Improvement of Power Efficiency in Organic Electroluminescent Devices

2001 ◽  
Vol 708 ◽  
Author(s):  
Chimed Ganzorig ◽  
Masamichi Fujihira
Keyword(s):  
Work Function ◽  
Indium Tin Oxide ◽  
Power Efficiency ◽  
Dipole Moments ◽  
Electron Injection ◽  
Charge Recombination ◽  
Fine Tuning ◽  
Hole Injection ◽  
Organic Electroluminescent ◽  
Benzoyl Chlorides

ABSTRACTIn order to improve power efficiency of organic electroluminescent (EL) devices, i.e., ITO/TPD/Alq3/Al, enhanced electron and hole injection at Alq3/Al and ITO/TPD interface, respectively, was attempted by designing proper charge injection at both interfaces. Enhanced charge recombination at TPD/Alq3 was also demonstrated. Here, ITO, TPD, Alq3, and Al are abbreviations for indium-tin-oxide, N,N'-diphenyl-N,N'-bis(3-methyl-phenyl)-1,1'-biphenyl-4,4'-diamine, tris(8-hydroxyquinoline) aluminum, and metal aluminum, respectively. Enhanced electron injection by introducing a thin layer of Li salts of fluoride, acetate, and benzoate was described. We have found that the electron injection was improved in the order of Li+, Na+, K+, Rb+, and Cs+, and Cs salts exhibited the best EL performance. Chemical modification of ITO has been attempted to fine-tuning the work function of ITO in order to reduce hole injection barrier height. EL characteristics were improved dramatically using ITO modified with H-, Cl-, and CF3-terminated benzoyl chlorides. By using reactive -COCl groups, ITO surfaces were covered quickly and the work function of ITO was changed widely depending upon permanent dipole moments introduced in para-position of benzoyl chlorides. Correlation between the change in the work function of ITO and the EL characteristics was examined. The improvement of charge recombination was attained by increasing the interfacial areas, i.e. introducing a mixed layer of TPD and Alq3, or inserting a thin film of rubrene with a higher recombination efficiency.

Chemical modification of indium-tin-oxide electrodes by surface molecular design

2001 ◽  
Vol 708 ◽  
Author(s):  
Chimed Ganzorig ◽  
Masamichi Fujihira
Keyword(s):  
Chemical Modification ◽  
Work Function ◽  
Tin Oxide ◽  
Indium Tin Oxide ◽  
Molecular Design ◽  
Dipole Moments ◽  
Transparent Electrode ◽  
Terminal Group ◽  
Hole Injection ◽  
Oxide Electrodes

ABSTRACTIndium-tin-oxide (ITO) is the most widely used material as a transparent electrode due to its excellent transparency and high conductivity. The devices based on bare ITO, however, exhibited inefficient hole injection due to insufficient high work function and required high drive voltages. Thus, various surface treatments of ITO have been attempted to change the work function of ITO in order to reduce the hole injection barrier height. Electroluminescent (EL) characteristics of devices were improved dramatically using ITO chemically modified with H-, Cl-, and CF3-terminated benzoyl chlorides. By the use of reactive -COCl groups, ITO surfaces were modified quickly and the work function of the modified ITO was changed widely depending upon the permanent dipole moments introduced in p-position of benzoyl chloride. We also compared the performance of the EL devices with ITO modified with different binding groups (-SO2Cl, -COCl, and -PO2Cl2) of p-chlorobenzene derivatives. Finally, we examined the correlation between the change in the work function and the performance of the EL devices by the chemical modification and found that the enormous increase in ITO work function up to 0.9 eV is possible using phenylphosphoryl dichloride with a CF3-terminal group in p-position.

Modulation of indium–tin oxide work function by a versatile self-assembled monolayer measured with the scanning Kelvin nanoprobe

2011 ◽  
Vol 89 (12) ◽  
pp. 1512-1518 ◽  
Author(s):  
Christophe Blaszykowski ◽  
Larissa-Emilia Cheran ◽  
Michael Thompson
Keyword(s):  
Work Function ◽  
Tin Oxide ◽  
Indium Tin Oxide ◽  
Light Emitting Diode ◽  
Critical Role ◽  
Protective Layer ◽  
Organic Coating ◽  
Hole Injection ◽  
Self Assembled Monolayer ◽  
Self Assembling

In molecular optoelectronics, high-quality contacts at electrode|organics interfaces are crucial for charge carriers to efficiently flow through and therefore play a critical role on device performance. Electrode surface morphology, adhesibility, wettability, and work function are thus many parameters that must be accurately controlled, which is achievable using self-assembling monolayer (SAM) surface chemistry. Herein, we employ this technique to alter the electronic and surface energy-related properties of indium–tin oxide (ITO). In comparison to unmodified ITO, the newly introduced SAM-derivatized surface exhibits limited wettability and considerably higher work function (ΔΦ = ~1.2 eV). Several applications are proposed for this organic coating, notably at the anode of organic light-emitting diode (OLED) devices for decreasing the hole injection barrier or as an atmospherically stable protective layer in the coatings industry.

Study on Work Function of Modified Indium Tin Oxide and Performance of Organic Electroluminescent Devices.

