Photon frequency management for efficient sunlight trapping

Weston, Andrew and Danos, Lefteris and Alderman, Nicholas and Fang, Liping and Markvart, Tomas (2014) Photon frequency management for efficient sunlight trapping. In: Next Generation Materials for Solar Photovoltaics, 2014-01-15, The Chemistry Centre, Burlington House.

Abstract

The ability to effectively capture the incident sunlight radiation presents one of the grand challenges of photovoltaics today1. A variety of existing solar cells rely on efficient light trapping to enhance optical absorption. This can be achieved by surface texturing which can increase light absorption and reduce the thickness of solar cells2. A light trapping method has been developed using photon frequency management where the frequency is changed between absorption of the incident light and its re-emission as fluorescence3. A similar idea is to employ a photon management structure to maximize the photon path length inside the solar cell and obtain a novel form of optical confinement, or light trapping when compared to the traditional scheme based on surface texturing. The exact mechanism of the light trapping scheme [FIG 1] is as follows. Once solar light travels through the photonic mirror, photons of corresponding wavelength to the band gap of the semiconductor (silicon) are absorbed for use in the single p-n junction of the solar cell. Due to the low absorption coefficient of silicon, not all photons are absorbed. The remaining photons travel to the absorbing/fluorescent layer, which consists of multilayers of dye, and are absorbed by the top layer of dye. The excited dye molecule (D*) exhibits fluorescence emission. The emitted photon (of a wavelength which is within the range of reflection of the photonic filter) is reflected back to the silicon by the hot mirror where it is reabsorbed for use in the p-n junction. We have prepared absorbing/fluorescent dye layers based on Langmuir-Blodgett films and spin coated perylene dyes, which have been shown to be suitable for light harvesting4. Optical characterisation of the dye layers was carried out using absorption and fluorescence spectroscopy and preliminary results are shown in Figure 2. We apply the optimised dye structure on a wafer bonded crystalline silicon layer on glass of only 200 nm thickness and estimate the increase in light trapping efficiency. Preliminary results are compared to modelling.