Electrical characterization of amorphous silicon MIS-based structures for HIT solar cell applications

A complete electrical characterization of hydrogenated amorphous silicon layers (a-Si:H) deposited on crystalline silicon (c-Si) substrates by electron cyclotron resonance chemical vapor deposition (ECR-CVD) was carried out. These structures are of interest for photovoltaic applications. Different growth temperatures between 30 and 200 °C were used. A rapid thermal annealing in forming gas atmosphere at 200 °C during 10 min was applied after the metallization process. The evolution of interfacial state density with the deposition temperature indicates a better interface passivation at higher growth temperatures. However, in these cases, an important contribution of slow states is detected as well. Thus, using intermediate growth temperatures (100–150 °C) might be the best choice.

Insulator-to-metal transition in vanadium supersaturated silicon: variable-range hopping and Kondo effect signatures

We report the observation of the insulator-to-metal transition in crystalline silicon samples supersaturated with vanadium. Ion implantation followed by pulsed laser melting and rapid resolidification produce high quality single-crystalline silicon samples with vanadium concentrations that exceed equilibrium values in more than 5 orders of magnitude. Temperature-dependent analysis of the conductivity and Hall mobility values for temperatures from 10K to 300K indicate that a transition from an insulating to a metallic phase is obtained at a vanadium concentration between 1.1 × 10^(20) and 1.3 × 10^(21) cm^(−3) . Samples in the insulating phase present a variable-range hopping transport mechanism with a Coulomb gap at the Fermi energy level. Electron wave function localization length increases from 61 to 82 nm as the vanadium concentration increases in the films, supporting the theory of impurity band merging from delocalization of levels states. On the metallic phase, electronic transport present a dispersion mechanism related with the Kondo effect, suggesting the presence of local magnetic moments in the vanadium supersaturated silicon material.

Blue-enhanced thin-film photodiode for dual-screen x-ray imaging

This article reports on a-Si:H-based low-leakage blue-enhanced photodiodes for dual-screen x-ray imaging detectors. Doped nanocrystalline silicon was incorporated in both the n- and p-type regions to reduce absorption losses for light incoming from the top and bottom screens. The photodiode exhibits a dark current density of 900 pA/cm(2) and an external quantum efficiency up to 90% at a reverse bias of 5 V. In the case of illumination through the tailored p-layer, the quantum efficiency of 60% at a 400 nm wavelength is almost double that for the conventional a-Si:H n-i-p photodiode.

High-performance flexible hybrid field-effect transistors based on cellulose fiber paper

IEEE Electron Device Letters, VOL. 29, NO. 9,

Kr-Ar laser Raman spectrometer for low temperature measurements /

A Czerny Mount double monochromator is used to measure Raman scattered radiation near 90" from a crystalline, Silicon sample. Incident light is provided by a mixed gas Kr-Ar laser, operating at 5145 A. The double monochromator is calibrated to true wavelength by comparison of Kr and Ar emission Une positions (A) to grating position (A) display [1]. The relationship was found to be hnear and can be described by, y = 1.219873a; - 1209.32, (1) where y is true wavelength (A) and xis grating position display (A). The Raman emission spectra are collected via C"*""*" encoded software, which displays a mV signal from a Photodetector and allows stepping control of the gratings via an A/D interface. [2] The software collection parameters, detector temperature and optics are optimised to yield the best quality spectra. The inclusion of a cryostat allows for temperatmre dependent capabihty ranging from 4 K to w 350 K. Silicon Stokes temperatm-e dependent Raman spectra, generally show agreement with Uterature results [3] in their frequency haxdening, FWHM reduction and intensity increase as temperature is reduced. Tests reveal that a re-alignment of the double monochromator is necessary before spectral resolution can approach literature standard. This has not yet been carried out due to time constraints.

