Bilayer Cathode Research Articles

Role of LiF in polymer light-emitting diodes with LiF-modified cathodes

Abstract We report on high-efficiency polymer light-emitting diodes (PLEDs) based on poly [2-methoxy-5-(3′,7′-dimethyloctyloxyl)]-1,4-phenylene vinylene (OC1C10) with LiF-modified cathodes. Devices with different cathodes are made and characterized by the electroabsorption technique to measure their built-in voltage. Devices with a LiF/Al bilayer cathode or a LiF:Al composite cathode, all show significantly improved performance as compared to those with bare Al cathodes. The improvement is correlated with enhanced electron injection due to a decrease of the electron injection barrier, which is also indicated by the electroabsorption measurements. The same effect is also observed with LiF(0.6 nm)/Mg cathodes. However, inserting the same LiF thin film between Ag and OC1C10 does not improve the device performance. Cathodes composed of ultra-thin films of LiF(0.6 nm)/Al(1 nm) or LiF:Al(2 nm) covered by Ag (100 nm) show the same performance as LiF(0.6 nm)/Al bilayer cathode or a LiF:Al composite cathode, indicating that the enhancement is specific to LiF and Al. Our experiments can be explained by assuming that Li-ions can dissociate from LiF and diffuse into the OC1C10 layer, leading to an n-type zone close to the polymer/cathode interface. This n-doped layer at the interface facilitates electron injection at the cathode/polymer interface and eventually leads to the formation of an Ohmic contact.

23.1: Efficient Electron Injection from Bilayer Cathode with High Work Function Metal

AbstractA novel bilayer cathode has been developed to variety of RGB emitting polymers to form an effective electron injector. An effective electron injection from high work function metal such as Al can be realized by incorporation of a thin layer of water‐ or alcohol‐soluble copolymer between metal and emitting layer. The device performance of polymer LEDs fabricated with such bilayer cathodes can be enhanced to levels comparable to or even higher than that obtained by using Ca and Ba cathode.

Highly efficient and substrate independent CsF/Yb/Ag cathodes for organic light-emitting devices

The application of a cathode system comprising of an ultrathin CsF layer and an overlaid ytterbium (Yb) metal in organic electroluminescent devices is reported. This cathode shows dramatically reduced operating voltage and improved current and power efficiencies, both in small molecule or polymer based devices. The improvement is attributed to the dissociation of CsF upon Yb deposition to liberate low-work function Cs which reduces the electron injection barrier at the cathode/organic interface. Thermodynamically, the CsF dissociation is a substrate-independent process, and should be applicable for most organic based devices. Devices with this bilayer cathode system also show good operational stability.

Role of ultrathin Alq3 and LiF layers in conjugated polymer light-emitting diodes

We report that an ultrathin layer of tris-(8-hydroxyquinoline) aluminum (Alq3) interposed between a bilayer cathode of LiF/Al and an emissive layer can significantly improve electroluminescence efficiency in a polymer light-emitting device (PLED). At a current density of 35 mA/cm2, a poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenyl-enevinylene] (MEH-PPV)-based PLED with a cathode of Alq3/LiF/Al has a quantum efficiency of 2.5%, considerably higher than that of a MEH-PPV-based PLED with a bare Al or a LiF/Al cathode. Interfacial characteristics of MEH-PPV/LiF/Al and MEH-PPV/Alq3/LiF/Al were investigated by photoelectron spectroscopy, and the results were used to explain the difference in device performance.

Efficient CsF/Yb/Ag cathodes for organic light-emitting devices

A high-performance cathode consisting of an ultrathin CsF layer and a rare-earth ytterbium (Yb) metal is reported for application in organic electroluminescent devices. Standard tris-(8-hydroxyquinoline) aluminum/α-napthylphenylbiphenyl diamine devices with this bilayer cathode showed dramatically reduced operating voltage and a low turn-on voltage of 2.42 V as compared to 3.75 and 2.95 V in devices using, respectively, the Mg:Ag and single-layer Yb cathodes. At a current density of 200 mA/cm2, devices with the CsF/Yb cathode exhibited high luminance efficiency of 3.45 cd/A and power efficiency of 1.27 lm/W. Analysis by x-ray photoemission spectroscopy suggested that the performance improvement is related to the substantial reduction of electron injection barrier at the cathode/organic interface. It was found that upon Yb deposition, CsF dissociates to liberate low work function Cs metal atoms resulting in a cathode with a lower electron injection barrier and thus a better balance of carriers in the device. Thermodynamically, this exothermic chemical reaction is expected to take place independent of the type of substrates and organic medium, indicating that this bilayer cathode system may be applicable in a wide range of organic electronic/optoelectronic devices.

