Metal nanostructures in optoelectronic devices [electronic resource]

Organic photovoltaic cells are considered a promising solar cell technology because of the tunability of the electronic and optical properties of organic semiconductors, the potential for low-cost roll-to-roll manufacturing and their compatibility with flexible substrates. However, low efficiencies and unproven environmental stability are two major roadblocks that stand in the way of commercialization. In this thesis, we analyze the enhancement in optical absorption of an absorbing medium when spherical metal nanoparticles are embedded. Our analysis uses the generalized Mie theory to calculate the absorbed optical power as a function of the distance from the metal nanoparticle. This analysis is used to evaluate the potential of enhancing optical absorption in thin-film solar cells by embedding spherical metal nanoparticles. We consider the trade-off between maximizing overall optical absorption and ensuring that a large fraction of the incident optical power is dissipated in the absorbing host medium rather than in the metal nanoparticle. We show that enhanced optical absorption results from strong scattering by the metal nanoparticle which locally enhances the optical electric fields. We also discuss the effect of a thin dielectric encapsulation of the metal nanoparticles. Transparent conductive electrodes are important components of thin-film solar cells, light-emitting diodes, and many display technologies. Doped metal oxides are commonly used, but their optical transparency is limited for films with low sheet resistance. Furthermore, they are prone to cracking when deposited on flexible substrates. They are also costly and require a high-temperature process to achieve the best performance. We demonstrate solution-processed transparent electrodes consisting of random meshes of metal nanowires that exhibit an optical transparency equivalent to or better than that of metal-oxide thin films for the same sheet resistance. Organic solar cells deposited on these electrodes show a performance equivalent to that of devices based on conventional metal-oxide transparent electrodes. We demonstrate semitransparent organic photovoltaic cells using laminated silver nanowire meshes as a transparent, conductive cathode layer. The lamination process does not damage the underlying solar cell and results in a transparent electrode with low sheet resistance and high optical transmittance without impacting photocurrent collection. The resulting semitransparent phthalocyanine/fullerene organic solar cell has a power conversion efficiency that is 57% of that of a device with a conventional metal cathode due to differences in optical absorption. Multijunction architectures in which multiple energy gaps are combined in a series-connected stack of solar cells are seen as a promising approach to increasing the power conversion efficiency of organic solar cells to commercially important values. Higher efficiencies can be obtained if the photocurrent-matching requirement of such multijunction cells is removed by the use of intermediate electrodes in multi-terminal multijunction cells. We demonstrate multi-terminal multijunction organic photovoltaic cells using laminated metal nanowire meshes as intermediate electrode. The multijunction cell combines a polymer bulk heterojunction front cell and a small-molecule bilayer back cell. Although the photocurrent densities of the subcells are significantly different, the overall current is not limited by the lowest current. Using simulations, we compare the efficiency potential of series-connected and multi-terminal multijunction cells. We demonstrate organic light emitting devices replacing indium-tin-oxide with silver nanowire mesh as a transparent conductor. Irregular surface which silver nanowire mesh creates helps to increase outcoupling efficiency of organic light emitting devices up to 30%. Electromagnetic simulations confirm our experimental results.

Keywords: Electrical Engineering.
Advisor: Peumans Peter
Brongersma Mark L. advisor. Thesis advisor
Wong Hon-Sum Philip