Enhancing solar cells with plasmonic nanovoids
This thesis explores the use of plasmonic nanovoids for enhancing the efficiency of thin-film solar cells. Devices are fabricated inside plasmonically resonant nanostructures, demonstrating a new class of plasmonic photovoltaics. Novel cell geometries are developed for both organic and amorphous silicon solar cell materials. An external-quantum efficiency rig was set up to allow simultaneous microscope access and micrometer-precision probe-tip control for optoelectronic characterisation of photovoltaic devices. An experimental setup for angle-resolved reflectance was extended to allow broadband illumination from 380 - 1500nm across incident angles 0 - 70 degrees giving detailed access to the energy-momentum dispersion of optical modes within nanostructured materials. A four-fold enhancement of overall power conversion efficiency is observed in organic nanovoid solar cells compared to flat solar cells. The efficiency enhancement is shown to be primarily due to strong localised plasmon resonances of the nanovoid geometry, with close agreement observed between experiment and theoretical simulations. Ultrathin amorphous silicon solar cells are fabricated on both nanovoids and randomly textured silver substrates. Angle-resolved reflectance and computational simulations highlight the importance of the spacer layer separating the absorbing and plasmonic materials. A 20% enhancement of cell efficiency is observed for nanovoid solar cells compared to flat, but with careful optimisation of the spacer layer, randomly textured silver allows for an even greater enhancement of up to 50% by controlling the coupling to optical modes within the device. The differences between plasmonic enhancement for organic and amorphous silicon solar cells are discussed and the balance of surface plasmon absorption between a semiconductor and a metal is analytically derived for a broad range of solar cell materials, yielding clear design principles for plasmonic enhancement. These principles are used to outline future directions of research for plasmonic photovoltaics.