Chemical structure of Al/LiF/Alq interfaces in organic light-emitting diodes

Al/LiF cathode/organic is known to form an excellent interface for electron injection into the organic active layer, resulting in excellent performing organic light-emitting diodes (OLEDs). Here, the chemical structure of the interface between the Al/LiF bilayer cathode and tris (8-hydroxyquinoline) aluminum (Alq) of working OLED devices was investigated by using x-ray photoelectron spectroscopy (XPS). Using a in situ peel-off technique, we are able to characterize the buried interface structure without disturbing the chemical states of each element probed. The data show that there are two types of F at the interface; one is attributed to LiF and the other to F attached to the Alq. This F-doped Alq layer could induce a downshift in molecular orbital levels and thus leads to a reduced electron injection barrier. XPS depth profile results show significant O diffusion through Al layer to the interface, and the diffusion of O ends abruptly at the Al/LiF interface.

Electrochemical properties of reduced-temperature SOFCs with mixed ionic–electronic conductors in electrodes and/or interlayers

We have investigated the electrochemical properties of two types of reduced-temperature solid oxide fuel cells (SOFCs) in which the mixed ionic–electronic conductors are used to improve their performances. Electrolyte-supported cells, in which doped LaGaO 3 strengthened by Al 2O 3 dispersion is used as the electrolyte, were prepared and tested. Samaria-doped ceria (SDC) interlayers of 0.3-μm thickness were fired onto both surfaces of the electrolyte of 0.2-mm thickness at 1523 K, before firing the Ni–Sm 0.1Ce 0.9O 1.95–(CeO 2) 0.1[(Y 2O 3) 0.08(ZrO 2) 0.92] 0.9 (Ni–SDC–CeYSZ) (10 mol% ceria-doped yttria-stabilized zirconia [YSZ]) cermet anode at 1723 K and La 0.7Sr 0.3Co 0.2Fe 0.8O 3–Sm 0.2Ce 0.8O 1.9 (LSCF–SDC) composite cathode at 1373 K. The cells have a nominal size of 60×60 mm 2 with an effective electrode area of 4 cm 2. The single cell thus prepared showed a high power density of 0.67 W cm −2 at 1073 K and long-term stability during the operation time of 1000 h. Anode-supported cells with a thin YSZ electrolyte film approximately 30 μm thick were also prepared by co-sintering of screen-printed YSZ paste on a compacted anode substrate. The cells have a nominal size of 50×50 mm 2 with an effective electrode area of 4 cm 2. The single cell with the LSCF–SDC composite cathode having SDC interlayer showed the maximum power density of 0.648 W cm −2 at 1023 K. The bilayer cathode also showed high resistance against degradation by Cr-poisoning.

Contact formation of LiF/Al cathodes in Alq-based organic light-emitting diodes

A systematic study has been carried out on the formation of an effective electron-injecting contact by depositing an LiF/Al bilayer on tris-(8-hydroxyquinoline) aluminium (Alq) in organic light-emitting diodes. Efficient electron injection is observed in both a LiF/Al bilayer cathode and an LiF-doped Al composite cathode. An analysis with ultraviolet photoelectron spectroscopy reveals a strong similarity in interface chemistry between LiF/Al and LiF-doped Al on Alq. Measurements with high-resolution electron energy loss spectroscopy show limited interfacial reaction of LiF on both Al and Alq, whereas a strong attenuation of the loss peak related to the Li-F stretch mode is observed after depositing an ultrathin Al film on Alq/LiF. The results indicate that the contact is formed as a consequence of chemical reaction with a reacted layer of 1 nm or less. Molecular orbital calculation suggests that the release of Li and subsequent reaction with Alq is thermodynamically allowed. The shallow-contact nature allows for much greater flexibility in the design of cathode structures and potential applications to various device configurations.

Electron injection and electroluminescence investigation of organic light-emitting devices based on a Sn/Al cathode

Organic light-emitting devices (OLEDs) with the Sn/Al cathode have been fabricated. The structure of the devices is ITO/CuPc(12 nm)/NPB(60 nm)/Alq 3(60 nm)/Sn(2.8 nm)/Al(120 nm). It is found that the presence of a thin tin (Sn) layer at the organic–aluminum (Al) interface can enhance both electron injection efficiency and electroluminescence when compared to OLEDs with only Al cathode. An Ag/Al bilayer cathode structure is also tested, and the similar results can be obtained. The improvement is believed to be due to two factors: (1) the enhancement of the electron density of the Sn or Ag layer; (2) the Sn or Ag layer in part prevents the Al atoms from diffusing into the organic layer. Al/Sn–Alq 3–Sn/Al and Al–Alq 3–Al devices have also been fabricated. It is shown that the nature of the interface is not equivalent for metal-on-Alq 3 and Alq 3-on-metal.

