11-10-2016, 11:42 AM
1458579769-AbstractDrArbeitMatthiasdelaRosaenglish.pdf (Size: 25.06 KB / Downloads: 109)
Abstract
Self-emissive organic light-emitting diodes (OLEDs) are a new promising technology with
high expected profitability on the display market, which is currently dominated by liquid
crystals. They show low driving voltages in combination with unrestricted viewing angles,
high color-brilliance, light weight, small film-thicknesses and low production costs. Because
of the plasticity of the materials they can be deposited on flexible substrates. Nowadays, there
are already OLED-display devices available such as PDAs, mp3 players, mobile phones,
navigation-systems etc. One can distinguish two material-classes used in OLED: OLEDs
based on small molecules, which are deposited by thermal evaporation and the low-cost
polymeric OLEDs, which are processed from solution. Initially, it was difficult to realise
polymeric multilayer OLEDs, since newly deposited layers dissolved the underlying layers,
but are now easily accessible by the use of direct lithography in combination with oxetanefunctionalised
polymers.
Despite intense research efforts during the last decade there are still improvements to be made
in OLED-lifetime and OLED-outcoupling. The light-outcoupling is limited by the refractive
indices of the OLED building layers. Simple ray-optics allows to estimate the external
quantum efficiency of a standard OLED to 20 % of the initially generated light. 80 %,
however, are lost to total internal reflection. To overcome this problem many approaches have
been introduced. They can be divided into modifications of the external OLED-architecture
(e.g. mesa structures, micro-lenses) and modifications of the internal layer structure. It was
demonstrated that diffraction elements, such as periodic structures, are highly suitable to
improve light-outcoupling. Doubling of the efficiency and luminescence enhancements up to
factors of five were reported with respect to the flat reference devices, but only in
combination with very poor overall efficiencies.
Subject of this thesis was to structure well-performing organic OLED-polymers by direct
lithography (DL) and investigate their applications in electro-optic devices as diffraction
elements. Additionally, photoembossing (PE) should be applied to access structured wellperforming
oxetane functionalised OLED-materials without any wet-development step. The
third method is a combination of direct lithography and photoembossing and is referred to as
“combined DL-PE structuring” in this thesis. Further applications of periodic modulated
organic semiconductors, showing amplified spontaneous emission (ASE) while optically
excited, are organic lasers.
Firstly, intense studies of the structuring techniques were conducted. The aim was to achieve
structures of high modulation (peak to valley distance) in combination with small grating
periods Λ (Λ < 400 µm). The direct lithography of oxetane-functionalized polymeric
semiconductors was studied interferometrically as well as by the use of shadow masks. The
final structures showed coupled dependencies on intensity, exposure time, initiator
concentration, curing, the atmosphere and the underlying layer. Another critical step in direct
lithography is the wet development. However, self-structured emitting layers could be
successfully tested in OLED applications: strong modifications of the emission characteristics
were detected. Since film thicknesses and grating dimensions are on the same scale as the
wavelength of the emission, diffractions of higher orders can be observed, which can to be
explained by the Raman-Nath-theory. Photoembossing was established, too. Now oxetanefunctionalized,
well-performing OLED-materials can be structured yielding films with
surface- and refractive index modulations. By choice of the cross-linking monomer the
resulting refractive index contrasts as well as the final electrical properties of the gratings can
be influenced giving the possibility to electro-optical design the photosensitive blend to the
desired application. All process-parameters were investigated in detail. Shadow-mask
experiments applying a liquid monomer revealed that mass-diffusion is takes place in the
millimetre range. Real-time diffraction experiments showed that the diffraction efficiency is a
function of the grating modulation as well as the shape of the grating.
All structuring techniques yielded an improvement of the outcoupling efficiency in OLED
applications. The improvement factors were typically between 1.2 and 2.0 with respect to the
flat reference devices. Even improvements of factors of 1.15 could be detected with respect to
conventional OLEDs without additives. In the chosen OLED layer-order emitter-thicknesses
between 180-200 nm were necessary to detect maximum diffraction efficiency of the gratings.
