J.M. Serra

University of Lisbon | UL · Departamento de Engenharia Geográfica, Geofísica e Energia (DGGE)

We present here a thorough study of a one-step metal-assisted chemical etching (MACE) method to reduce the reflectivity of monocrystalline silicon (mono c-Si) wafers, thus increasing their light capture efficiency. The method uses hydrogen peroxide (H2O2) and hydrofluoric acid (HF) as etchants and silver (Ag) as reaction catalyst. In this study, we...

The Silicon on Dust Substrate (SDS) is a gas-to-wafer process that produces multicrystalline silicon ribbons directly from gaseous feedstock (silane), avoiding the standard industry steps of polysilicon deposition, crystal growth, and wafering. The SDS technique consists of three main steps: (i) micrometric-sized silicon powder production by grindi...

In monolithically integrated 2-terminal silicon/perovskite tandem solar cells, one of the critical steps is the formation of the layer(s) that interconnects the two sub-cells. Of the range of possibilities for monolithic integration, crystalline silicon tunnel junctions are a potential candidate. The crystalline tunnel junction has the advantage of...

Vehicle-to-grid (V2G) and dynamic charging have been pointed out as potential assets to provide grid ancillary services and enable higher levels of renewable generation. However, such strategies may require that EV chargers operate below their nominal power, which can raise power quality (PQ) concerns. This work aims to propose a PQ characterizatio...

This work proposes to demonstrate a workflow to access the optical performance of randomly texturized silicon substrates using a numerical 3D image reconstruction algorithm followed by Finite-Difference Time-Domain (FDTD) simulations based on direct integration of Maxwell's time-dependent equations. The characterized samples were obtained by metal-...

We present here an optimisation study of a method to reduce the reflectivity of monocrystaline silicon wafers. The method is based a metal-assisted chemical etching (MACE) technique that uses hydrogen peroxide (H2O2) and hydrofluoric (HF) as etchants, and silver as catalyst. During this study, several etching times and etchant concentrations were t...

: Plug-in electric vehicles (PEVs) are expected to play a role as power grid ancillary service providers through vehicleto-grid (V2G) chargers, enabling higher levels of renewable electricity penetration. However, to fully exploit the storage capacity of PEVs and fast responsiveness, it is crucial to understand their operational characteristics. Th...

International CO2 reduction commitments are pushing increasing penetration levels of renewable energy sources and electrification of the transports sector. The expected growth for electric vehicles (EVs) will surely have a tremendous impact on the electricity distribution system. Despite the challenges ahead, EVs are able to make bidirectional ener...

We present the results achieved with an optical zone melting recrystallization (ZMR) system, which concentrates the radiation of two halogen lamps on the surface of a microcrystalline silicon (µc-Si) ribbon sample, creating a long, 2 mm width molten region (∼1414° C). µc-Si ribbon samples measuring up to 25×100 mm² were previously obtained using an...

This chapter reviews the applications of luminescence-based techniques in the photovoltaic industry, with special focus on crystalline silicon-based devices – the dominant technology in the market.

As the installation of PV systems continues to decrease, the weight of the operation and maintenance in the cost structure of systems will become an ever increasing factor. The significant challenges are to maximise electrical energy production by reducing system downtime for planned and unplanned maintenance. Electroluminescence (EL) is becoming a...

Silicon Ribbons The silicon on dust substrate (SDS) process is a two‐step technique to obtain multicrystalline silicon (mc‐Si) ribbons for photovoltaics directly from gaseous feedstock. Silicon pre‐ribbons with a porous structure and dimensions up to 25×100 mm², are produced with an inline optical chemical vapor deposition (CVD) system. Using silan...

The silicon on dust substrate process is a two‐step technique to produce multicrystalline silicon (mc‐Si) ribbons directly from gaseous feedstock. Silicon pre‐ribbons of very small grain size (ranging from nano to microcrystalline), with a porous structure and dimensions up to 25 × 100 mm2, are obtained using an inline optical chemical vapor deposi...

In this work, we report recent results of solar cells fabricated on silicon foils obtained by the Stress induced LIft-off Method (SLIM)-cut technique using an epoxy stress-inducing layer. Indeed, the use of silicon foils for the production of solar cells offers the ability to reduce material costs while allowing potentially a higher conversion effi...

This paper presents the results achieved with an inline optical chemical vapor deposition (CVD) system operating at atmospheric pressure and at low temperatures (<600° C). Using silane as the gaseous precursor, microcrystalline silicon layers were grown on compressed silicon micrometric powder substrates, moving at constant speed, and crossing thre...

