Recolección de luz mediante cristales fotónicos para aplicaciones fotovoltaicas

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Óptica, leída el 14/12/2016Photovoltaic solar cells base their operation on the efficient light absorption and the subsequent conversion into electricity by separation of electric charges. Generally, solar cells use interferencial layers and/or thick absorbers to minimize the optical losses. In recent years, the photovoltaic community has a growing interest in using various types of nanostructures to increase the efficiency, minimizing either the reflectivity and/or increasing the absorption. These techniques are known as light trapping. The use of nanostructures with periodic permittivity, i.e. photonic crystals, can be very beneficial compared to the conventional interferential layers and this enhancement justifies the possible disadvantage of requiring a more complex fabrication. Indeed, photonic crystals have great flexibility in designing the optical response of a system, namely the reflection, transmission and absorption. This flexibility allows to improve efficiency, either by reducing the reflection of the cell and/or increasing the absorption by increasing the effective optical path. This thesis focuses on the design of photonic crystals for 111-V solar cells. These cells achieve the greatest efficiency in converting light to electricity. There is a high interest in improving the already high efficiency to reduce the cost of the produced electricity in terms of kWh/$. These materials are generally used with optical concentration systems, with the consequence that the surface of the semiconductor layer can be reduced three orders of magnitude in comparison to conventional solar cells. This factor obviously lowers the cost of using nanostructures in concentration technology...Las células fotovoltaicas basan su funcionamiento en el atrapamiento eficiente de luz para su posterior conversión en energía eléctrica mediante la separación de cargas. Habitualmente, los sistemas usados paraminimizar las perdidas por fotones no absorbidos se basan en láminas delgadas interferenciales y/o en aumentar el espesor del medio activo. En los últimos años dentro de la comunidad fotovoltaica existe un interés creciente en usar diversos tipos de nanoestructuras para aumentar la eficiencia, ya sea minimizando la reflexión o aumentando la absorción. Estas técnicas son conocidas como light-trapping o de atrapamiento de luz. El uso de nanostructuras ópticas periódicas, es decir cristales fotónicos, puede ser muy beneficioso frente a las laminas interferenciales convencionales y así justificar las posibles desventajas derivadas de necesitar una fabricación más compleja. De hecho, los cristales fotónicos presentan una mayor flexibilidad a la hora de diseñar la respuesta óptica del sistema: reflexión, transmisión y absorción. Esto permite mejorar la eficiencia, ya sea reduciendo la reflexión del sistema y/o incrementando la absorción mediante el aumento del camino óptico efectivo. Esta tesis se centra en el diseño de cristales fotónicos para células basadas en materiales III-V. Estos materiales son los que alcanzan una mayor eficiencia en la conversión de luz a electricidad. Existe un alto interés en mejorar la ya de por si elevada eficiencia de esta tecnología, con el objetivo de reducir el coste de la electricidad producida en terminos de kWh/$. Una característica importante a tener en cuenta es que estos materiales son usados de forma habitual en sistemas ópticos de concentración, con la consecuencia de que la superficie de la capa semiconductora puede reducirse tres ordenes de magnitud con respecto a la de módulos convencionales. Esto obviamente abarata el coste de introducir nanoestructuras en el proceso de fabricación...Depto. de ÓpticaFac. de Ciencias FísicasTRUEunpu

Light harvesting using photonic crystals for photovoltaic applications

Photovoltaic solar cells base their operation on the efficient light absorption and the subsequent conversion into electricity by separation of electric charges. Generally, solar cells use interferencial layers and/or thick absorbers to minimize the optical losses. In recent years, the photovoltaic community has a growing interest in using various types of nanostructures to increase the efficiency, minimizing either the reflectivity and/or increasing the absorption. These techniques are known as light trapping. The use of nanostructures with periodic permittivity, i.e. photonic crystals, can be very beneficial compared to the conventional interferential layers and this enhancement justifies the possible disadvantage of requiring a more complex fabrication. Indeed, photonic crystals have great flexibility in designing the optical response of a system, namely the reflection, transmission and absorption. This flexibility allows to improve efficiency, either by reducing the reflection of the cell and/or increasing the absorption by increasing the effective optical path. This thesis focuses on the design of photonic crystals for 111-V solar cells. These cells achieve the greatest efficiency in converting light to electricity. There is a high interest in improving the already high efficiency to reduce the cost of the produced electricity in terms of kWh/$. These materials are generally used with optical concentration systems, with the consequence that the surface of the semiconductor layer can be reduced three orders of magnitude in comparison to conventional solar cells. This factor obviously lowers the cost of using nanostructures in concentration technology..

