  Luminescent solar concentrator (LSC) can absorb large-area incident sunlight,
then emit luminescence with high quantum efficiency, which finally be collected
by a small photovoltaic (PV) system. The light-harvesting area of the PV system
is much smaller than that of the LSC system, potentially improving the
efficiency and reducing the cost of solar cells. Here, based on Fermi-golden
rule, we present a theoretical description of the luminescent process in
nanoscale LSCs where the conventional ray-optics model is no longer applicable.
As an example calculated with this new model, we demonstrate that a slot
waveguide consisting of a nanometer-sized low-index slot region sandwiched by
two high-index regions provides a broadband enhancement of light harvesting by
the luminescent centers in the slot region. This is because the slot waveguide
can (1) greatly enhance the spontaneous emission due to the Purcell effect, (2)
dramatically increase the effective absorption cross-section of luminescent
centers, and (3) strongly improve the quantum efficiency of luminescent
centers. It is found that about 80% solar photons can be ultimately converted
to waveguide-coupled luminescent photons even for a low luminescent quantum
efficiency of 0.5. This LSC is potential to construct a tandem structure which
can absorb nearly full-spectrum solar photons, and also may be of special
interest for building integrated nano-PV applications.
