Jürgen Simmerer

A quantitative numerical model of multilayer vapor-deposited organic light emitting diodes

A one-dimensional numerical model for the quantitative simulation of multilayer organic light emitting diodes (OLEDs) is presented. It encompasses bipolar charge carrier drift with field-dependent mobilities and space charge effects, charge carrier diffusion, trapping, bulk and interface recombination, singlet exciton diffusion and quenching effects. Using field-dependent mobility data measured on unipolar single layer devices, reported energetic levels of highest occupied and lowest unoccupied molecular orbitals, and realistic assumptions for experimentally not direct accessible parameters, current density and luminance of state-of-the-art undoped vapor-deposited two- and three-layer OLEDs with maximum luminance exceeding 10000 cd/m2 were successfully simulated over 4 orders of magnitude. For an adequate description of these multilayer OLEDs with energetic barriers at interfaces between two adjacent organic layers, the model also includes a simple theory of charge carrier barrier crossing and recombinati...

Electroluminescence and electron transport in a perylene dye

Charge carrier transport in vapor-deposited films of 1,6,7,12-tetraphenoxy-N,N′-bis-(2,6-diisopropylphenyl)-perylene-3,4,9,10-bis(dicarboximide) was investigated using two different methods, the time-of-flight (TOF) technique and time-resolved electroluminescence. Electron mobilities of 10−5 cm2/V s were measured in the bulk using a time-of-flight technique. Hole transport was found to be dispersive and, thus, a transit time for holes could not be obtained. The above dye was also used to fabricate single layer light emitting diodes showing clearly visible red electroluminescence under ambient conditions. Our experiments on transit electroluminescence confirmed the measured electron mobility and ruled out the possibility that the transit time of holes is shorter than the time range investigated in our time-of-flight experiments.

Work function of ITO substrates and band-offsets at the TPD/ITO interface determined by photoelectron spectroscopy

Abstract Surface compositions and work functions ( Φ ) of commercially available indium tin oxide (ITO) substrates were measured by photoelectron spectroscopy (UPS/XPS). Whereas substrates cleaned by organic solvents are significantly contaminated and have low Φ values (3.9–4.2±0.1 eV), substrates cleaned by Ar + sputtering typically have values of Φ =4.3±0.1 eV. Even higher Φ values (up to 4.7±0.1 eV) are obtained by reactive ion etching with oxygen, likely related to oxygen-containing surface impurities. Evaporated TPD is physisorbed on ITO, but causes a drop of the vacuum potential by 0.2–0.4 eV (depending on the ITO pretreatment) directly at the TPD/ITO interface, in contradiction to the common-vacuum level rule. The TPD highest occupied molecular orbital (HOMO) is found 1.1–1.3 eV below the Fermi level of the ITO, which indicates the presence of a significant barrier for hole injection.

Space-charge-limited electron currents in 8-hydroxyquinoline aluminum

We investigate electron injection and transport in single-layer devices of 8-hydroxyquinoline aluminum sandwiched between two electrodes. Electrodes comprising a thin lithium fluoride layer are compared with co-evaporated magnesium–silver cathodes and with pure aluminum cathodes. By employing both transient and quasistatic current measurements, the impact of the LiF-layer thickness on electron injection is investigated. It is demonstrated that contacts comprising 0.1–0.2 nm LiF and an aluminum capping layer are able to sustain space-charge-limited currents in 8-hydroxyquinoline aluminum. Further, steady-state current–voltage measurements as a function of temperature are discussed with respect to trap distributions in 8-hydroxyquinoline aluminum.

Electron injection and transport in 8-hydroxyquinoline aluminum

Abstract We have measured the current–voltage characteristics and device efficiency of organic light emitting diodes (OLEDs) based on 8-hydroxyquinoline aluminum (Alq 3 ) in combination with several cathode layer setups. The electron injection properties of cathode metals evaporated under high vacuum (HV) and ultra-high vacuum (UHV) conditions are compared. Further, cathodes incorporating a thin layer of lithium fluoride, which is covered with a metal capping layer, are investigated. It will be shown that aluminum is an outstanding capping metal and significantly improves both electron injection and device efficiency. Quasi-static and transient current–voltage measurements on single-layer devices will be presented. It will be demonstrated that cathodes, comprising 0.2 nm LiF and aluminum, are able to sustain space charge limited currents in Alq 3 . Additionally, the efficiency and lifetime data of multi-layer devices using this cathode layer setup are discussed.

