Thomas Dobbertin

Low-voltage organic electroluminescence device with an ultrathin, hybrid structure

We have prepared organic light-emitting diodes with a narrow recombination zone confined by an organic double-heterojunction structure using both polymer and small molecules (a hybrid structure). In these light-emitting diodes, we used very thin small molecule layers, down to a total thickness of 40 nm, to achieve an exponential forward characteristic. These layers were evaporated on a highly conductive layer of PEDT:PSS for a high-yield process and for good charge injection at the anode. Although no doping processes were applied during device fabrication, either at the injecting electrodes or in the Alq3 layer, the diodes attained high brightness at very low voltage, for instance, 10.000 cd/m2 at voltage of 4.7 V.

OLED matrix displays: in-line process technology and fundamentals

Abstract For more than a decade considerable effort has been put into the development of light emitting devices based on evaporated layers of organic semiconductors. To date, the properties of matrix displays consisting of organic light emitting diodes (OLEDs) basically meet automotive and consumer product requirements. OLED matrix displays offer high contrast, wide viewing angle and a broad temperature range at low power consumption. Due to the sensitivity of organic thin films, device structuring by conventional etching techniques is not feasible and alternative structuring techniques were developed. To meet industrial demands for device fabrication an in-line-OMBD was developed. For the first time ever, a fully in-line processed OLED is presented. Electrical current in organic devices is limited by the low conductivity of organic semiconductors and by energy barriers at the metal–organic semiconductor interface. Photoelectric measurements facilitate the determination of barrier height differences between various electrode setups. Further insight in the energy band alignment at organic heterointerfaces is gained by ultraviolet photoelectron spectroscopy (UPS). In addition to widely employed electrical ( I – V , C – V ) and optical ( P – I ) measurements, thermally stimulated current (TSC) and luminescence (TSL) allow the characterization and a more detailed understanding of carrier traps and charge transport in organic devices. Energy transfer in a doped OLED emitting layer can be investigated by time-resolved photoluminescence measurements.

OLED matrix displays: technology and fundamentals

For more than a decade, considerable effort has been put into the development of light emitting devices based on evaporated layers of organic semiconductors. To date, the properties of matrix displays consisting of organic light emitting diodes (OLEDs) basically meet automotive and consumer product requirements. OLED matrix displays offer high contrast, wide viewing angle and a broad temperature range at low power consumption. In contrast to polymer devices, OLEDs are processed in ultrahigh vacuum systems. The organic source materials are sublimated from effusion cells. Due to the sensitivity of organic thin films, device structuring by conventional etching techniques is not feasible and alternative structuring techniques were developed. Electrical current in organic devices is limited by the low conductivity of organic semiconductors and by energy barriers at the metal-organic semiconductor interface. Photoelectric measurements facilitate the determination of barrier height differences between various electrode set-ups. Further insights into the energy band alignment at organic heterointerfaces are gained by ultraviolet photoelectron spectroscopy (UPS). In addition to widely employed electrical (I-V, C-V) and optical (PI) measurements, thermally stimulated current (TSC) and luminescence (TSL) allow the characterization and a more detailed understanding of carrier traps and charge transport in organic devices. Energy transfer in a doped OLED emitting layer can be investigated by time-resolved photoluminescence measurements.

A novel patterning technique for high-resolution RGB-OLED-displays: Laser induced local transfer (LILT)

A novel patterning technique for high-resolution full-color OLED-displays will be discussed. Currently applied production systems for OLED-displays incorporate a shadow masking system for patterning of single red, green and blue pixels. Due to its limited scalability, alternative techniques, which can be applied to larger substrate sizes, have to be developed. One approach can be the laser induced local transfer of organic materials. An infrared absorbing substrate (target) is coated with either a red, green or blue light-emitting organic material and placed in a short distance (below 50 νm) of the OLED-substrate onto which the organic material is to be patterned. The laser beam is deflected by a scanner onto the target in single lines. If the scanning speed and the laser power are adjusted properly, the target locally heats up to a temperature at which the organic material sublimes and condenses on the opposing OLED-substrate. By repeating this process for each colour red, green and blue stripes can be deposited. Line widths below 70 νm have been achieved.

A laser induced local transfer for patterning of RGB-OLED-displays

RGB-OLED-displays can be realized by at least three different approaches: Color from white, color from blue or patterning of red, green and blue OLEDs, which is favorable for reasons of higher efficiency and lower costs. Common patterning techniques like photolithography cannot be applied due to the degradation of the OLEDs after the exposure to solvents. Shadow masking which is currently widely applied is not applicable for bigger substrate sizes of future mass production tools. Therefore a novel approach for patterning of organic semiconductors will be demonstrated. The laser induced local transfer (LILT) of organic small molecule materials allows for mass production of high resolution RGB-OLED-displays. An infrared absorbing target is coated with the desired emitting material, which is placed in a short distance in front of an OLED substrate. A scanner deflects and focuses an infrared laser beam onto the target. By adjusting scanning speed and laser power accurately the target locally heats up to a temperature where the organic material sublimes and will be deposited on the opposite OLED substrate. By repeating this for red, green and blue emitting materials a RGB-OLED-display can be realized. For process evaluation and development a LILT-module has been built, incorporating two custom vacuum chambers, several lift and transfer stages, a high-speed high-precision scanner and an infrared continuous-wave laser (cw). This module is designed to be part of a future inline deposition system for full-color OLED displays. In the first experiments it could be observed, that the pattern resolution is strongly dependent on the scanning speed, exhibiting minimum feature sizes of 40μm. It can be deducted that this is due to the lasers beam profile (TEM00), which allows for the smallest focus possible, but may not allow for rugged process conditions suitable for production. Rectangular steep-edged beam profiles may overcome this problem.