Efficient polymer light emitting diodes with metal fluoride/Al cathodes

We reported efficient polymer light emitting diodes with alkali and alkaline earth metal fluoride Al cathodes. The quantum efficiencies of these devices are more than two orders of magnitude higher than those in devices without fluoride layer, comparable to the best data reported with similar electroluminescent polymers. The mechanism of the metal fluoride/Al bilayer cathodes was also discussed based on photovoltaic, ac impedance, and photoluminescence efficiency measurement.

Alkali metal acetates as effective electron injection layers for organic electroluminescent devices

The effect of alkali metal (Li +, Na +, K +, Rb +, and Cs +) acetates and fluorides at an aluminum/tris(8-hydroxyquinoline) aluminum (Alq 3) interface on performance of organic electroluminescent (EL) devices is described. We also studied the effect of acetates with different metals, such as Mg 2+ and Al 3+, on the performance of the EL devices. EL characteristics of the devices with Al/alkali metal acetates were compared with those of devices with Al/alkali metal fluorides or an Al cathode itself. We found that the bilayer cathodes with alkali metal acetates exhibited better device performance than the bilayer cathodes with alkali metal fluorides. The improvement can be attributed to lowering of the work function of the Al cathode and doping of Alq 3 with alkali metals that were formed by the higher reactivity of alkali metal acetates than alkali metal fluorides with hot Al atoms impinging on these salts during the vapor deposition. The enhanced electron injection resulted in more balanced charge-carrier injection and thus higher EL efficiencies.

Efficient electron injection in blue-emitting polymer light-emitting diodes with LiF/Ca/Al cathodes

We report electroabsorption and electroluminescence investigations of polymer light-emitting diodes featuring a LiF/Ca/Al cathode, for efficient electron injection into the electroluminescent polymer layer. Our measurement of the built-in potential gives direct evidence of a sizeable reduction of the cathodic barrier height not only with respect to Ca, but also versus LiF/Al or CsF/Al bilayer cathodes, currently amongst the most efficient electron injectors for low electron affinity polymers. In blue-emitting (∼2.7 at peak) polyfluorene-based LEDs, with poly(ethylenedioxythiophene)/poly(styrene sulphonic acid) anodes and LiF/Ca/Al cathodes, we measure a built-in potential of 2.7 V, a luminance of ∼1600 cd/m2 (the highest among the devices studied here) at a driving voltage of 5 V, and efficiencies as high as ∼3 lm/W. We also find that the turn-on voltage essentially coincides with the built-in potential within the experimental error.

Synthesis of Pyrene-Bearing Oligothiophenes as Novel Light Emitting Materials and Their Application to Single-Organic-Layer Electroluminescence Devices

Abstract Ter- and quaterthiophenes bearing pyrenes at the terminal α or β positions have been synthesized as novel emitting materials in organic electroluminescence devices. The efficient single-layer EL devices of these compounds are fabricated by use of an Al/ultrathin LiF bilayer cathode. One of them exhibits a maximum luminance of 1860 cd/m2 and a luminous efficiency of 0.15 lm/W, which are of the highest performance class among single-organic-layer EL devices.

Energy level line-up in polymer light-emitting diodes via electroabsorption spectroscopy

The influence of carrier-injecting layers on the built-in potential of polymer light-emitting diodes (PLEDs) is analysed by means of electroabsorption spectroscopy. Two promising materials on which research has recently focused are: first, poly(ethylene-dioxythiophene)/poly(styrene sulphonic acid) (PEDOT:PSS), a doped conducting polymer employed as hole injector; and secondly, lithium fluoride (LiF), which has been shown to enhance electron injection of Al-based cathodes. In the first samples LEDs with an indium tin oxide (ITO) anode were compared to those incorporating a PEDOT:PSS layer (in an ITO/PEDOT:PSS bilayer anode). In the second set of samples the comparison was between LEDs with an aluminium cathode and those incorporating a LiF layer (in an Al/LiF bilayer cathode) as a function of LiF thickness. Electroabsorption spectroscopy results supported by work function measurements clearly prove that both PEDOT:PSS and LiF introduce a large reduction in the barrier height at the anode and cathode interfaces, respectively. The ensuing enhancement in carrier injection is considered to be responsible for the marked improvement in device performance (lower operating voltages, higher luminous efficiencies and longer lifetimes).