TPD Layer Research Articles

Solution-processed double-layered hole transport layers for highly-efficient cadmium-free quantum-dot light-emitting diodes.

The search for heavy-metal-free quantum-dot light-emitting diodes (QD-LEDs) has greatly intensified in the past few years because device performance still falls behind that of CdSe-based QD-LEDs. Apart from the effects of nanostructures of the emitting materials, the unbalanced charge injection and transport severely affects the performance of heavy-metal-free QD-LEDs. In this work, we presented solution-processed double hole transport layers (HTLs) for improving the device performance of heavy-metal-free Cu-In-Zn-S(CIZS)/ZnS-based QD-LEDs, in which N,N'-Bis(3-methylphenyl)-N,N'-bis(phenyl)benzidine (TPD) as an interlayer was incorporated between the emitting layer and the HTL. Through optimizing the thickness of poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenyl-amine (TFB) and TPD layers, a maximum external quantum efficiency (ηEQE) of 3.87% and a current efficiency of 9.20 cd A-1 were achieved in the solution-processed QD-LEDs with double-layered TFB/TPD as the HTLs, which were higher than those of the devices with pristine TFB, TPD and TFB:TPD blended layers. The performance enhancement could be attributed to the synergistic effects of the reduction of the hole injection barrier, the increase of the hole mobility and suppressed charge transfer between the HTL and the emitting layer. Furthermore, the best ηEQE of 5.61% with a mean ηEQE of 4.44 ± 0.73% was realized in the Cu-In-Zn-S-based QD-LEDs by varying the annealing temperature of TPD layer due to the more balanced charge injection and transport as well as smooth surface of TPD layer.

상부 발광 유기 발광 소자에서 두께와 시야각에 따른 마이크로 캐비티 특성

Top-emission device has a merit of high aperture ratio and narrow emission spectrum compared to that of bottom-emission one. Emission spectra of top-emission organic light-emitting diodes depending on a layer thickness and view angle were analyzed using a theory of microcavity. Device structure was manufactured to be Al (100 nm)/TPD//LiF (0.5 nm)/Al (2 nm)/Ag (30 nm). N,N`-diphenyl-N,N`- di (m-tolyl)-benzidine (TPD) and tris (8-hydroxyquinoline) aluminium (Alq3) were used as a hole-transport layer and emission layer, respectively. And a thickness of TPD and Alq3 layer was varied in a range of 40 nm~70 nm and 60 nm~110 nm, respectively. Angle-dependent emission spectrum out of the device was measured with a device fixed on a rotating plate. Since the top-emission device has a property of microcavity, it was observed that the emission spectrum shift to a longer wavelength region as the organic layer thickness increases, and to a shorter wavelength region as the view angle increases. Layer thickness and view-angle dependent emission spectra of the device were analyzed in terms of microcavity theory. A reflectivity of semitransparent cathode and optical path length were deduced.

TCNQ 분자를 이용한 유기 발광 소자의 전기적 특성

Electrical properties of organic light-emitting diodes were studied in a device with 7,7,8,8-tetracyano-quinodimethane (TCNQ) to see how the TCNQ affects on the device performance. Since the TCNQ has a high electron affinity, it is used for a charge-transport and injection layer. We have made a reference device in a structure of ITO(170 nm)/TPD(40 nm)/<TEX>$Alq_3$</TEX>(60 nm)/LiF(0.5 nm)/Al(100 nm). And two types of devices were manufactured. One type of device is the one made by doping 5 and 10 vol% of TCNQ to N,N'-diphenyl-N,N'-di(m-tolyl)-benzidine (TPD) layer. And the other type is the one made with TCNQ layer inserted in between the ITO anode and TPD organic layer. Organic layers were formed by thermal evaporation at a pressure of <TEX>$10^{-6}$</TEX> torr. It was found that for the TCNQ doped devices, turn-on voltage of the device was reduced by about 20 % and the current efficiency was improved by about three times near 6 V. And for devices with TCNQ layer inserted in between the ITO anode and TPD layer, it was found that the current efficiency was improved by more than three times even though there was not much change in turn-on voltage.

