High-rate deposition of nano-crystalline silicon thin films on plastics

Nanocrystalline silicon (nc-Si:H) is commonly used in the bottom cell of tandem solar cells. With an indirect bandgap, nc-Si:H requires thicker (∼1 µm) films for efficient light harvesting than amorphous Si (a-Si:H) does. Therefore, thin-film high deposition rates are crucial for further cost reduction of highly efficient a–Si:H based photovoltaic technology. Plastic substrates allow for further cost reduction by enabling roll-to-roll inline deposition. In this work, high nc-Si:H deposition rates on plastic were achieved at low substrate temperature (150 °C) by standard Radio-frequency (13.56 MHz) Plasma Enhanced Chemical Vapor Deposition. Focus was on the influence of deposition pressure, inter-electrode distance (1.2 cm) and high power coupled to the plasma, on the hydrogen-to-silane dilution ratios (HD) necessary to achieve the amorphous-to-nanocrystalline phase transition and on the resulting film deposition rate. For each pressure and rf-power, there is a value of HD for which the films start to exhibit a certain amount of crystalline fraction. For constant rf-power, this value increases with pressure. Within the parameter range studied the deposition rate was highest (0.38 nm/s) for nc-Si:H films deposited at 6 Torr, 700 mW/cm2 using HD of 98.5 %. Decreasing the pressure to 3 Torr (1.5 Torr) and rf-power to 350 mW/cm2 using HD – 98.5 % deposition rate is 0.12 nm/s (0.076 nm/s). Raman crystalline fraction of these films is 72, 62 and 53 % for the 6, 3 and 1.5 Torr films, respectively.Fundação para a Ciência e a Tecnologia (FCT)DREBM/PICS_CNRS/201

Piezoresistive silicon thin film sensor array for biomedical applications

N-type hydrogenated nanocrystalline silicon thin film piezoresistors, with gauge factor −28, were deposited on rugged and flexible polyimide foils by Hot-wire chemical vapor deposition using a tantalum filament heated to 1750 °C. The piezoresistive response under cyclic quasi-static and dynamical (up to 100 Hz) load conditions is reported. Test structures, consisting of microresistors having lateral dimensions in the range from 50 to 100 μm and thickness of 120 nm were defined in an array by reactive ion etching. Metallic pads, forming ohmic contacts to the sensing elements, were defined by a lift-off process. A readout circuit for the array consisting in a mutiplexer on each row and column of the matrix is proposed. The digital data will be processed, interpreted and stored internally by an ultra low-power micro controller, also responsible for the communication of two-way wireless data, e.g. from inside to outside the human body.© 2011 Elsevier B.V. All rights reservedFundação para a Ciência e a Tecnologia (FCT

Deposition of silicon nitride thin films by hot-wire CVD at 100ºC and 250ºC

Silicon nitride thin films for use as passivation layers in solar cells and organic electronics or as gate dielectrics in thin-film transistors were deposited by the Hot-wire chemical vapor deposition technique at a high deposition rate (1-3 Ǻ/s) and at low substrate temperature. Films were deposited using NH3/SiH4 flow rate ratios between 1 and 70 and substrate temperatures of 100º C and 250ºC. For NH3/SiH4 ratios between 40 and 70, highly transparent (T ~ 90%), dense films (2.56 - 2.74 g/cm3) with good dielectric properties and refractive index between 1.93 and 2.08 were deposited on glass substrates. Etch rates in BHF of 2.7 Ǻ/s and 10 MV cm−1.Fundação para a Ciência e Tecnologia (FCT) - FCT/CNRS programa com o contracto no. 20798, bolsa de investigaçao e projecto PTDC-CTM-66558-200

