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Numerical modeling and experimental investigation of laser-assisted machining of silicon nitride ceramicsShen, Xinwei January 1900 (has links)
Doctor of Philosophy / Department of Industrial & Manufacturing Systems Engineering / Shuting Lei / Laser-assisted machining (LAM) is a promising non-conventional machining technique for advanced ceramics. However, the fundamental machining mechanism which governs the LAM process is not well understood so far. Hence, the main objective of this study is to explore the machining mechanism and provide guidance for future LAM operations. In this study, laser-assisted milling (LAMill) of silicon nitride ceramics is focused.
Experimental experience reveals that workpiece temperature in LAM of silicon nitride ceramics determines the surface quality of the machined workpiece. Thus, in order to know the thermal features of the workpiece in LAM, the laser-silicon nitride interaction mechanism is investigated via heating experiments. The trends of temperature affected by the key parameters (laser power, laser beam diameter, feed rate, and preheat time) are obtained through a parametric study. Experimental results show that high operating temperature leads to low cutting force, good surface finish, small edge chipping, and low residual stress. The temperature range for brittle-to-ductile transition should be avoided due to the rapid increase of fracture toughness.
In order to know the temperature distribution at the cutting zone in the workpiece, a transient three-dimensional thermal model is developed using finite element analysis (FEA) and validated through experiments. Heat generation associated with machining is considered and demonstrated to have little impact on LAM. The model indicates that laser power is one critical parameter for successful operation of LAM. Feed and cutting speed can indirectly affect the operating temperatures.
Furthermore, a machining model is established with the distinct element method (or discrete element method, DEM) to simulate the dynamic process of LAM. In the microstructural modeling of a β-type silicon nitride ceramic, clusters are used to simulate the rod-like grains of the silicon nitride ceramic and parallel bonds act as the intergranular glass phase between grains. The resulting temperature-dependent synthetic materials for LAM are calibrated through the numerical compression, bending and fracture toughness tests. The machining model is also validated through experiments in terms of cutting forces, chip size and depth of subsurface damage.
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Remote plasma sputtering for silicon solar cellsKaminski, Piotr M. January 2013 (has links)
The global energy market is continuously changing due to changes in demand and fuel availability. Amongst the technologies considered as capable of fulfilling these future energy requirements, Photovoltaics (PV) are one of the most promising. Currently the majority of the PV market is fulfilled by crystalline Silicon (c-Si) solar cell technology, the so called 1st generation PV. Although c-Si technology is well established there is still a lot to be done to fully exploit its potential. The cost of the devices, and their efficiencies, must be improved to allow PV to become the energy source of the future. The surface of the c-Si device is one of the most important parts of the solar cell as the surface defines the electrical and the optical properties of the device. The surface is responsible for light reflection and charge carrier recombination. The standard surface finish is a thin film layer of silicon nitride deposited by Plasma Enhanced Chemical Vapour Deposition (PECVD). In this thesis an alternative technique of coating preparation is presented. The HiTUS sputtering tool, utilising a remote plasma source, was used to deposit the surface coating. The remote plasma source is unique for solar cells application. Sputtering is a versatile process allowing growth of different films by simply changing the target and/or the deposition atmosphere. Apart from silicon nitride, alternative materials to it were also investigated including: aluminium nitride (this was the first use of the material in solar cells) silicon carbide, and silicon carbonitride. All the materials were successfully used to prepare solar cells apart from the silicon carbide, which was not used due to too high a refractive index. Screen printed solar cells with a silicon nitride coating deposited in HiTUS were prepared with an efficiency of 15.14%. The coating was deposited without the use of silane, a hazardous precursor used in the PECVD process, and without substrate heating. The elimination of both offers potential processing advantages. By applying substrate heating it was found possible to improve the surface passivation and thus improve the spectral response of the solar cell for short wavelengths. These results show that HiTUS can deposit good quality ARC for silicon solar cells. It offers optical improvement of the ARC s properties, compared to an industrial standard, by using the DL-ARC high/low refractive index coating. This coating, unlike the silicon nitride silica stack, is applicable to encapsulated cells. The surface passivation levels obtained allowed a good blue current response.
