Transparent electrodes represent a critical component in a wide range of optoelectronic devices such as high-speed photodetectors and solar cells. Fundamentally, the presence of any conductive structures in the optical path leads to dissipation and reflection, which adversely affects device performance. Many different approaches have been attempted to minimize such shadowing losses, including the use of transparent conductive oxides (TCOs), metallic nanowire mesh grids, graphene-based contacts, and high-aspect ratio metallic wire arrays. In this dissertation I discuss a conceptually different approach to achieve transparent electrodes, which involves recapturing photons initially reflected by highly conductive electrode lines. To achieve this, light-redirecting metallic wires are embedded in a thin dielectric layer. Incident light is intentionally reflected toward large internal angles, which enables trapping of reflected photons through total internal reflection (TIR). Light trapping transparent electrodes could potentially reach the holy grail of transparent electrodes: the simultaneous achievement of high conductivity and near-complete optical transparency. We numerically and experimentally investigate several light trapping electrode structures. First, we study the spectral and angular optical transmission of embedded interdigitated metallic electrodes with inclined wire surfaces and demonstrate efficient broadband angle-insensitive polarization-independent light trapping. Proof-of-principle experiments are carried out, demonstrating several of the features observed in our numerical studies. Second, a novel type of grating-based light trapping transparent electrode is discussed. In this approach, diffraction from metal wires covered with nanoscale silicon gratings is used to achieve total internal reflection. We show that careful grating optimization achieves strong suppression of specular reflection, enabling a more than fivefold reduction of shadowing losses. The realization of a high light-trapping efficiency in a coplanar structure makes the design a promising candidate for integration in real-world optoelectronic devices. Finally, the transmission of high-index metasurfaces is investigated. Such structures may enable efficient light redirection around metallic contacts, if reflection losses by the metasurface can be suppressed. We demonstrate that the traditional anti-reflection coating approach fails for such structures, and present an improved design approach that reduces reflection losses over a broad range of structural parameters.
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Doctor of Philosophy (Ph.D.)
College of Optics and Photonics
Optics and Photonics
Optics and Photonics
Length of Campus-only Access
Doctoral Dissertation (Open Access)
Sun, Mengdi, "Light Trapping Transparent Electrodes" (2022). Electronic Theses and Dissertations, 2020-. 1094.