Efficient Electron Injection Layer Research Articles

Investigation of Organic LED Materials Using a Transparent Cathode for Improved Efficiency

This paper proposes a flexible transparent organic light-emitting diode (OLED) which is notable due to its high electron injection and transmittance. Though graphene-based OLEDs are promising candidates for flexible displays due to the enormous merits offered, a significant decrease in its device efficiency was reported in comparison to the other conventional indium tin oxide-based OLEDs. The device illustrated enhances the performance of the graphene-based flexible OLEDs through balanced charge recombination. A theoretical analysis has been done through performing optical simulations. The structure of the graphene-based OLED is optimized through the employment of efficient cathode and electron injection layer materials to boost the electron injection into the device. With the aid of simulations, it was found that optimum results are obtained with aluminum zinc oxide (Al:ZnO) as the cathode and Al/LiF/Alq3 as the electron injection layer. The proposed structure exhibits high transmittance of 87.14% with a current efficiency of 61.17 cd/A, and external quantum efficiency (EQE) of 21.2% at 1000 cd/m2 for the bottom side. An evident improvement in the device performance with 32.9% increase in the current efficiency, and 45.2% increase in the EQE was observed.

Graphene Doped Cs2CO3 as an Efficient Electron Injection Layer for Organic Light-Emitting Diodes

Graphene Doped Cs₂CO₃ as an Efficient Electron Injection Layer for Organic Light-Emitting Diodes

Solution processed alkali-metal and alkaline-earth-metal compounds as the efficient electron injection layer in organic light-emitting diodes

To enable the use of high work-function metals as the cathode in organic light emitting diodes (OLEDs), different types of alkali-metal and alkaline-earth-metal based compounds are examined as the electron injection layer (EIL). All the studied compounds can improve the device performance for aluminum, gold, and silver cathodes. Potassium hydroxide (KOH) and potassium carbonate (K2CO3) are employed as EIL for the first time in OLEDs. The performance of the device with sodium hydroxide (NaOH)/silver cathode almost doubles the conventional barium/aluminum device’s efficiency. By examining the work-function of EIL-modified metals through Kelvin Probe, a general correlation between the device performance and cathode work-function is revealed. The image-charge effect is found to be a contributor to the reduction of the metal work-function.

Organometallic compound derivates as a novel efficient electron injection layer for hybrid light-emitting diodes

In this study we have presented the use of a organometallic compounds derivates (Keggin type polyoxometalates) for the production of powerful electron injection layer between the emissive polymer layer (PFO) and cathode layer (Al) . Electron transport studies in organometallic compounds layers showed efficient electron transport in very thin structures. High efficiency solution-processed organic light emitting diodes (OLEDs) are achieved by organometallic compounds such as Keggin type polyoxometalates used as electron injection or transport layers with superb electron mobilities and hole blocking capacities. These results indicate the potential for organometallic compounds derivates as a variable, emerging new class of efficient electron injection/transport molecular materials for high performance air-stable HyLEDs. The Keggin type polyoxometalates show unique features such as superior ionization energy and low electron affinity which conversion them as efficient electron injection/hole blocking layers and, immensely high electron mobility.

Efficient Color-Stable Inverted White Organic Light-Emitting Diodes with Outcoupling-Enhanced ZnO Layer

Inverted organic light-emitting diode (OLED) has attracted extensive attention due to the demand in active-matrix OLED display panels as its geometry enables the direct connection with n-channel transistor backplane on the substrate. One key challenge of high-performance inverted OLED is an efficient electron-injection layer with superior electrical and optical properties to match the indium tin oxide cathode on substrate. We here propose a synergistic electron-injection architecture using surface modification of ZnO layer to simultaneously promote electron injection into organic emitter and enhance out-coupling of waveguided light. An efficient inverted white OLED is realized by introducing the nanoimprinted aperiodic nanostructure of ZnO for broadband and angle-independent light out-coupling and inserting an n-type doped interlayer for energy level tuning and injection barrier lowering. As a result, the optimized inverted white OLEDs have an external quantum efficiency of 42.4% and a power efficiency of 85.4 lm W1-, which are accompanied by the superiority of angular color stability over the visible wavelength range. Our results may inspire a promising approach to fabricate high-efficiency inverted OLEDs for large-scale display panels.

