneously, an efficient tandem white OLED showing high color stability and warm white emission over a wide range of luminance has been also fabricated.

Highly efficient tandem organic light-emitting devices adopting a nondoped charge-generation unit and ultrathin emitting layers

Abstract Highly efficient non-doped tandem phosphorescent organic light-emitting devices (PhOLEDs) have been demonstrated by employing LiF/AL/1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) as a charge generation unit (CGU) and ultrathin phosphorescent dyes as emitting layers (EMLs). It was found that the performance of the tandem PhOLEDs was significantly dependent on the Al thickness in CGU. Analyses regarding the current density-voltage characteristics of the CGU only devices indicate that the charge carriers are generated at the HAT-CN/NPB interface, and Al/LiF layers work as assistant electron injection layers to facilitate electron extraction from CGU and injection into the adjacent electron transport layer. From the capacitance-voltage and optical transparency characteristics of CGU with different thicknesses of Al interlayer, the thick Al layer is beneficial to charge separation, while it weakens the light output of the tandem device. Under the optimal Al thickness of 5 nm, more than twofold enhancement of current efficiency is achieved for the non-doped tandem blue device. Furthermore, the non-doped tandem white PhOLED was also developed showing a maximum current efficiency of 94.9 cd/A and maximum external quantum efficiency of 31.6%.

Diphenanthroline Electron Transport Materials for the Efficient Charge Generation Unit in Tandem Organic Light-Emitting Diodes

In this paper, we report two new phenanthroline-based compounds, 1,4-bis(2-phenyl-1,10-phenanthrolin-4-yl)benzene (p-bPPhenB) and 1,3-bis(2-phenyl-1,10-phenanthrolin-4-yl)benzene (m-bPPhenB), for the charge generation unit of tandem organic light-emitting diodes (OLEDs). These two compounds exhibited high electron mobility of (5.8–4.4) × 10–3 cm2/(V s), a very small injection barrier at the p–n junction interface, a high glass transition temperature of 123.9–182.1 °C, and exceptionally good operational stability. Because of such excellent characteristics, a single-stack red phosphorescent OLED (PhOLED) with p-bPPhenB showed a low driving voltage (2.7 V) and significantly improved maximum power efficiency (56.8 lm/W), external quantum efficiency (30.8%), and device lifetime (LT95, 130 h) compared to those of the control device using bathophenanthroline (Bphen) (3.7 V, 39 lm/W, 27.1%, and 13 h). Furthermore, a two-stack (tandem) red PhOLED using p-bPPhenB in the charge generation unit exhibited superior cha...

Electron-transporting layer doped with cesium azide for high-performance phosphorescent and tandem white organic light-emitting devices

Cesium azide was employed as an effective n-dopant in the electron-transporting layer (ETL) of organic light-emitting devices (OLEDs) owing to its low deposition temperature and high ambient stability. By doping cesium azide onto 4,7-diphenyl-1,10-phenanthroline, a green phosphorescent OLED having best efficiencies of 66.25 cd A−1, 81.22 lm W−1 and 18.82% was realized. Moreover, the efficiency roll-off from 1000 cd m−2 to 10 000 cd m−2 is only 12.9%, which is comparable with or even lower than that of devices utilizing the co-host system.Physical mechanisms for the improvement of device performance were studied in depth by analyzing the current density–voltage (J–V) characteristics of the electron-only devices. In particular, by comparing the J–V characteristics of the electron-only devices instead of applying the complicated ultraviolet photoelectron spectrometer measurements, we deduced the decrease in barrier height for electron injection at the ETL/cathode contact. Finally, an efficient tandem white OLED utilizing the n-doped layer in the charge generation unit (CGU) was constructed. As far as we know, this is the first report on the application of this CGU for fabricating tandem white OLEDs. The emissions of the tandem device are all in the warm white region from 1213 cd m−2 to 10870 cd m−2, as is beneficial to the lighting application.

Effects of Aluminum Interlayer in Charge Generation Unit for Tandem Organic Light-Emitting Diodes

Effects of Aluminum Interlayer in Charge Generation Unit for Tandem Organic Light-Emitting Diodes

Investigations of solution-processed charge generation unit with low concentration of small molecule doped in p-type/HAT-CN6 for tandem OLED

We investigated the charge generation and injection mechanism in solution processed charge generation unit (CGU) used in our high performance tandem organic light emitting diode (OLED) via capacitance–voltage (C–V) and current density–voltage (J–V) measurements. By doping 2wt% of small molecule 1,1-bis-(4-bis(4-tolyl)-aminophenyl) cyclohexene (TAPC) into Poly (N-vinylcarbazole) (PVK) as p-type layer of the CGU, we obtained more than two folds improvement in the tandem device efficiency compared to single device. The performance improvement of the TAPC doped CGU could be attributed to low built-in potential, large vacuum level shift as well as high charge density for efficient charge generation.

Inverted Tandem Phosphorescence Organic Light-Emitting Diodes Based on <formula formulatype="inline"><tex Notation="TeX">$\hbox{MoO}_{3}/\hbox{Al/Cs}_{2}\hbox{CO}_{3}$</tex> </formula> Charge Generation Unit

A multilayer charge generation unit (CGU) $\hbox{MoO}_{3}/\hbox{Al/Cs}_{2}\hbox{CO}_{3}$ has been developed for inverted tandem organic light-emitting diodes (OLEDs). The current efficiency of inverted tandem phosphorescence OLED with $\hbox{MoO}_{3}$ (8 nm)/Al (1 nm)/ $\hbox{Cs}_{2}\hbox{CO}_{3}$ (0.5 nm) CGU is almost twice as high as that of individual electroluminescent (EL) device. Charges generate at hole-transporting host material/ $\hbox{MoO}_{3}$ interface and $\hbox{MoO}_{3}$ bulk layer, and electrons are extracted by $\hbox{Cs}_{2}\hbox{CO}_{3}$ /electon-transporting layer (ETL) interface. The significantly enhanced performance of the tandem device with $\hbox{MoO}_{3}/\hbox{Al/Cs}_{2}\hbox{CO}_{3}$ CGU compared with a $\hbox{MoO}_{3}/\hbox{Cs}_{2}\hbox{CO}_{3}$ -based one may be attributed to preventing chemical reaction of $\hbox{MoO}_{3}$ and $\hbox{Cs}_{2}\hbox{CO}_{3}$ by the inserted ultrathin Al layer. Moreover, the current efficiency and EL spectrum of another inverted tandem OLED combining red and green stack further prove the effective charge generation and separation with the multilayer $\hbox{MoO}_{3}/\hbox{Al/Cs}_{2}\hbox{CO}_{3}$ CGU. It is expected that such a CGU is a promising candidate for inverted tandem white OLED for active matrix organic light-emitting diode display driven by an n-type thin-film transistor backplane.