Cavity Optics in Organic Semiconductors: From Light-Extraction of OLEDs to Exciton-Polaritons

Abstract

Organic optoelectronics has been an active topic of research and development over the past decades. While organic photovoltaic cells, transistors and other organic electronics are still in transition to commercialization, organic light-emitting devices have revolutionized displays of mobile phones and TVs. Due to the intrinsic properties of organic materials, electron-hole pairs, called excitons, are responsible for optical transitions and, thus, are crucial to organic optoelectronics. Understanding exciton-photon interactions and managing photons are critical to high performance light-emitting devices. This thesis aims at understanding this topic and providing potential solutions. The first part of this thesis focuses on the optical power distribution in organic light-emitting diodes and providing practical solutions to the limited light-extraction efficiencies. We begin by reviewing the operation and optics of light-emitting devices and modeling methods for device optics. Based on our calculations, we identify the problem of extracting light trapped in high refractive index regions of the devices, and propose principles of designs for light-extraction structures. Different light-extraction methods are demonstrated for both bottom and top-emitting devices. The second part of the thesis deals with the physics and application of the strong coupling of exciton and photons in organic semiconductors. A new particle, called the exciton-polariton, emerges as a result from the strong coupling between Frenkel excitons in organic materials and photons. We review the progress of organic exciton-polariton research in the topic of polariton lasing and long-range transport. We demonstrate the polariton laser threshold dependence on temperature and on amplified spontaneous emission. Additionally, we show Frenkel excitons in an amorphous organic film couple with Bloch surface wave. The coupling strength reaches the ultra-strong coupling regime by controlling the organic film thickness. The propagation of the exciton-polaritons with different coupling strengths shows that the more photonic fraction results in longer transport.