Cesium Azide—An Efficient Material for Green Light-Emitting Diodes With Giant Quantum Dots

A novel efficient and air-stable electron injection layer (EIL) of cesium azide (CsN3) was compared with conventional ones including CsF, Cs2CO3, LiF and without EIL in type-II quantum dot light-emitting diodes (QLEDs) with both organic electron and hole transport layers. Via directly decomposing to pristine cesium (Cs), the low-temperature evaporated CsN3 provided a better interfacial energy level alignment without damaging the underneath organic layer. Consequently, the current efficiencies of 7.45 cd/A was achieved in the CsN3-based green QLEDs consisting of giant CdSe@ZnS/ZnS quantum dots at 544 nm, which was 310% (at 10 mA/cm2) improvement over the LiF-based QLEDs. Moreover, the light turn-on voltage in CsN3-devices significantly decreased $\sim 5.5$ V in comparison with LiF-devices.

Highly efficient and stable electron injection layer for inverted organic light-emitting diodes.

A highly efficient and stable electron injection layer (EIL) for inverted organic light-emitting diodes (OLEDs) is developed. A 1 nm-thick Al is deposited between indium tin oxide cathode and commonly used Cs2CO3 EIL, which can significantly improve the stability. The Al may react with evaporated Cs2CO3 and form a much stabler Al-O-Cs complex, avoiding Cs oxidization by air according to X-ray photoemission spectroscopy measurement. When the Al is evaporated after Cs2CO3 layer, although such a Al-O-Cs complex also forms, the inferior electron injection at Al/4,7-diphenyl-1,10-phenanthroline interface leads to a joule heat-induced resistance that adversely affects the air stability of the device. It is expected that the developed Al/Cs2CO3 EIL promotes high efficiency and stable active-matrix OLEDs based on n-type thin film transistor.

Ethoxylated polyethylenimine as an efficient electron injection layer for conventional and inverted polymer light emitting diodes

We report inverted light emitting devices using ethoxylated polyethylenimine (PEIE) as a single electron injection layer for indium tin oxide cathode, which possess comparable efficiency to those using ZnO/PEIE double electron injection layers. Implementation of a PEIE layer between light emitting polymer layer and aluminum has been shown to significantly enhance device efficiency as well. Improvement of device efficiency can be attributed to increased electron injection due to the reduced work function of PEIE modified cathode as well as the hole blocking effect of PEIE layer. Furthermore, PEIE serves as an efficient electron injector for a range of light emitting polymers with wide distribution of energy levels.

The utilization of low‐temperature evaporable CsN3‐doped NBphen as an alternative and efficient electron‐injection layer in OLED

Cesium azide (CsN3) doped 2,9‐bis(naphthalen‐2‐yl)‐4,7‐diphenyl‐1,10‐phenanthroline (NBphen) has been demonstrated as an efficient electron‐injection layer (EIL) in this work. Advantageous for its low evaporation temperature and air stability, CsN3 is employed as an n‐dopant to replace the reactive alkali metals or alkali metal compounds. Organic light‐emitting diodes utilizing this composite EIL exhibit highly improved current density–luminance–voltage characteristics compared to the control device with LiF. When C545T‐doped Alq3 is used as an emitting layer, a maximum current efficiency of 10.4 cd A−1 can be reached. Our results indicate that this CsN3‐doped NBphen composite layer has great potential as an alternative EIL in organic electronic devices.

Enhanced performances in inverted bottom-emission organic light-emitting diodes with KBH4-doped electron-injection layer

We report that a potassium borohydride (KBH4)-doped 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) layer can be utilized as an efficient electron-injection layer (EIL) for the inverted bottom-emission organic light-emitting diodes (IBE-OLEDs). The KBH4-doped EIL in the IBE-OLEDs having tris(2-phenylpyridine)iridium(III) as a green phosphorescent dopant show a lower turn-on voltage (∼7 V) and about two times higher efficiency (∼13.9% and 44.5 cd A−1 at 0.63 mA cm−2) compared with the device without the KBH4-doped EIL. Furthermore, the KBH4-doped EIL rarely affects the electroluminescence spectrum of the device. The KBH4-doped TmPyPB films with different doping ratios are also analyzed by using electron-only devices (EODs) and X-ray photoemission spectroscopy. As the doping ratio of the KBH4 increases, the current of the EOD and the amounts of potassium increase. This result indicates that the potassium improves the electron-injection ability of an organic electron-transport layer. Therefore, the KBH4-doped EIL can be utilized as an effective EIL in the inverted structure OLEDs.