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

© 2014 AIP Publishing LLC. 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.This work has been supported by Spanish MINECO (Grant Nos. ENE2012-37804-C02-02 and TEC2011-29120-C05-04).Peer Reviewe

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. © 2013 AIP Publishing LLC.We acknowledge the SGAI-CSIC for the allocation of computational resource and the financial support by MINECO (ENE2012-37804-C02-02, FPI grant) and CAM (S2009/ENE-1477).Peer Reviewe

Photonics Crystals on high efficiency III-V Solar Cells

Póster presentado en la 28th European PV Solar Energy Conference and Exhibition (EU PVSEC 2013), celebrada en París del 30 de septiembre al 4 de octubre de 2013.We have explored the following photon management options for increasing the efficiency of concentrator solar cells: increasing the voltage by recycling the luminescence, increasing the current reducing reflection and increasing the optical path. We find that a 1m m thick GaAs solar cell with optimized nanostructured front and back surfaces has significantly higher efficiency than a similar cell with a perfect back side mirror and a state of the art bilayer antireflective coating. We have also designed a multilayer coating with high reflectivity at angles away from the surface normal that results in luminescence trapping, and thus increases the open circuit voltage and the cell efficiency.Peer Reviewe

Engineering the reciprocal space for ultrathin GaAs solar cells

III-V solar cells dominate the high efficiency charts, but with significantly higher cost than other solar cells. Ultrathin III-V solar cells can exhibit lower production costs and immunity to short carrier diffusion lengths caused by radiation damage, dislocations, or native defects. Nevertheless, solving the incomplete optical absorption of sub-micron layers presents a challenge for light-trapping structures. Simple photonic crystals have high diffractive efficiencies, which are excellent for narrow-band applications. Random structures a broadband response instead but suffer from low diffraction efficiencies. Quasirandom (hyperuniform) structures lie in between providing high diffractive efficiency over a target wavelength range, broader than simple photonic crystals, but narrower than a random structure. In this work, we present a design method to evolve a simple photonic crystal into a quasirandom structure by modifying the spatial-Fourier space in a controlled manner. We apply these structures to an ultrathin GaAs solar cell of only 100 nm. We predict a photocurrent for the tested quasirandom structure of 25.3 mA/cm$^2$, while a planar structure would be limited to 16.1 mA/cm$^2$. The modified spatial-Fourier space in the quasirandom structure increases the amount of resonances, with a progression from discrete number of peaks to a continuum in the absorption. The enhancement in photocurrent is stable under angle variations because of this continuum. We also explore the robustness against changes in the real-space distribution of the quasirandom structures using different numerical seeds, simulating variations in a self-assembly method

Characterization of multiterminal tandem photovoltaic devices and their subcell coupling

Three-terminal (3T) and four-terminal (4T) tandem photovoltaic (PV) devices using various materials have been increasingly reported in the literature, but measurement standards are lacking. Here, multiterminal devices measured as functions of two load variables are characterized unambiguously as functions of three device voltages or currents on hexagonal plots. We demonstrate these measurement techniques using two GaInP/GaAs tandem solar cells, with a middle contact between the two subcells, as example 3T devices with both series-connected and reverse-connected subcells. Coupling mechanisms between the subcells are quantified within the context of a simple equivalent optoelectronic circuit. Electrical and optical coupling mechanisms are most clearly revealed using coupled dark measurements. These measurements are sensitive enough to observe very small luminescent coupling from the bottom subcell to the top subcell in the prototype 3T device. Quick simplified measurement techniques are also discussed within the context of the complete characterization

Polarization conversion on nanostructured metallic surfaces fabricated by LIPSS

Conference on Laser-Based Micro-and Nanoprocessing (LBMP) at Photonics West Conference (13ª. 2019. San Francisco, California). ISBN: 978-1-5106-2455-9 © 2019 SPIE. This work is part of the following projects: ECOGRAB, funded by the Government of Spain under the RETOS COLABORACIÓN I+D+i programme. LASER4SURF has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 768636.Waveplates modify polarization by generating a phase change. Laser Induced Periodic Surface Structures (LIPSS) have recently started to be studied as waveplates due to the birefringence in-duced by the nanoripples, easily fabricated in a one-step process by laser, where LIPSS morphology is defined by the characteristics of the laser process parameters and the substrate material. The optical properties of these waveplates are defined by LIPSS parameters such as period, depth or width of the ripples. In this work we have deposited thin film coatings on stainless steel samples containing LIPSS for different coating thickness and composition. Results show that thin film coatings are a good candidate for the tunability of LIPSS birefringence since the coating modifies the induced polarization change and reflectivity of the sample depending on coating thickness and composition, as expected from numerical simulations.Depto. de ÓpticaFac. de Ciencias FísicasTRUEUnión Europea. H2020Ministerio de Ciencia e Innovación (MICINN)pu