Microcavity Effect in Compliance with Interference of Light in Alq3 Based Top-Emission Organic Light-Emitting Diodes

Since top-emission organic light-emitting diodes have a structure of microcavity, there is a change in emission spectrum compared to that of noncavity such as a full width at half maximum and peak wavelength of the spectrum. Optical properties of top-emission organic light-emitting diodes were studied depending on an organic layer thickness and view angle. A structure of the manufactured device was Al(100 nm)/TPD/Alq3/LiF(0.5 nm)/Al(2 nm)/Ag(30 nm). An Al(100 nm) and LiF(0.5 nm)/Al(2 nm)/Ag(30 nm) were used as an anode and semitransparent cathode, respectively. A thickness of hole injection layer of N,N′-diphenyl-N,N′-bis(3-metylphenyl)-1,1-biphenyl-4,4′-diamine (TPD) was varied from 40 nm to 70 nm, and that of an emission layer of tris(8-hydroxyquinoline) aluminum(III) (Alq3) was varied from 60 nm to 110 nm. At each device, a thickness ratio of TPD and Alq3 layer was kept to be about 2:3. It was observed that the emission spectrum of top-emission organic light-emitting diodes shows a microcavity effect when the organic layer thickness and view angle were varied. This microcavity effect of the device was analyzed in terms of theoretical interpretation based on an interference of light. Through these analyses, we were able to understand a shift of peak wavelength in spectrum, and deduce a refractive index and a thickness of organic layer. These deduced values are in agreement with known values.

Improved Performance of Top-Emission Organic Light Emitting Device With a Mixed Single Layer

Top-emission organic light emitting devices (TEOLED) with a mixed single layer by mixing of electron transport materials, hole transport material and dope material were investigated. With an optimized mixing ratio of PyPySPyPy:Alq3:α-NPD:Rubrene [25:50:25:1], maximum power efficiency, 3.45 lm/W, and maximum luminance, 7,000 cd/m2 was obtained. Compared to the conventional heterostructure device, the performance was improved with a factor of more than two. The dependence of organic layer thickness on device performance was also investigated and the result showed that 100 nm was an appropriate thickness for good performance of mixed single layer TEOLED. In addition, the maximum luminance and power efficiency were obtained at 20,000 cd/m2 and 7.5 lm/W, respectively, when TPD layer of 45 nm was capped onto top metal electrode.

High response deep ultraviolet organic photodetector with spectrum peak focused on 280 nm

A high response organic deep ultraviolet (DUV) photodetector (PD) with 280 nm as the response spectrum peak was demonstrated. A maximum photoresponse of 309 mA/W under 280 nm UV illumination with an intensity of 0.428 mW/cm2 and a detectivity of 1×1012 cmHz1/2/W at −8 V were achieved, respectively. The optimized PD diode has a structure of ITO/m-MTDATA (10 nm)/m-MTDATA:Bphen(1:1, 60 nm)/Bphen (10 nm)/Cs2CO3: Bphen (30 wt %,10 nm)/Al(12 nm)/TPD(40 nm). Under 280 nm constant and shuttered illumination conditions with an intensity of 0.18 mW/cm2 the operational time is longer than 440 min when its response respectively decreases to 50% and 83% of its original value. The realization of the DUV detection is attributed to the stronger absorption of shorter UV wavelengths of Bphen acceptor and covering UV waveband longer than 300 nm by the TPD layer. The more detailed mechanism of harvesting the high PD performance is also discussed.

High efficiency rubrene based inverted top-emission organic light emitting devices with a mixed single layer

Inverted top-emission organic light emitting devices (TEOLEDs) with a mixed single layer by mixing of electron transport materials (PyPySPyPy and Alq 3), hole transport material (α-NPD) and dope material (rubrene) were investigated. Maximum power efficiency of 3.5 lm/W and maximum luminance of 7000 cd/m 2 were obtained by optimizing the mixing ratio of PyPySPyPy:Alq 3:α-NPD:rubrene=25:50:25:1. Luminance and power efficiency of mixed single layer device were two times improved compared to bi-layer heterojunction device and tri-layer heterojunction device. Lifetime test also shows that the mixed single layer device exhibits longer operational lifetimes of 343 h, which is three times longer than the 109 h for tri-layer device, and two times longer than the 158 h for bi-layer device. In addition, the maximum luminance and power efficiency were obtained at 20,000 cd/m 2 and 7.5 lm/W, respectively, when a TPD layer of 45 nm was capped onto the top metal electrode.

