Charge Generation Layer Research Articles

Highly efficient tandem white organic light-emitting diodes based upon C60/NaT4 organic heterojunction as charge generation layer

We present a high-efficiency tandem white organic light-emitting diode based upon a C60/5,5′′′-bis(naphth-2-yl)-2,2′:5′,2′′:5′′,2′′′-quaterthiophene (NaT4) organic heterojunction as a charge generation layer. A maximum current efficiency, maximum power efficiency, and maximum external quantum efficiency of 111.3 cd A−1, 50.5 lm W−1, and 38.7%, respectively, are obtained. More importantly, the efficiency roll-off is greatly improved with only a slight roll-off to109 cd A−1, 40.3 lm W−1, and 38% at a luminance of 1000 cd m−2. Surprisingly, the electroluminescent spectrum is highly stable at all levels of luminance. Furthermore, our tandem device shows better operational stability than a tandem device employing a conventional charge generation layer. It is concluded that the excellent charge generation, transport, and extraction in the C60/NaT4 organic heterojunction-based CGL is responsible for the high performances.

Solution-processed organic light-emitting devices with two polymer light-emitting units connected in series by a charge-generation layer

We present a solution-based process to fabricate stacked OLEDs consisting of two polymer light-emitting units (LEUs), connected in series by a charge generation layer (CGL). We used Cs2CO3-doped ZnO nanoparticles as an EIL on the LE-polymer to improve the electron injection from the cathode. The surface morphology of a spin-coated metal oxide nanoparticle appears to be rough, with many gaps due to agglutination of nanoparticles. We chose poly(4-vinyl pyridine) (PVPy) as a binder to improve the film morphology of the ZnO:Cs2CO3 mixture and facilitate the formation of a uniform and dense film to prevent the solvent from soaking into the 1st LEU. The efficient solution-based processing of EILs in the 1st CGL containing MoO3/poly-TPD bilayers was employed for the construction of an MPE device. The device exhibited a sum current efficiency of 10 cd A−1, with 4 cd A−1 contributed by the 1st unit and 6 cd A−1 by the 2nd unit.

P-Type Doping of Small-Molecule Organic Semiconductors using Organic Vapor Phase Deposition (OVPD)

ABSTRACTIn the past few years, organic vapor phase deposition (OVPD) has been demonstrated to be an effective deposition method for high-performance monochrome and white organic light emitting diodes (OLEDs) [1-4]. OVPD provides good material utilization efficiency and large achievable deposition rates.An application of p-type doping is the improvement of hole injection either from the anode contact or from a charge generation layer in stacked OLEDs [5]. Nevertheless, no reports on p-type doping using OVPD can be found in literature, in part due to the thermal instability and high chemical sensitivity of organic dopants.In this work, p-type doping using an AIXTRON Gen-1 OVPD tool with two different show-erhead designs is examined. NDP-2 (NOVALED) and N,N‘-diphenyl-N,N‘-bis(1-naphthylphenyl)-1,1‘-biphenyl-4,4‘-diamine (NPB) were used as p-type dopant (guest) and hole-conducting host, respectively. p-Type doped hole-only devices were fabricated and compared with undoped ones.Two different showerhead designs (made either of aluminum or stainless steel) were investi-gated with respect to OLED performance to determine possible side reactions.Highly efficient monochrome red OLEDs including a p-type doped hole transport layer were demonstrated exhibiting a current efficiency of 31 cd/A, a power efficiency of 26 lm/W and a driving voltage of 3.7 V without improved light outcoupling (all values at 1000 cd/m2).

Semi-Analytical Model of Charge Image Formation in Electrophotography

High resolution latent image formation is an important step in electrophotography to achieve high quality prints. Presently, analytical models exist for the other important steps: photoconductor charging, development (both single and dual component), and transfer. We present here an analytical model of charge image generation on the surface of an organic photoconductor due to laser exposure. The inputs to the model include laser intensity profile as a function of time, quantum efficiency of the charge generation layer, hole mobility of the charge transport layer, and initial charge density on the surface of the PC. The output of the model is the PC surface charge map as it evolves in time, which is a key input to the toner development process in electrophotography.

