Understanding phosphorus diffusion into silicon in a MOVPE environment for III-V on silicon solar cells

Dual-junction solar cells formed by a GaAsP or GaInP top cell and a silicon bottom cell seem to be attractive candidates to materialize the long sought-for integration of III?V materials on silicon for photovoltaic applications. When manufacturing a multi-junction solar cell on silicon, one of the first processes to be addressed is the development of the bottom subcell and, in particular, the formation of its emitter. In this study, we analyze, both experimentally and by simulations, the formation of the emitter as a result of phosphorus diffusion that takes place during the first stages of the epitaxial growth of the solar cell. Different conditions for the Metal-Organic Vapor Phase Epitaxy (MOVPE) process have been evaluated to understand the impact of each parameter, namely, temperature, phosphine partial pressure, time exposure and memory effects in the final diffusion profiles obtained. A model based on SSupremIV process simulator has been developed and validated against experimental profiles measured by ECV and SIMS to calculate P diffusion profiles in silicon formed in a MOVPE environment taking in consideration all these factors.

Guide to intermediate band solar cell research

The intermediate band solar cell (IBSC) is a solar cell that, in order to increase its efficiency over that of single gap solar cells, takes advantage of the absorption of below-bandgap energy photons by means of an intermediate band (IB) located in the semiconductor bandgap. For this process to improve the solar cell performance, the belowbandgap photon absorption has to be effective and the IB cannot limit the open-circuit voltage of the cell. In this paper we provide a guide to the new researcher interested in the idea in order he can quickly become familiar with the concept and updated with the most relevant experimental results.

Nano-imprinted rear-side diffraction gratings for absorption enhancement in solar cells

As wafer-based solar cells become thinner, light-trapping textures for absorption enhancement will gain in importance. In this work, crystalline silicon wafers were textured with wavelength-scale diffraction grating surface textures by nanoimprint lithography using interference lithography as a mastering technology. This technique allows fine-tailored nanostructures to be realized on large areas with high throughput. Solar cell precursors were fabricated, with the surface textures on the rear side, for optical absorption measurements. Large absorption enhancements are observed in the wavelength range in which the silicon wafer absorbs weakly. It is shown experimentally that bi-periodic crossed gratings perform better than uni-periodic linear gratings. Optical simulations have been made of the fabricated structures, allowing the total absorption to be decomposed into useful absorption in the silicon and parasitic absorption in the rear reflector. Using the calculated silicon absorption, promising absorbed photocurrent density enhancements have been calculated for solar cells employing the nano-textures. Finally, first results are presented of a passivation layer deposition technique that planarizes the rear reflector for the purpose of reducing the parasitic absorption.

1000x shadow-free mirror-based K��hler concentrator

A new design for a photovoltaic concentrator, the most recent advance based on the Kohler concept, is presented. The system is mirror-based, and with geometry that guaranties a maximum sunlight collection area (without shadows, like those caused by secondary stages or receivers and heat-sinks in other mirror-based systems). Designed for a concentration of 1000x, this off axis system combines both good acceptance angle and good irradiance uniformity on the solar cell. The advanced performance features (concentration-acceptance products ?CAP- about 0.73 and affordable peak and average irradiances) are achieved through the combination of four reflective folds combined with four refractive surfaces, all of them free-form, performing K��hler integration 2 . In K��hler devices, the irradiance uniformity is not achieved through additional optical stages (TIR prisms), thus no complex/expensive elements to manufacture are required. The rim angle and geometry are such that the secondary stage and receivers are hidden below the primary mirrors, so maximum collection is assured. The entire system was designed to allow loose assembly/alignment tolerances (through high acceptance angle) and to be manufactured using already well-developed methods for mass production, with high potential for low cost. The optical surfaces for K��hler integration, although with a quite different optical behavior, have approximately the same dimensions and can be manufactured with the same techniques as the more traditional secondary optical elements used for concentration (typically plastic injection molding or glass molding).