A hydrophilic monodisperse conjugated starburst macromolecule with multidimensional topology as electron transport/injection layer for organic electronics

A hydrophilic monodisperse conjugated starburst macromolecule and its excellent function as efficient electron injection layer have been demonstrated.

A facile solution-processed alumina film as an efficient electron-injection layer for inverted organic light-emitting diodes

Inverted organic light-emitting diodes (IOLEDs) can effectively improve device stability because they concentrate the air-stable anode and high work function (WF) metal oxide hole-injection layer (HIL) at the top of the devices. In this work, we report a facile solution-processed ultra-thin alumina film used as an electron-injection layer in IOLEDs and present significantly improved device performance. We achieved a high current efficiency of 5.12 cd A−1 at 10 mA cm−2 and the best current efficiency approaching 5.5 cd A−1 at 40 mA cm−2 without doping of an emission layer (EML) for a single Alq3-based green fluorescent IOLED, and a high current efficiency for a green phosphorescent IOLED as well. Furthermore, the extrapolated 50% decay lifetime (t50) shows that our Alq3-based green fluorescent IOLED is about 5 times more stable than the conventional OLED.

Water/alcohol soluble electron injection material containing azacrown ether groups: synthesis, characterization and application to enhancement of electroluminescence

Using an environmentally stable metal as the cathode in a polymer light-emitting diode (PLED) is an essential requirement for its practical application. We present the preparation of a water/alcohol soluble copoly(p-phenylene) (P1) containing pendant azacrown ether and ethylene glycol ether groups as a highly efficient electron injection layer (EIL) for PLEDs, allowing the use of environmentally stable aluminum as the cathode. Multilayer PLEDs [ITO/PEDOT:PSS/PF-Green-B/EIL/Al] using P1 as EIL exhibit significantly enhanced device performance, particularly in the presence of K2CO3 or Cs2CO3. The maximum luminous power efficiency and maximum luminance of the device with Cs2CO3-doped P1 as EIL were enhanced to 9.16 lm W(-1) and 17,050 cd m(-2), respectively, compared with those without EIL (0.16 lm W(-1), 890 cd m(-2)). The turn-on voltage was also significantly reduced from 5.7 V to 3.7 V simultaneously. The performance enhancement has been attributed to improved electron injection which has been confirmed by the rise in open-circuit voltage (Voc) obtained from photovoltaic measurements. The incorporation of such an electron injection layer significantly enhances device performance for PLEDs with an environmentally stable metal as the cathode.

Efficient inverted bottom-emission blue phosphorescent organic light-emitting diodes with a ytterbium-doped electron injection layer

An efficient electron injection layer (EIL) for inverted bottom-emission organic light-emitting diodes (IBE-OLEDs) is developed by doping ytterbium (Yb) into an organic electron transport material of 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB). The Yb-doped EIL in the IBE-OLEDs having iridium(III) bis(4,6-(difluorophenyl)pyridinato-N,C2′) picolinate (FIrpic) as a blue phosphorescent dopant shows a lower turn-on voltage (∼4.5 V) and about a 6.6 times higher efficiency (∼8.6% and 15.3 cd/A at 0.15 mA/cm2) compared with the device without the Yb-doped EIL. Furthermore, the electroluminescence spectrum of the device with the Yb-doped EIL is about the same as that of the device without the Yb-doped EIL. This result indicates that the Yb-doped EIL improves the electron injection from the ITO cathode to the organic electron transport layer. Therefore, the Yb-doped EIL can be utilized as an effective electron injection layer in the OLEDs.