Etude théorique des propriétés électroniques et optiques des super-réseaux de Si/SiO2

FRANCAIS: L'observation d'une intense luminescence dans les super-réseaux de Si/SiO2 a ouvert de nouvelles avenues en recherche théorique des matériaux à base de silicium, pour des applications éventuelles en optoélectronique. Le silicium dans sa phase cristalline possède un gap indirect, le rendant ainsi moins intéressant vis-à-vis d'autres matériaux luminescents. Concevoir des matériaux luminescents à base de silicium ouvrira donc la voie sur de multiples applications. Ce travail fait état de trois contributions au domaine. Premièrement, différents modèles de super-réseaux de Si/SiO2 ont été conçus et étudiés à l'aide de calculs ab initio afin d'en évaluer les propriétés structurales, électroniques et optiques. Les deux premiers modèles dérivés des structures cristallines du silicium et du dioxyde de silicium ont permis de démontrer l'importance du rôle de l'interface Si/SiO2 sur les propriétés optiques. De nouveaux modèles structurellement relaxés ont alors été construits afin de mieux caractériser les interfaces et ainsi mieux évaluer la portée du confinement sur les propriétés optiques. Deuxièmement, un gap direct dans les modèles structurellement relaxés a été obtenu. Le calcul de l'absorption (par l'application de la règle d'or de Fermi) a permis de confirmer que les propriétés d'absorption (et d'émission) du silicium cristallin sont améliorées lorsque celui-ci est confiné par le SiO2. Un décalage vers le bleu avec accroissement du confinement a aussi été observé. Une étude détaillée du rôle des atomes sous-oxydés aux interfaces a de plus été menée. Ces atomes ont le double effet d'accroître légèrement le gap d'énergie et d'aplanir la structure électronique près du niveau de Fermi. Troisièmement, une application directe de la théorique des transitions de Slater, une approche issue de la théorie de la fonctionnelle de la densité pour des ensembles, a été déterminée pour le silicium cristallin puis comparée aux mesures d'absorption par rayons X. Une très bonne correspondance entre cette théorie et l'expérience est observée. Ces calculs ont été appliqués aux super-réseaux afin d'estimer et caractériser leurs propriétés électroniques dans la zone de confinement, dans les bandes de conduction.

Étude de la formation et de l'évolution de nanostructures par méthodes Monte Carlo

Cette thèse, composée de quatre articles scientifiques, porte sur les méthodes numériques atomistiques et leur application à des systèmes semi-conducteurs nanostructurés. Nous introduisons les méthodes accélérées conçues pour traiter les événements activés, faisant un survol des développements du domaine. Suit notre premier article, qui traite en détail de la technique d'activation-relaxation cinétique (ART-cinétique), un algorithme Monte Carlo cinétique hors-réseau autodidacte basé sur la technique de l'activation-relaxation nouveau (ARTn), dont le développement ouvre la voie au traitement exact des interactions élastiques tout en permettant la simulation de matériaux sur des plages de temps pouvant atteindre la seconde. Ce développement algorithmique, combiné à des données expérimentales récentes, ouvre la voie au second article. On y explique le relâchement de chaleur par le silicium cristallin suite à son implantation ionique avec des ions de Si à 3 keV. Grâce à nos simulations par ART-cinétique et l'analyse de données obtenues par nanocalorimétrie, nous montrons que la relaxation est décrite par un nouveau modèle en deux temps: "réinitialiser et relaxer" ("Replenish-and-Relax"). Ce modèle, assez général, peut potentiellement expliquer la relaxation dans d'autres matériaux désordonnés. Par la suite, nous poussons l'analyse plus loin. Le troisième article offre une analyse poussée des mécanismes atomistiques responsables de la relaxation lors du recuit. Nous montrons que les interactions élastiques entre des défauts ponctuels et des petits complexes de défauts contrôlent la relaxation, en net contraste avec la littérature qui postule que des "poches amorphes" jouent ce rôle. Nous étudions aussi certains sous-aspects de la croissance de boîtes quantiques de Ge sur Si (001). En effet, après une courte mise en contexte et une introduction méthodologique supplémentaire, le quatrième article décrit la structure de la couche de mouillage lors du dépôt de Ge sur Si (001) à l'aide d'une implémentation QM/MM du code BigDFT-ART. Nous caractérisons la structure de la reconstruction 2xN de la surface et abaissons le seuil de la température nécessaire pour la diffusion du Ge en sous-couche prédit théoriquement par plus de 100 K.

