Design of antireflective nanostructures and optical coatings for next-generation multijunction photovoltaic devices

Optical Performance Assessment of Nanostructured Alumina Multilayer Antireflective Coatings Used in III-V Multijunction Solar Cells

The optical performance of a multilayer antireflective coating incorporating lithography-free nanostructured alumina is assessed. To this end, the performance of single-junction GaInP solar cells and four-junction GaInP/GaAs/GaInNAsSb/GaInNAsSb multijunction solar cells incorporating the nanostructured alumina is compared against the performance of similar solar cells using conventional double-layer antireflective coating. External quantum efficiency measurements for GaInP solar cells with the nanostructured coating demonstrate angle-independent operation, showing only a marginal difference at 60° incident angle. The average reflectance of the nanostructured antireflective coating is ∼3 percentage points smaller than the reflectance of the double-layer antireflective coating within the operation bandwidth of the GaInP solar cell (280-710 nm), which is equivalent of ∼0.2 mA/cm² higher current density at AM1.5D (1000 W/m²). When used in conjunction with the four-junction solar cell, the nanostructured coating provides ∼0.8 percentage points lower average reflectance over the operation bandwidth from 280 to 1380 nm. However, it is noted that only the reflectance of the bottom GaInNAsSb junction is improved in comparison to the planar coating. In this respect, since in such solar cells the bottom junction typically is limiting the operation, the nanostructured coating would enable increasing the current density ∼0.6 mA/cm² in comparison to the standard two-layer coating. The light-biased current-voltage measurements show that the fabrication process for the nanostructured coating does not induce notable recombination or loss mechanisms compared to the established deposition methods. Angle-dependent external quantum efficiency measurements incline that the nanostructured coating excels in oblique angles, and due to low reflectance at a 1000-1800 nm wavelength range, it is very promising for next-generation broadband multijunction solar cells with four or more junctions.

Use of nanostructured alumina thin films in multilayer anti-reflective coatings

A new method for modification of planar multilayer structures to create nanostructured aluminum oxide anti-reflection coatings is reported. The method is non-toxic and low-cost, being based on treatment of the coating with heated de-ionized water after the deposition of aluminum oxide. The results show that the method provides a viable alternative for attaining a low reflectance ARC. In particular, a low average reflectivity of ∼3.3% is demonstrated in a broadband spectrum extending from 400 nm to 2000 nm for ARCs deposited on GaInP solar-cells, the typical material used as top-junction in solar cell tandem architectures. Moreover, the process is compatible with volume manufacturing technologies used in photovoltaics, such as ion beam sputtering and electron beam evaporation.

Wide-Angle Broadband Antireflection Coatings Prepared by Atomic Layer Deposition

A novel broadband antireflective coating with ultra-low residual reflectance for light incidence angles from 0° up to 60° is presented. The system consists of an interference multilayer coating made by atomic layer deposition (ALD) combined with a low- n nanoporous SiO₂ top-layer obtained by wet-chemical etching of an atomically mixed SiO₂/Al₂O₃ ALD composite. The average residual reflectance measured at normal incidence for double-sided coated B270 glass substrates is only 0.5% in a broad spectral range from 400 to 1100 nm. The average reflectance of the substrate considering both front and rear sides decreased in the visible spectral range of 420-680 nm from 9.9 and 15.8 to 0.4 and 1.8% at an oblique angle of incidence (AOI) of 45° and 60°, respectively, by applying the hybrid ALD antireflection coatings. The measured average transmittance reaches 99.5% at AOI 6° in the 400-950 nm spectral range. Measurements three weeks after preparation show only a small reduction of the average transmittance to 99.3% in this spectral range spanning 550 nm. Ten months later, the average transmittance is still 99.0%, whereby the sample handling might have also affected the performance. The hybrid ALD system shows excellent conformal AR performance on a strongly curved B270 aspheric lens with a diameter of 50 mm and a height of 25 mm. The presented process is a promising route toward omnidirectional AR coatings on complex 3D optics, which are increasingly important for consumer and high-performance optical systems.

