Fluorescent concentrators concentrate both diffuse and direct radiation without requiring tracking of the sun. In
fluorescent concentrators, luminescent materials embedded in a transparent matrix absorb sunlight and emit radiation
with a different wavelength. Total internal reflection traps the emitted light and guides it to solar cells attached to the
concentrator's edges. The escape cone of total internal reflection, however, limits the light collection efficiency.
Spectrally selective photonic structures, which transmit light in the absorption range of the luminescent material and
reflect the emitted light, reduce these losses. In this paper, we review different realizations of such structures and show
that they increase collection efficiency by 20%. However, light emitted into steep angles in respect to the front surface,
which would be lost without the photonic structures, has a very long effective path inside the concentrator until it reaches
a solar cell. Therefore it suffers from path length dependent losses. We discuss how emission into the unfavorable
directions can be suppressed by integrating the luminescent material into photonic structures, thus reducing these losses.
We present possible realizations both for the concentrator design and for the solar cells used in such systems.