In my dissertation, I introduce different types of anisotropy to Organic Light Emitting Diodes (OLED) and demonstrate how these can be used to increase the efficiency of the device. A material is anisotropic when it has a physical property
that has a different value when measured in different directions. OLED devices are a type of Solid State Lighting (SSL) which consist, for the most part, of amorphous organic semiconductors. OLEDs have been studied extensively ever since the first demonstration of a device with a useful efficiency by Tang and Van Slyke in 1987 at Kodak. Since then, the technology has matured to a point where it has found
widespread commercial applications. OLEDs are the technology behind many state of the art displays and, to a lesser extent, lighting solutions and are gaining market
share at a rapid pace. There is still a lot of room for improvement in terms of the efficiency of these devices. Improved efficiency will result in lower power usage as
well as longer lifetime which, as it turns out, are the two main challenges that OLED faces as a technology.
An important part of my doctoral work is the installation, development and characterization of equipment to fabricate, encapsulate and characterize OLED devices. A summary of this practical work is presented in chapter 2. I successfully
installed a thermal evaporation system and a glove box to fabricate high efficiency OLEDs and developed a tool that is capable of measuring the polarization- and wavelength-dependent angular emission from these devices with high accuracy. In
addition to this practical work, I have conducted research into different forms of anisotropy through both simulation and experimentation.
A first type of anisotropy that I showed that was capable of increasing the efficiency of OLEDs was geometric anisotropy and is detailed in chapter 4. A
periodic structure with a pitch of 600 nm was incorporated into a working OLED device. Light that would otherwise remain trapped within the layers of the device and be lost through absorption was extracted by this periodic structure, thus
increasing the efficiency of the device. The efficiency of the device was increased by 20% as a direct result of this internal periodic structure. The goal of this work, however, was not to achieve record efficiency but rather to demonstrate that we
could model these structures accurately using two simulation methods, namely namely Finite Difference Time-Domain Method (FDTD) and Rigorous Coupled Wave
Analysis (RCWA). I was able to show that we could model these integrated periodic structures accurately, paving the way for further optimization through simulation which eliminates the need to do extensive proto-typing of these internal periodic
structures which are notoriously hard to include in the stack. The results of this work were published in a peer reviewed journal. [1]
The second type of anisotropy that I investigated was anisotropy in the refractive index of the organic layers. A first step in this process was to validate the simulation tools that I had. This was done by modeling the polarization- and
wavelength-dependent angular transmission through a stack of anisotropic layers and fabricating that exact device and experimentally measuring the polarizationand wavelength-dependent angular transmission through it. I was able to show that we could fabricate a stack of 11 anisotropic organic layers and simulate the transmission spectrum with very high accuracy. This serves as a validation of the
model that is used throughout the rest of the thesis. In this process, I was able to demonstrate that multi-layer stacks of organic anisotropic layers can have optical properties that are desirable and are impossible to achieve with isotropic layers.
The details of this study are presented in chapter 3.
With the simulation method validated, I proceeded to do an extensive study of how anisotropic layer can be used in Bottom emitting Organic Light Emitting Diode (BOLED)s (in which the generated emission passes through the transparent
substrate on which the device is deposited). I was the first researcher to publish on this design space and found that the anisotropy of the organic layers can account for a 10 % change in efficiency, depending on whether or not the anisotropy is used advantageously. This study assumes that the anisotropy is 0.2 (difference between ordinary and extra-ordinary refractive index), which is a degree of anisotropy that is commonly found in organic semiconductors. Higher gains could be achieved with stronger anisotropy, although this would need further thorough analysis. I demonstrate that the optimal anisotropy is strongly dependent on the orientation
of the emitting molecules in the emitting layer of the OLED stack. I have put forward simple guidelines for each of the fundamental emitter orientations so that others can use these guidelines to optimize their specific devices. Even though these
simple guidelines can guide researcher in selecting materials, it is essential that the interference effects are optimized by tuning the nanometer scale thicknesses of each layer, which can only be achieved by a full simulation which takes anisotropy into account. These results are presented in chapter 5 and were published in both a peer reviewed article and a patent application.
In order to further strengthen the confidence in the validity of the simulation tool, a number of devices with anisotropic layers were fabricated using the equipment that I installed. Detailed analysis of the polarization- and wavelength-dependent
angular emission reveals that the model can accurately predict the optical behavior of anisotropic light emitting devices. This is an impressive result since the effect of anisotropy can only be observed at (hard to probe) high emission angles. The
effect of anisotropy in the refractive index is strongest at large angles, at which the emissionremains trapped inside the device and can therefore not be detected.
As mentioned at the start of this summary, OLEDs are rapidly gaining market share in the display industry. In the this industry, the organic layers are typically deposited on top of an opaque Thin-Film-Transistor (TFT) backplane which prohibits
the generated light from passing through the substrate. These types of devices are termed Top-emitting Organic Light Emitting Diode (TOLED)s and the optics of these stacks are very different from BOLEDs. Recognizing the importance of the
TOLED geometry, I conducted a study of anisotropic layers in TOLEDs and developed guidelines for them. The results are fundamentally different from the results pertaining to BOLEDs since the loss channels in these devices have fundamentally
different stacks. The main difference being the presence of metal on both sides of the stack for the TOLED. I found that, assuming a maximum anisotropy of 0.2, the difference in efficiency between the worst-case use of anisotropic layers and
the best-case use is 7.3 %. These results can have an important impact on the efficiency of this emerging display technology and, assuming widespread adoption,
responsible use of energy across the globe. The results of this study are presented in chapter 7.