Doped Hole Transport Layer Research Articles

Doped hole transport layers processed from solution: Planarization and bridging the voids in noncontinuous silver nanowire electrodes

We report about solution-processing of a doped small molecule hole transport layer (s-HTL) comprising of N,N′-((diphenyl-N,N′-bis)9,9,-dimethyl-fluoren-2-yl)-benzidine (BF-DPB) as matrix and the p-dopant “NDP9” in the non-halogenated solvent tetrahydrofuran (THF). We show that the doping process is already happening in solution and stays effective after coating. Conductivities achieved with this process are comparable to those reached by thermal co-evaporation under high vacuum, which is the usual deposition method for this material. Applied as planarization layer onto AgNW films with best performance values of 15Ω/□ and 83.5% total transmission including the substrate, the s-HTL is proving its ability to sufficiently smoothen the initially rough AgNW topography. We analyze the necessary lateral conductivity to bridge micrometer-large voids in the mesh, as they are inherent in nanowire network electrodes. In combination with zinc phtalocyanine:C60 organic solar cells, a s-HTL conductivity less than 1 × 10−4 S/cm can lead to decreased device performance with a loss greater than 10% for nanowires with 90 nm diameter and the associated mesh width. Furthermore, we demonstrate more efficient vacuum-deposited p-i-n solar cells with an oligothiophene (DCV5T-Me) as donor, C60 as acceptor. They exhibit power conversion efficiencies up to 4.4% on AgNW bottom electrodes with s-HTL, compared to 4.1% on ITO with s-HTL as reference device.

Improvement of Performance of Organic Light-Emitting Diodes with Both a MoO3 Hole Injection Layer and a MoO3 Doped Hole Transport Layer**Supported by the National Natural Science Foundation of China under Grant No 11274402, the National Basic Research Program of China under Grant No 2012CB933704, the Natural Science Foundation of Guangdong Province under Grant No S2012020011003, and the Program for Changjiang

We improve the performance of organic light-emitting diodes (OLEDs) with both a MoO3 hole injection layer (HIL) and a MoO3 doped hole transport layer (HTL), and present a systematical and comparative investigation on these devices. Compared with OLEDs with only MoO3 HIL or MoO3 doped HTL, OLEDs with both MoO3 HIL and MoO3 doped HTL show superior performance in driving voltage, power efficiency, and stability. Based on the typical NPB/Alq3 heterojunction structure, OLEDs with both MoO3 HIL and MoO3 doped HTL show a driving voltage of 5.4V and a power efficiency of 1.41 lm/W for 1000 cd/m2, and a lifetime of around 0.88 h with an initial luminance of 5268 cd/m2 under a constant current of 190 mA/cm2 operation in air without encapsulation. While OLEDs with only MoO3 HIL or MoO3 doped HTL show higher driving voltages of 6.4V or 5.8V and lower power efficiencies of 1.20 lm/W or 1.34 lm/W for 1000 cd/m2, and a shorter lifetime of 0.33 or 0.60 h with an initial luminance of around 5122 or 5300 cd/m2 under a constant current of 200 or 216 mA/cm2 operation. Our results demonstrate clearly that using both MoO3 HIL and MoO3 doped HTL is a simple and effective approach to simultaneously improve both the hole injection and transport efficiency, resulting from the lowered energy barrier at the anode interface and the increased hole carrier density in MoO3 doped HTL.

Controlled formation of charge depletion zones by molecular doping in organic pin-diodes and its description by the Mott-Schottky relation

Molecular doping of organic semiconductors is a key technology for highly efficient organic light-emitting diodes. Nevertheless, the underlying fundamental mechanisms are under discussion. This is because of the complex situation of structural disorder and strong polaronic coupling in such systems. We provide for the first time a systematic study of the formation of charge depletion zones in organic pin-diodes comprising molecular doped hole and electron transport layers. Impedance spectroscopy is employed to study the capacitance of these depletion zones. In particular, we show that the voltage dependent capacitance function obeys the Mott-Schottky relation concerning the influence of doping and the effect of an additional depletion zone given by the intrinsic interlayer. From temperature dependent measurements of the depletion capacitance, we can deduce the amount of active dopant states, their activation energy, and the spatial field distribution within the junction. The measured activation energy of the dopant states and the overall value of active dopant molecules suggest a highly efficient doping process, where dopant molecules act as shallow acceptor states.

Organic pin-diodes approaching ultra-high-frequencies

Organic pin-diodes employing molecular doped hole and electron transport layers working at ultra-high-frequencies (UHF) are presented. In comparison to undoped organic Schottky diodes reported for ultra-high-frequency applications, the pin-concept is superior since the doping concentration and the intrinsic interlayer thickness can be adjusted in order to control the performance of such pin-diodes. We investigate the influence of both parameters on basic diode parameters like forward resistance and rectification ratio. In particular, we discuss the role of molecular doping and its influence on the AC signal response and we present an optimized doping ratio in order to accomplish highest charge carrier mobility and conductivity. Optimized devices are working as rectifiers up to 300MHz at a small operating voltage of 2V. The frequency limit is estimated to be at 1GHz.