Efficient 1535 nm light emission from an all-Si-based optical micro-cavity containing Er3+ and Yb3+ ions

This work reports on the construction and spectroscopic analyses of optical micro-cavities (OMCs) that efficiently emit at ~1535 nm. The emission wavelength matches the third transmission window of commercial optical fibers and the OMCs were entirely based on silicon. The sputtering deposition method was adopted in the preparation of the OMCs, which comprised two Bragg reflectors and one spacer layer made of either Er- or ErYb-doped amorphous silicon nitride. The luminescence signal extracted from the OMCs originated from the 4I13/2→4I15/2 transition (due to Er3+ ions) and its intensity showed to be highly dependent on the presence of Yb3+ ions.According to the results, the Er3+-related light emission was improved by a factor of 48 when combined with Yb3+ ions and inserted in the spacer layer of the OMC. The results also showed the effectiveness of the present experimental approach in producing Si-based light-emitting structures in which the main characteristics are: (a) compatibility with the actual microelectronics industry, (b) the deposition of optical quality layers with accurate composition control, and (c) no need of uncommon elements-compounds nor extensive thermal treatments. Along with the fundamental characteristics of the OMCs, this work also discusses the impact of the Er3+-Yb3+ ion interaction on the emission intensity as well as the potential of the present findings.

Numerical simulation of interdigitated back contact hetero-junction solar cells

The goal of this thesis is the application of an opto-electronic numerical simulation to heterojunction silicon solar cells featuring an all back contact architecture (Interdigitated Back Contact Hetero-Junction IBC-HJ). The studied structure exhibits both metal contacts, emitter and base, at the back surface of the cell with the objective to reduce the optical losses due to the shadowing by front contact of conventional photovoltaic devices. Overall, IBC-HJ are promising low-cost alternatives to monocrystalline wafer-based solar cells featuring front and back contact schemes, in fact, for IBC-HJ the high concentration doping diffusions are replaced by low-temperature deposition processes of thin amorphous silicon layers. Furthermore, another advantage of IBC solar cells with reference to conventional architectures is the possibility to enable a low-cost assembling of photovoltaic modules, being all contacts on the same side. A preliminary extensive literature survey has been helpful to highlight the specific critical aspects of IBC-HJ solar cells as well as the state-of-the-art of their modeling, processing and performance of practical devices. In order to perform the analysis of IBC-HJ devices, a two-dimensional (2-D) numerical simulation flow has been set up. A commercial device simulator based on finite-difference method to solve numerically the whole set of equations governing the electrical transport in semiconductor materials (Sentuarus Device by Synopsys) has been adopted. The first activity carried out during this work has been the definition of a 2-D geometry corresponding to the simulation domain and the specification of the electrical and optical properties of materials. In order to calculate the main figures of merit of the investigated solar cells, the spatially resolved photon absorption rate map has been calculated by means of an optical simulator. Optical simulations have been performed by using two different methods depending upon the geometrical features of the front interface of the solar cell: the transfer matrix method (TMM) and the raytracing (RT). The first method allows to model light prop-agation by plane waves within one-dimensional spatial domains under the assumption of devices exhibiting stacks of parallel layers with planar interfaces. In addition, TMM is suitable for the simulation of thin multi-layer anti reflection coating layers for the reduction of the amount of reflected light at the front interface. Raytracing is required for three-dimensional optical simulations of upright pyramidal textured surfaces which are widely adopted to significantly reduce the reflection at the front surface. The optical generation profiles are interpolated onto the electrical grid adopted by the device simulator which solves the carriers transport equations coupled with Poisson and continuity equations in a self-consistent way. The main figures of merit are calculated by means of a postprocessing of the output data from device simulation. After the validation of the simulation methodology by means of comparison of the simulation result with literature data, the ultimate efficiency of the IBC-HJ architecture has been calculated. By accounting for all optical losses, IBC-HJ solar cells result in a theoretical maximum efficiency above 23.5% (without texturing at front interface) higher than that of both standard homojunction crystalline silicon (Homogeneous Emitter HE) and front contact heterojuction (Heterojunction with Intrinsic Thin layer HIT) solar cells. However it is clear that the criticalities of this structure are mainly due to the defects density and to the poor carriers transport mobility in the amorphous silicon layers. Lastly, the influence of the most critical geometrical and physical parameters on the main figures of merit have been investigated by applying the numerical simulation tool set-up during the first part of the present thesis. Simulations have highlighted that carrier mobility and defects level in amorphous silicon may lead to a potentially significant reduction of the conversion efficiency.

