A nanostructure inspired by the corneas of night–flying moths provided a triple-junction solar cell with broadband (400 nm to 1000 nm) and omnidirectional (at any incident angle) harvesting of sunlight.
Antireflection coatings are applied on top of solar cells in order to indulge them of harvesting all photons available for optimum energy conversion. These coatings, as the name implies, hinder reflection of sun’s rays from the surface of solar cells, confining them beneath for higher energy conversion.
However, standard antireflective coating is for limited range of wavelength of light absorbed and incident angle of sunlight, which is the angle the light rays make with the cell’s surface. This angle of incidence continuously changes throughout the day, making it difficult for the solar cells difficult to absorb optimum amount of sunlight. Solar cell panels surpass this directional dependence of photovoltaic cells by using automated sun-tracking device.
Nanobiomimicry is a technique employed in research engineering that studies as an inspiration and mimics an organism’s system and structure at the nanoscale level. For some time now, scientists have been applying the moth’s eye concept to antireflection.
Moth’s eye reflects very little light at night due to its arrayed nanopillars that collectively behave as an intermediate refractive index at the air-medium interface. This structure causes the moth’s eye to be antireflective for a broad range of wavelength and for any incident angle of light. These antireflective capabilities of moth’s eye nanostucture are the very features desired in solar cells that aim to optimally collect sunlight of broad spectrum and at incident angle that continuously changes the entire day.
Photovoltaic cell researchers have been using the moth-eye nanostructure, but not for triple-junction solar cells (TJSC’s), which are composed of a series of three connected diodes, with each subcell engineered to absorb different wavelength ranges of sunlight. This type of solar cells is also commonly called tandem cells.
Researchers from the Department of Photonics, National Chiao Tung University in Taiwan have successfully fabricated a moth-eye-inspired TJSC. Their tandem cell is composed of indium gallium phosphide, indium gallim arsenide, and germanium.
How did they fabricate these cells? First, they coated the cell’s front surface dielectric, silicon nitride, with a monolayer of polystyrene nanospheres via spin-coating. These nanospheres arrange themselves into a closely-packed hexagonal lattice structure as seen in Figure (a). This monolayer then served as the sacrificial mask or covering for etching of the silicon nitride via a plasma-based technique.
The silicon nitride is etched while the monolayer of nanospheres is on top of it. This resulted to shrinking of the nanospheres while etching down the silicon nitride forming a two-dimensional parabolic array as shown in Figures (b) and (c). The structure and antireflection concept of these moth-eye-inspired nanopillars are actually somewhat similar to Natcore’s black solar cell.
The final TJSC resulted to low reflectance at wavelengths ranging from 400 nm to 1000 nm and at incident angles ranging from 0o to 80o, a photocurrent equal to 11.6 mA/cm2, and power conversion efficiency of 25.3%.
The research group’s future work is to replace silicon nitride (n ~ 1.8) with a material of higher refractive index to match that of the top-cell materials (n ~ 3.5).