Full-Wafer Roller-NIL Processes for Silicon Solar Cell Texturisation

The highest solar cell efficiencies both for c-Si and mc-Si were reached using template based texturing processes. Especially for mc-Si the benefit of a defined texture, the so called honeycomb texture, was demonstrated impressively. However, up until now, no industrially feasible process has been available to pattern the necessary etching masks with the sufficient resolution. Roller-Nanoimprint Lithography (Roller-NIL) has the potential to overcome these limitations and to allow high quality pattern transfers, even in the sub-micron regime, in continuous in-line processes. Therefore, this etch-mask patterning technique is a suitable solution to bring such elaborate features like the honeycomb texture to an industrial realization. Beyond that, this fast printing-like technology opens up new possibilities to introduce promising concepts like photonic structures into solar cells.

Damage reduction of the laser drilling process on back contact solar cells by chemical treatment

Production of back contact solar cells requires holes generations on the wafers to keep both positive and negative contacts on the back side of the cell. This drilling process weakens the wafer mechanically due to the presence of the holes and the damage introduced during the process as microcracks. In this study, several chemical processes have been applied to drilled wafers in order to eliminate or reduce the damage generated during this fabrication step. The treatments analyzed are the followings: alkaline etching during 1, 3 and 5 minutes, acid etching for 2 and 4 minutes and texturisation. To determine mechanical strength of the samples a common mechanical study has been carried out testing the samples by the Ring on Ring bending test and obtaining the stress state in the moment of failure by FE simulation. Finally the results obtained for each treatment were fitted to a three parameter Weibull distribution

Structural challenges in multijunction solar cells for ultra-high CPV

The paths towards high efficiency multijunction solar cells operating inside real concentrators at ultra high concentration (>1000 suns) are described. The key addressed factors comprehend: 1) the development of an optimized tunnel junction with a high peak current density (240 A/cm2) to mitigate the non-uniform light profiles created by concentrators, 2) the inclusion of highly conductive semiconductor lateral layers to minimize the effects of the non-uniform light profiles in general, and the chromatic aberration in particular; and 3) an adequate design of reliability studies to test multijunction solar cells for real operation conditions in order to determine the fragile parts in the device and improve them. These challenges are faced by means of experimental and theoretical investigation using a quasi-3D distributed circuital model.

Crack origin and detection in thin cristallyne solar cells in a production line

In order to reduce cost and make up for the rising price of silicon, silicon wafers are sliced thinner and wider,eading to weaker wafers and increased breakage rates during fabrication process. In this work we have analysed different cracks origins and their effect on wafer���s mechanical strength. To enhance wafer���s strength some etching methods have been tested. Also, we have analysed wafers from different points of an entire standard production process. Mechanical strength of the wafers has been obtained via the four line bending test and detection of cracks has been tested with Resonance Ultrasonic Vibration (RUV) system, developed by the University of South Florida.

New laser-based approaches to improve the passivation and rear contact quality in high effiency crystalline silicon solar cells

Laser processing has been the tool of choice last years to develop improved concepts in contact formation for high efficiency crystalline silicon (c-Si) solar cells. New concepts based on standard laser fired contacts (LFC) or advanced laser doping (LD) techniques are optimal solutions for both the front and back contacts of a number of structures with growing interest in the c-Si PV industry. Nowadays, substantial efforts are underway to optimize these processes in order to be applied industrially in high efficiency concepts. However a critical issue in these devices is that, most of them, demand a very low thermal input during the fabrication sequence and a minimal damage of the structure during the laser irradiation process. Keeping these two objectives in mind, in this work we discuss the possibility of using laser-based processes to contact the rear side of silicon heterojunction (SHJ) solar cells in an approach fully compatible with the low temperature processing associated to these devices. First we discuss the possibility of using standard LFC techniques in the fabrication of SHJ cells on p-type substrates, studying in detail the effect of the laser wavelength on the contact quality. Secondly, we present an alternative strategy bearing in mind that a real challenge in the rear contact formation is to reduce the damage induced by the laser irradiation. This new approach is based on local laser doping techniques previously developed by our groups, to contact the rear side of p-type c-Si solar cells by means of laser processing before rear metallization of dielectric stacks containing Al2O3. In this work we demonstrate the possibility of using this new approach in SHJ cells with a distinct advantage over other standard LFC techniques.

