Optical resonances in TiO2 Mie scatterers on SiN and a-Si thin films for solar cells

Abstract

Thin film solar cells are an efficient alternative to conventional thick, SI wafer based, solar cells. The small thickness however leads to substantially lower light absorption. Ongoing research for efficiency optimization investigates concepts such as plasmonics and elongated structures. A novel approach involves the use of nanoparticles that can efficiently couple light into the solar cell via Mie scattering. A Mie scatterer, usually a high refractive index material, favors forward scattering of light i.e. in the direction of propagation by causing light to resonate inside it. Light is then redirected to the layers below in a way that its optical path length can be increased and therefore have a higher probability of being absorbed. Such a process can facilitate the effective and wavelength selective harvesting of photons with energies close to the band gap of the solar cell. Current research involving nanoparticles acting as Mie scatterers for light trapping in solar cells is based on nanolithography techniques. These processes are very cost inefficient therefore it is necessary to seek alternative fabrication methods. In this thesis we present the fabrication of a light trapping layer consisting of TiO2 Mie scatterers deposited on amorphous Silicon and Silicon Nitride thin films. The TiO2 nanoparticles are fabricated using a gas-phase aggregation nanocluster source. The nanoparticles are cubic in shape and are able to support Mie resonances that can couple light into the solar cell. The nanoparticles range in size from less than 10 nm to greater than 160 nm, of which the latter is comparable to the wavelength. Optical measurements of refection and transmission show peaks and valleys in the wavelength range between 350 nm and 900 nm creating asymmetrical line shapes, which strongly correspond to the so called Fano-type resonance, which is caused by the interference of a continuous back ground and single transition. Therefore, this work opens a new highly promising research path that will involve the use of nanocluster sources for increased efficiency of thin film solar cells.