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Development of an Experimentally Validated Finite Element Model for Spark Plasma Sintering of High Temperature CeramicsNeff, Paul K., Neff, Paul K. January 2016 (has links)
Spark plasma sintering (SPS) is a powder consolidation technique used to rapidly densify a variety of material systems. SPS is capable of precisely controlling material microstructures and achieving non-equilibrium phases due to rapid heating and cooling rates through the simultaneous application of pressure and direct current. Due to these characteristics, SPS is an ideal processing technique for high temperature ceramics which require processing at temperatures greater than 1500°C. Due to the desirable properties obtained on small diameter materials processed by SPS, larger and more complex geometries are desired while maintaining sample microstructures. In order to accurately scale ceramics produced by SPS, a finite element model must be developed that can be used as a predictive tool. My research focuses on developing a finite element model for the spark plasma sintering furnace at the University of Arizona and validating modeled results using experimentally obtained data. Electrical and thermal conductivity as functions of temperature vary widely among different grades of commercially available electrode grade graphite at constant density. Modeled material properties are optimized in order to calibrate modeled results to experimentally obtained data (i.e. measured current, voltage, and temperature distributions). Sensitivity analysis is performed on the model to better understand model physics and predictions. A calibrated model is presented for 20mm ZrB2 and Si3N4 discs. Sample temperature gradients are experimentally confirmed using grain size and β-Si3N4 phase composition. The model is used to investigate scale up from 20mm to 30mm discs and 30mm rings as well as effects of processing conditions on β-Si3N4 content.
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Ultra-baixo coeficiente de atrito entre o par cerâmico Si3N4-Al2O3 em água. / Ultra-low friction coefficient between Si3N4-Al2O3 in water.Ferreira, Vanderlei 08 September 2008 (has links)
Neste trabalho, foi investigado o comportamento tribológico dos pares cerâmicos aluminanitreto de silício no deslizamento em água e em uma suspensão de sílica coloidal em água (hidrosol). O objetivo foi verificar a possibilidade de atingir um coeficiente de atrito da ordem de unidades de milésimos, aqui chamado de ultra-baixo coeficiente de atrito (UBCA), verificar se a mudança do meio, de água para hidrosol, diminui o running-in do coeficiente de atrito, e verificar o efeito da variação da rugosidade inicial da alumina no comportamento do atrito. Os ensaios foram realizados na configuração de teste esferasobre- disco, no qual a esfera foi de nitreto de silício e o disco de alumina, sob carga normal de 54 N e velocidade de 1 m/s. A água utilizada nos ensaios foi destilada e deionizada, e a sílica coloidal amorfa, hidrofílica, sem porosidade e de tamanho médio de partícula de 12 nm foi a Aerosil® 200, e o hidrosol foi preparado com pH 8,5 num eletrólito de NaCl de 1 mM. A esfera de nitreto de silício, adquirida comercialmente, e a alumina, sinterizada em laboratório, foram caracterizadas quanto a densidade, as fases foram determinadas por difração de raios X, microscopia eletrônica de varedura (MEV) observada em amostras ceramográficas atacadas. Algumas propriedades mecânicas como dureza, módulo de elasticidade e tenacidade à fratura foram determinadas. Duas condições de rugosidade dos discos de alumina foram utilizadas nos ensaios tribológicos, 350 nm e 10 nm RMS. Em todos os ensaios, em água, em hidrosol e independentemente da rugosidade inicial do disco o coeficiente de atrito no regime permanente apresentou pequena dispersão de valores de 0,002 a 0,006, e não foi possível estabelecer diferença entre elas. A menor rugosidade do disco de alumina acarretou menor desgaste e menor período de running-in de coeficiente de atrito, tanto em água quanto em hidrosol. Os ensaios em meio de hidrosol acarretaram menor desgaste das cerâmicas e apresentaram menor running-in de coeficiente de atrito, comparados aos ensaios com água. O disco de alumina apresentou menor desgaste do que a esfera de nitreto de silício, em todas as condições estudadas. Com a análise das perdas volumétricas, da rugosidade final das superfícies desgastadas, das curvas de coeficiente de atrito e das espessuras mínimas de filme lubrificante, calculadas com uso de modelo da literatura, foi possível relacionar a diminuição do desgaste e do running-in de coeficiente de atrito em meio de hidrosol, com a presença da sílica na superfície ou próxima dela. / In this work, the tribological behavior of the alumina-silicon nitride couple was