In this paper we report recent developments on high power blue laser chips. Reduction of internal losses as well as optimized thermal management had been essential to increase optical output power. R and D samples with average performance of 3W optical output at junction temperatures of 130°C are demonstrated. The chips are suitable for use in a novel multi chip housing: For the first time up to 20 blue laser chips have been packaged into one compact housing resulting in the first InGaN laser device with optical output > 50W. The highly integrated package offers a unique small size. The outer dimensions of the package are 25.5mm x 35mm with an emitting surface of 16mm x 16.5mm. Therefore the complexity of optical alignment is dramatically reduced and only a single sheet multi lens array is required for beam collimation. Besides the unique technical performance the multi-die package offers significantly lower assembly costs because of the reduced complexity and assembly time. The butterfly package contains 4 bars with up to 5 multimode laser chips in series connection on each bar operating at 2.3A. The typical module wavelength is 450nm +/- 10nm. At a case temperature of 50°C the R and D samples achieve efficiencies of typ. 30% and an optical output power of 50W corresponding to an electrical power consumption of ~165W. This new technology can be used for high performance light engines of high brightness projectors.
We focus on the determination of the internal luminescence quantum efficiency of a green-emitting organic light-emitting diode (OLED). By considering different geometrical configurations of OLED thin-film stacks, we elucidate the role of the internal luminescence quantum efficiency of the emitter in the thin-film microcavity. Combining optical simulations with experimental results, a comprehensive efficiency analysis is performed. Here the electroluminescence of a set of OLEDs is characterized. Additionally, the devices are characterized using time-resolved photoluminescence measurements. The experimental data are analyzed using optical simulations. This analysis leads to a quantification of internal luminescence quantum efficiency and allows conclusions about competing mechanisms resulting in nonradiative recombination of charge carriers.
Apart from usage of organic light emitting diodes for flat panel display applications OLEDs are a potential candidate for
the next solid state lighting technology. One key parameter is the development of high efficient, stable white devices. To
realize this goal there are different concepts. Especially by using highly efficient phosphorescent guest molecules doped
into a suitable host material high efficiency values can be obtained. We started our investigations with a single dopant
and extended this to a two phosphorescent emitter approach leading to a device with a high power efficiency of more
than 25 lm/W @ 1000 cd/m2. The disadvantage of full phosphorescent device setups is that esp. blue phosphorescent
emitters show an insufficient long-term stability. A possibility to overcome this problem is the usage of more stable
fluorescent blue dopants, whereas, due to the fact that only singlet excitons can decay radiatively, the efficiency is lower.
With a concept, proposed by Sun et al.1 in 2006, it is possible to manage the recombination zone and thus the
contribution from the different dopants. With this approach stable white color coordinates with sufficient current
efficiency values have been achieved.
One of the unique selling propositions of OLEDs is their potential to realize highly transparent devices over the
visible spectrum. This is because organic semiconductors provide a large Stokes-Shift and low intrinsic absorption
losses. Hence, new areas of applications for displays and ambient lighting become accessible, for instance, the
integration of OLEDs into the windshield or the ceiling of automobiles. The main challenge in the realization of
fully transparent devices is the deposition of the top electrode. ITO is commonly used as transparent bottom
anode in a conventional OLED. To obtain uniform light emission over the entire viewing angle and a low series
resistance, a TCO such as ITO is desirable as top contact as well. However, sputter deposition of ITO on top of
organic layers causes damage induced by high energetic particles and UV radiation. We have found an efficient
process to protect the organic layers against the ITO rf magnetron deposition process of ITO for an inverted
OLED (IOLED). The inverted structure allows the integration of OLEDs in more powerful n-channel transistors
used in active matrix backplanes. Employing the green electrophosphorescent material Ir(ppy)3 lead to IOLED
with a current efficiency of 50 cd/A and power efficiency of 24 lm/W at 100 cd/m2. The average transmittance
exceeds 80 % in the visible region. The on-set voltage for light emission is lower than 3 V. In addition, by vertical
stacking we achieved a very high current efficiency of more than 70 cd/A for transparent IOLED.
"Optical Technologies have conquered the world" - their economic key data showed an impressive growth in the past
couple of years, and the predictions for the up-coming years keep the expectations high1, 2. In the case of OLED (Organic
Light Emitting Diode) lighting, e.g. IDTechEx is predicting a worldwide market growth from 50 million USD in 2009 to
3.3 billion USD in 20123.
LED and OLED technology, although both being referred to as solid state lighting, are rather complementary in their
characteristics. Whereas LEDs are high efficient point light sources, OLEDs cover large area, diffuse lighting applications
which can follow the increased awareness for creation of personalized atmosphere. Ambience and mood lighting
can be perfectly realized by the means of OLED large area illumination which will pave the way for applications that up
to now could not have been realized.
OLED lighting technology rests on three pillars at the same time, the basic performance like efficiency and lifetime, the
unique features, and costs. These key challenges and their impact on various applications will be discussed.
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 laser's 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 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.
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.
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.
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.