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).

Electric Field Induced Electron Transfer between a Single-Walled Carbon Nanotube and a Molecularly Doped Hole Transport Layer

In this work, we report a novel, electric field induced hole injection reaction between thin films of single-walled carbon nanotubes (CNT) and a molecularly doped charge (hole) transport layer (CTL) comprising a hole transport molecule (TPD) doped in polycarbonate in the bilayer device configuration. The CNT bilayer device was shown to be able to charge and then discharge in the dark, whereas the controlled bilayer device, constructed by replacing the CNT film with a metal (TiZr) layer, remains charged up after charging. The discharge of the charged CNT bilayer device suggests that hole injection from the CNT film to the CTL (or electron transfer from the CTL to the CNT film) occurs and that the injected holes are neutralized by a series of isoenergetic electron transfer across the CTL. Results show that the rate of the initial discharge increases as the surface conductivity of the CNT film increases. Current–voltage and time-of-flight measurements suggest that the discharge process is limited by the mobility of the CTL when the surface resistivity of the CNT film is ≤250 ohm/sq. For bilayer devices formulated with more resistive CNT films, the discharge process is limited by the hole injection efficiency. Since conductive CNT films are an essentially dense network of carbon nanotubes on Mylar, the decrease in the hole injection efficiency for bilayer devices fabricated from more resistive CNT films can be attributed to the increase in porosity in the CNT network in these films. The increased porosity limits hole injection between the carbon nanotube and the CTL. Since micrometer size pixels of CNT film can be made on a flexible substrate by inkjet printing and a nano imprinting technique, we suggest to couple this electric field induced hole injection reaction with an active-matrix backplane for a novel digital printing application.

Polymer light‐emitting diodes with doped hole‐transport layers

AbstractWe demonstrate a solution processed bi‐layer PLED based on poly(p‐phenylene vinylene) derivatives using orthogonal solvents. To lower the voltage drop the hole transport layer (HTL) based on poly[2,5‐bis(2′‐ethylhexyloxy)‐co‐2,5‐bis(butoxy)‐1,4‐phenylenevinylene] (BEH/BB‐PPV (1:3)) is doped with tetracyano‐tetrafluoro‐quinodimethane (F4TCNQ). The conductivity of BEH/BB‐PPV (1:3) was observed to increase by two orders of magnitude upon doping with F4TCNQ. The doped HTL was observed to lower the operating voltage of a double layer PLED, but suffers from additional quenching by the dopant at higher voltages due to the lack of an electron blocking functionality.

Efficiency Enhancement in Organic Light-Emitting Devices With a Magnetic Doped Hole-Transport Layer

Magnetic field effects on tris-(8-hydroxyquinoline) aluminum-based organic light-emitting devices (OLEDs) by employing Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> as a magnetic dopant in the hole-transport layer (HTL) have been studied. The magnetic doped OLEDs exhibit efficient injection and transport of holes, and its performances are further enhanced after a magnetic field is applied. The enhancement of luminance and current efficiency of 20% and 24% has been obtained from the magnetic doped devices, while they are only 8% and 9%, respectively, for the nondoped devices under an applied magnetic field of 500 mT. Organic magnetoresistance induced by the magnetic doped HTL is the main origin of increased electroluminescence for the magnetic doped OLEDs.

Efficiency Variations in Organic Light-Emitting Devices Utilizing the 4,4′,4″-tris(N-(2-naphthyl)-Nphenylamino) triphenylamine Hole Transport Layer Doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane

The electrical and the optical properties of organic light emitting devices (OLEDs) with a 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4-TCNQ) doped 4,4′,4″-tris(N-(2-naphthyl)-Nphenylamino)triphenylamine (2-TNATA) hole transport layer (HTL) were investigated. In comparison with that of the OLEDs containing an undoped 2-TNATA layer, the hole transport efficiency of OLEDs with a F4-TCNQ doped 2-TNATA HTL was significantly enhanced due to an increase in the doping concentration and resulted in enhancement of the luminance. The electroluminescence spectra for the OLEDs with a doped HTL showed a dominant emission peak corresponding to the Alq3 layer. This peak was not affected by the existence of the F4-TCNQ doped HTL.