Electroluminescent imaging is increasingly used to detect defects in silicon solar cells. However, the cost of the conventional luminescence systems is a limiting factor for generalized use. A simple and reliable low-cost electroluminescence setup is presented. The developed system was tested on commercial silicon solar cells for the acquisition of...

In this paper, we report on the first solar cells fabricated on silicon foils employing the stress-induced liftoff method-cut technique and using an epoxy stress-inducing layer. The latter is a 900-μm-Thick epoxy layer, which was manually dispensed on the surface of a monocrystalline silicon sample and cured at 150 °C for 1 h. The crack propagation...

The major barrier for PV penetration is cost. And the single most important cost factor in silicon technology is the wafer (≈ 35% of the module cost). Although tremendous progress on cell processing has been reported in recent years, a much smaller evolution is seen on what should be the key point to address – the wafer. The ingot-slicing process i...

The development of high temperature gas sensors for the monitoring and determination of thermophysical properties of complex process mixtures at high temperatures faces several problems, related with the materials compatibility, active sensing parts sensitivity, and lifetime. Ceramic/thin metal films based sensors, previously developed for the dete...

This manuscript describes and presents results obtained with an inline optical CVD system operating at low temperature (<1100K) and at atmospheric pressure. Nanocrystalline silicon layers were grown on top of a moving (>10mm/min) compressed silicon powder substrate. The deposition parameters, together with the silicon powder substrate feature, prov...

Abstract A method to dope silicon ribbons is presented. The method consists on the spraying of the ribbons with a phosphoric acid solution followed by a recrystallization in an optical heating furnace. During the sample heating, as phosphoric acid is dehydrated the resulting phosphorous compounds are either evaporated or serve as source for phospho...

The stress induced lift-off method (SLIM) -cut technique allows the detachment of thin silicon foils using a stress inducing layer. In this work, results of SLIM-cut foils obtained using an epoxy stress inducing layer at room temperature are presented. Numerical analyses were performed in order to study and ascertain the important experimental para...

An important factor for cost reduction of solar electricity is the reduction of silicon material quantity used for making a solar cell. It is well known in the PV industry that kerf losses associated with wafering are a limiting factor in the efforts to reduce material usage and some techniques have been developed to address this issue. The kerfles...

We present 5 × 5 cm2 SLIM-cut foils obtained by cooling form curing temperatures of 150 °C to room temperature using an epoxy stress inducing layer. Numerical simulations were performed to help the definition of an optimum geometry and we demonstrate the capability to obtain several thin foils from the same substrate. The evolution of minority carr...

An inline optical CVD process operating at low temperature (<873 K) and at atmospheric pressure is presented here to grow silicon films on top of crystalline, sintered silicon and pressed silicon powder substrates moving at constant speed (>10 mm/min) inside the furnace. Solid silicon substrates were laser textured to reduce the reflectivity, lever...

The SLIM-cut technique provides a way to obtain thin silicon foils without a standard sawing step, thus avoiding kerf losses. This process consists of three steps: depositing a stress-inducing layer on top of the silicon surface; stress activation by heating and cooling, resulting in crack propagation in the silicon and detachment of a thin silicon...

A new method for molten zone crystallization is presented. The method is based on the formation of a molten capillary by applying an electric current. Since the power is delivered directly to the liquid, the technique has the potential for low energy budget. On the other hand, being a floating molten zone method, the liquid silicon never contacts f...

An electric molten capillary in a silicon ribbon was for the first time created and maintained using laser scanning as a localized thermal stimulation to promote the electric current concentration in the desired place on the ribbon. Since this type of molten capillary can potentially be used to grow a silicon ribbon, a Computational Fluid Dynamics...

The Solar Schools project was a solar electrification project involving over 30 primary and secondary schools in São Tomé and Principe. The project approach was based on standardized PV solutions for DC lighting for classrooms and AC power for school management activities. A relevant component of the project was the development of local skills for...

The microwave reflection photoconductivity decay (MW-PCD) is a well known technique to measure lifetime in silicon wafers by the analysis of the decay of the reflected incident microwaves on the sample. However this decay can be affected by for instance surface properties, namely surface recombination. Besides HF, iodine in an ethanol solution has...

The Solar Schools project was a solar electrification project involving over 30 primary schools in São Tomé and Principe. The project approach was based on standardized PV solutions for DC lighting for classrooms and AC power for school management activities. A relevant component of the project was the development of local skills for installation a...

The decrease in wafer thickness seen as a route to cost reductions has raised a growing interest in techniques that allow the preparation of thin wafers without kerf loss. The Slim-cut process [1] is one of these new techniques and comprises mainly three stages: a stress layer deposition step on the top of a monocrystalline silicon sample, a heatin...

