Photonic crystals are widely known for their light-trapping capabilities. This is often associated with the occurrence of a photonic band gap or other suppression in the electromagnetic density of states [1-3]. This enables guiding of light on an optical micro-chip and unprecedented forms of strong-coupling between light and matter. In the past, practical applications of these effects have focused on information technology. More recently, an important opportunity has emerged in the area of energy technology. This arises from lighttrapping in the higher bands of a photonic crystal, where the electromagnetic density of states is enhanced rather than suppressed . This enables unprecedented strong absorption of sunlight in materials with weak intrinsic absorption [4-15]. We apply this to solar cells based in silicon [5-9, 11, 13, 14], GaAs , dye-sensitized TiO2  and low-bandgap polymers .