Air stable and low temperature evaporable Li 3N as a n type dopant in organic light-emitting diodes

N doped organic light-emitting diodes were developed by using Li 3N as a n type dopant in electron transport layer. Driving voltage was greatly lowered by using Li 3N doped electron transport layer and combination of MoO 3 doped hole transport layer with Li 3N doped electron transport layer gave high quantum efficiency of 15% and low driving voltage of 4 V at 1000 cd/m 2 in green phosphorescent organic light-emitting diodes. Decomposition of Li 3N during evaporation into Li and N 2 was found to be responsible for n doping effect of Li 3N.

Efficient hole injection by doping of hexaazatriphenylene hexacarbonitrile in hole transport layer

Hexaazatriphenylene-hexacarbonitrile (HAT) was used as a dopant in hole transport layer and hole injection properties of the HAT doped hole transport layer were studied. Hole injection was greatly improved by doping of the HAT in the hole transport material and a driving voltage was lowered by more than 3 V. Lifetime of green devices was also improved by more than three times. In addition, a high quantum efficiency of 16.8% and a power efficiency of 47 lm/W were obtained at 1000 cd/m 2 in green phosphorescent devices.

Dicyanovinyl-quinquethiophenes with varying alkyl chain lengths: Investigation of their performance in organic devices

We compare between two derivatives of dicyanovinyl-quinquethiophenes with different alkyl side chain lengths. Both materials show comparable open circuit voltages Voc in organic solar cells with fullerene C60 as acceptor, as expected since they have the same highest occupied molecular orbital energy. However, differences in the current-voltage-characteristics, particularly in the fill factor, are observed. We analyze both derivatives in hole-only devices and find a difference in the hole injection between the doped hole transport layer and the oligothiophenes. Additionally, we determine the hole mobility of the two materials and explain the different behaviors of the two materials in solar cells.

Low-Driving-Voltage, Long-Lifetime Organic Light-Emitting Diodes with Molybdenum-Oxide (MoO3)-Doped Hole Transport Layers

This work was supported by \Samsung SDI - Seoul National University Display Innovation Program" and \New Growth Engine Display Center", Ministry of Com- merce, Industry and Energy (MOCIE), Korea.

Energy level alignment of electrically doped hole transport layers with transparent and conductive indium tin oxide and polymer anodes

Using ultraviolet photoemission spectroscopy, we investigated the energy level alignment at the interfaces of typical anodes used in organic electronics, indium tin oxide (ITO) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), with the oligomeric hole transport material N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD), and studied the influence of electrical interface doping by the strong electron acceptor tetrafluoro tetracyanoquinodimethane (F4-TCNQ). The fundamentally different anode materials with work functions of 4.40eV (ITO) and 4.85eV (PEDOT:PSS) show different hole injection barriers, which also depend on the thickness of the F4-TCNQ interface dopant layer. PEDOT:PSS anodes exhibit a consistently lower hole injection barrier to MeO-TPD compared to ITO by 0.1eV. We attribute this low hole injection barrier to additional charge transfer reactions at the PEDOT:PSS/MeO-TPD interface. In contrast, the deposition of the electron acceptor at the interface helps significantly to lower the hole injection barrier for ITO anodes.

The exciplex emission of organic light emitting diodes with 5, 6, 11, 12-tetraphenylnaphthacene (Rubrene)-doped hole transport layer

Exciplex electroluminescence of indium-tin-oxide glass (ITO)/[4',4-tris[3-methylphenyl (phenyl) amino]triphenylarnine](m-MTDATA)/[Naphthyl-substituted benzidine derivative] (NPB)/NPB: rubrene/BCP/LiF/Al devices have been investigated. The ITO/MTDATA/NPB/NPB:Rubrene/BCP/LiF/Al devices show emission color adiustable by the hybrid exciplex interaction of Rubrene with NPB, and this exciplex becomes the primary recombination center at a critical Rubrene doping concentration. We obtained its emission was green-white light and the green emission is exciplex emission of the binary-blend NPB:rubrene layer. It is demonstrated at the PL spectra.

P-186: Enhanced Efficiency and Stability of Organic Light-Emitting Diodes with Copper Hexadecafluorophthalocyanine Doped Hole Transport Layer

We report the enhancement of the electroluminescence (EL) efficiency and the device stability of organic light-emitting diodes (OLEDs) by inserting a copper hexadecafluorophthalocyanine (F16CuPC)-doped N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine (α-NPD) layer between an indium-tin oxide and α-NPD. Compared with the reference device consisting of the bilayer structure of α-NPD and tris(8-hydroxyquinoline) aluminum (Alq3), the devices having F16CuPC-doped α-NPD show not only an increased EL efficiency (∼4.5 cd/A compared with ∼3.8 cd/A for the reference device) and lower driving voltage but also a longer lifetime and smaller operating voltage variation over time. It is suggested that the improved device performance can be attributed to the higher hole-injection efficiency and thermal stability of the F16CuPC-doped α-NPD layer.