Effect of soiling in CPV systems

The effect of soiling in flat PV modules has been already studied, causing a reduction of the electrical output of 4% on average. For CPV's, as far as soiling produces light scattering at the optical collector surface, the scattered rays should be definitively lost because they cannot be focused onto the receivers again. While the theoretical study becomes difficult because soiling is variable at different sites, it becomes easier to begin the monitoring of the real field performance of concentrators and then raise the following question: how much does the soiling affect to PV concentrators in comparison with flat panels?? The answers allow to predict the PV concentrator electrical performance and to establish a pattern of cleaning frequency. Some experiments have been conducted at the IES-UPM and CSES-ANU sites, consisting in linear reflective concentration systems, a point focus refractive concentrator and a flat module. All the systems have been measured when soiled and then after cleaning, achieving different increases of ISC. In general, results show that CPV systems are more sensitive to soiling than flat panels, accumulating losses in ISC of about 14% on average in three different tests conducted at IESUPM and CSES-ANU test sites in Madrid (Spain) and Canberra (Australia). Some concentrators can reach losses up to 26% when the system is soiled for 4 months of exposure.

Free-form optical systems for nonimaging applications : Sistemas ��pticos anam��rficos para aplicaciones anid��licas

La ��ptica anid��lica es una rama de la ��ptica cuyo desarrollo comenz�� a mediados de la d��cada de 1960. Este relativamente nuevo campo de la ��ptica se centra en la transferencia eficiente de la luz, algo necesario en muchas aplicaciones, entre las que destacamos los concentradores solares y los sistemas de iluminaci��n. Las soluciones de la ��ptica cl��sica a los problemas de la transferencia de energ��a de la luz s��lo son adecuadas cuando los rayos de luz son paraxiales. La condici��n paraxial no se cumple en la mayor��a de las aplicaciones para concentraci��n e iluminaci��n. Esta tesis contiene varios dise��os free-form (aquellos que no presentan ninguna simetr��a, ni de rotaci��n ni lineal) cuyas aplicaciones van destinadas a estos dos campos. El t��rmino nonimaging viene del hecho de que estos sistemas ��pticos no necesitan formar una imagen del objeto, aunque no formar la imagen no es una condici��n necesaria. Otra palabra que se utiliza a veces en lugar de nonimaging es la palabra anid��lico, viene del griego "an+eidolon" y tiene el mismo significado. La mayor��a de los sistemas ��pticos dise��ados para aplicaciones anid��licas no presentan ninguna simetr��a, es decir, son free-form (anam��rficos). Los sistemas ��pticos free-form est��n siendo especialmente relevantes durante los ��ltimos a��os gracias al desarrollo de las herramientas para su fabricaci��n como m��quinas de moldeo por inyecci��n y el mecanizado multieje. Sin embargo, solo recientemente se han desarrollado t��cnicas de dise��o anid��licas capaces de cumplir con estos grados de libertad. En aplicaciones de iluminaci��n el m��todo SMS3D permite dise��ar dos superficies free-form para controlar las fuentes de luz extensas. En los casos en que se requiere una elevada asimetr��a de la fuente, el objeto o las restricciones volum��tricos, las superficies free-form permiten obtener soluciones de mayor eficiencia, o disponer de menos elementos en comparaci��n con las soluciones de simetr��a de rotaci��n, dado que las superficies free-form tienen m��s grados de libertad y pueden realizar m��ltiples funciones debido a su naturaleza anam��rfica. Los concentradores anid��licos son muy adecuados para la captaci��n de energ��a solar, ya que el objetivo no es la reproducci��n de una imagen exacta del sol, sino sencillamente la captura de su energ��a. En este momento, el campo de la concentraci��n fotovoltaica (CPV) tiende hacia sistemas de alta concentraci��n con el fin de compensar el gasto de las c��lulas solares multi-uni��n (MJ) utilizadas como receptores, reduciendo su ��rea. El inter��s en el uso de c��lulas MJ radica en su alta eficiencia de conversi��n. Para obtener sistemas competitivos en aplicaciones terrestres se recurre a sistemas fotovoltaicos de alta concentraci��n (HCPV), con factores de concentraci��n geom��trica por encima de 500x. Estos sistemas se componen de dos (o m��s) elementos ��pticos (espejos y/o lentes). En los sistemas presentados a lo largo de este trabajo se presentan ejemplos