Solar-cell efficiency enhancements using 2D photonic crystals

The performance of solar cells (SCs) can be enhanced significantly by employing 2D photonic crystals (2D-PCs). They can either be used as omnidirectional, broadband antireflection coatings (ARCs) or applied to thin-film SCs (where PC effects are much more significant than in generic SCs). Although multilayer ARCs are commonly used, problems related to material selection, thermal mismatch, sensitivity to thickness variations, and instability of the thin-film stacks persist. Tapered, submicrometer patterned structures may offer amore practical method for ultrabroadband, omnidirectional, antireflection operation.1 We have developed an optimized PC ARC that combines good optical properties (low reflectivity over a wide range of wavelengths) with high mechanical stability based on application of a thin dielectric layer perforated with the PC pattern. (A slight patterning of the PC inside the indium phosphide—InP— layer is required for optimum performance.) Thus, mixed, thin dielectric-layer/bulk patterning can be used for best optical and mechanical results compared to alternatives such as motheye-like fabrication, nanorods, and nanostructures employing oblique-angle deposition methods, which may be damaged seriously even by smooth contact with other surfaces. Our results show that our fabricated nanoholes/PC structures can be used as effective ARCs. Changing the lattice symmetry (square and triangular) does not introduce drastic changes in reflectivity, although the reflectivity is reduced for smaller lattice parameters (a). Figure 1 shows the reflectivity measured at normal incidence for our two best-performing coatings, one corresponding to a triangular and the other to a square lattice with a D 470 and 400nm, respectively. For comparison, we also show the reflectivity of a nonpatterned InP bulk sample. The values obtained are comparable to those characterizing other tapered, Figure 1. Reflectivity (in arbitrary units, a.u.) measured at normal incidence for our best-performing coatings, corresponding to triangular and square lattices with lattice parameters a = 470 and 400nm, respectively. For comparison, the corresponding reflectivity of an indium phosphide (InP) bulk sample is also included. ARC: Antireflection coating.