Influence of co‐deposited active layers on carrier transport and luminescent properties in organic light emitting diodes

AbstractWe have investigated the influence of a co‐deposited active layer in organic light‐emitting diodes (OLEDs) on carrier transport and optical properties to improve radiative characteristics of OLEDs. The co‐deposited layer consists of two organic materials; one is a hole transport material (TPD) and the other is an electron transport/emissive material (Alq3). We evaluated current‐voltage characteristics and electroluminescence (EL) properties of various samples in which the thicknesses and compound ratios of the co‐deposited layers are different. The results indicate that the devices consisting of TPD:Alq3 co‐deposited layer sandwiched between TPD and Alq3 layers exhibit lower starting voltages for the light emission than the sample of simple TPD/Alq3 heterojunction structure. In addition, the starting voltage is independent of the thickness of TPD:Alq3 co‐deposited layer. These samples have two interfaces at both surfaces of TPD:Alq3 co‐deposited layer. Thus, we estimated the radiative recombination occurs at the interfaces. Nevertheless, we found that the radiative recombination occurs only at the interface of TPD:Alq3 co‐deposited layer and Alq3 layer. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Polycrystallization phenomena of organic alloy and multilayer structures on naphthyl‐substituted diamine‐based hole transport layer for organic light‐emitting diode

AbstractWe demonstrated the polycrystallization process of N,N'‐di(1‐naphthyl)‐N,N'‐diphenyl‐1,1'‐diphenyl‐1,4'‐diamine (NPD) and N,N'‐diphenyl‐N,N'‐bis(3‐methylphenyl)‐1,1'‐diphenyl‐4,4'‐diamine (TPD), which is well known as a hole transport material, and the availability of the organic alloy method fabricated by co‐evaporation method. We fabricated multi‐layer structure of NPD and TPD: especially bilayer as a basic and typical structure. One is TPD layer on NPD layer (T/N specimen) and the other is NPD layer on TPD layer (N/T specimen). The polycrystallization of bilayer structure specimens is suppressed as compared with organic NPD:TPD=1:1 alloy specimen. Especially the T/N specimen is found to be stable in an amorphous region. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Exciplex or electroplex emissions from the interface between aromatic diamine and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline?

Organic light-emitting diodes (OLEDs) have been fabricated which consist of N, N′-diphenyl- N, N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine) (TPD), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), and tris(8-hydroxyquinoline) aluminum (Alq 3). Four emission peaks located at about 401 nm, 425 nm, 452 nm and 480 nm have been obtained in the electroluminescence (EL) spectra of these devices. The former two emissions originate from the exciton emission of TPD molecular. The last two emissions could be attributed to local (LOC) exiplex emission and charge transfer (CT) exiplex emission at the interface between TPD and BCP layers, respectively.

Influence of heterojunction interface on exciplex emission from organic light-emitting diodes under electric fields

In this paper, electroluminescence from organic light-emitting diodes based on 2-(4′-biphenyl)-5-(4′′-tert-butylphenyl)-1,3,4-oxadiazole (PBD) and N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) is reported. Based on the exciplex emission from the TPD/PBD interface under high electric fields, the influence of the TPD/PBD interface on exciplex emission was investigated by increasing the number of TPD/PBD interfaces while keeping both the total thickness of the TPD layer and the PBD layer constant in the multiple quantum-wells (MQW) device ITO/TPD/[PBD/TPD]n/PBD/Al (n is the well number that was varied from 0 to 3). Our experimental data shows that exciplex emission can be enhanced by suitably increasing the well number of this kind of MQW-like device.

The electroluminescent investigation of double layer Eu-complex organic electronic luminescence diodes

Double layer organic electronic luminescence diodes (OLEDs) based on europium(dibenzoylmethanato)3monophenanthroline [Eu(DBM)3bath], ITO/N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD)/Eu(DBM)3bath/LiF/Al have been fabricated. With increasing the thickness of hole transporting layer, the maximum EL efficiency was increased, and the EL efficiency of 10cd/A was achieved when the thickness of TPD layer was 80nm; however, at high current density, the EL efficiency of all devices was decreased drastically. Besides, the evolution of EL emission spectra with increasing operating voltage was found, the mechanisms of the symmetry around the ion improved and the annihilation of excited state of Eu(DBM)3bath were discussed in explaining this phenomenon.