Enhancement of the efficiency and the color stabilization of organic light-emitting devices fabricated utilizing stepwise doped hole transport layers

The electrical and the optical properties of organic light-emitting devices (OLEDs) consisting of aluminum (Al)/lithium quinolate/tris (8-hydroxyquimoline) Al/5,6,11,12-tetraphenylnaphthacene (rubrene)-doped N, N′-bis-(1-naphthyl)- N, N′-diphenyl-1,1-biphenyl-4,4′-diamine (NPB)/indium-tin-oxide/glass structures fabricated with uniformly doped and stepwise-doped hole transport layers (HTLs) were investigated. The turn-on voltage of the OLEDs fabricated utilizing a stepwise-doped HTL was smaller than that of the OLEDs fabricated with a uniformly doped HTL, and the corresponding luminance at the same voltage was higher. The Commission Internationale de l'Eclairage (CIE) chromaticity coordinates of the OLEDs fabricated utilizing a stepwise-doped HTL became stabilized, and the CIE chromaticity coordinates of the OLEDs at 12 V was (0.43, 0.53), indicative of a yellow emission corresponding to the rubrene layer. The luminescence mechanisms of the OLEDs fabricated utilizing a stepwise-doped HTL are described on the basis of the experimental results.

Opposing influence of hole blocking layer and a doped transport layer on the performance of heterostructure OLEDs

Abstract This paper reports on heterostructure small molecule organic light emitting devices (OLEDs), the design of which includes doped hole and electron transport layer (HTL and ETL) and a hole blocking layer (HBL) which can be either doped or not. Doped transport layers are expected to lower the operating voltage of devices. Insertion of a hole blocking layer increases the carrier and exciton confinement which consequently improves the recombination rate and the device efficiency. Nevertheless, an HBL tends to increase the threshold voltage. The opposing influence of doped transport layers and HBL is evidenced in this study and compromise structures are presented. The doped HTL material is N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4-TCNQ). A comparison with undoped TPD and poly(N-vinylcarbazole) (PVK) HTL is given. Devices with doped HTL show a lowering of the operating voltage from 6.5 (PVK) down to 4 V (these voltages refer to those necessary to achieve a luminance L=10 Cd/m2). A constant current efficiency higher than 2 Cd/A is obtained in the voltage range 5–9 V with doped HTL. Insertion of a 5–20 nm thick HBL made of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (Bathocuproine BCP) between an 8-(hydroquinoline) aluminum (Alq3) electron transport layer (ETL) and a DCM doped Alq3 emitting layer (EML) induces both a detrimental effect (increase of the operating voltage to 6 V attributed to a low electron mobility in BCP) and beneficial effect (strong increase of the luminance and doubling of the current efficiency which reach to about 4.5 Cd/A thanks to improved carrier and exciton confinement). An optimization of the thickness of the doped EML and of BCP is also reported. The doping of the HBL and of the ETL with 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) leads to devices with luminance as high as 1000 Cd/m2 at 5.3 V and a maximum efficiency of 1 Cd/A at 4 V.

Dramatic improving luminous efficiency of organic light emitting diodes under low driving current using nitrogen doped hole transporter

In this letter, output luminance, current efficiency and power efficiency of the organic light emitting diodes (OLEDs) with N 2 doped hole transport layer (HTL) have been studied in detail. Experimental results show that the current efficiency and the power efficiency thus in turn the output luminance of OLEDs prepared with HTL evaporated in the optimum N 2 gas ambient pressure of 1×10 −4 Torr are improved about 13, 9 and 12 times, respectively, under 2.7 mA/cm 2 driving current. The significant improving mechanism has been illustrated comprehensively with a series of schematic models.

Study of organic light emitting devices with a 5,6,11,12-tetraphenylnaphthacene (rubrene)-doped hole transport layer

We investigated the stability of an organic light emitting device (OLED) with structure of indium–tin–oxide (ITO) anode/N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine hole transport layer (HTL)/tris(8-hydroxyquinoline) aluminum (AlQ3) electron transport layer/Mg:Ag cathode, in which different portions of the HTL were doped with 5,6,11,12-tetraphenylnaphthacene (rubrene). Compared to undoped devices, the stability of OLEDs in which HTL doping was limited to only a thin interfacial layer at either the ITO or AlQ3 interface was essentially the same, whereas the stability of OLEDs in which a substantial portion of the HTL was doped was about an order of magnitude higher, and approached that of devices in which the whole HTL was doped. The color of the emission depended only on the material in immediate contact with AlQ3. The results demonstrate that increasing the OLED’s stability by means of doping the HTL is associated with changes in bulk HTL properties rather than interfacial properties, and is consistent with the OLED degradation mechanism based on the instability of AlQ3 cationic species.