Effect of organic bulk heterojunction as charge generation layer on the performance of tandem organic light-emitting diodes

Organic bulk heterojunction comprising a pentacene and C60 blend can be used as the charge generation layer (CGL) in tandem organic light-emitting diodes (OLEDs) for significantly improved power efficiency. Herein, we systematically study the effects of the ratio and thickness of the CGL on the performance of tandem OLEDs. We have found that the efficiency of tandem OLEDs not only is related to the thickness of the CGL but also strongly depends on the ratio of pentacene to C60 in the CGL. Investigations regarding the capacitance-voltage and atomic force microscopy measurements clearly indicate that different thicknesses of the CGL lead to different behaviors of the charge distributions, and different ratios of pentacene to C60 in the CGL result in different probabilities of charge transfer from pentacene to C60 and imbalanced charge transport, thereby leading to completely different device performance.

Tandem white phosphorescent organic light-emitting diodes based on interface-modified C60/pentacene organic heterojunction as charge generation layer

The significant improvement of power efficiency and efficiency roll-off in tandem white organic light-emitting diodes (WOLEDs) was achieved using interface-modified C60/pentacene organic heterojunction as charge generation layer (CGL). The resulting tandem WOLED exhibits a maximum power efficiency of 53.8 lm W−1 and a maximum current efficiency of 101.5 cd A−1 without any out-coupling techniques. More importantly, the efficiency roll-off is greatly improved, yet remaining power efficiency of 53 lm W−1 at 100 cd m−2 and 45 lm W−1 at 1000 cd m−2, which outperforms that of the conventional CGL-based tandem WOLED. This clearly demonstrates that the organic heterojunction is a promising CGL for high-performance tandem WOLEDs.

Organic semiconductor heterojunction charge generation layer and its application in tandem organic light-emitting diodes

Tandem organic light-emitting diodes (OLEDs) have attracted a great deal of attention from researchers because of their high luminance, efficiency and stability. This paper summarizes past advances in the study of tandem OLEDs. Then, we present a bilayer organic semiconductor heterojunction charge generation layer made of p- and n-type organic semiconductors. A low operational voltage, high power efficiency tandem OLED was fabricated using this charge generation layer as a connecting unit. We also analyze the physical mechanisms that guide the operation of the device, and discuss methods for reducing the voltage and enhancing the power efficiency. Our design concept provides a template for the fabrication of high performance tandem OLEDs. Finally, we discuss the future of tandem OLED technologies.

High power efficiency tandem organic light-emitting diodes based on bulk heterojunction organic bipolar charge generation layer

The significant enhancement in power efficiency of tandem organic light-emitting diodes (OLEDs) was achieved using bulk heterojunction organic bipolar charge generation layer (CGL), consisted of zinc phthalocyanine (ZnPc): fullerene (C60) blend. In addition to a significant enhancement in luminance and current efficiency this work yielded a maximum power efficiency of 21 lm W−1 for a tandem OLED, notably almost two times higher than that of a single-unit device, with a maximum power efficiency of 10.1 lm W−1. The enhancement in power efficiency at higher luminance is also over 50%. The remarkable enhancement in power efficiency has been attributed to the effective charge generation, transport and extraction due to the presence of interface-modified ZnPc:C60 CGL in tandem OLEDs. Our results demonstrate that the bulk heterojunction, consisting of two matched n- and p-type organic semiconductors, is a promising bipolar CGL for high power efficiency tandem OLEDs.

