Effective Electron Injection Layer Research Articles

Fabrication of efficient polymer light-emitting diodes using water/alcohol soluble poly(vinyl alcohol) doped with alkali metal salts as electron-injection layer

An effective electron-injection layer (EIL) is crucial to efficient polymer light-emitting diodes (PLEDs) with high work-function metal as cathode. This work presents the use of water/alcohol soluble poly(vinyl alcohol) (PVA), especially doped with alkali metal salts, as a highly effective EIL to fabricate efficient multilayer PLEDs, allowing the use of stable aluminum as the cathode. Using neat PVA as EIL, the maximum brightness and maximum current efficiency of the device [ITO/PEDOT:PSS/SY/PVA/Al(90 nm)] were significantly enhanced to 5518 cd/m2 and 2.64 cd/A (from 395 cd/m2 and 0.06 cd/A without the EIL) due to promoted electron-injection and hole-blocking. The device performance is further enhanced by doping the PVA with alkali metal salts (M2CO3 or CH3COOM; M: Na, K, Cs), and the enhancement is increased with increasing dopant concentration. Particularly, the PVA doped with 30 wt% alkali metal carbonates revealed the best performance (20214–25163 cd/m2, 5.83–6.83 cd/A). This has been attributed to improved electron-injection from aluminum cathode, which has been confirmed by the corresponding increase in the open-circuit voltages (V oc) obtained from photovoltaic measurements. Current results indicate that commercially available PVA are promising electron-injection layer for PLEDs when doped with appropriate alkali metal salts.

Hydroxyethyl cellulose filled with M2+ chelate complexes with ethylenediaminetetraacetic acid (EDTA) as an effective electron-injection layer for polymer light-emitting diodes

An effective electron-injection layer (EIL) is crucial to the development of highly efficient polymer light-emitting diodes (PLEDs) using stable, high work-function aluminium as the cathode. This work presents the first investigation using hydroxyethyl cellulose (HEC), filled with chelate complexes [(CH3COO)2-M, EDTA-M; M: Ca2+, Mg2+], as an electron-injection layer (EIL) to fabricate multilayer polymer light-emitting diodes (ITO/PEDO:PSS/HY-PPV/EIL/Al) by spin-coating processes. Devices based on HEC doped with EDTA-M provided the best performance. The maximum luminance and maximum current efficiency of polymer light-emitting diodes with EDTA-Ca in an HEC layer were 7502cd/m2 and 2.85cd/A, respectively, whereas those with EDTA-Mg were 8443cd/m2 and 3.12cd/A, which was approximately seven- to eight-fold of that without EIL. This performance enhancement was attributed to electron donation from the chelator that reduces metal cations to a “pseudo-metallic state”, enabling it to act as an intermediate step to facilitate electron injection. The results demonstrate that chelates of bivalent cations with EDTA can potentially serve as electron-injection materials for optoelectronic applications.

Semi-transparent polymer light emitting diodes with multiwall carbon nanotubes as cathodes

Semi-transparent polymer light emitting diodes (PLEDs) with double side emission using multiwall carbon nanotubes (MWCNT) sheets as top transparent cathode are described. The combination of an effective electron injection layer (EIL) and a hole blocking layer (HBL): PFN/Cs2CO3, allowed an enhancement of the electron injection from the MWCNT sheets. Those semitransparent devices exhibited a brightness and an efficiency of 220 Cd/m2 and 0.54 Cd/A, respectively, which were measured on the glass side. Our results indicate that the new architecture of laminated top-MWCNT could be useful for the development of transparent displays using a vacuum free simple dry fabrication process.

A fluorene-based material containing triple azacrown ether groups: synthesis, characterization and application in chemosensors and electroluminescent devices

We design a novel multifunctional fluorene-based material containing triple azacrown ether (FTC) not only for application in aqueous solution as a chemosensor towards Fe(3+) but also to enhance the electroluminescence of PLEDs using an environmentally stable aluminum cathode. The photo-physical and sensing properties were investigated by absorption and photoluminescence (PL) spectroscopy. The FTC exhibited specific selectivity and high sensitivity toward Fe(3+), with the Stern-Volmer coefficients (Ksv) being 1.59 × 10(5) M(-1) in a solvent mixture of tetrahydrofuran and water (THF-H2O = 9/1, v/v). The FTC maintained high selectivity toward Fe(3+) in the presence of ten interfering metal cations. The HOMO and LUMO levels were estimated to be -5.88 eV and -2.88 eV, respectively. The FTC significantly enhances the emission performance of PLEDs [ITO/PEDOT:PSS/MEH-PPV/EIL/Al] when used as an electron injection layer (EIL), especially in the presence of metal carbonates. Particularly, the device using K2CO3 doped FTC as the electron-injection layer (EIL) exhibited significantly enhanced performance compared to the one without EIL. The performance was significantly enhanced to 11 630 cd m(-2) and 1.47 cd A(-1), respectively, from 230 cd m(-2) and 0.03 cd A(-1) of the non-FTC device. Current results indicate that multifunctional fluorene-based material FTC is a potential candidate for selective detection of Fe(3+) and as an effective electron injection layer to enhance the performance of MEH-PPV.