Nanometer-size anisotropy of injection-molded polymer micro-cantilever arrays

Understanding and controlling the structural anisotropies of injection-molded polymers is vital for designing products such as cantilever-based sensors. Such micro-cantilevers are considered as cost-effective alternatives to single-crystalline silicon-based sensors. In order to achieve similar sensing characteristics,structure and morphology have to be controlled by means of processing parameters including mold temperature and injection speed. Synchrotron radiation-based scanning small- (SAXS) and wide-angle x-ray scattering techniques were used to quantify crystallinity and anisotropy in polymer micro-cantilevers with micrometer resolution in real space. SAXS measurements confirmed the lamellar nature of the injection-molded semi-crystalline micro-cantilevers. The homogenous cantilever material exhibits a lamellar periodicity increasing with mold temperature but not with injection speed. We demonstrate that micro-cantilevers made of semi-crystalline polymers such as polyvinylidenefluoride, polyoxymethylene, and polypropylene show the expected strong degree of anisotropy along the injection direction.

A numerical study of Bi-periodic binary diffraction gratings for solar cell applications

In this paper, a numerical study is made of simple bi-periodic binary diffraction gratings for solar cell applications. The gratings consist of hexagonal arrays of elliptical towers and wells etched directly into the solar cell substrate. The gratings are applied to two distinct solar cell technologies: a quantum dot intermediate band solar cell (QD-IBSC) and a crystalline silicon solar cell (SSC). In each case, the expected photocurrent increase due to the presence of the grating is calculated assuming AM1.5D illumination. For each technology, the grating period, well/tower depth and well/tower radii are optimised to maximise the photocurrent. The optimum parameters are presented. Results are presented for QD-IBSCs with a range of quantum dot layers and for SSCs with a range of thicknesses. For the QD-IBSC, it is found that the optimised grating leads to an absorption enhancement above that calculated for an ideally Lambertian scatterer for cells with less than 70 quantum dot layers. In a QD-IBSC with 50 quantum dot layers equipped with the optimum grating, the weak intermediate band to conduction band transition absorbs roughly half the photons in the corresponding sub-range of the AM1.5D spectrum. For the SSC, it is found that the optimised grating leads to an absorption enhancement above that calculated for an ideally Lambertian scatterer for cells with thicknesses of 10 ?m or greater. A 20um thick SSC equipped with the optimised grating leads to an absorption enhancement above that of a 200um thick SSC equipped with a planar back reflector.

