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

Femtosecond laser fabrication of LIPSS-based waveplates on metallic surfaces

A fast and reliable method for the fabrication of polarization modifying devices using femtosecond laser is reported. A setup based on line focusing is used for the generation of LIPSS on stainless steel, processing at different speeds between 0.8 and 2.4 mm/s with constant energy per pulse of 1.4 mJ. SEM and AFM characterizations of the LIPSS show a progressive increase in period as the processing speed increases, while height remains approximately constant in the studied range. Optical characterization of the devices shows an induced change in the polarization of the reflected beam that varies with the processing speed, which allows a controlled fabrication of these devices

Femtosecond laser fabrication of LIPSS-based waveplates on metallic surfaces

A fast and reliable method for the fabrication of polarization modifying devices using femtosecond laser is reported. A setup based on line focusing is used for the generation of LIPSS on stainless steel, processing at different speeds between 0.8 and 2.4 mm/s with constant energy per pulse of 1.4 mJ. SEM and AFM characterizations of the LIPSS show a progressive increase in period as the processing speed increases, while height remains approximately constant in the studied range. Optical characterization of the devices shows an induced change in the polarization of the reflected beam that varies with the processing speed, which allows a controlled fabrication of these devices

Light harvesting using photonic crystals for photovoltaic applications

Tesis llevada a cabo para conseguir el grado de Doctor por la Universidad Complutense de Madrid.--2016-11-14.--Sobresaliente Cum Laude[ES] 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/$. Una característica importante a tener en cuenta es que estos materiales son usados de forma habitual en sistemas ópticos de concentración, con la consecuencia de que la superficie de la capa semiconductora puede reducirse tres ordenes de magnitud con respecto a la de módulos convencionales. Esto obviamente abarata el coste de introducir nanoestructuras en el proceso de fabricación...[EN] Photovoltaic solar cells base their operation on the efficient light absorption and the subsequent conversion into electricity by separation of electric charges. Generally, solar cells use interferencial layers and/or thick absorbers to minimize the optical losses. In recent years, the photovoltaic community has a growing interest in using various types of nanostructures to increase the efficiency, minimizing either the reflectivity and/or increasing the absorption. These techniques are known as light trapping. The use of nanostructures with periodic permittivity, i.e. photonic crystals, can be very beneficial compared to the conventional interferential layers and this enhancement justifies the possible disadvantage of requiring a more complex fabrication. Indeed, photonic crystals have great flexibility in designing the optical response of a system, namely the reflection, transmission and absorption. This flexibility allows to improve efficiency, either by reducing the reflection of the cell and/or increasing the absorption by increasing the effective optical path. This thesis focuses on the design of photonic crystals for 111-V solar cells. These cells achieve the greatest efficiency in converting light to electricity. There is a high interest in improving the already high efficiency to reduce the cost of the produced electricity in terms of kWh/$. 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...Peer reviewe

Dependence of multijunction optimal gaps on spectral variability and other environmental and device parameters

Trabajo presentado en la 46th IEEE Photovoltaic Specialist Conference (PVSC), celebrada en Chicago, Illinois (Estados Unidos), del 16 al 21 de junio de 2019We present a method to calculate the yearly energy production of multijunctions including spectral variations. We use it to find the optimal band gaps yielding maximum energy production. The band gaps found are different to those previously reported when using the standard efficiency as the optimization target. Our calculations predict that novel hybrid tandems in combination with bifacial silicon can lead to energy yields near 1 MWh m -2 year -1 at most locations of interest. We also discuss the effects of changing parameters such as the external radiative efficiency, series resistance, sub-cell thickness, temperatures, and location.Funding was provided by the Goverment of Spain (TEC2015-64189-C3-2-R, ENE2017-91092-EXP, RYC-2014- 15621) and Comunidad de Madrid (P2018/EMT-4308

Machine learning for realistic yearly averaged photovoltaic efficiency calculations

Trabajo presentado en el 35th EUPVSEC: European Solar Energy Conference, celebrada en Bruselas (Bélgica), del 24 al 28 de septiembre de 2018We demonstrate a practical method based on machine learning techniques to obtain the yearly averaged efficiency or energy yield with uncertainties that are a small fraction of those resulting from previous proposals. The full yearly spectral sets are reduced by three orders of magnitude to sets of characteristic spectra. The yearly energy yield is calculated as a function of the band gaps including the effects of series resistance, window layer surface recombination, temperature and spectral variability, external radiative efficiency, radiative coupling from top junctions and photon recycling at bottom junctions with a back mirror. Multiple terminal devices are also considered. When graphically representing the efficiencies as a function of the band gaps, multiple local maxima are found within 2% of the absolute maximum, and which of these local maxima has the highest efficiency depends very critically on a number of assumptions. Identifying the absolute maxima is thus not as relevant as the general trends observed when graphically representing the whole multidimensional parameter space. Yearly energy yields of up to 1 MWh/m2 are shown to be attainable with advanced multijunction devices making use of both the direct and diffuse components of solar radiation.Peer reviewe