Light absorption enhancement and radiation hardening for triple junction solar cell through bioinspired nanostructures.

Multi-junction solar cells constitute the main source of power for space applications. However, exposure of solar cells to the space radiation environment significantly degrades their performance across the mission lifetime. Here, we seek to improve the radiation hardness of the triple junction solar cell, GaInP/Ga(In)As/Ge, by decreasing the thickness of the more sensitive middle junction. Thin junctions facilitate the collection of minority carriers and show slower degradation due to defects. However, thinning the junction decreases the absorption, and consequently, the expected photocurrent. To compensate for this loss, we examined two bioinspired surface patterns that exhibit anti-reflective and light-trapping properties: (a) the moth-eye structure which enables vision in poorly illuminated environments and (b) the patterns of the hard cell of a unicellular photosynthetic micro-alga, the diatoms. We parametrize and optimize the biomimetic structures, aiming to maximize the absorbed light by the solar cell while achieving significant reduction in the middle junction thickness. The density of the radiation-induced defects is independent of the junction thickness, as we demonstrate using Monte Carlo simulations, allowing the direct comparison of different combinations of middle junction thicknesses and light trapping structures. We incorporate the radiation effects into the solar cell model as a decrease in minority carrier lifetime and an increase in surface recombination velocity, and we quantify the gain in efficiency for different combinations of junction thickness and the light-trapping structure at equal radiation damage. Solar cells with thin junctions compensated by the light-trapping structures offer a promising approach to improve solar cell radiation hardness and robustness, with up to 2% higher end-of-life efficiency than the commonly used configuration at high radiation exposure.

Optically-thick 300 nm GaAs solar cells using adjacent photonic crystals.

Ultra-thin photovoltaics offer the potential for increasing efficiency while minimizing costs. However, a suitable light trapping strategy is needed to reach the optically thick regime for otherwise thin-film structures. III-V materials can benefit from simple adjacent light trapping structures, if correctly designed. Here we present three strategies for a 300 nm thick GaAs cell using front photonic crystals, back photonic crystals, and both front and back combined, predicting a maximum photocurrent, Jsc=29.9 mA/cm2 under the radiative limit, including an enhanced absorption in the Urbach-tail. We analyze the increased absorption isolating the Fabry-Perot resonances, the single pass absorption and the scattered contribution from the incident light.

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.

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.

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.

Ultrasonographic autopsy (echopsy): a new autopsy technique.

Autopsy has been one of the most important techniques for the development of modern medicine, mainly during the nineteenth century and the first half of last century. However, in the last few years, the number of autopsies performed in hospitals has dramatically decreased all over the world. This loss of interest can be attributed both to important advances in other diagnostic and therapeutic techniques and to the fear of malpractice suits. Several groups have tried to overcome this problem, developing different autopsy techniques, one of which is needle autopsy. Most authors using this technique have acknowledged that it is difficult to obtain material from certain organs and lesions, which makes its diagnostic reliability worse than that of conventional autopsy. To overcome this drawback, our team has recently developed a modification of needle autopsy, called ultrasonographic autopsy or echopsy, in which samples are obtained under ultrasonographic control. We report the results of the first 100 cases of echopsy performed in our hospital, comparing this technique with conventional autopsy performed on all the corpses. The concordance rate for the cause of death and the main pathological diagnosis between echopsy and classical autopsy was 83% in our series, which makes echopsy a feasible and reliable alternative to conventional autopsy in cases in which families refuse to give their consent for classical autopsy or in cases of infectious diseases.