Manipulating Light in Organic Thin-film Devices.

Abstract

Optoelectronic devices based on organic semiconductors have been an active topic of research for more than two decades. While organic photovoltaic cells, organic semiconductor lasers, photodetectors and other organic electronics are still working to transition from the laboratory to commercialized products, organic light-emitting diodes (OLEDs) have found wide acceptance in small and medium, high-resolution displays, with signs of near-future adoption in TV panels and large-area lighting. The fundamentally different properties of the materials and principles of device operation offer great new possibilities in terms of energy-savings, color gamut, ease and cost of manufacturing, and novel form factors.

In the first part of this thesis, we review the operation and optics of OLEDs, focusing on the problem of extracting light trapped in the high refractive index regions of the device. Since nearly 80% of generated light is lost before exiting in the forward viewing direction, detailed understanding of the underlying effects and methods to remedy the issue are necessary. We use 3D finite-element modeling to investigate techniques to outcouple light in a typical OLED. Furthermore, we demonstrate a method of fabricating an embedded dielectric grid with an ultra-low refractive index as an effective means of enhancing outcoupling. Lastly, we present progress in fabricating planarized scattering structures for light extraction.

The second half of this thesis deals with the physics and applications of the strong-coupling regime in organic semiconductor microcavities, where a new quasiparticle (the polariton) emerges due to the strong interaction of light and matter. We review the progress of organic polaritonic lasers and present experimental evidence of Bose-Einstein statistics underlying their principle of operation. We show that the polariton lasing threshold in anthracene can be reduced by an order of magnitude as the temperature is decreased, in contrast to the behavior of conventional organic lasers. Additionally, we exploit the strong-coupling regime to engineer a hybrid organic-inorganic excited state at room temperature. Such photon-mediated hybridization of disparate Frenkel and Wannier-Mott excited states may allow new devices with tailored optical properties.