de concentradores HCPV con elementos reflexivos como etapa primaria, as�� como concentradores con elementos refractivos (lente de Fresnel). Con la necesidad de aumentar la eficiencia de los sistemas HCPV reales y con el fin de proporcionar la divisi��n m��s eficiente del espectro solar, c��lulas conteniendo cuatro o m��s uniones (con un potencial de alcanzar eficiencias de m��s del 45% a una concentraci��n de cientos de soles) se exploran hoy en d��a. En esta tesis se presenta una de las posibles arquitecturas de divisi��n del espectro (spectrum-splitting en la literatura anglosajona) que utilizan c��lulas de concentraci��n comercial. Otro campo de aplicaci��n de la ��ptica nonimaging es la iluminaci��n, donde es necesario proporcionar un patr��n de distribuci��n de la iluminaci��n espec��fico. La iluminaci��n de estado s��lido (SSL), basada en la electroluminiscencia de materiales semiconductores, est�� proporcionando fuentes de luz para aplicaciones de iluminaci��n general. En la ��ltima d��cada, los diodos emisores de luz (LED) de alto brillo han comenzado a reemplazar a las fuentes de luz convencionales debido a la superioridad en la calidad de la luz emitida, elevado tiempo de vida, compacidad y ahorro de energ��a. Los colimadores utilizados con LEDs deben cumplir con requisitos tales como tener una alta eficiencia, un alto control del haz de luz, una mezcla de color espacial y una gran compacidad. Presentamos un colimador de luz free-form con microestructuras capaz de conseguir buena colimaci��n y buena mezcla de colores con una fuente de LED RGGB. Una buena mezcla de luz es importante no s��lo para simplificar el dise��o ��ptico de la luminaria sino tambi��n para evitar hacer binning de los chips. La mezcla de luz ��ptica puede reducir los costes al evitar la modulaci��n por ancho de pulso y otras soluciones electr��nicas patentadas para regulaci��n y ajuste de color. Esta tesis consta de cuatro cap��tulos. Los cap��tulos que contienen la obra original de esta tesis son precedidos por un cap��tulo introductorio donde se presentan los conceptos y definiciones b��sicas de la ��ptica geom��trica y en el cual se engloba la ��ptica nonimaging. Contiene principios de la ��ptica no formadora de imagen junto con la descripci��n de sus problemas y m��todos de dise��o. Asimismo se describe el m��todo de Superficies M��ltiples Simult��neas (SMS), que destaca por su versatilidad y capacidad de controlar varios haces de rayos. Adicionalmente tambi��n se describe la integraci��n K��hler y sus aplicaciones en el campo de la energ��a fotovoltaica. La concentraci��n fotovoltaica y la iluminaci��n de estado s��lido son introducidas junto con la revisi��n de su estado actual. El Segundo y Tercer Cap��tulo contienen dise��os ��pticos avanzados con aplicaci��n en la concentraci��n solar principalmente, mientras que el Cuarto Cap��tulo describe el colimador free-form con surcos que presenta buena mezcla de colores para aplicaciones de iluminaci��n. El Segundo Cap��tulo describe dos concentradores ��pticos HCPV dise��ados con el m��todo SMS en tres dimensiones (SMS3D) que llevan a cabo integraci��n K��hler en dos direcciones con el fin de proporcionar una distribuci��n de irradiancia uniforme libre de aberraciones crom��ticas sobre la c��lula solar. Uno de los dise��os es el concentrador XXR free-form dise��ado con el m��todo SMS3D, donde el espejo primario (X) y la lente secundaria (R) se dividen en cuatro sectores sim��tricos y llevan a cabo la integraci��n K��hler (proporcionando cuatro unidades del array K��hler), mientras que el espejo intermedio (X) presenta simetr��a rotacional. Otro concentrador HCPV presentado es el Fresnel-RXI (FRXI) con una lente de Fresnel funcionando como elemento primario (POE) y una lente RXI como elemento ��ptico secundario (SOE), que presenta configuraci��n 4-fold con el fin de realizar la integraci��n K��hler. Las lentes RXI son dispositivos nonimaging conocidos, pero su aplicaci��n como elemento secundario es novedosa. Los concentradores XXR y FRXI K��hler son ejemplos acad��micos de muy alta concentraci��n (m��s de 2,000x, mientras que los sistemas convencionales hoy en d��a no suelen llegar a 1,000x) preparados para las c��lulas solares N-uni��n (con N>3), que probablemente requerir��n una mayor concentraci��n y alta uniformidad espectral de irradiancia con el fin de obtener sistemas CPV terrestres eficientes y rentables. Ambos concentradores est��n dise��ados maximizando funciones de m��rito como la eficiencia ��ptica, el producto concentraci��n-aceptancia (CAP) y la