Voltage-Induced Infrared Spectra from the Organic Field-Effect Transistor Based on N,N′-bis(3-methylphenyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (TPD)

The field-effect transistor fabricated with N,N′-bis(3-methylphenyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (TPD) on a silicon substrate has shown a p-channel organic field-effect transistor (OFET) operating in the accumulation mode. The hole mobility in the TPD layer has been obtained to be 7.2 × 10−4 cm2 V−1s−1. Gate-voltage-induced infrared absorption from the OFET has been measured in the transmission-absorption configuration by the FT-IR difference-spectrum method. The observed spectra depend on the gate voltage applied. The vibrational Stark effect and the bands originating from the carriers injected into the TPD layer have been observed.

The effect of doping iodine on organic light-emitting diode

The performances of organic light-emitting diodes (OLEDs) with the configuration Al/Alq3 (Aluminum Tris-(8-hydroxygninoline))/TPD(N,N′-diphenyl-N,N′bis-(3-methylphenyl)-1,l′-bipheny-4,4′-diamine)/ITO have been significantly improved by doping iodine (I2) on both Alq3 and TPD layers. The luminance is promoted from 2800 cd/m2 without doping to 8000 cd/m2 with I2 doping under bias 10 V. Additionally, the driving voltage (@100 cd/m2) was reduced from 7.5 V without doping to 5.2 V with I2 doping. We attribute the promotions to the reduction of the electron and hole injection energy barrier at Al/Alq3 and TPD/ITO interfaces and the expansion of trap energy states beneath the LUMO of Alq3 generated by I2 doping. The mechanism is illustrated comprehensively with a schematic energy diagram model and nicely supported with photoluminescence (PL), electroluminescence (EL) spectra and other experimental results.

Organic light emitting diodes with nanostructured ultrathin layers at the interface between electron- and hole-transport layers

Organic light emitting diodes (OLEDs) with nanostructured ultrathin layers inserted at the interface between electron- and hole-transport layers were investigated. The fundamental structure of the OLEDs fabricated by a vacuum evaporation method was indium-tin-oxide (ITO) anode/copper phthalocyanine (CuPc)/N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-diphenyl-4,4′-diamine (TPD)/8-hydroxyquinoline aluminum (Alq3)/LiF/Al cathode. Fullerene (C60) and rhodamine B (RhB) molecules were used as the nanosutructured ultrathin layers inserted at the interface between the Alq3 and TPD layers. The electroluminescent (EL) properties have been measured for the OLEDs with C60 and RhB ultrathin layers and the dependences on the thickness and the position of the inserted layers were examined. For the OLEDs with the C60 ultrathin layer, the improvements of the drive voltage and EL efficiency were observed. The OLED with the inserted C60 ultrathin film of a monolayer thickness showed the highest efficiency, which was twice as large as that without C60 layer. On the contrary, the improvements were not observed for the OLEDs with the RhB ultrathin layer.

Preparation and Characterization of Organic Electroluminescent Devices Using Fluorescent Polyimides as a Light-Emitting Layer

The electroluminescence (EL) properties of the organic electroluminescent devices (OELDs) using three kinds of fluorescent polyimides, ODPA/DCHM (HFPI-1), 10FEDA/DCHM (HFPI2), and P2FDA/DCHM (HFPI-3), for light emitting layers were investigated. The HOMO energy levels of HFPI-1 and HFPI-2 were determined as 6.7 and 6.9 eV, respectively, using cyclic voltammetry. Single-layer EL devices having the ITO/HFPI/Al structure were fabricated using HFPI-1 and HFPI-2, but these devices did not exhibit any current and EL properties. This is due to the quite large energy gaps between ITO (work function: 4.7 eV) and the HOMO levels of polyimides (>2.0 eV). Further, multilayer devices were fabricated using carrier transport materials. For these devices, the carrier injection properties from both electrodes were expected to be improved. However, no current and EL properties were observed for all devices. These experimental facts indicate that fluorescent polyimides have little carrier transportability. The EL properties of the devices using TPD-dispersed fluorescent poly-imides as hole transport layers were also investigated. In case of using HFPI-3 whose excitation spectrum is overlapped with the emission of Alq3, the device emits a distinct EL at 520 nm under the operation at 15 V. Further, the emission was displaced to 560 nm under the operation at 30 V, which can be explained by the energy transfer to HFPI-3 or the displacement of the recombination zone of electrons and holes from Alq3 to HFPI-3+TPD layer. This device mechanism might be promising as a novel wavelength-tunable OELD.

