摘要： Currently, the efficiency of thin silicon film solar cells is constrained by a limited intrinsic absorption of silicon. The efficiency could be significantly increased by using a highly efficient back reflector that sends unabsorbed light back into the solar cell [1]. As an interesting candidate for a back reflector, we study here a thin 3D photonic band gap crystal [2, 3] that forbids light within the band gap for all directions and for all polarizations. We consider a dispersive complex refractive index obtained from experiments [4]. We find by extensive finite-element computations of the 3D time-harmonic Maxwell equations that even a very thin silicon 3D photonic band gap crystal slab reflects broadband visible light omnidirectionally for all polarizations. Our results show a nearly 2.6 times enhanced angle- and polarization-averaged absorption between 680 nm and 890 nm compared to a 2400 nm thin silicon film. We observe that a 3D inverse woodpile photonic crystal enhances the absorption of a thin silicon film by (i) behaving as a perfect reflector exhibiting nearly 100% reflectivity in the stop bands, and by (ii) generating guided resonant modes at many discrete wavelengths (as illustrated in Figs. 1(a, b)). We find that the absorption is enhanced by positioning an inverse woodpile back reflector at the back side of a thin silicon film, which will keep the length of the solar cell unchanged and also reduce the mass of the thin film solar cell. For a very thin sub-wavelength absorbing layer with a photonic crystal back reflector, as shown in Fig. 1(c), we identify two physical mechanisms that cause the large enhancement at discrete wavelengths: (i) a guided resonance due to the Bragg attenuation length of the photonic crystal [5] and (ii) confinement due to the effective surface defect on the photonic crystal.