uniformidad de irradiancia sobre la c��lula libre de la aberraci��n crom��tica (integraci��n K��hler). El Tercer Cap��tulo presenta una arquitectura para la divisi��n del espectro solar basada en un m��dulo HCPV con alta concentraci��n (500x) y ��ngulo de aceptancia alto (���>���1��) que tiene por objeto reducir ambas fuentes de p��rdidas de las c��lulas triple uni��n (3J) comerciales: el uso eficiente del espectro solar y la luz reflejada de los contactos met��licos y de la superficie de semiconductor. El m��dulo para la divisi��n del espectro utiliza el espectro solar m��s eficiente debido a la combinaci��n de una alta eficiencia de una c��lula de concentraci��n 3J (GaInP/GaInAs/Ge) y una de contacto posterior (BPC) de concentraci��n de silicio (Si), as�� como la t��cnica de confinamiento externo para la recuperaci��n de la luz reflejada por la c��lula 3J con el fin de ser reabsorbida por la c��lula. En la arquitectura propuesta, la c��lula 3J opera con su ganancia de corriente optimizada (concentraci��n geom��trica de 500x), mientras que la c��lula de silicio trabaja cerca de su ��ptimo tambi��n (135x). El m��dulo de spectrum-splitting consta de una lente de Fresnel plana como POE y un concentrador RXI free-form como SOE con un filtro paso-banda integrado en ��l. Tanto POE como SOE realizan la integraci��n K��hler para producir homogeneizaci��n de luz sobre la c��lula. El filtro paso banda env��a los fotones IR en la banda 900-1,150nm a la c��lula de silicio. Hay varios aspectos pr��cticos de la arquitectura del m��dulo presentado que ayudan a reducir la complejidad de los sistemas spectrum-splitting (el filtro y el secundario forman una sola pieza s��lida, ambas c��lulas son coplanarias simplific��ndose el cableado y la disipaci��n de calor, etc.). Prototipos prueba-de-concepto han sido ensamblados y probados a fin de demostrar la fabricabilidad del filtro y su rendimiento cuando se combina con la t��cnica de reciclaje de luz externa. Los resultados obtenidos se ajustan bastante bien a los modelos y a las simulaciones e invitan al desarrollo de una versi��n m��s compleja de este prototipo en el futuro. Dos colimadores s��lidos con surcos free-form se presentan en el Cuarto Cap��tulo. Ambos dise��os ��pticos est��n dise��ados originalmente usando el m��todo SMS3D. La segunda superficie ��pticamente activa est�� dise��ada a posteriori como una superficie con surcos. El dise��o inicial de dos espejos (XX) est�� dise��ado como prueba de concepto. En segundo lugar, el dise��o RXI free-form es comparable con los colimadores RXI existentes. Se trata de un dise��o muy compacto y eficiente que proporciona una muy buena mezcla de colores cuando funciona con LEDs RGB fuera del eje ��ptico como en los RGB LEDs convencionales. Estos dos dise��os son dispositivos free-form dise��ados con la intenci��n de mejorar las propiedades de mezcla de colores de los dispositivos no aplan��ticos RXI con simetr��a de revoluci��n y la eficiencia de los aplan��ticos, logrando una buena colimaci��n y una buena mezcla de colores. La capacidad de mezcla de colores del dispositivo no-aplan��tico mejora a��adiendo caracter��sticas de un aplan��tico a su hom��logo sim��trico sin p��rdida de eficiencia. En el caso del dise��o basado en RXI, su gran ventaja consiste en su menor coste de fabricaci��n ya que el proceso de metalizaci��n puede evitarse. Aunque algunos de los componentes presentan formas muy complejas, los costes de fabricaci��n son relativamente insensibles a la complejidad del molde, especialmente en el caso de la producci��n en masa (tales como inyecci��n de pl��stico), ya que el coste del molde se reparte entre todas las piezas fabricadas. Por ��ltimo, las ��ltimas dos secciones son las conclusiones y futuras l��neas de investigaci��n. ABSTRACT Nonimaging optics is a branch of optics whose development began in the mid-1960s. This rather new field of optics focuses on the efficient light transfer necessary in many applications, among which we highlight solar concentrators and illumination systems. The classical optics solutions to the problems of light energy transfer are only appropriate when the light rays are paraxial. The paraxial condition is not met in most applications for the concentration and illumination. This thesis explores several free-form designs (with neither rotational nor linear symmetry) whose applications are intended to cover the above mentioned areas and more. The term nonimaging comes from the fact that these optical systems do not need to form an image of the object, although it is not a