investigated under water and hydrosol (colloidal silica suspensions in water) lubricated sliding. The purposes were to study how an ultra-low friction coefficient can be achieved and to analyze the effects of the environment, lubricant and alumina roughness changes on the friction behavior. Ball-on-disk tests with a normal load of 54 N and a sliding speed of 1 m/s were carried out, using a silicon nitride ball and an alumina disk. The water used as lubricant was distilled and deionized. The silica was amorphous colloidal and hydrophilic, without porous and with a 12 nm medium particle diameter, commercially named Aerosil ® 200. The hydrosol was obtained with a pH value of 8,5 and a 1mM NaCl electrolyte. To estimate the minimum film thickness, formed during the lubricated sliding tests, a theory model was used. The commercial silicon nitride balls and the alumina disks, which were conformed and sintered in laboratory, were characterized by density, X-ray diffraction and scanning electron microscopy measurements. The mechanical properties such as hardness, Young modulus and fracture toughness were determined. The friction coefficient values obtained in the steady state regime showed low standard deviations (0,002 to 0,006) under all conditions. A shorter period of running-in was observed with the lower disk roughness, both in water and hydrosol lubrication. The hydrosol lubricated sliding produced a lower wear and friction running-in comparing with the tests under water lubrication. The alumina disk always showed lower wear than the silicon nitride ball. The volume loss, friction coefficients, worn surfaces roughness and minimum film thickness results suggest that the wear and friction coefficient running-in decrease was caused by the presence of silica on the sliding surfaces or on the near surface regions.
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Strategies for high efficiency silicon solar cellsDavidson, Lauren Michel 01 May 2017 (has links)
The fabrication of low cost, high efficiency solar cells is imperative in competing with existing energy technologies. Many research groups have explored using III-V materials and thin-film technologies to create high efficiency cells; however, the materials and manufacturing processes are very costly as compared to monocrystalline silicon (Si) solar cells. Since commercial Si solar cells typically have efficiencies in the range of 17-19%, techniques such as surface texturing, depositing a surface-passivating film, and creating multi-junction Si cells are used to improve the efficiency without significantly increasing the manufacturing costs. This research focused on two of these techniques: (1) a tandem junction solar cell comprised of a thin-film perovskite top cell and a wafer-based Si bottom cell, and (2) Si solar cells with single- and double-layer silicon nitride (SiNx) anti-reflection coatings (ARC).
The perovskite/Si tandem junction cell was modeled using a Matlab analytical program. The model took in material properties such as doping concentrations, diffusion coefficients, and band gap energy and calculated the photocurrents, voltages, and efficiencies of the cells individually and in the tandem configuration. A planar Si bottom cell, a cell with a SiNx coating, or a nanostructured black silicon (bSi) cell can be modeled in either an n-terminal or series-connected configuration with the perovskite top cell. By optimizing the bottom and top cell parameters, a tandem cell with an efficiency of 31.78% was reached.
Next, planar Si solar cells were fabricated, and the effects of single- and double-layer SiNx films deposited on the cells were explored. Silicon nitride was sputtered onto planar Si samples, and the refractive index and thicknesses of the films were measured using ellipsometry. A range of refractive indices can be reached by adjusting the gas flow rate ratios of nitrogen (N2) and argon (Ar) in the system. The refractive index and thickness of the film affect where the minimum of the reflection curve is located. For Si, the optimum refractive index of a single-layer passivation film is 1.85 with a thickness of 80nm so that the minimum reflection is at 600nm, which is where the photon flux is maximized. However, using a double-layer film of SiNx, the Si solar cell performance is further improved due to surface passivation and lowered surface reflectivity. A bottom layer film with a higher refractive index passivates the Si cell and reduces surface reflectivity, while the top layer film with a smaller refractive index further reduces the surface reflectivity. The refractive indices and thicknesses of the double-layer films were varied, and current-voltage (IV) and external quantum efficiency (EQE) measurements were taken. The double-layer films resulted in an absolute value increase in efficiency of up to 1.8%.
