Imperfectly geometric shapes of nanograting structures as solar absorbers with superior performance for solar cells

摘要

The expectation of perfectly geometric shapes of subwavelengthgrating (SWG) structures such as smoothness of sidewalls and sharpcorners and nonexistence of grating defects is not realistic due tomicro/nanofabrication processes. This work numerically investigates opticalproperties of an optimal solar absorber comprising a single-layered silicon(Si) SWG deposited on a finite Si substrate, with a careful considerationgiven to effects of various types of its imperfect geometry. The absorptancespectra of the solar absorber with different geometric shapes, namely, thegrating with attached nanometer-sized features at the top and bottom ofsidewalls and periodic defects within four and ten grating periods areinvestigated comprehensively. It is found that the grating with attachedfeatures at the bottom absorbs more energy than both the one at the top andthe perfect grating. In addition, it is shown that the grating with defects ineach fourth period exhibits the highest average absorptance (91%)compared with that of the grating having defects in each tenth period(89%), the grating with attached features (89%), and the perfect one (86%).Moreover, the results indicate that the absorptance spectrum of theimperfect structures is insensitive to angles of incidence. Furthermore, theabsorptance enhancement is clearly demonstrated by computing magneticfield, energy density, and Poynting vector distributions. The resultspresented in this study prove that imperfect geometries of the nanogratingstructure display a higher absorptance than the perfect one, and providesuch a practical guideline for nanofabrication capabilities necessary to beconsidered by structure designers.