2012 ◽  
Vol 1402 ◽  
Author(s):  
Battulga Munkhbat ◽  
Bolormaa Gendensuren ◽  
Ganbaatar Tumenulzii ◽  
Rentsenmyadag Dashzeveg ◽  
Sarangerel Davaasambuu ◽  
...  
Keyword(s):  
Work Function ◽  
Tin Oxide ◽  
Indium Tin Oxide ◽  
Self Assembly ◽  
Organic Layer ◽  
Self Assembled Monolayer ◽  
Indium Tin Oxide Electrode ◽  
Function Change ◽  
Work Function Change ◽  
Organic Electroluminescent

ABSTRACTThe work function of the indium tin oxide electrode (ITO) modified by a self-assembled monolayer (SAM), which is used as an electrode in organic electroluminescent (EL) devices, was investigated in this study. It is revealed that chemical modification of ITO with p-substituted with different terminal groups (NH2−, Cl−, and CF3−) benzoic acids as a SAMs material with carboxyl binding group is caused to increase the work function of the ITO electrode. Through a self-assembly process, the transmittance of the ITO with a SAM was not changed. The work function change of ITO with various SAM was measured by using cyclic voltammetry. Characteristics of EL devices were increased because the energy barrier was decreased in an interface between the ITO and an organic layer in the EL devices. The correlation of the work function change and the performance of the chemically modified EL devices was estimated.

Energy Level Alignment at Bebq2/PEI/ITO Interfaces Studied by UV Photoemission Spectroscopy

MRS Advances ◽  
2017 ◽  
Vol 2 (42) ◽  
pp. 2261-2266
Author(s):  
Kohei Shimizu ◽  
Hirohiko Fukagawa ◽  
Katsuyuki Morii ◽  
Hiroumi Kinjo ◽  
Tomoya Sato ◽  
...  
Keyword(s):  
Energy Level ◽  
Work Function ◽  
Indium Tin Oxide ◽  
Electron Injection ◽  
Photoemission Spectroscopy ◽  
Level Shift ◽  
Kelvin Probe ◽  
Vacuum Level ◽  
Energy Level Alignment ◽  
Improvement Effect

ABSTRACTA polyethyleneimine (PEI) interlayer has been applied on indium tin oxide (ITO) to improve electron injection in organic devices including inverted organic light-emitting diodes (OLEDs). To understand the improvement effect by PEI insertion, the energy level alignment at bis(10-hydroxybenzo[h]quinolinato)beryllium (Bebq2)/PEI/ITO interfaces was investigated by UV photoemission spectroscopy (UPS). The deposition of a PEI layer was found to reduce the absolute work function of ITO by 1.4 eV. The vacuum level shifts at Bebq2/ITO and Bebq2/PEI interfaces were also determined as 0.3 eV and 0.1 eV in the direction to reduce the electron injection barrier, respectively. Thus the work function reduction by PEI and downward vacuum level shift at the Bebq2/PEI interface can contribute to the improvement effect. Kelvin probe measurement revealed the weak orientation polarization in Bebq2 film with the bottom side positively polarized. This polarization polarity is also advantageous for electron injection in inverted devices.

Controlled Injection of Holes into ALQ3 Based Oleds by Means of An Oxidized Transport Layer

2001 ◽  
Vol 708 ◽  
Author(s):  
Mathew K. Mathai ◽  
Keith A. Higginson ◽  
Bing R. Hsieh ◽  
Fotios Papadimitrakopoulos
Keyword(s):  
Indium Tin Oxide ◽  
Oxidative Degradation ◽  
Transport Layer ◽  
Hole Injection ◽  
Carrier Injection ◽  
Salt Loading ◽  
Light Emitting ◽  
Hole Transporting ◽  
Intrinsic Degradation ◽  
Variable Conductivity

ABSTRACTIn this paper we report a method for tuning the extent of hole injection into the active light emitting tris- (8-hydroxyquinoline) aluminum (Alq3) layer in organic light emitting diodes (OLEDs). This is made possible by modifying the indium tin oxide (ITO) anode with an oxidized transport layer (OTL) comprising a hole transporting polycarbonate of N,N'-bis(3-hydroxymethyl)-N,N'-bis(phenyl) benzidine and diethylene glycol (PC-TPB-DEG) doped with varying concentrations of antimonium hexafluoride salt of N,N,N',N'-tetra-p-tolyl-4,4'-biphenyldiamine (TMTPD+ SbF6-). The conductivity of the OTL can be changed over three orders of magnitude depending on salt loading. The analysis of hole and electron current variations in these devices indicates that optimizing the conductivity of the OTL enables the modulation of hole injection into the Alq3 layer. The bipolar charge transport properties for OLEDs in which the interfacial carrier injection barriers have been minimized, are governed by the conductivities of the respective layers and in this case it is shown that the variable conductivity of the OTL does allow for better control of the same. Accordingly, varying the concentration of holes in the device indicates that beyond an optimum concentration of holes, further hole injection results in the formation of light quenching cationic species and the initiation of oxidative degradation processes in the Alq3 layer, thus accelerating the intrinsic degradation of these devices. The variable conductivity of the OTL can hence be used to minimize the occurrence of these processes.

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