The role of copper-phthalocyanine (CuPc) has intermediate layer between the anode and the hole-transport layer in multilayer organic light-emitting devices (OLEDs) was studied. The OLEDs consisted of CuPc, N,N'-di(naphtalene-1-yl)-N,N'-diphenyl-benzidine (NPB) as hole-transport layer and tris-(8-hydroxyquinolinato)-aluminum (Alq3) as electron-transport and emitting layer sandwiched between a high-work-function metal and a semi-transparent calcium cathode. A combinatorial approach that allows the simultaneous fabrication of 10 x 10 individual devices was used to vary the thicknesses of CuPc and NPB over a broad range from 0 to 45 nm and from 10 to 100 nm, respectively. Systematic current-voltage and impedance measurements revealed a redistribution of the internal electric field of the CuPc/NPB/Alq3 three-layer structure compared to that of the NPB/Alq3 bilayer OLED. It was demonstrated that the hole transport is mainly controlled by the internal energy barrier at the CuPc/NPB interface. The fact that CuPc strongly impedes hole injection into NPB also has a significant impact on the frequency-dependent behavior of the capacitance, especially the cutoff frequency.
In order to get a detailed understanding of organic light-emitting devices (OLEDs), optimize their performance and provide reliable data for device modeling, we have developed an ultra-high vacuum (UHV) evaporation system for combinatorial studies. Our system allows the simultaneous fabrication of 10 x 10 individual devices on one substrate enabling a systematic variation of material combinations and electrodes as well as device parameters such as layer thickness, layer sequence, dye dopant concentrations. Here, we present an overview of the capabilities of combinatorial methods for electrical and electro-optical device optimization. We show results on multilayer OLEDs ranging from the conventional copper-phthalocyanine (CuPc)/N,N'-di(naphtalene-1-yl)-N,N'-diphenyl-benzidine (NPB)/ and tris-(8-hydroxyquinolinato)aluminum (Alq) trilayer device to double-doped, red-emitting OLEDs with efficiencies up to 1.5 cd/A at 20 mA/cm2 measured through a semitransparent metal electrode and CIE color coordinates of x=0.65, y=0.34.
The methods of thermally stimulated currents (TSC) and thermally stimulated luminescence (TSL) were employed to reveal the trap structure of the most prominent organic semiconductors materials such as tris-8-(hydroxyquinoline) (Alq3), N-N'-di(1-naphtyl)-N-N'-diphenylbenzidine ((alpha) -NPD), and 4,4',4'-tris-(N-2-naphtyl)-N-phenylamino- triphenylamine (1-Naph-DATA). The energetic trap depths and a lower limit of the trap densities were derived for all investigated materials by means of the initial-rise method and curve fitting techniques. Typical activation energies range between 0.1 and 0.6 eV and trap concentrations differ between 1014 and 1017 cm-3. Most materials exhibit trap levels with a single activation energy, however, in Alq3 a brought distribution of trap depths will be reported. In addition, the polarity of the dominant trap levels was determined by a comparison of TSC spectra from optically and electrically filled traps. Besides the trap detection and characterization the effect of doping and accelerated aging on the trap structure will be shown. TSC and TSL results on rubrene doped Alq3 reveals a characteristic shift in the trap depth indicating new rubrene related trapping site. The effect of aging on the trap structure of organic semiconductors in 'potentially harmful' atmospheres such as oxygen and humidity and their correlation to I-V characteristics will also be reported.
The influence of interfacial charges on the device characteristics of multilayer organic light-emitting diodes (OLEDs) is investigated, and a concept to improve device performance is presented. We studied devices consisting of copper phthalocyanine (CuPc) as hole injection and buffer layer, N, N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB) as hole transport layer, and tris(8- hydroxyquinolinato)aluminum (Alq3) as electron transport and emitting layer sandwiched between a high-work-function metal and a semi-transparent calcium electrode. Detailed current-voltage measurements show that the device characteristics in negative bias direction and at low positive bias below the built-in voltage depend strongly on the bias sweep direction, indicating that interfacial charges have a pronounced influence on the device characteristics. Low-frequency capacitance-voltage experiments reveal a voltage-independent capacitance in negative bias direction and a significant increase between 0 and 2 V, evidence of a redistribution of the internal electric field in this device configuration. Time-resolved electroluminescence (EL) measurements proved that also the EL response time at low voltages is governed by the accumulation of charge carriers inside the device rather than by their transport. Optimizing the device structure by grading the organic-organic interfaces results in an enhanced current flow, an improved brightness, and a faster EL response time. Our investigations clearly indicate that the abrupt CuPc-NPB as well as the NPB-Alq3 interface significantly influence the performance of our multilayer OLED.
Polymer light-emitting diodes, based for example on MEH-PPV, are known to be susceptible to oxidative degradation. This leads to loss of conjugation, i.e. lower carrier mobility and higher operating voltage, and to the formation of carbonyl species, i.e. to luminescence quenching. In-situ FTIR has revealed that ITO can act as the source of oxygen. In order to explore further the mechanism of oxidation and to provide guidance for its elimination, we have studied the behavior of MEH-PPV LEDs prepared with a variety of conducting polymer anodes including polyaniline and polythiophene derivatives cast from various solvents and with various molecular and polymeric dopants. In all cases examined, it is found that polymer anodes lead to significant improvement in lifetime over devices with ITO as the anode contact. Moreover, in contrast to the variability observed for ITO anodes, conducting polymers with polymers with polymeric dopants yield consistently good devices with power efficiencies of about 0.5 percent at 5 volts and brightness in excess of 1000 cd/m2. Anodes prepared with small molecule dopants are more variable and exhibit short term behavior which suggests interfacial electrochemistry. We describe the device characteristics in the context of a model of hole-dominated bipolar charge injection with Langevin recombination.