Laser-fired contact optimization in c-Si solar cells

In this work we study the optimization of laser-fired contact (LFC) processing parameters, namely laser power and number of pulses, based on the electrical resistance measurement of an aluminum single LFC point. LFC process has been made through four passivation layers that are typically used in c-Si and mc-Si solar cell fabrication: thermally grown silicon oxide (SiO2), deposited phosphorus-doped amorphous silicon carbide (a-SiCx/H(n)), aluminum oxide (Al2O3) and silicon nitride (SiNx/H) films. Values for the LFC resistance normalized by the laser spot area in the range of 0.65–3 mΩ cm2 have been obtained

Dissolution and gettering of iron during contact co-firing

The dissolution and gettering of iron is studied during the final fabrication step of multicrystalline silicon solar cells, the co-firing step, through simulations and experiments. The post-processed interstitial iron concentration is simulated according to the as-grown concentration and distribution of iron within a silicon wafer, both in the presence and absence of the phosphorus emitter, and applying different time-temperature profiles for the firing step. The competing effects of dissolution and gettering during the short annealing process are found to be strongly dependant on the as-grown material quality. Furthermore, increasing the temperature of the firing process leads to a higher dissolution of iron, hardly compensated by the higher diffusivity of impurities. A new defect engineering tool is introduced, the extended co-firing, which could allow an enhanced gettering effect within a small additional time

Microprocesos láser para la mejora de dispositivos fotovoltaicos basados en silicio cristalino