Cathode-induced luminescence quenching in polyfluorenes

We investigate the impact of the deposition of low work function metals such as calcium on thin layers of fluorene-type polymers by time-of-flight secondary ion mass spectroscopy. An implantation process rather than a slow metal diffusion is found to be the most probable source of metal contamination within the polymer layers. This contamination extends to a range of several tens of nanometers in the organic layers. Photoluminescence and electroluminescence measurements are performed with varying calcium layer thicknesses. The luminescence efficiency exhibits a strong correlation with the depth profile of the calcium present within the polymer. The results are discussed with respect to the exciton diffusion length in the fluorene polymer. A numerical model including exciton formation, migration, and quenching is proposed in order to describe the observed phenomena.

REDUCED OPERATING VOLTAGE OF ORGANIC ELECTROLUMINESCENT DEVICES BY PLASMA TREATMENT OF THE INDIUM TIN OXIDE ANODE

The impact of oxygen plasma treatment of indium tin oxide anodes on performance and durability of vapor-deposited organic electroluminescent devices is shown. Investigations focused on the long-term stability using driving conditions suitable for passive matrix driven displays. Reliability studies of solvent only cleaned samples indicate the presence of a predominating degradation process at the interface between indium tin oxide and the hole injection layer which results in a drastic rise of the operating voltage. This voltage increase could be reduced to 0.31 mV/h by oxygen plasma treatment. As hole injection layer copper phthalocyanine is compared with a star-shaped amine derivative.

Comparison of mobility and hole current activation energy in the space charge trap-limited regime in a starburst amine

Using two complementary methods, we have investigated the individual contribution of the space charge-limited hole transport in vapor-deposited films of 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (m-MTDATA) to the temperature behavior of organic light-emitting diodes. In single-layer indium tin oxide/m-MTDATA/Ag structures, we have measured the activation energies of the current density and of the hole mobility as a function of the applied electric field. Both activation energies obtained under steady-state and pulsed conditions are comparable, which confirms that the temperature behavior of the current density is predominantly governed by the hole mobility.

Activation energies in organic light emitting diodes comprising ohmic contacts both for electron and hole injection

Abstract The lowest obtainable operating voltage for organic light emitting diodes (OLEDs) utilising a predefined organic layer setup can only be achieved with ohmic contacts both for electron and hole injection. We have investigated dark current transients of unipolar single-layer samples, and we have found ohmic contacts both for hole injection at indium tin oxide (ITO)/4,4′,4″-tris{N-(1-naphtyl)-N-phenylamino}-triphenylamine (1-Naphdata) interfaces and for electron injection at 8-hydroxyquinoline aluminum (Alq3)/LiF/Al interfaces. Therefore, the properties of OLEDs comprising these two interfaces are governed only by bulk material properties and internal organic/organic interfaces. In order to identify the dominating mechanisms concerning the temperature-dependent behaviour of prototypical double layer OLEDs, we have measured (with respect to the applied electric field) the activation energies of the charge carrier mobility and of the steady state current density in 1-Naphdata (holes) and Alq3 (electrons), the activation energies of the steady state current density and of the luminance in OLEDs comprising an 1-Naphdata/Alq3 heterojunction, plus the activation energy of the luminance onset. These experimentally activation energies are discussed with respect to device performance in the typical operating temperature range of flat panel displays including implications for further device optimisation.

Charge injection barrier modification in organic LEDs

Balanced injection of positive and negative charge carriers is a key issue for the operation of highly efficient organic light emitting devices at low operating voltage. In this article, we will give an overview of our investigations on the optimization of charge carrier injection from the anode and the cathode into organic semiconductors. These investigations include proper pretreatment of the indium–tin oxide substrate, and stacking of several organic hole transporting layers to increase the hole injection current from the transparent anode into the emissive layer. On the cathode side, binary metal alloys, and the effect of an insulating layer between the respective metal cathode and the first electron transporting layer are investigated.