This paper describes a method for measuring the dopant concentration in a semiconductor using the Seebeck effect at a given point of a sample using a transient thermal gradient. The system was tested with high-purity metallic samples and several silicon samples with different concentrations of boron. The dopant concentrations of these samples obtai...

We investigate the relationships between growth rate, time-temperature profile, residual stress,dislocation density, and electrical performance of silicon ribbons grown via optical zone melting.The time-temperature profiles of ribbons grown at different velocities were investigated using direct measurements and computational fluid dynamics (CFD) mo...

In this work, we describe a new process of fabrication of low-cost silicon wafers which are produced by sintering silicon powder followed by a Zone Melting Recrystallization (ZMR) step. The influence of the ZMR step on the physical and chemical characteristics of the substrate is reported. The evolution of the impurities content along the different...

In the present work, a recrystallization process of sintered silicon wafers was studied to produce photovoltaic solar cells. The recrystallization step is carried out by Zone Melting Recrystallization (ZMR) or Full Wafer Recrystallization (FWR). Structural analysis and impurity content of the processed material are presented. Electrical characteriz...

It is well known that impurity contamination is a very important factor in silicon processing. Contamination arises from crucibles or graphite susceptors during crystal growth, from the use of fibers or dies as in the case of String ribbon or EFG technologies respectively. Furthermore, every hot part in a crystal growth system will contribute to in...

An electric current is used to create and maintain a linear molten zone in silicon. The electrical signature of the electric molten zone (EMZ) is described in detail. The measurement of the EMZ width as a function of operational parameters has shown good agreement with published data on the electrical conductivity of liquid silicon, and suggests a...

The ?Stress induced LIft-off Method? (SLIM-cut) is a new process of fabrication of thin silicon wafers, that may contribute for the reduction of photovoltaic costs by avoiding the standard wafering step. The SLIM-cut process comprises three stages: a metallic layer deposition step by screen printing on top of a monocrystalline silicon slab, a heati...

The major barrier for PV penetration is cost. And the most important cost factor in silicon technology is the wafer (50% of the module cost). Although tremendous progress on cell processing has been reported in recent years, a much smaller evolution is seen on what should be the key point to address – the wafer. The ingot-slicing process is reachin...

The major challenge to PV penetration is cost. Although remarkable progress on solar cell processing has been reported in recent years, a much smaller evolution has been seen on the production of the wafer itself, which represents nearly half of the module cost for silicon based technologies. This paper presents the status of silicon ribbon technol...

In a budgetary assessment of photovoltaic (PV) plants, land related costs can be a critical decision factor. Besides the cost of the land itself, the density of the installed PV systems determines other costs such as those associated to wiring, fencing and/or civil works. Furthermore, the available land area can also be a limiting factor in terms o...

This paper describes a new method for the crystallization of silicon sheet using an electrically generated molten line. A thin molten line is created in the sample by applying an electric current and then, using a suitable external thermal gradient, it is forced to travel across the sample crystallizing it. The crystallized sample has columnar cm-s...

The sprayed boric acid (SBA) method for bulk doping of silicon ribbons is investigated. Experimental procedures and main results are reviewed. Computational fluid dynamics and experimental tests using partial spraying suggest the role of gas transported evaporated boron oxide to explain the boron incorporation profiles along the sample. The industr...

This article describes a process for generating and controlling a closed molten zone with an induced electrical current, and using it for silicon ribbon tube recrystallization. The silicon tube is the secondary loop of a transformer in which the current is generated by electrical induction. The Joule heat caused by the induced current generates a c...

This paper reports the first results with both intrinsic and p-doped multicrystalline silicon ribbons obtained by the Continuous Mode Silicon over Dust Substrate process (SDS). The SDS is a two-step process in which the ribbons are obtained directly from a gaseous source, silane. In the first step the pre-ribbon is deposited by continuous optical f...

A new method for boron bulk doping of silicon ribbons is developed. The method is based on the spraying of the ribbons with a boric acid solution and is particularly suited for silicon ribbons that require a zone-melting recrystallization step. To analyse the quality of the material thus obtained, multicrystalline silicon samples doped with this do...

In this paper we present the progress on the development of the Continuous Optical Fast CVD (COFCVD) system based on the Silicon on Dust Substrate (SDS) process. The SDS process is a ribbon technology, where ribbons are grown on top of a silicon dust substrate, directly from a gas precursor, silane. Besides the advantages of avoiding kerf losses, t...

This paper presents a study to integrate photovoltaic (PV) production larger than micro-generation in urban areas, using the school roofs as installation sites. This initiative is part of a more comprehensive effort to increase dispersed electricity production as opposition to a conventional policy of centralized electricity production often locate...