En los últimos años la tecnología láser se ha convertido en una herramienta imprescindible en la fabricación de dispositivos fotovoltaicos, ayudando a la consecución de dos objetivos claves para que esta opción energética se convierta en una alternativa viable: reducción de costes de fabricación y aumento de eficiencia de dispositivo. Dentro de las tecnologías fotovoltaicas, las basadas en silicio cristalino (c-Si) siguen siendo las dominantes en el mercado, y en la actualidad los esfuerzos científicos en este campo se encaminan fundamentalmente a conseguir células de mayor eficiencia a un menor coste encontrándose, como se comentaba anteriormente, que gran parte de las soluciones pueden venir de la mano de una mayor utilización de tecnología láser en la fabricación de los mismos. En este contexto, esta Tesis hace un estudio completo y desarrolla, hasta su aplicación en dispositivo final, tres procesos láser específicos para la optimización de dispositivos fotovoltaicos de alta eficiencia basados en silicio. Dichos procesos tienen como finalidad la mejora de los contactos frontal y posterior de células fotovoltaicas basadas en c-Si con vistas a mejorar su eficiencia eléctrica y reducir el coste de producción de las mismas. En concreto, para el contacto frontal se han desarrollado soluciones innovadoras basadas en el empleo de tecnología láser en la metalización y en la fabricación de emisores selectivos puntuales basados en técnicas de dopado con láser, mientras que para el contacto posterior se ha trabajado en el desarrollo de procesos de contacto puntual con láser para la mejora de la pasivación del dispositivo. La consecución de dichos objetivos ha llevado aparejado el alcanzar una serie de hitos que se resumen continuación: - Entender el impacto de la interacción del láser con los distintos materiales empleados en el dispositivo y su influencia sobre las prestaciones del mismo, identificando los efectos dañinos e intentar mitigarlos en lo posible. - Desarrollar procesos láser que sean compatibles con los dispositivos que admiten poca afectación térmica en el proceso de fabricación (procesos a baja temperatura), como los dispositivos de heterounión. - Desarrollar de forma concreta procesos, completamente parametrizados, de definición de dopado selectivo con láser, contactos puntuales con láser y metalización mediante técnicas de transferencia de material inducida por láser. - Definir tales procesos de forma que reduzcan la complejidad de la fabricación del dispositivo y que sean de fácil integración en una línea de producción. - Mejorar las técnicas de caracterización empleadas para verificar la calidad de los procesos, para lo que ha sido necesario adaptar específicamente técnicas de caracterización de considerable complejidad. - Demostrar su viabilidad en dispositivo final. Como se detalla en el trabajo, la consecución de estos hitos en el marco de desarrollo de esta Tesis ha permitido contribuir a la fabricación de los primeros dispositivos fotovoltaicos en España que incorporan estos conceptos avanzados y, en el caso de la tecnología de dopado con láser, ha permitido hacer avances completamente novedosos a nivel mundial. Asimismo los conceptos propuestos de metalización con láser abren vías, completamente originales, para la mejora de los dispositivos considerados. Por último decir que este trabajo ha sido posible por una colaboración muy estrecha entre el Centro Láser de la UPM, en el que la autora desarrolla su labor, y el Grupo de Investigación en Micro y Nanotecnologías de la Universidad Politécnica de Cataluña, encargado de la preparación y puesta a punto de las muestras y del desarrollo de algunos procesos láser para comparación. También cabe destacar la contribución de del Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, CIEMAT, en la preparación de experimentos específicos de gran importancia en el desarrollo del trabajo. Dichas colaboraciones se han desarrollado en el marco de varios proyectos, tales como el proyecto singular estratégico PSE-MICROSIL08 (PSE-iv 120000-2006-6), el proyecto INNDISOL (IPT-420000-2010-6), ambos financiados por el Fondo Europeo de Desarrollo Regional FEDER (UE) “Una manera de hacer Europa” y el MICINN, y el proyecto del Plan Nacional AMIC (ENE2010-21384-C04-02), cuya financiación ha permitido en gran parte llevar a término este trabajo. v ABSTRACT. Last years lasers have become a fundamental tool in the photovoltaic (PV) industry, helping this technology to achieve two major goals: cost reduction and efficiency improvement. Among the present PV technologies, crystalline silicon (c-Si) maintains a clear market supremacy and, in this particular field, the technological efforts are focussing into the improvement of the device efficiency using different approaches (reducing for instance the electrical or optical losses in the device) and the cost reduction in the device fabrication (using less silicon in the final device or implementing more cost effective production steps). In both approaches lasers appear ideally suited tools to achieve the desired success. In this context, this work makes a comprehensive study and develops, until their implementation in a final device, three specific laser processes designed for the optimization of high efficiency PV devices based in c-Si. Those processes are intended to improve the front and back contact of the considered solar cells in order to reduce the production costs and to improve the device efficiency. In particular, to improve the front contact, this work has developed innovative solutions using lasers as fundamental processing tools to metalize, using laser induced forward transfer techniques, and to create local selective emitters by means of laser doping techniques. On the other side, and for the back contact, and approached based in the optimization of standard laser fired contact formation has been envisaged. To achieve these fundamental goals, a number of milestones have been reached in the development of this work, namely: - To understand the basics of the laser-matter interaction physics in the considered processes, in order to preserve the functionality of the irradiated materials. - To develop laser processes fully compatible with low temperature device concepts (as it is the case of heterojunction solar cells). - In particular, to parameterize completely processes of laser doping, laser fired contacts and metallization via laser transfer of material. - To define such a processes in such a way that their final industrial implementation could be a real option. - To improve widely used characterization techniques in order to be applied to the study of these particular processes. - To probe their viability in a final PV device. Finally, the achievement of these milestones has brought as a consequence the fabrication of the first devices in Spain incorporating these concepts. In particular, the developments achieved in laser doping, are relevant not only for the Spanish science but in a general international context, with the introduction of really innovative concepts as local selective emitters. Finally, the advances reached in the laser metallization approached presented in this work open the door to future developments, fully innovative, in the field of PV industrial metallization techniques. This work was made possible by a very close collaboration between the Laser Center of the UPM, in which the author develops his work, and the Research Group of Micro y Nanotecnology of the Universidad Politécnica de Cataluña, in charge of the preparation and development of samples and the assessment of some laser processes for comparison. As well is important to remark the collaboration of the Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, CIEMAT, in the preparation of specific experiments of great importance in the development of the work. These collaborations have been developed within the framework of various projects such as the PSE-MICROSIL08 (PSE-120000-2006-6), the project INNDISOL (IPT-420000-2010-6), both funded by the Fondo Europeo de Desarrollo Regional FEDER (UE) “Una manera de hacer Europa” and the MICINN, and the project AMIC (ENE2010-21384-C04-02), whose funding has largely allowed to complete this work.

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.