We will present efficient semi-transparent bulk-heterojunction [regioregular of poly(3-hexylthiophene):
(6,6)-phenyl C61 butyric acid methyl ester] solar cells with an inverted device architecture. Highly transparent ZnO
and TiO2 films prepared by Atomic Layer Deposition are used as cathode interlayers on top of ITO. The topanode
consists of a RF-sputtered ITO layer. To avoid damage due to the plasma deposition of this layer, a
sputtering buffer layer of MoO3 is used as protection. This concept allows for devices with a transmissivity
higher than 60 % for wavelengths 650 nm. The thickness of the MoO3 buffer has been varied in order to
study its effect on the electrical properties of the solar cell and its ability to prevent possible damage to the
organic active layers upon ITO deposition. Without this buffer or for thin buffers it has been found that device
performance is very poor concerning the leakage current, the fill factor, the short circuit current and the power
conversion efficiencies. As a reference inverted solar cells with a metal electrode (Al) instead of the ITO-top
contact are used. The variation between the PCE of top versus conventional illumination of the semi-transparent
cells was also examined and will be interpreted in view of the results of the optical simulation of the dielectric
device stack with and without reflection top electrode. Power conversion efficiencies of 2-3 % for the opaque
inverted solar cells and 1.5-2.5 % for the semi-transparent devices were obtained under an AM1.5G illumination.
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.
Compared to established LCD and plasma technologies displays based on organic light emitting diodes (OLEDs)
promise more brilliant images, less energy consumption and lower production costs. Furthermore, the organic
layers that make up an OLED can be engineered to be transparent in the visible part of the spectrum. In
combination with transparent conductive oxides like Indium-Tin-Oxide (ITO) or Aluminum doped Zinc-Oxide
(AZO) as contacts OLEDs may be built entirely transparent. One major issue to be addressed in the fabrication
of these devices is the deposition of the top transparent contact without damaging the organic layers.
Transparent OLED pixels can be arranged to form entirely transparent OLED displays. For the active matrix
addressing of the individual OLED pixels, we use TFTs which are transparent themselves. Rather than silicon,
they are based on the wide-bandgap semiconductor Zinc-Tin-Oxide (ZTO) and transmit about 80 % of the visible
light (400-750 nm). The transistors typically have field-effect mobilities of 13 cm2/Vs (an order of magnitude
larger than a-Si TFTs) and an on-off ratio of 106. The OLED pixel which needs to be driven may be positioned
directly on top of the driver circuit. The pixels fabricated accordingly have an overall transmittance > 70 %
in the visible spectrum. The brightness of the OLED pixels could be varied from 0 to 700 cd/m2 via the gate
bias of the driving TFTs. These devices state the initial building blocks of future, large-area, high-resolution
transparent OLED displays. More complex transparent driving circuits, required to compensate eventual device
variations 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.
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.