Plasmonic enhancement of organic optoelectronic devices

Abstract

Plasmonics can be applied in a wide range of optoelectronic devices and it is induced by the interaction between incident light and conduction electrons. Resonance is induced by matching the photon energy and the frequency of electrons, which can cause the surface charge distribution and strengthens the electromagnetic field. Generally, plasmonics can be classified into surface plasmon resonance (SPR) and localized surface plasmon resonance (LSPR). SPR is the propagating wave, which occurs at interface between the dielectric and metal. LSPR is the non-propagating wave, which is the interaction between the metal nanoparticles (NPs) and incident light when the NP size is smaller than the light wavelength. In this thesis, plasmonic enhancement is studied to improve the performance of organic solar cells (OSCs) and light emission of organic semiconductors.

OSCs are low cost, light weight, flexibility, and solution process ability at room temperature. Short exciton diffusion length limits the thickness of active layer, which causes low photon absorption and consequently low current generation. In this part, gold nanoparticles (Au NPs) are blended into OSCs to enhance photovoltaic performance. Au NPs can induce the localized surface plasmon resonance (LSPR) which enhances the light absorption due to electromagnetic field generation. Also, light can be trapped by scattering to increase the optical path and thus enhance the charge carrier generation.

Film structure and 1D nanostructure of organic semiconductor are studied by their photoluminescence (PL) intensity. Generally, the PL intensity can be enhanced by SPR. Excitation energy can induce the surface plasmon (SP) instead of photon, which can amplify the spontaneous emission and stimulated emission. Compared to thin films, 1D organic structures achieve higher PL enhancement because they can trap the light more efficiently by Fabry-Pérot cavity. Different morphologies of organic semiconductor are synthesized and it is found that hexagonal plates can obtain better PL enhancement because of the Fabry-Pérot cavity mode.