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Surface, Emitter and Bulk Recombination in Silicon and Development of Silicon Nitride Passivated Solar CellsKerr, Mark John, Mark.Kerr@originenergy.com.au January 2002 (has links)
[Some symbols cannot be rendered in the following metadata please see the PDF file for an accurate version of the Abstract]
¶
Recombination within the bulk and at the surfaces of crystalline silicon has been
investigated in this thesis. Special attention has been paid to the surface passivation achievable
with plasma enhanced chemical vapour deposited (PECVD) silicon nitride (SiN) films due to
their potential for widespread use in silicon solar cells. The passivation obtained with thermally
grown silicon oxide (SiO2) layers has also been extensively investigated for comparison.
¶
Injection-level dependent lifetime measurements have been used throughout this thesis to
quantify the different recombination rates in silicon. New techniques for interpreting the
effective lifetime in terms of device characteristics have been introduced, based on the physical
concept of a net photogeneration rate. The converse relationships for determining the effective
lifetime from measurements of the open-circuit voltage (Voc) under arbitrary illumination have
also been introduced, thus establishing the equivalency of the photoconductance and voltage
techniques, both quasi-static and transient, by allowing similar possibilities for all of them.
¶
The rate of intrinsic recombination in silicon is of fundamental importance. It has been
investigated as a function of injection level for both n-type and p-type silicon, for dopant
densities up to ~5x1016cm-3. Record high effective lifetimes, up to 32ms for high resistivity
silicon, have been measured. Importantly, the wafers where commercially sourced and had
undergone significant high temperature processing. A new, general parameterisation has been
proposed for the rate of band-to-band Auger recombination in crystalline silicon, which
accurately fits the experimental lifetime data for arbitrary injection level and arbitrary dopant
density. The limiting efficiency of crystalline silicon solar cells has been re-evaluated using this
new parameterisation, with the effects of photon recycling included.
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Surface recombination processes in silicon solar cells are becoming progressively more
important as industry drives towards thinner substrates and higher cell efficiencies. The surface
recombination properties of well-passivating SiN films on p-type and n-type silicon have been
comprehensively studied, with Seff values as low as 1cm/s being unambiguously determined.
The well-passivating SiN films optimised in this thesis are unique in that they are stoichiometric
in composition, rather than being silicon rich, a property which is attributed to the use of dilute
silane as a process gas. A simple physical model, based on recombination at the Si/SiN interface
being determined by a high fixed charge density within the SiN film (even under illumination),
has been proposed to explain the injection-level dependent Seff for a variety of differently doped
wafers. The passivation obtained with the optimised SiN films has been compared to that
obtained with high temperature thermal oxides (FGA and alnealed) and the limits imposed by
surface recombination on the efficiency of SiN passivated solar cells investigated. It is shown
that the optimised SiN films show little absorption of UV photons from the solar spectrum and
can be easily patterned by photolithography and wet chemical etching.
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The recombination properties of n+ and p+ emitters passivated with optimised SiN films
and thermal SiO2 have been extensively studied over a large range of emitter sheet resistances.
Both planar and random pyramid textured surfaces were studied for n+ emitters, where the
optimised SiN films were again found to be stoichiometric in composition. The optimised SiN
films provided good passivation of the heavily doped n+-Si/SiN interface, with the surface
recombination velocity increasing from 1400cm/s to 25000cm/s as the surface concentration of
electrically active phosphorus atoms increased from 7.5x1018cm-3 to 1.8x1020cm-3. The
optimised SiN films also provided reasonable passivation of industrial n+ emitters formed in a
belt-line furnace. It was found that the surface recombination properties of SiN passivated p+
emitters was poor and was worst for sheet resistances of ~150./ . The hypothesis that
recombination at the Si/SiN interface is determined by a high fixed charge density within the
SiN films was extended to explain this dependence on sheet resistance. The efficiency potential
of SiN passivated n+p cells has been investigated, with a sheet resistance of 80-100./ and a
base resistivity of 1-2.cm found to be optimal. Open-circuit voltages of 670-680mV and
efficiencies up to ~20% and ~23% appear possible for SiN passivated planar and textured cells
respectively. The recombination properties measured for emitters passivated with SiO2, both n+
and p+, were consistent with other studies and found to be superior to those obtained with SiN
passivation.