Solution-Processed Inorganic–Organic Hybrid Electron Injection Layer for Polymer Light-Emitting Devices

A lithium quinolate complex (Liq) has high solubility in polar solvents such as alcohols and can be spin-coated onto emitting polymers, resulting in a smooth surface morphology. A polymer light-emitting device fabricated with spin-coated Liq as an electron injection layer (EIL) exhibited a lower turn-on voltage and a higher efficiency than a device with spin-coated Cs₂CO₃ and a device with thermally evaporated Ca. The mixture of ZnO nanoparticles and Liq served as an efficient EIL, resulting in a lower driving voltage even in thick films (∼10 nm), and it did not require a high-temperature annealing process.

Sodium triphosphate as an efficient electron injection layer in organic light-emitting diodes

A multilayer organic light-emitting diode (OLED) with favorable performance was fabricated via using sodium triphosphate (Na3PO4) material as an efficient electron injection layer. Luminance and efficiency of the devices are notably improved due to a thin Na3PO4 layer inserted between electron transport layer (ETL) and aluminum cathode. The optimal thickness of electron injection layer is confirmed to be 1nm, and the electroluminescent efficiency of OLED reaches 3.5cd/A. The role of Na3PO4 was investigated on the enhanced performance of OLED through X-ray photoelectron spectroscopy (XPS). The effect of different positions of Na3PO4 layer inside Alq3 layer on the performance of OLED was systematically studied. By comparing with the devices based on ETL/Na3PO4/Al, ETL/LiF/Al and ETL/NaCl/Al, the electroluminescent efficiency of Na3PO4-based device is the highest. Na3PO4 as electron injection material has a promising potential application in OLED.

P‐112: Highly Efficient Electron Injection Layer of LiF/Yb Bilayer for Top‐emitting Organic Light Emitting Diodes

AbstractWe report highly efficient electron injection layer (EIL) of LiF/Yb bilayer for top‐emitting organic light emitting diodes. The device with the LIF/Yb bilayer shows reduced operating voltage and enhanced efficiencies compared to other two devices with Yb and LiF/Al due to the reduced electron injection barrier, resulting in better charge balance. The device with the LiF/Yb bilayer shows high luminous efficiency of 53.2 cd/A and external quantum efficiency of 16.9%.

Bilayer graphene anode for small molecular organic electroluminescence

We demonstrate that bilayer graphene can be used as the anode of a small molecule organic light-emitting diode (OLED). In our OLEDs, bilayer graphene was used as the anode, Sm/Au as the cathode and Alq3 as the emitter. By applying Cs2CO3-doped 4,7-diphenyl-1,10-phenanthroline to partly substitute Alq3 as the electron injection and transport layer, the electron current injected from Sm/Au was enhanced to match the hole current injected from the bilayer graphene anode and consequently improved the light emission efficiency. The maxima of luminance efficiency and power efficiency reached 1.18 cd A−1 and 0.41 lm W−1, respectively. We think that the efficiency of the bilayer graphene anode OLED can be further optimized by finding a more efficient electron injection and transport layer and/or reducing the hole density of the graphene anode.

An organic p–n junction as an efficient and cathode independent electron injection layer for flexible inverted organic light emitting diodes

We demonstrate an organic p–n junction as an efficient electron injection layer for green inverted bottom-emission organic light emitting diodes (IBOLEDs). The organic p–n junction composed of a p-CuPc/n-Bphen layer showed very efficient charge generation under a reverse bias reaching to 100mA/cm2 at 0.3V, and efficient electron injection from indium tin oxide (ITO) when adopted in IBOLEDs. Moreover, the organic p–n junction resulted in the same current density–voltage–luminance characteristics independent of the work function of the cathode, which is a valuable advantage for flexible displays.

Efficient electron injection layer based on thermo-cleavable materials for inverted bottom-emission polymer light emitting diodes

The electron injection material PF-EP was found to be thermo-cleavable. Its solubility in chlorobenzene can be adjusted by thermal treatment at different temperatures. By analyzing the results of TGA, FT-IR, and 13C solid-state NMR, we interpret that the solubility transition is caused by partial acidification, crosslinking through a hydrogen-bonded network and coordination of O, P and Li. Based on this thermo-cleavable approach, efficient fully solution-processed IBPLEDs were successfully fabricated. The maximum current efficiency of the device with 2.5% Li2CO3 doped PF-EP as the EIL reaches nearly 1.7 times of that of a conventional device. We attribute the high performance to the good electron injection and hole blocking abilities of PF-EP and Li2CO3.