Thin-film silicon solar cells and sensors deposited on flexible substrates

Tese de doutoramento do Programa Doutoral em Física (MAP-fis)Células solares flexíveis de filmes finos de silício são geralmente fabricadas a baixa temperatura sobre substratos de plástico ou a mais elevadas temperaturas sobre folhas de aço. Esta tese reporta o estudo da deposição de filmes finos sobre diferentes substratos de plástico, transparentes e coloridos, para células solares do tipo sobrestrato e substrato, respectivamente. Como objetivo co-lateral, os filmes dopados depositados sobre plástico foram usados como sensores de deformação, utilizando as suas propriedades piezo-resistivas. Elevadas taxas de deposição dos filmes de silício depositados sobre plástico foram obtidas a baixa temperatura do substrato (150ºC) por rf-PECVD. A influência de diferentes parâmetros de deposição sobre as propriedades e taxa de deposição dos filmes resultantes foram estudados e correlacionados. Células solares de filmes finos de silício amorfo e microcristalino foram desenvolvidas a baixas temperaturas sobre plásticos. Eficiências de 5 – 6.5% foram alcançadas para as células amorfas e 7.5% para as células microcristalinas. Efeitos de aprisionamento da luz foram estudados através da texturização por ablação laser de substratos de plástico e corrosão úmida de TCO sobre plástico. Filmes finos de silício microcristalino, depositados por HW-CVD, com fator piezoresistivo de -32.2, foram usados para fabricar sensores de deformação em uma membrana plástica muito fina (15 μm). Estruturas de teste em têxtil e a miniaturização dos sensores piezoresistivos depositados sobre substratos flexíveis de poliimida foram abordados.Flexible thin film silicon photovoltaics are usually done on low temperature plastic substrates or on stainless steel foil. This thesis reports on the study of thin film deposition on different plastic substrates, both transparent and colored, for superstrate and substrate solar cells, respectively. Some of the optimized doped layers deposited on plastics were used as strain gauges based on their piezo-resistive properties. High-rate deposited silicon films on plastic were achieved at low substrate temperature (150oC) by standard Radio-frequency (13.56 MHz) Plasma Enhanced Chemical Vapor Deposition (rf-PECVD). The influence of different deposition parameters on the resulting film properties and deposition rate were studied and correlated. Thin film silicon solar cells were developed at low temperatures on plastics. Efficiencies of 5 – 6.5% were achieved for amorphous cells deposited by rf-PECVD, and 7.5% for microcrystalline cells deposited at Very High Frequency (81.36 MHz) Plasma Enhanced Chemical Vapor Deposition (VHF-PECVD). Light trapping effects were studied by laser texturing of plastic substrates and wet etching of Transparent Conductive Oxide (TCO) on plastic. Microcrystalline silicon thin films, prepared by hot-wire chemical vapor deposition, with a piezoresistive gauge factor of -32.2, were used to manufacture a thin skin-like piezo-resistor strain-sensing membrane. Test structures on textile and the miniaturization of the piezoresistive sensors deposited on flexible polyimide substrates were addressed

Effect of hot-filament annealing in a hydrogen atmosphere on the electrical and structural properties of nb-doped TiO2 sputtered thin films

In this work Nb-doped TiO2 thin films were deposited by d.c.-pulsed reactive magnetron sputtering at 500 °C from a composite target with weight fractions of 96% Ti and 4% Nb, using oxygen as reactive gas. In order to enhance the conductive properties, the as-deposited samples were treated in vacuum with atomic hydrogen at a substrate temperature of 500 °C. The atomic hydrogen flow was generated by a hot filament, inside a high-vacuum chemical vapour deposition reactor, at a temperature of 1750 °C. In order to optimise the hydrogen hot-wire treatments, the H2 pressure was varied between 1.3 and 67 Pa, the treatment time was monitored between 1 and 5 min and the hot-filament current was changed between 12 and 17 A. Dark conductivity was measured as a function of temperature and its value at room temperature was extrapolated and used to assess the effect of the hydrogen annealing on the charge transport properties. A two-order of magnitude increase in dark conductivity was typically observed for optimised hydrogen treatments (10 Pa), when varying the hydrogen pressure, resulting in a minimum resistivity of ~3×10−3 Ω cm at room temperature. The maximum amount of atomic H incorporation in oxygen vacancies was determined to be ~5.7 at.%. Carrier mobility and resistivity were also investigated using Hall effect measurements. Correlations between structural and electrical properties and the hydrogen treatment conditions are discussed. The purpose of these films is to provide a transparent and conductive front contact layer for a-Si based photovoltaics, with a refractive index that better matches that of single and tandem solar cell structures. This can be achieved by an appropriate incorporation of a very small amount of cationic doping (Nb5+) into the titanium dioxide lattice.Fundação para a Ciência e a Tecnologia (FCT