necessary condition not to form an image. Another word sometimes used instead of nonimaging is anidolic, and it comes from the Greek ���an+eidolon��� and has the same meaning. Most of the optical systems designed for nonimaging applications are without any symmetry, i.e. free-form. Free-form optical systems become especially relevant lately with the evolution of free-form tooling (injection molding machines, multi-axis machining techniques, etc.). Nevertheless, only recently there are nonimaging design techniques that are able to meet these degrees of freedom. In illumination applications, the SMS3D method allows designing two free-form surfaces to control very well extended sources. In cases when source, target or volumetric constrains have very asymmetric requirements free-form surfaces are offering solutions with higher efficiency or with fewer elements in comparison with rotationally symmetric solutions, as free-forms have more degrees of freedom and they can perform multiple functions due to their free-form nature. Anidolic concentrators are well suited for the collection of solar energy, because the goal is not the reproduction of an exact image of the sun, but instead the collection of its energy. At this time, Concentration Photovoltaics (CPV) field is turning to high concentration systems in order to compensate the expense of multi-junction (MJ) solar cells used as receivers by reducing its area. Interest in the use of MJ cells lies in their very high conversion efficiency. High Concentration Photovoltaic systems (HCPV) with geometric concentration of more than 500x are required in order to have competitive systems in terrestrial applications. These systems comprise two (or more) optical elements, mirrors and/or lenses. Systems presented in this thesis encompass both main types of HCPV architectures: concentrators with primary reflective element and concentrators with primary refractive element (Fresnel lens). Demand for the efficiency increase of the actual HCPV systems as well as feasible more efficient partitioning of the solar spectrum, leads to exploration of four or more junction solar cells or submodules. They have a potential of reaching over 45% efficiency at concentration of hundreds of suns. One possible architectures of spectrum splitting module using commercial concentration cells is presented in this thesis. Another field of application of nonimaging optics is illumination, where a specific illuminance distribution pattern is required. The Solid State Lighting (SSL) based on semiconductor electroluminescence provides light sources for general illumination applications. In the last decade high-brightness Light Emitting Diodes (LEDs) started replacing conventional light sources due to their superior output light quality, unsurpassed lifetime, compactness and energy savings. Collimators used with LEDs have to meet requirements like high efficiency, high beam control, color and position mixing, as well as a high compactness. We present a free-form collimator with microstructures that performs good collimation and good color mixing with RGGB LED source. Good light mixing is important not only for simplifying luminaire optical design but also for avoiding die binning. Optical light mixing may reduce costs by avoiding pulse-width modulation and other patented electronic solutions for dimming and color tuning. This thesis comprises four chapters. Chapters containing the original work of this thesis are preceded by the introductory chapter that addresses basic concepts and definitions of geometrical optics on which nonimaging is developed. It contains fundamentals of nonimaging optics together with the description of its design problems, principles and methods, and with the Simultaneous Multiple Surface (SMS) method standing out for its versatility and ability to control several bundles of rays. K��hler integration and its applications in the field of photovoltaics are described as well. CPV and SSL fields are introduced together with the review on their background and their current status. Chapter 2 and Chapter 3 contain advanced optical designs with primarily application in solar concentration; meanwhile Chapter 4 portrays the free-form V-groove collimator with good color mixing property for illumination application. Chapter 2 describes two HCPV optical concentrators designed with the SMS method in three dimensions (SMS3D). Both concentrators represent K��hler integrator arrays that provide uniform irradiance distribution free from chromatic aberrations on the solar cell. One