Jerónimo Buencuerpo

Optical absorption enhancement in a hybrid system photonic crystal – thin substrate for photovoltaic applications

A hybrid approach for light trapping using photonic crystal nanostructures (nanorods, nanopillars or nanoholes) on top of an ultra thin film as a substrate is presented. The combination of a nanopatterned layer with a thin substrate shows an enhanced optical absorption than equivalent films without patterning and can compete in performance with nanostructured systems without a substrate. The designs are tested in four relevant materials: amorphous silicon (a-Si), crystalline silicon (Si), gallium arsenide (GaAs) and indium phosphide (InP). A consistent enhancement is observed for all of the materials when using a thin hybrid system (300 nm) even compared to the non patterned thin film with an anti-reflective coating (ARC). A realistic solar cell structure composed of a hybrid system with a layer of indium tin oxide (ITO) an ARC and a back metal layer is performed, showing an 18% of improvement for the nanostructured device.

Absorption features of the zero frequency mode in an ultra-thin slab

The optical absorption in a homogeneous and non-dispersive slab is governed by the well-known Fabry-Perot resonances. We have found that below the lowest order Fabry-Perot resonance, there is another absorption maximum due to the zero frequency mode whose peak frequency is given not by the real part of the complex resonance frequency, as it is the case for all other resonances, but by the imaginary part. This result is of interest, among other applications, for ultra thin solar cells, as tuning the zero frequency mode peak with the maximum of solar irradiance results in an increased efficiency.

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.

Light-trapping in photon enhanced thermionic emitters.

A series of photonic crystal structures are optimized for a photon enhanced thermionic emitter. With realistic parameter values to describe a p-type GaAs device we find an efficiency above 10%. The light-trapping structures increases the performance by 2% over an optimal bilayer anti-reflective coating. We find a device efficiency very close to the case of a Lambertian absorber, but below its maximum performance. To prevent an efficiency below 10% the vacuum gap must be dimensioned according to the concentration factor of the solar irradiance.

Photon management with nanostructures on concentrator solar cells

Optimizing the feature sizes of dielectric nanostructures on the top (ZnS) and bottom (SiO2) surfaces of a 1 μm thick GaAs solar cell, we obtain a higher efficiency (34.4%) than a similar cell with a state of the art bilayer antireflection coating and a planar mirror (33.2%). The back side nanostructure increases the photocurrent due to enhanced optical path length inside the semiconductor, while the nanostructure on the front side increases the photocurrent due to lower reflectance losses.

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.

Cloaking of solar cell contacts at the onset of Rayleigh scattering

Electrical contacts on the top surface of solar cells and light emitting diodes cause shadow losses. The phenomenon of extraordinary optical transmission through arrays of subwavelength holes suggests the possibility of engineering such contacts to reduce the shadow using plasmonics, but resonance effects occur only at specific wavelengths. Here we describe instead a broadband effect of enhanced light transmission through arrays of subwavelength metallic wires, due to the fact that, in the absence of resonances, metal wires asymptotically tend to invisibility in the small size limit regardless of the fraction of the device area taken up by the contacts. The effect occurs for wires more than an order of magnitude thicker than the transparency limit for metal thin films. Finite difference in time domain calculations predict that it is possible to have high cloaking efficiencies in a broadband wavelength range, and we experimentally demonstrate contact shadow losses less than half of the geometric shadow.

Optical absorption enhancement in a hybrid system photonic crystal — Thin film for photovoltaic applications

A hybrid approach for light trapping using photonic crystal (PC) nanostructures (nanorods, nanopillars or nanoholes) on top of an ultra-thin film is presented. The combination of a nanopatterned layer with a thin substrate shows an enhanced optical absorption than equivalent films without patterning and can compete in performance with nanostructured systems without a substrate. The designs are tested in four relevant materials: amorphous silicon (a-Si), crystalline silicon (Si), gallium arsenide (GaAs) and indium phosphide (InP). A consistent enhancement is observed for all of the materials when using a thin hybrid system (300 nm) even compared to the non-patterned thin film with an anti-reflective coating (ARC). A realistic solar cell structure composed of a hybrid system with a layer of indium tin oxide (ITO) an ARC and a back metal layer is simulated, showing an 13% of improvement for the nanostructured device.

3D-FDTD Analysis of Absorption Enhancement in Nanostructured Thin Film Solar Cells

We investigate 1D-2D photonic crystals for light absorption enhancement on thin film photovoltaics (Si, GaAs an InP) by FDTD. A comparison with RCWA and TMM is presented. The absorption is increased substantially for these systems.

Antireflective nanostructures for CPV

We have optimized a periodic antireflective nanostructure. The optimal design has a theoretical broadband reflectivity of 0.54% on top of GaInP with an AlInP window layer. Preliminary fabrication attempts have been carried out on top of GaAs substrates. Due to the lack of a window layer, and the need to fine tune the fabrication process, the fabricated nanostructures have a reflectivity of 3.1%, but this is already significantly lower than the theoretical broadband reflectance of standard MgF2/ZnS bilayers (4.5%).