Enhancing light absorption in silicon solar cells by fluorescent molecules

Abstract

This project aims to harness photon transport in planar solar converters via fluorescent molecules to enhance light absorption in silicon and hence reduce the material requirements and the cost of crystalline silicon solar cells. To accomplish this aim two approaches has been investigated: concentrating the far field radiation of the excited fluorescent molecules on a small area of silicon solar cell by using a fluorescent solar collector; directly injecting the excitonic energy of the excited fluorescent molecules to the waveguide modes in a proximal thin crystalline silicon solar cell via near field interactions.
An analytical model has been developed to characterise photon reabsorption in fluorescent solar collectors. This model is able to predict the spectrum of the incident photon flux on the optically coupled edge solar cell, which is not easy to measure experimentally. In the limit of high reabsorption, a useful simple expression has been found for the reabsorption probability limit, which only depends on the étendue of the photon flux emitted at the edge of the collector, the absorption coefficient of the dye molecule, and the refractive index of the collector matrix.
Perceiving the solar cell as a waveguide, the highly oscillating behaviour of the quantum efficiency of a 200 nm thick crystalline silicon solar cell has been linked to the waveguide modes supported by the thin solar cell, by studying the analytical properties of the solar cell absorbance in the complex plain of the wavenumber of light.
Efficient energy injection into a 25 nm thick thin crystalline silicon film has been demonstrated by studying molecular fluorescence and energy transfer of a carbocyanine dye deposited as Langmuir-Blodgett monolayers at different distance to the surface of copper, bulk crystalline silicon and 25 nm thick crystalline silicon films. Via the time correlated single photon counting technique, the dependence of fluorescence lifetime on the distance to the 25 nm thick crystalline silicon films has been found to fit quantitatively with an analytical expression for the injection rate of waveguide modes or simply the photon tunnelling rate from an excited molecule to a nearby thin waveguide obtained by a complex variable analysis.

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