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Since thin-film silicon solar cells have limited optical absorption, weexplore the effect of a nanostructured back reflector to recycle the unabsorbedlight. As a back reflector we investigate a 3D photonic band gap crystal madefrom silicon that is readily integrated with the thin films. We numericallyobtain the optical properties by solving the 3D time-harmonic Maxwell equationsusing the finite-element method, and model silicon with experimentallydetermined optical constants. The absorption enhancement relevant forphotovoltaics is obtained by weighting the absorption spectra with the AM 1.5standard solar spectrum. We study thin films either thicker ($L_{Si} = 2400$nm) or much thinner ($L_{Si} = 80$ nm) than the wavelength of light. At $L_{Si}= 2400$ nm, the 3D photonic band gap crystal enhances the spectrally averaged($\lambda = 680$ nm to $880$ nm) silicon absorption by $2.22$x (s-pol.) to$2.45$x (p-pol.), which exceeds the enhancement of a perfect metal backreflector ($1.47$ to $1.56$x). The absorption is enhanced by the (i) broadbandangle and polarization-independent reflectivity in the 3D photonic band gap,and (ii) the excitation of many guided modes in the film by the crystal'ssurface diffraction leading to enhanced path lengths. At $L_{Si} = 80$ nm, thephotonic crystal back reflector yields a striking average absorptionenhancement of $9.15$x, much more than $0.83$x for a perfect metal, which isdue to a remarkable guided mode confined within the combined thickness of thethin film and the photonic crystal's Bragg attenuation length. The broadbandwidth of the 3D photonic band gap leads to the back reflector's Braggattenuation length being much shorter than the silicon absorption length.Consequently, light is confined inside the thin film and the absorptionenhancements are not due to the additional thickness of the photonic crystalback reflector.

Enhanced Absorption in thin and ultrathin silicon films by 3D photonic band gap back reflectors