¶
Stoichiometric SiN films were used to passivate the front and rear surfaces of various
solar cell structures. Simplified PERC cells fabricated on 0.3.cm p-type silicon, with either a
planar or random pyramid textured front surface, produced high Vocs of 665-670mV and
conversion efficiencies up to 19.7%, which are amongst the highest obtained for SiN passivated
solar cells. Bifacial solar cells fabricated on planar, high resistivity n-type substrates (20.cm)
demonstrated Vocs up to 675mV, the highest ever reported for an all-SiN passivated cell, and
excellent bifaciality factors. Planar PERC cells fabricated on gettered 0.2.cm multicrystalline
silicon have also demonstrated very high Vocs of 655-659mV and conversion efficiencies up to
17.3% using a single layer anti-reflection coating. Short-wavelength internal quantum efficiency
measurements confirmed the excellent passivation achieved with the optimised stoichiometric
SiN films on n+ emitters, while long-wavelength measurements show that there is a loss of
short-circuit current at the rear surface of SiN passivated p-type cells. The latter loss is
attributed to parasitic shunting, which arises from an inversion layer at the rear surface due to
the high fixed charge (positive) density in the SiN layers. It has been demonstrated that that a
simple way to reduce the impact of the parasitic shunt is to etch away some of the silicon from
the rear contact dots. An alternative is to have locally diffused p+ regions under the rear
contacts, and a novel method to form a rear structure consisting of a local Al-BSF with SiN
passivation elsewhere, without using photolithography, has been demonstrated.
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The Influence of Adjacent Segment on the Reliability of Cu Dual Damascene InterconnectsChang, Choon Wai, Choi, Z.-S., Thompson, Carl V., Gan, C.L., Pey, Kin Leong, Choi, Wee Kiong, Hwang, N. 01 1900 (has links)
Three terminal âdotted-I’ interconnect structures, with vias at both ends and an additional via in the middle, were tested under various test conditions. Mortalities (failures) were found in right segments with jL value as low as 1250 A/cm, and the mortality of a dotted-I segment is dependent on the direction and magnitude of the current in the adjacent segment. Some mortalities were also found in the right segments under a test condition where no failure was expected. Cu extrusion along the delaminated Cu/Si₃N₄ interface near the central via region was believed to cause the unexpected failures. From the time-to-failure (TTF), it is possible to quantify the Cu/Si₃N₄ interfacial strength and bonding energy. Hence, the demonstrated test methodology can be used to investigate the integrity of the Cu dual damascene processes. As conventionally determined critical jL values in two-terminal via-terminated lines cannot be directly applied to interconnects with branched segments, this also serves as a good methodology to identify the critical effective jL values for immortality. / Singapore-MIT Alliance (SMA)
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Production Of Hydrogenated Nanocrystalline Silicon Based Thinfilm TransistorAliyeva, Tamila 01 July 2010 (has links) (PDF)
The instability under bias voltage stress and low mobility of hydrogenated amorphous
silicon (a-Si:H) thin film transistor (TFT), produced by plasma enhanced chemical vapor deposition
(PECVD) technique, are the main problems impeding the implementation of active
matrix arrays for light emitting diode display panels and their peripheral circuitry. Replacing
a-Si:H by hydrogenated nanocrystalline silicon film (nc-Si:H) seems a solution due to its
higher mobility and better stability. Therefore nc-Si:H TFT was produced and investigated in
this thesis.
All TFT layers (doped nc-Si:H, intrinsic nc-Si:H and insulator films) were produced separately,
characterized by optical (UV-visible and FTIR spectroscopies, XRD) and electrical
(current-voltage, I-V) methods, and optimized for TFT application. Afterwards the non
self-aligned bottom-gate TFT structure was fabricated by the photolithographic method using
2-mask set.
The n+ nc-Si:H films, used for TFT drain/source ohmic contacts, were produced at high
H2 dilution and at several RF power densities (PRF). The change of their lateral resistivity
(rho) was measured by reducing the film thickness via reactive ion etching. The rho values rise
below a critical film thickness, indicating the presence of the disordered and less conductive
incubation layer. The optimum PRF for the lowest incubation layer was determined.