Inverted topside-emitting organic light-emitting diodes for active-matrix OLED displays

Top-emitting organic light-emitting diodes (OLEDS) fornext-generation active-matrix OLED-displays (AM-OLEDs) arediscussed. The emission of light via the conductive transparent top-contact is considered necessary in terms of integrating OLED-technology to standard Si-based driver circuitry. The inverted OLED configuration (IOLED) in particular allows for the incorporation of more powerful n-channel field-effect transistors preferentially used for driver backplanes in AM-OLED displays. The use of the highly conductive polymer PEDOT:PSS as hole injection layer yields anodes with an extremely low contact resistance. The non-destructive spin-coating is enabled by a hydrophobic buffer layer such as pentacene. The overlying transparent electrode was realized employing low-power radio-frequency magnetron sputter-deposition of indium-tin-oxide (ITO). Additionally, a cathode with an interfacially metal-doped electron-injecting layer is proposed. Hybrid inverted OLEDs utilizing the fluorescent emitter system Alq3:Ph-QAD allowed efficiencies of 2.7 lm/W around 150 cd/m2. Device efficiencies are increased by employing a phosphorescent dye Ir(ppy)3 doped into the hole-transporter TCTA. Such phosphorescent hybrid IOLEDs exhibit peak efficiencies of 19.6 cd/A and 5.8 lm/W at 127 cd/m2. Thus, the main requirements for a use of hybrid inverted IOLEDs in AM-OLED-displays are satisfied.

Highly efficient phosphorescent guest-host-systems for hybrid inverted organic light-emitting diodes with sputtered indium-tin-oxide anodes

Inverted organic light-emitting diodes showing light emission from the top are discussed. Top-emitting organic light-emitting diodes are required for next-generation active-matrix organic light-emitting displays , as Si-driving circuitry has to be incorporated into the display itself. We focus on hybrid anodes, thereby giving a simple model for spin-coating of PEDOT:PSS on top of an organic layer-stack, LiF-based cathodes and phosphorescent emitters, allowing for highly efficient inverted organic light emitting diodes. A maximum current efficiency of 55.4 cd/A at 140 cd/m2 and a maximum luminous efficiency of 17.2 lm/W at 50 cd/m2 has been obtained.

Inverted topside-emitting organic light-emitting diodes

Top-emitting organic light-emitting diodes (OLEDS) for next-generation active-matrix OLED-displays (AM-OLEDs) are discussed. The emission of light via the conductive transparent top-contact is considered necessary in terms of integrating OLED-technology to standard Si-based driver circuitry. The inverted OLED configuration (IOLED) in particular allows for the incorporation of more powerful n-channel field-effect transistors preferentially used for driver backplanes in AM-OLED displays. To obtain low series resistance the overlying transparent electrode was realized employing low-power radio-frequency magnetron sputter-deposition of indium-tin-oxide (ITO). The devices introduce a two-step sputtering sequence to reduce damage incurred by the sputtering process paired with the buffer and hole transporting material pentacene. Systematic optimization of the organic growth sequence focused on device performance characterized by current and luminous efficiencies is conducted. Apart from entirely small-molecule-based IOLED that yield 9.0 cd/A and 1.6 lm/W at 1.000 cd/m2 a new approach involving highly conductive polyethylene dioxythiophene-polystyrene sulfonate (PEDOT:PSS) as anode buffers is presented. Such hybrid IOLEDs show luminance of 1.000 cd/m2 around 10 V at efficiencies of 1.4 lm/W and 4.4 cd/A.

Laser threshold analysis of first- and second-order organic solid state distributed feedback laser

Optically pumped organic semiconductor thin-films have been processed on first and second order distributed feedback gratings. The organic thin-films were made by co-evaporation of tris-(8-hydroxy quinoline)aluminium (Alq3) and the laser dye 4-(Dicyanomethylene)-2-methyl-6-(julolidin-4-yl-vinyl)-4H-pyran (DCM2). The DFB laser wavelength varied depending on the grating period between 647.8 nm and 668.6 nm for first order operation and between 626.7 nm and 640.1 nm for second order operation. By evaporating the same organic film on both resonator designs we could compare first and second order laser parameters. We measured laser output characteristics and determined threshold energy values for different wavelengths and for first and second order of the Bragg grating. The laser threshold energy of the first order organic DFB laser was reduced by a factor 8 compared to the second order laser. Minimum threshold energy density was measured for a first order sample with 13.8 μJ/cm2. Reducing the laser threshold value is especially important for future applications like electrically driven organic solid-state lasers, where it will be more difficult to reach the laser threshold excitation.