One of the factors limiting solar cell efficiency is light reflection at the surface and at the multiple interfaces of the device stack. Suppressing reflections increases the amount of light transmitted into the actual absorber layer. In the recent years, many different approaches have been proposed to employ antireflection coatings that can be adapted to any material. Unfortunately, exotic materials, metals, or a fine tuning of nanostructures size and shape are usually required. As a consequence, large scale application is not easy to achieve yet. In this work, we propose the use of optical metamaterials as antireflection coatings. Solar cells will be one of the possible applications.
We develop the 1D and 2D Maxwell-Garnett theory to design effective layers with arbitrary refractive index. We use the transfer matrix method and full-field simulations to explore the optical behavior of different types of metamaterial designs. In particular, we obtain complete suppression of light reflection from a high index material by tuning the filling fraction and thickness of our layer. Moreover, we explore the practical realization of such metamaterials by using Focused Ion Beam to fabricate sub-wavelength linear grating on a silicon wafer. We measure optical reflectivity over the entire visible spectrum from our samples, demonstrating good antireflection properties. The effect of light polarization is also investigated. In fact, the effective refractive index is strongly dependent on the incident polarization. We propose a double layer linear grating in order to achieve zero reflectivity at both polarizations.
In addition, we discuss the transition between deep sub-wavelength and resonant regime. If resonant modes are supported, the effective refractive index deviates from the Maxwell-Garnett theory. We elucidate the role of light trapping in the efficiency of the antireflection properties.
Finally, we extend our approach to a realistic aSi solar cell. We propose the patterning of the transparent conductive oxide (TCO) layer atop the absorbing material, while keeping the rest of the fabrication process flow unaltered. We show an increase of the solar cell efficiency by more than 10%, reaching the single-pass absorption limit of the TCO.