Design and Fabrication of Wafer-Level Microlens Array with Moth-Eye Antireflective Nanostructures

Wafer-level packaging (WLP) based camera module production has attracted widespread industrial interest because it offers high production efficiency and compact modules. However, suppressing the surface Fresnel reflection losses is challenging for wafer-level microlens arrays. Traditional dielectric antireflection (AR) coatings can cause wafer warpage and coating fractures during wafer lens coating and reflow. In this paper, we present the fabrication of a multiscale functional structure-based wafer-level lens array incorporating moth-eye nanostructures for AR effects, hundred-micrometer-level aspherical lenses for camera imaging, and a wafer-level substrate for wafer assembly. The proposed fabrication process includes manufacturing a wafer lens array metal mold using ultraprecise machining, chemically generating a nanopore array layer, and replicating the multiscale wafer lens array using ultraviolet nanoimprint lithography. A 50-mm-diameter wafer lens array is fabricated containing 437 accurate aspherical microlenses with diameters of 1.0 mm; each lens surface possesses nanostructures with an average period of ~120 nm. The microlens quality is sufficient for imaging in terms of profile accuracy and roughness. Compared to lenses without AR nanostructures, the transmittance of the fabricated multiscale lens is increased by ~3% under wavelengths of 400-750 nm. This research provides a foundation for the high-throughput and low-cost industrial application of wafer-level arrays with AR nanostructures.

Guided-mode resonance gratings for enhanced mid-infrared absorption in quantum dot intermediate-band solar cells

Achieving strong absorption of low-energy photons is one of the key issues to demonstrate quantum dot solar cells working in the intermediate band regime at practical concentration factors and operating temperatures. Guided-mode resonance effects may enable large enhancement of quantum dot intraband optical transitions. We propose quantum dot thin-film cells designed to have significant field waveguiding in the quantum dot stack region and patterned at the rear-side with a sub-wavelength diffraction grating. Remarkable increase of the optical path length at mid-infrared wavelengths is shown owing to guided-mode resonances. Design guidelines are presented for energy and strength of the second-photon absorption for III-V quantum dots, such as InAs/GaAs and GaSb/GaAs, whose intraband and intersubband transitions roughly extends over the 2 - 8 µm range. The proposed design can also be applied to quantum dot infrared detectors. Angle-selectivity is discussed in view of applications in concentrator photovoltaic systems and infrared imaging systems.

Nano-cones for broadband light coupling to high index substrates

The moth-eye structure has been proposed several times as an antireflective coating to replace the standard optical thin films. Here, we experimentally demonstrate the feasibility of a dielectric moth-eye structure as an antireflective coating for high-index substrates, like GaAs. The fabricated photonic crystal has Si₃N₄ cones in a square lattice, sitting on top of a TiO₂ index matching layer. This structure attains 1.4% of reflectance power losses in the operation spectral range of GaAs solar cells (440–870 nm), a 12.5% relative reduction of reflection power losses in comparison with a standard bilayer. The work presented here considers a fabrication process based on laser interference lithography and dry etching, which are compatible with solar cell devices. The experimental results are consistent with scattering matrix simulations of the fabricated structures. In a broader spectral range (400–1800 nm), the simulation estimates that the nanostructure also significantly outperforms the standard bilayer coating (3.1% vs. 4.5% reflection losses), a result of interest for multijunction tandem solar cells.

Broadband antireflective nano-cones for tandem solar cells

Broadband solar cell antireflection coatings made of nano-cones are studied in square lattices of ZnS, TiO(2) and Si(3)N(4). In the best case, the spectrally integrated transmittance (accounting for both reflection and dielectric absorption losses) for direct solar radiation is 99 %, which represents a four-fold decrease in transmission losses in comparison to a standard antireflective coating bilayer. The dependence of the transmission as a function of nanostructure dimensions is studied, showing a wide maximum, thus leading to a high tolerance for manufacturing errors. This high transmittance is also robust against deviations from normal incidence. Our analysis suggests that the high transmittance is due not only to an effective gradual index effect, but is also due to light coupling to quasiguided modes in the photonic crystal leaking mostly towards the substrate.

Surface structured optical coatings with near-perfect broadband and wide-angle antireflective properties

Optical thin-film coatings are typically limited to designs where the refractive index varies in only a single dimension. However, additional control over the propagation of incoming light is possible by structuring the other two dimensions. In this work, we demonstrate a three-dimensional surface structured optical coating that combines the principles of thin-film optical design with bio-inspired nanostructures to yield near-perfect antireflection. Using this hybrid approach, we attain average reflection losses of 0.2% on sapphire and 0.6% on gallium nitride for 300-1800 nm light. This performance is maintained to very wide incidence angles, achieving less than 1% reflection at all measured wavelengths out to 45° for sapphire. This hybrid design has the potential to significantly enhance the broadband and wide-angle properties for a number of optical systems that require high transparency.