Parallel polarization of the emission relative to the OLED-grating-fringes was found.
Detection of the angle dependent Emission revealed grating influences on the TE-modes as
well as on the TM-modes of the gratings. Which mode obtained stronger modification
depended on the grating diffraction efficiencies and the dipole-orientations relative to the
plane of the substrate. An orientated dipole emits light, which can be diffracted out of the
structure and thus be observed outside the device. The polarisation of the emission depends
on that orientation. Dipoles orientated parallel to the plane of the substrate mainly contribute
to TM-modes, whereas vertically orientated dipoles mainly lead to TE-modes. The diffraction
efficiency of a mode is a function of the coupling of that mode to the structure. The
dimensions of the grating should match the vertical diffraction conditions for the most lossy
wavelength to obtain maximum outcoupling improvement.
By implementation of the periodical structures the device obtains electric modifications, too.
At a given applied electric field, the surface relief, i.e. the modulation of the film-thickness,
leads to an enhancement of the luminescence in the “valleys” by the E-field improved charge
carrier injection. The efficiency improvement of that effect was determined to a factor of
1.058 (5.8%) of the overall maximum improvement (factor of 2), and is therefore considered
to be negligible. The effect of the cathode-surface-increase by the relief was also classified to
be marginal.
Important electric modifications result from the material re-distributions in the corrugated
films by applying photoembossing. Material enrichment is taking place at the peaks, which is
followed by a depletion of that material in the valleys. The result is a modulation of the
electrical properties, e.g. conductance and mobility, according to the electrical properties of
the blend components. In turn, that leads to modulated recombination zones and thus to
modified ray optics. A refractive index modulation is also found if the polymers and
monomers exhibit different refractive indices, benefiting the outcoupling efficiency of the
diffraction-grating. The higher the contrast of the refractive indices the higher is the
diffraction efficiency of the structure.
The collectivity of all optical and electrical effects characterizes the external emission of
corrugated OLEDs. Theoretical calculations of the Institut für Optik und Feinmechanik (IOF)
in Jena confirm these interpretations.
The other application is the implementation of self-structured polymer resonators in organic
laser devices. Self-emitting DFB-resonators were fabricated for the first time. By the use of
the excellent performing OLED-materials electrically driven organic lasers become more
accessible. Two differently structured laser-systems were examined: a photoembossed- and a
directly structured system. Both laser-systems exhibited very low optical-excitation
thresholds, a crucial requirement for electrical pumping. The photoembossed system could be
characterized by its stopband to a mixed gain- and index mode-coupling to the DFBresonator,
thus achieving lowest thresholds of 1.45µJ/cm². The direct-structured lasers are
based on pure index-coupling-mechanisms. Here, no stopband could be detected. These lasers
exhibit higher excitation thresholds of 5.5 µJ/cm2
. Because of the poor electrical performance
of the PS-lasers caused by the addition of a non-conductive low refracting monomer, the
system is limited to optical applications. For electrical applications the DS-resonators with
their slightly higher threshold are more suitable.
In this thesis, the concept could be realised successfully. It was demonstrated that polymeric
materials can be structured, while maintaining the electro-optical properties. Application of
the periodic structures leads to outcoupling improvements in OLED-devices and the
successful fabrication of conducting organic laser-resonators. Furthermore, advanced
knowledge of ray-optics and mode-coupling to grating structures was obtained.
In the near future the implementation of 2-dimensional periodic structures in OLEDs and laser
devices is planned. We expect further OLED outcoupling improvements as well as lower
lasing-thresholds since more light will be collected and thus diffracted out of the devices.
A significant parameter for the fabrication of corrugated devices by photoembossing and
crossed-structuring was the choice of monomer. The monomer properties mainly determined
exposure conditions as well as electro-optical properties of the final gratings. Advantageous
would be a perfect match of the materials to the desired application and device architecture.
Further applications of periodic structures could be their use in organic solar cells to improve
incoupling of sunlight into the device. The application is currently under investigation in our group.