Hole Injection Enhancement by a WO3 Interlayer in Inverted Organic Light-Emitting Diodes and Their Interfacial Electronic Structures

The interfacial energy level alignment of hole injection layers with WO3 insertion in an inverted organic light-emitting diode structure and its influence on electroluminescence have been studied using in situ X-ray, ultraviolet photoelectron spectroscopy, and device measurements. The hole injection barrier for N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) upon Al deposition was estimated to be 1.37 eV, which makes an Al anode unfavorable for hole injection. With a thin WO3 layer deposited onto the NPB, the NPB highest occupied molecular orbital (HOMO) level bends dramatically to 0.15 eV below the Fermi level. This NPB HOMO level and WO3 conduction band close to the Fermi level form a charge generation layer without an interfacial chemical reaction. This is why the WO3 interlayer dramatically helps charge injection even with a low work-function Al metal anode. Indeed, the electroluminescence and current efficiency from this structural device were greatly enhanced by 3 orders of magni...

Ultra-high efficiency by multiple emission from stacked organic light-emitting devices

We investigated the charge generation characteristics of intermediate layer, consisting of an organic acceptor material 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN 6) and an organic donor material N, N′-di(naphthalene-1-yl)- N, N′-diphenylbenzidine (NPD), for a stacked organic light-emitting device (OLED). A stack consisting of an ultra-thin 1 nm of LiF and of Al was used as an electron-injection layer (EIL) from HAT-CN 6 to an adjacent electron transporting layer (ETL). The orange-emitting fluorescent stacked OLED with the charge generation layer and the electron injection layer exhibited twice higher current efficiencies and a longer operational lifetime than those of the corresponding unstacked device under a high luminance. We also fabricated a green phosphorescent OLED showing an extremely high current efficiency of 256 cd/A.

A novel charge generation layer for stacked organic light-emitting devices

We report on a novel charge generation layer(CGL) of LiF/Al/V2O5 for a stacked organic light-emitting device(SOLED). The two emitting units of SOLED based on the CGL are independent as if they are individual devices. The CGL has an ability to inject electrons and holes into two adjacent emitting units. The luminance efficiency of the SOLED based the CGL is 1.7 times of the control device at the same current density. Moreover, the novel CGL can be prepared only with thermal evaporation without sputtering TIO or co-evaporation of metal and organic material during device fabrication, thus making the device processing.

Organic heterojunctions as a charge generation layer in tandem organic light-emitting diodes: the effect of interfacial energy level and charge carrier mobility

A homologous series of p-type thiophene organic semiconductors NaTn (naphthyl end-capped oligothiophenes, n = 2–6 represents the number of thiophene units) were employed to form organic heterojunctions with n-type organic semiconductor C60 (C60/NaTn), applied for high-performance tandem organic light-emitting diodes (OLEDs). The effect of organic heterojunctions as a charge generation layer on the performance of tandem OLEDs has been well demonstrated. We found that not only are the highest occupied molecular orbital levels of NaTn close to the lowest unoccupied molecular orbital of C60, but also that a high charge carrier mobility is very important for constructing an effective charge generation layer to achieve high power efficiency in tandem OLEDs. Our results offer a design/selection rule for organic semiconductors used to construct effective organic heterojunction charge generation layers, which will be useful in future high-performance tandem OLEDs.

Enhancement of Electron Injection in Organic Light-Emitting Diodes with Photosensitive Charge Generation Layer

We describe the enhancement of electron injection by external light irradiation in organic light-emitting diodes (OLEDs) with a charge generation layer (CGL). Under He–Ne laser irradiation, photon-induced charge generation under electric field occurred in the CGL. Electrons and holes were generated in the CGL and the electrons were directly injected into adjacent electron transport layers. As a result, the enhancement of current density under light irradiation was observed. A thousand fold enhancement of current density was obtained for light irradiation under forward biased condition. Current density was determined by the intensity of light irradiation and was saturated under the higher voltage regions. Up-converted light emission was also observed under the light irradiation.

高效叠层有机电致发光器件

By employing cesium carbonate doped tris-(8-hydroxyquinoline) aluminum/MoO3 as the effective charge generation layer(CGL),high efficiency two-unit series connected tandem organic light-emitting diodes were fabricated.It is found the performance of the two-unit tandem devices can be controlled by the thickness of the MoO3 layer.The 30 nm-thick MoO3 CGL tandem device exhibits the best device performance with the current efficiency of 14.5 cd/A,which is above 2.0 times than that of the control device over a wide range of current densities.It's suggested that the connecting layer broke down under a high current density,leading to the decrease of current efficiency.The improvement of tandem device performance is attributed to the efficient electron and hole injection from charge generation layer to two adjacent emission units.

