Improved efficiency of microcrystalline silicon thin film solar cells with wide band-gap CdS buffer layer

In this paper, we have reported a new structure based upon an optical simulation of maximum light trapping and management in microcrystalline silicon thin film solar cells by using multi texture schemes and introducing an n-type cadmium sulphide (CdS) buffer layer with the goal of extreme light coupling and absorption in silicon absorber layer. Photon absorption was improved by optimising the front and back texturing of transparent conductive oxide (TCO) layers and variation in buffer layer thickness. We have demonstrated that light trapping can be improved with proposed geometry of 1μm thick crystalline silicon absorber layer below a thin layer of wide band gap material. We have improved the short circuit current densities by 1.35mA/cm2 resulting in a total short circuit current of 25 mA/cm2 and conversion efficiency of 9% with the addition of CdS buffer layer and multi textures, under global AM1.5 conditions. In this study, we have used 2 Dimensional Full Vectorial Finite Element (2DFVFEM) to design and optimize the proposed light propagation in solar cell structure configuration. Our simulation results show that interface morphology of CdS layer thickness and textures with different aspect and ratios have the most prominent influence on solar cell performance in terms of both short circuit current and quantum efficiency

Thin-film active device investigations Final status report, 1 Dec. 1967 - 31 Aug. 1968

Developing evaporated gallium arsenide to determine electrical and structural transport properties for coplanar electrode, space charge limited triode

Effects of target composition on the optical constants of DC sputtered ZnO: Al thin films

Al-doped ZnO thin films were deposited from ZnO:Al ceramic and Zn:Al metal alloy targets in Ar and Ar + O2 atmospheres respectively, using Direct Current (DC) magnetron sputtering. The samples exhibited transmittance T > 80% in visible region with good NIR shielding. The results indicated that, band gap energy ranged from 3.34 to 3.44 eV and 3.39 to 3.46 eV for films prepared from alloy and ceramic targets, respectively. Films obtained from alloy target at a substrate temperature of 200 oC showed low electrical sheet resistance of 10 Ω/sq, and highest values of mobility (15.9 cm2/Vs) and carrier concentration (2.98 × 1021 cm-3). However, films prepared from ceramic target at a substrate temperature of 300 oC revealed the highest sheet resistance of 32 Ω/sq, with lower values of mobility (14.1 cm2/Vs) and carrier concentration (1.92 × 1020 cm-3). The increase in sheet resistance and decrease in mobility as well as carrier concentration might be due to increased scattering centers for carriers, resulting to increased sheet resistance. Optical spectra of the films were fitted to SCOUT software in order to determine the refractive index, n and extinction coefficient, k. Generally, the calculated n and k in the visible part of the solar spectrum for different samples, ranged from 1.59 to 2.2 and 0.00013 to 0.0194 respectively, which are in agreement with results obtained using other methods. In general, the findings of this study shows that alloy target is suitable for deposition of ZnO:Althin films for devices/applications where low deposition temperature is required.Keywords: DC Magnetron Sputtering, Optical Constants, Transparent Conducting Oxides (TCO

New strategies in laser processing of TCOs for light management improvement in thin-film silicon solar cells

Light confinement strategies play a crucial role in the performance of thin-film (TF) silicon solar cells. One way to reduce the optical losses is the texturing of the transparent conductive oxide (TCO) that acts as the front contact. Other losses arise from the mismatch between the incident light spectrum and the spectral properties of the absorbent material that imply that low energy photons (below the bandgap value) are not absorbed, and therefore can not generate photocurrent. Up-conversion techniques, in which two sub-bandgap photons are combined to give one photon with a better matching with the bandgap, were proposed to overcome this problem. In particular, this work studies two strategies to improve light management in thin film silicon solar cells using laser technology. The first one addresses the problem of TCO surface texturing using fully commercial fast and ultrafast solid state laser sources. Aluminum doped Zinc Oxide (AZO) samples were laser processed and the results were optically evaluated by measuring the haze factor of the treated samples. As a second strategy, laser annealing experiments of TCOs doped with rare earth ions are presented as a potential process to produce layers with up-conversion properties, opening the possibility of its potential use in high efficiency solar cells