Among the deposition parameters only increased NH3/SiH4 flow rate ratio improved the
insulating properties of the amorphous silicon nitride (a-SiNx:H) films, chosen as the TFT
gate dielectric. The electrical characteristics of two TFTs with a-SiNx:H having low leakage
current, fabricated at different NH3/SiH4 ratios (~19 and ~28) were compared and discussed.
The properties (such as crystallinity, large area uniformity, etc.) of the nc-Si:H film as
TFT channel layer, were found to depend on PRF. For the films deposited at the center of
the PECVD electrode the change from an amorphous dominant structure to a nanocrystalline
phase took place with increasing PRF, whereas those at the edge had always nanocrystalline
nature, independent of PRF. The two different TFTs produced at the center of the electrode
with a-Si:H and nc-Si:H grown at low and high PRF, respectively, were compared through
their I-V characteristics and electrical stability under the gate bias voltage stress.
Finally, nc-Si:H TFT structure, produced and optimized in this work, was analyzed through
gate-insulator-drain/source capacitor by capacitance-voltage (C-V) measurements within
106-10-2 Hz frequency (F) range. The inversion regime was detected at low F without any
external charge injection. Besides, ac hopping conductivity in the nc-Si:H bulk was extracted
from the fitting results of the C-F curves.
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Nanocrystal Silicon Based Visible Light Emitting Pin DiodesAnutgan, Mustafa 01 December 2010 (has links) (PDF)
The production of low cost, large area display systems requires a light emitting material
compatible with the standard silicon (Si) based complementary metal oxide semiconductor
(CMOS) technology. The crystalline bulk Si is an indirect band semiconductor with very
poor optical properties. On the other hand, hydrogenated amorphous Si (a-Si:H) based wide
gap alloys exhibit strong visible photoluminescence (PL) at room temperature, owing to the
release of the momentum conservation law. Still, the electroluminescence (EL) intensity from
the diodes based on these alloys is weak due to the limitation of the current transport by the
localized states.
In the frame of this work, first, the luminescent properties of amorphous silicon nitride
(a-SiNx:H) thin films grown in a plasma enhanced chemical vapor deposition (PECVD) system
were analyzed with respect to the nitrogen content. Then, the doping effciency of p- and
n-type hydrogenated nanocrystalline Si (nc-Si:H) films was optimized via adjusting the deposition
conditions. Next, the junction quality of these doped layers was checked and further
improved in a homojunction pin diode.
Heterojunction pin light emitting diodes (LEDs) were fabricated with a-SiNx:H as the
luminescent active layer. The EL effciency of the fresh diodes was very low, as expected.
As a solution, the diodes were electro-formed under high electric field leading to nanocrystallization
accompanied by a strong visible light emission from the whole diode area. The
current-voltage (I-V) and EL properties of these transformed diodes were investigated in detail.
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Development of high-efficiency solar cells on thin silicon through design optimization and defect passivationSheoran, Manav 24 March 2009 (has links)
The overall goal of this research is to improve fundamental understanding of the hydrogen passivation of defects in low-cost silicon and the fabrication of high-efficiency solar cells on thin crystalline silicon through low-cost technology development. A novel method was developed to estimate the flux of hydrogen, released from amorphous silicon nitride film, into the silicon. Rapid-firing-induced higher flux of hydrogen was found to be important for higher defect passivation. This was followed by the fabrication of solar cell efficiencies of ~ 17% on low-cost, planar cast multicrystalline silicon. Solar cell efficiencies and lifetime enhancement in the top, middle, and bottom regions of cast multicrystalline silicon ingots were explained on the basis of impurities and defects generally found in those regions. In an attempt to further reduce the cost, high-efficiency solar cells were fabricated on thin crystalline silicon wafers with full area aluminum-back surface field. In spite of loss in efficiency, wafer thinning reduced the module cost. Device modeling was performed to establish a roadmap towards high-efficiency thin cells and back surface recombination velocity and back surface reflectance were identified as critical parameters for high-efficiency thin cells. Screen-printed solar cells on float zone material, with efficiencies > 19% on 300 μm and > 18% on 140 μm were fabricated using a novel low-cost fabrication sequence that involved dielectric rear passivation along with local contacts and back surface field.
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