Hole-transporting properties of organosilanylene–diethynylpyrene and diethynylanthracene alternating polymers. Applications to patterning of light-emitting images

Diethylsilanylene- and tetramethyldisilanylene-1,6-diethynylpyrene alternating polymers ( 1 and 2) were prepared by the reactions of 1,6-di(lithioethynyl)pyrene and the respective dichloroorganosilanes. The hole-transporting properties of the resulting polymers were evaluated by the performance of electroluminescent (EL) devices with the structure of indium–tin–oxide/polymer/Alq/Mg–Ag, in comparison with those of an organosilanylene-9,10-diethynylanthracene alternating polymer ( 3), reported previously. Among them, the device with polymer 1 exhibited the best performance with a maximum luminescence of 6000 cd m −2, which increased to 16 000 cd m −2 when electron-blocking TPD layer was introduced between the polymer and Alq layers. Polymers 1– 3 were found to be photoactive and irradiation of the polymer films with a UV lamp led to a drastic drop of the luminance of the device, being applicable to the patterning of EL images.

Organic Light-Emitting Diodes with a Nanostructured Fullerene Layer at the Interface between Alq3 and TPD Layers

Organic light-emitting diodes (OLEDs) with a nanostructured fullerene (C60) layer at and near the interface between the electron- and hole-transport layers were fabricated using a vacuum evaporation method. The OLEDs had a fundamental structure of indium-tin-oxide (ITO) anode/copper phtalocyanine (CuPc)/N,N'-dephenyl-N,N'-bis(3-methylphenyl)-1,1'-diphenyl-4,4'-diamine (TPD)/8-hydroxyquinoline aluminum (Alq3)/LiF/Al cathode. The electroluminescent (EL) properties have been investigated for the OLEDs with a nanostructured C60 layer at the TPD/Alq3 interface and at the different positions in the Alq3 layer near the TPD/Alq3 interface. The dependences of the EL properties on the thickness of the inserted C60 layer and on the position from the interface were examined. The OLEDs with a C60 monolayer inserted at the TPD/Alq3 interface showed the highest efficiency.

Synthesis of novel bithiophene derivatives with an organosilanylene bridge, and their applications to electron-transporting materials in EL devices

Compounds having two bithiophene units linked by an organosilicon bridge were prepared. Their UV spectra and cyclic voltammograms indicated that the compounds have the low-lying LUMO. The electron transporting properties of the present silicon-bridged bithiophene derivatives were evaluated by the performance of electroluminescent (EL) devices having vapor deposited silicon-bridged bithiophenes, Alq, and TPD layers, as the electron-transport, emitter and hole-transport, respectively. By applying the bias voltage on the devices, the devices emitted green EL originated from the Alq emission with the maximum luminance of 8500 cd m −2.

Metallophthalocyanine films as hole-transport layer in organic light-emitting devices

Electroluminescence (EL) characteristics of the double-MPc-layered devices consisting of ITO/MPc1/MPc2/Alq 3/Mg–Ag were investigated, where MPc1 and MPc2 represent two metallophthalocyanines and Alq 3 stands for tris(8-quinolinolato)aluminum(III). A compatible double-MPc-layered structure led to a decreased hole-injection barrier at the ITO/MPc1 interface and an increased electron-blocking ability of the MPc2 layer in the device. The EL efficiency of the device was improved further by using a 4,4′-bis( m-tolyphenylamino) biphenyl (TPD) layer as an additional hole-transport layer due to its good electron-blocking ability. For the double-MPc-layered devices with a TPD layer, the EL performances were dependent primarily on the electronic structure of ITO/MPc1 interface rather than that of MPc1/MPc2 or MPc2/TPD interface.