Optimization of processes for the rear side of monocrystalline silicon solar cells

En esta tesis, se ha estudiado la formación de un campo local superficial trasero (LBSF) de aluminio mediante el uso de diferentes pastas de dicho metal, las cuales contienen fritas en su composición para una mejor sinterización. Para la impresión de la pasta sobre los diferentes sustratos de silicio monocristalino de tipo p se han empleado dos técnicas diferentes. Por un lado, la serigrafía, técnica ampliamente usada en la industria fotovoltaica tanto para la formación de contactos frontales como posteriores. Por el otro lado, se ha utilizado también la técnica de dispensado, la cual permite una impresión de la pasta evitando el contacto con el sustrato, posibilitando la impresión sobre obleas muy finas de silicio evitando su rotura. Se han obtenido las resistencias específicas de contacto para las diferentes estructuras de aluminio creadas y los resultados se han comparado para diferentes pastas y sustratos. De esta manera, previa comprobación mediante imágenes microscópicas de cortes transversales de las muestras, se garantiza la formación del campo local superficial trasero. Con el objetivo de una mejora de la eficiencia de las células solares de silicio, se han depositado diferentes capas finas sobre sustratos de silicio mediante las técnicas de magnetrón sputtering y PECVD (Plasma Enhanced Chemical Vapor Deposition), con el fin de darle características pasivantes, como SiO2, SiNx y SiriON (silicon rich oxynitride); antirreflejantes, como SiNx; o de óxido conductor transparente para células de heterounión, como el caso de la capa de ZnO:Al. Para el estudio de dichas propiedades, se han empleado diversas técnicas, como QSSPC (Quasi-Steady State Photoconductance) para analizar la calidad de la pasivación y espectrofotometría para la medida de las propiedades ópticas. También se han empleado técnicas de caracterización de capa delgada para analizar su estructura y morfología. Sobre las capas finas depositadas sobre silicio, se ha realizado el depósito de las pastas de aluminio para, mediante el método FTC (fire through contact), formar un campo local superficial trasero. Este método se ha utilizado como alternativa a los que se suelen usar en la industria fotovoltaica, los cuales utilizan una serie de pasos que incrementan el coste del proceso total, como láseres o el depósito de resinas para la posterior eliminación por método químico antes de la impresión metálica. Se incluye un capítulo dedicado a la optimización del emisor de aluminio para células de contacto-trasero unión-trasera desarrollado en el Fraunhofer ISE (Alemania), con el objeto de aplicar las mismas técnicas de recubrimiento para el desarrollo de este nuevo tipo de celda solar y su posible transferencia a la producción industrial, atendiendo al proyecto de plan nacional de título Transferencia de las estructuras de alta eficiencia a la producción industrial

INCREASING SOLAR ENERGY CONVERSION EFFICIENCY IN HYDROGENATED AMORPHOUS SILICON PHOTOVOLTAIC DEVICES WITH PLASMONIC PERFECT META – ABSORBERS