of the systems is the XXR free-form concentrator designed with the SMS3D method. The primary mirror (X) of this concentrator and secondary lens (R) are divided in four symmetric sectors (folds) that perform K��hler integration; meanwhile the intermediate mirror (X) is rotationally symmetric. Second HCPV concentrator is the Fresnel-RXI (FRXI) with flat Fresnel lens as the Primary Optical Element (POE) and an RXI lens as the Secondary Optical Element (SOE). This architecture manifests 4-fold configuration for performing K��hler integration (4 array units), as well. The RXI lenses are well-known nonimaging devices, but their application as SOE is novel. Both XXR and FRXI K��hler HCPV concentrators are academic examples of very high concentration (more than 2,000x meanwhile conventional systems nowadays have up to 1,000x) prepared for the near future N-junction (N>3) solar cells. In order to have efficient and cost-effective terrestrial CPV systems, those cells will probably require higher concentrations and high spectral irradiance uniformity. Both concentrators are designed by maximizing merit functions: the optical efficiency, concentration-acceptance angle (CAP) and cell-irradiance uniformity free from chromatic aberrations (K��hler integration). Chapter 3 presents the spectrum splitting architecture based on a HCPV module with high concentration (500x) and high acceptance angle (���>���1��). This module aims to reduce both sources of losses of the actual commercial triple-junction (3J) solar cells with more efficient use of the solar spectrum and with recovering the light reflected from the 3J cells��� grid lines and semiconductor surface. The solar spectrum is used more efficiently due to the combination of a high efficiency 3J concentration cell (GaInP/GaInAs/Ge) and external Back-Point-Contact (BPC) concentration silicon (Si) cell. By employing external confinement techniques, the 3J cell���s reflections are recovered in order to be re-absorbed by the cell. In the proposed concentrator architecture, the 3J cell operates at its optimized current gain (at geometrical concentration of 500x), while the Si cell works near its optimum, as well (135x). The spectrum splitting module consists of a flat Fresnel lens (as the POE), and a free-form RXI-type concentrator with a band-pass filter embedded in it (as the SOE), both POE and SOE performing K��hler integration to produce light homogenization. The band-pass filter sends the IR photons in the 900-1,150nm band to the Si cell. There are several practical aspects of presented module architecture that help reducing the added complexity of the beam splitting systems: the filter and secondary are forming a single solid piece, both cells are coplanar so the heat management and wiring is simplified, etc. Two proof-of-concept prototypes are assembled and tested in order to prove filter manufacturability and performance, as well as the potential of external light recycling technique. Obtained measurement results agree quite well with models and simulations, and show an opened path to manufacturing of the Fresnel RXI-type secondary concentrator with spectrum splitting strategy. Two free-form solid V-groove collimators are presented in Chapter 4. Both free-form collimators are originally designed with the SMS3D method. The second mirrored optically active surface is converted in a grooved surface a posteriori. Initial two mirror (XX) design is presented as a proof-of-concept. Second, RXI free-form design is comparable with existing RXI collimators as it is a highly compact and a highly efficient design. It performs very good color mixing of the RGGB LED sources placed off-axis like in conventional RGB LEDs. Collimators described here improve color mixing property of the prior art rotationally symmetric no-aplanatic RXI devices, and the efficiency of the aplanatic ones, accomplishing both good collimation and good color mixing. Free-form V-groove collimators enhance the no-aplanatic device's blending capabilities by adding aplanatic features to its symmetric counterpart with no loss in efficiency. Big advantage of the RXI design is its potentially lower manufacturing cost, since the process of metallization may be avoided. Although some components are very complicated for shaping, the manufacturing costs are relatively insensitive to the complexity of the mold especially in the case of mass production (such as plastic injection), as the cost of the mold is spread in many parts. Finally, last two sections are conclusions and future lines of investigation.