Electronic, redox and charge transport properties of an unusual hybrid structure: a bis(septithiophene) bridged by a fused tetrathiafulvalene (TTF)

A hybrid tetrathiafulvalene–oligothiophene compound has been synthesised, in which the fulvalene unit is fused on both sides to an end-capped septithiophene oligomer. The compound (1) has been studied by cyclic voltammetry, UV-vis spectroelectrochemistry and X-ray crystallography. The properties of this material are compared to the half-unit (9), which lacks the TTF core and contains only one septithiophene chain. In the case of the larger molecule, there are multiple and complex redox processes leading to the loss of 6–8 electrons per molecule. Charge generation layer time-of-flight measurements give maximum hole mobilities of ca. 1 × 10−5 cm2 V−1 s−1.

Enhancement of Electron Injection in Organic Light-Emitting Diodes with Photosensitive Charge Generation Layer

Enhancement of Electron Injection in Organic Light-Emitting Diodes with Photosensitive Charge Generation Layer

Role of the Charge Generation Layer in Tandem Organic Light-Emitting Diodes Investigated by Time-Resolved Electroluminescence Spectroscopy

In the tandem organic light-emitting diodes (tandem OLEDs), the charge generation layer (CGL) plays an essential role in the performance, which is composed of an n-doped layer and a p-doped layer f...

Organic Photovoltaic Cells Using Group 10 Metallophthalocyanine Electron Donors

Organic planar heterojunction solar cells using as an electron acceptor and group 10 metal phthalocyanines (nickel, palladium, and platinum) as electron donors have been fabricated and evaluated for their device characteristics. Upon changing the metal center, these devices exhibit interesting trends, such as variations in the open-circuit voltage, short-circuit current density, and power-conversion efficiency. Devices based on palladium phthalocyanine (PdPc) exhibited the best performance, achieving power-conversion efficiencies of 2.4% under AM1.5G ( ) illumination. This is due to the higher ionization potential of PdPc and greater efficiency of charge photogeneration. Nickel phthalocyanine (NiPc) has the highest hole mobility, but cells using NiPc have the weakest charge-generation efficiency. This is due to poor exciton diffusion to the organic heterojunction that can be overcome by using a composite NiPc: charge-generation layer. An optimized NiPc-based cell has a power-conversion efficiency of 1.9%.

Fabrication of Various ZnO Nanostructures and their Photoelectric Properties

Various morphologies of ZnO nanostructures, including nanoparticle (NP), nanopetal and nanorod (NR) array, were fabricated via solution-phase approaches. The morphology and crystallization of resulting Nano-ZnO were investigated by Field-emission scanning electron microscope (FESEM), High resolution-transmission electronic microscope (HR-TEM), and X-ray diffraction (XRD), respectively. A dual-layered photoreceptor containing the nanostructures film as the charge generation layer (CGL) were designed and fabricated. It was found that the photoreceptor based on ZnO NR array as CGL exhibited better photoelectric performance than that from ZnO NP or nanopetal.

Electronic structures of MoO3-based charge generation layer for tandem organic light-emitting diodes

The role of MoO3 in charge generation layers for tandem organic light-emitting diodes is investigated. The electronic structure of a typical MoO3-based charge generation layer, consisting of N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine, MoO3, and Mg doped 4,7-diphenyl-1,10-phenanthroline (NPB/MoO3/Mg:Bphen) is identified to be a p/n/n junction. It is shown that MoO3 can pronouncedly modify the energy level alignment, beneficial to charge separation at the NPB/MoO3 interface and electron injection at the MoO3/Mg:Bphen interface from MoO3 into suitable molecular energy levels of adjacent emission units. Moreover, Mg:Bphen is favorable to block holes flowing from the anode side directly into the adjacent emission unit.