Solar photovoltaic (PV) devices are an established, technically-viable and sustainable solution to society’s energy needs, however, in order to reach mass deployment at the terawatt scale, further decreases in the levelized cost of electricity from solar are needed. This can be accomplished with thin-film PV technologies by increasing the conversion efficiency using sophisticated light management methods. This ensures absorption of the entire solar spectrum, while reducing semiconductor absorber layer thicknesses, which reduces deposition time, material use, embodied energy and greenhouse gas emissions, and economic costs. Recent advances in optics, particularly in plasmonics and nanophotonics provide new theoretical methods to improve the optical enhancement in thin-film PV. The project involved designing and fabricating a plasmonic perfect meta-absorber integrated with hydrogenated amorphous silicon (a-Si:H) solar PV device to exhibit broadband, polarization-independent absorption and wide angle response simultaneously in the solar spectrum. First, recent advances in the use of plasmonic nanostructures forming metamaterials to improve absorption of light in thin-film solar PV devices is reviewed. Both theoretical and experimental work on multiple nanoscale geometries of plasmonic absorbers and PV materials shows that metallic nanostructures have a strong interaction with light, which enables unprecedented control over the propagation and the trapping of light in the absorber layer of thin-film PV device. Based on this, the geometry with the best potential for the proposed device is identified and used for device modelling and, finally the plasmonic enhanced n-i-p a-Si:H solar cell with top surface silver (Ag) metallic structure is proposed. In order for the plasmonic enhanced PV device to be commercialized the means of nanoparticle deposition must also be economical and scalable. In addition, the method to fabricate silver nanoparticles (AgNPs) must be at lower temperatures than those used in the fabrication process for a a-Si:H PV device (less than 180 0C). The results indicate the potential of multi-disperse self-assemble nanoparticles (SANPs) to achieve broadband resonant response for a-Si:H PV devices. Finally a plasmonic enhanced a-Si:H PV using multi-disperse SANPs is realized when AgNPs are integrated to the commercially fabricated nip-a-Si:H PV devices. The devices are characterized for both quantum efficiency and light I–V to evaluate the cell parameters (Jsc, Voc, FF and η). Real–time spectroscopic ellipsometry (RTSE) data is used to model the device performance and the theoretical parameters are compared with the experimental data. Conclusions are drawn and recommendations and future work is suggested

Structural, optical and vibrational properties of ZnO:M (M=Al3+ and Sr2+) nano and micropowders grown by hydrothermal synthesis

Powders of ZnO and ZnO:M (M = Al3+ and Sr2+) with 1 and 4% of M nominal content were synthetized by a hydrothermal method in a diethanolamine (DEA) medium. The samples were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), micro-Raman and photoluminescence (PL). The powder particles were spherical with average radius decreasing from 1 μm down to 70 nm with increasing Al3+ nominal content but nearly independent on the Sr2+ nominal content. The XRD and micro-Raman results indicate that both Al3+ and Sr2+ mostly incorporated substitutionally into the ZnO lattice, giving rise to compressive and tensile strain, respectively, as a result of ionic radii differences. The PL spectra for ZnO:Al exhibit a dopant-induced contribution at ∼3.1 eV, which is not observed for ZnO:Sr, due to radiative transitions involving trapping of photocarriers at theoretically expected substitutional Al3+ donor states or at Zn interstitial defects.Fil: Marín Ramírez, Oscar Alonso. Universidad Nacional de Tucumán. Instituto de Física del Noroeste Argentino. - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet Noa Sur. Instituto de Física del Noroeste Argentino; Argentina. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Laboratorio de Física del Sólido; ArgentinaFil: Soliz, Tania. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología; ArgentinaFil: Gutierrez, Jorge Andrés. Universidad del Quindío. Facultad de Ciencias Básicas y Tecnológicas; ColombiaFil: Tirado, Monica Cecilia. Universidad Nacional de Tucumán. Instituto de Física del Noroeste Argentino. - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet Noa Sur. Instituto de Física del Noroeste Argentino; Argentina. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Física. Departamento de Nanomateriales y Propiedades Dieléctricas; ArgentinaFil: Figueroa, Carlos. Universidad Nacional de Tucumán. Instituto de Física del Noroeste Argentino. - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet Noa Sur. Instituto de Física del Noroeste Argentino; Argentina. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Laboratorio de Física del Sólido; ArgentinaFil: Comedi, David Mario. Universidad Nacional de Tucumán. Instituto de Física del Noroeste Argentino. - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet Noa Sur. Instituto de Física del Noroeste Argentino; Argentina. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Laboratorio de Física del Sólido; Argentin