Interband absorption of photons by extended states in intermediate band solar cells

This paper considers sub-bandgap photon absorption in an InAs/GaAs quantum dot matrix. Absorption coefficients are calculated for transitions from the extended states in the valence band to confined states in the conduction band. This completes a previous body of work in which transitions between bound states were calculated. The calculations are based on the empirical k��p Hamiltonian considering the quantum dots as parallelepipeds. The extended states may be only partially extended?in one or two dimensions?or extended in all three dimensions. It is found that extended-to-bound transitions are, in general, weaker than bound-to-bound transitions, and that the former are weaker when the initial state is extended in more coordinates. This study is of direct application to the research of intermediate band solar cells and other semiconductor devices based on light absorption in semiconductors nanostructured with quantum dots.

Realistic detailed balance study of the quantum efficiency of quantum dot solar cells

An attractive but challenging technology for high efficiency solar energy conversion is the intermediate band solar cell (IBSC), whose theoretical efficiency limit is 63%, yet which has so far failed to yield high efficiencies in practice. The most advanced IBSC technology is that based on quantum dots (QDs): the QD-IBSC. In this paper, k��p calculations of photon absorption in the QDs are combined with a multi-level detailed balance model. The model has been used to reproduce the measured quantum efficiency of a real QD-IBSC and its temperature dependence. This allows the analysis of individual sub-bandgap transition currents, which has as yet not been possible experimentally, yielding a deeper understanding of the failure of current QD-IBSCs. Based on the agreement with experimental data, the model is believed to be realistic enough to evaluate future QD-IBSC proposals.

In-situ and Ex-situ characterization of III-V semiconductor materials and solar cells upon 10 MEV proton irradiation

In this work we present the results and analysis of a 10 MeV proton irradiation experiment performed on III-V semiconductor materials and solar cells. A set of representative devices including lattice-matched InGaP/GaInAs/Ge triple junction solar cells and single junction GaAs and InGaP component solar cells and a Ge diode were irradiated for different doses. The devices were studied in-situ before and after each exposure at dark and 1 sun AM0 illumination conditions, using a solar simulator connected to the irradiation chamber through a borosilicate glass window. Ex-situ characterization techniques included dark and 1 sun AM0 illumination I-V measurements. Furthermore, numerical simulation of the devices using D-AMPS-1D code together with calculations based on the TRIM software were performed in order to gain physical insight on the experimental results. The experiment also included the proton irradiation of an unprocessed Ge solar cell structure as well as the irradiation of a bare Ge(100) substrate. Ex-situ material characterization, after radioactive deactivation of the samples, includes Raman spectroscopy and spectral reflectivity.

Operational costs and reliability in a large rural electrification programme based on solar home systems

Experiences in decentralized rural electrification programmes using solar home systems have suffered difficulties during the operation and maintenance phase, due in many cases, to the underestimation of the maintenance cost, because of the decentralized character of the activity, and also because the reliability of the solar home system components is frequently unknown. This paper reports on the reliability study and cost characterization achieved in a large photovoltaic rural electrification programme carried out in Morocco. The paper aims to determinate the reliability features of the solar systems, focusing in the in-field testing for batteries and photovoltaic modules. The degradation rates for batteries and PV modules have been extracted from the in-field experiments. On the other hand, the main costs related to the operation and maintenance activity have been identified with the aim of establishing the main factors that lead to the failure of the quality sustainability in many rural electrification programmes.