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.

P‐166: An Effective Injection Layer for Inverted Top Emitting Organic Light Emitting Diodes

AbstractOne major challenge in inverted top‐emitting OLEDs is the requirement of good electron injection from the bottom reflective aluminum cathode to the organic electron‐transport layer (such as Alq3). We report the development of an effective electron injection layer (EIL) in inverted top emitting OLEDs that not only improves the ability of electron injection but also its efficiency. The device with this EIL achieved a current efficiency of 6.0 cd/A which is 50% higher than that of the device without this EIL.

Improvement of electron injection in inverted bottom-emission blue phosphorescent organic light emitting diodes using zinc oxide nanoparticles

We fabricated highly efficient iridium(III) bis[(4,6-di-fluorophenyl)-pyridinato-N,C2′] picolinate doped inverted bottom-emission blue phosphorescent organic light-emitting diodes, with an electron injection layer of zinc oxide (ZnO) nanoparticles (NPs). The ZnO NPs layer lowers the turn-on voltage by about 4 V and significantly enhances the efficiency. The device with ZnO NPs shows peak efficiencies of 16.5 cd/A and 8.2%, about three times higher than those of the device without ZnO NPs. Since the ZnO NPs layer has a wide band gap, good electron transporting properties and low work function, it can be utilized as an effective electron injection layer with good transparency.

Highly efficient inverted top-emitting organic light-emitting diodes using a lead monoxide electron injection layer

By introducing an effective electron injection layer (EIL) material, i.e., lead monoxide (PbO), combined with the optical design in device structure, a high efficiency inverted top-emitting organic light-emitting diode (ITOLED) with saturated and quite stable colors for different viewing angles is demonstrated. The green ITOLED based on 10-(2-benzothiazolyl)-1, 1, 7, 7-tetramethyl-2, 3, 6, 7-tetrahydro-1H, 5H, 11H-[1] benzopyrano [6, 7, 8-ij] quinolizin-11-one exhibits a maximum current efficiency of 33.8 cd/A and a maximum power efficiency of 16.6 lm/W, accompanied by a nearly Lambertian distribution as well as hardly detectable color variation in the 140 degrees forward viewing cone. A detailed analysis on the role mechanism of PbO in electron injection demonstrates that the insertion of the PbO EIL significantly reduces operational voltage, thus greatly improving the device efficiency. More importantly, the optically optimized device structure by setting the resonant wavelength at the peak wavelength of the intrinsic emission of the emitter and adding an effective outcoupling layer further enhances the device efficiency, at the same time, also reduces the color shift with viewing angles, leading to the simultaneous optimization in efficiency and angular emission characteristics in the fabricated ITOLEDs.

Efficient organic light-emitting diodes with zinc acetate as an effective electron injection layer

We report on the fabrication of organic light-emitting diodes (OLEDs) using a zinc acetate ((CH 3COO) 2Zn) layer as the cathode buffer layer. The results show that the device containing a (CH 3COO) 2Zn interlayer shows improved luminance and efficiency due to the Zn–N bond formation resulting in the occurrence of Alq 3 anion and also due to the band bending at the Alq 3/Al interface, which is beneficial to electron injection by lowering electron injection barrier. And the devices with structured cathodes (CH 3COO) 2Zn/LiF/Al and LiF/(CH 3COO) 2Zn/Al have a higher luminance and efficiency than the LiF/Al cathode-based device.

Cesium hydroxide doped tris-(8-hydroxyquinoline) aluminum as an effective electron injection layer in inverted bottom-emission organic light emitting diodes

We demonstrate highly efficient inverted bottom-emission organic light-emitting diodes (IBOLEDs) by using cesium hydroxide (CsOH) doped tris-(8-hydroxyquinoline) aluminum (Alq3) as the electron injection layer on indium tin oxide cathode, which could significantly enhance the electron injection, resulting in a large increase in luminance and efficiency. The maximum luminance, current efficiency, and power efficiency reach 21000cd∕cm2, 6.5cd∕A, and 3.5lm∕W, respectively, which are 40%–50% higher in efficiency than that of IBOLEDs with cesium carbonate (Cs2CO3) doped Alq3 as the electron injection layer, where the efficiencies are only 4.5cd∕A and 2.2lm∕W. Our results indicate that CsOH doped Alq3 should be an effective electron injection layer on a wide range of electrodes to fabricate high performance OLEDs.