The linear electric molten zone produced by passing an electric current in a silicon ribbon may be used to create a floating molten zone specially suited for a recrystallization process. A one-dimensional heat transfer numerical model of a linear electric molten zone is developed. Simulation results show the electric current concentration mechanism...

The silicon wafer accounts for about half of the photovoltaic module cost. We believe two aspects are essential to reduce considerably the "wafer" costs: the feedstock issue and ribbon technology. In the SDS process the pre-ribbons are grown directly from silane by fast CVD process and followed by zone melting recrystallization (ZMR). In this paper...

In this paper, we present a silicon on dust substrate (SDS) process, a new method for the growth of silicon ribbons. As a demonstration of the concept, we also present results on solar cells made of these new silicon ribbons. SDS ribbons were obtained directly from a gaseous feedstock by a fast CVD step using silane. The resulting self-supported in...

The deployment of photovoltaics in large scale, in particular PV microgeneration, requires the development of a numerous workforce trained for PV systems installation and maintenance. Since there is an obvious lack of local training opportunities for PV professionals, the University of Lisbon and INETI have promoted a new Training Course for PV Sys...

The method for boron bulk doping of silicon ribbons based on the use of sprayed boric acid as a doping source is particularly suitable for producing p-type silicon ribbons that require zone melting recrystallisation (ZMR). This paper reports on a test of the application of this method for the doping of multicrystalline material that was used as the...

This paper presents a new method for bulk boron doping of silicon ribbons for solar cells. The method is based on the spraying of the ribbon with a solution of boric acid followed by zone melting recrystallization in an argon atmosphere. Dopant incorporation is evaluated by measuring the spreading resistance of the silicon ribbon.A numerical model...

This letter describes how the temperature dependence of the electrical conductivity in semiconductors may be used to produce a linear floating molten zone which is intrinsically stable and uniform along its length. An analytical model and an experimental demonstration of such electric molten zone are both presented. This effect may be of particular...

This paper reports on the measurement of residual stress in EFG silicon ribbons for solar cell applications using the phase-shifting infrared (IR) photoelastic method. The samples analysed were wafers cut from EFG octagons with 100mm face width and from EFG 125mm face-width octagon under development. Experimental results show that the distribution...

This article reports on a method for the measurement of residual stress in multicrystalline silicon ribbons, based on the infrared photoelastic technique. This self-calibrating method allows the in situ determination of the photoelastic coefficients and can thus be used for any crystal orientation. The method was validated by the experimental deter...

A low cost differential profilometer based on standard commercial displacement transducers is fully described. Unlike most common profilometers this device can be used to measure the thickness profile of samples having both surfaces irregular. A sensitivity of about 0.2 μm, independent of the sample thickness is achieved.

The study of electromagnetic wave propagation in a coaxial cable can be a powerful approach to the study of waves at an undergraduate level. This study can explore different experimental situations, going from those where the finite velocity of propagation must be considered (distributed or transmission line behaviour), to those where this velocity...

Nonlinear effects were observed in a forced vibrating string. The motion of the string becomes elliptic as the amplitude of the vibration increases. The fundamental resonance frequency depends on the amplitude of the vibration. At sufficient high amplitudes the system exhibits hysterisis: the resonance frequency is different when measured increasin...

We report on the characterisation of silicon tubes recrystallised by closed molten zone, a technique developed as a step to a possible process for thin silicon sheet production. The tube faces are quite flat and have a smooth surface. For the electrical characterisation, samples were cut from the tube faces and simple photovoltaic solar cells were...

A process to obtain self-supported thin silicon films is being developed. Films are grown by optical chemical vapour deposition (CVD) (using halogen lamps as heating system) from silane, at low temperature and relatively high growth rates, on silicon substrates with a sacrificial layer of porous Si (PS), which allows film detachment. The PS layer w...

Non uniform n+ diffused layers on p-silicon are usually obtained by a uniform doping process followed by etch back in areas defined by a photolitographic mask applied onto the sample. We present a study on an alternative technique to obtain nonuniform n+ doped layers using a photoselective etching process, thus avoiding the photolitographic mask. T...

We present a simple technique for photodefined etching of p-type silicon. The technique involves wet photoselective corrosion of silicon by an aqueous HF solution containing a strong oxidising agent (bromine). The etching process can be largely inhibited by light, allowing the shaping of photodefined 3-D structures on the surface of p-type silicon...

Most thin films produced by a wide variety of methods, either physical or chemical (PVD, CVD, sputtering, etc.) for temperature sensor applications, can be used only in very narrow ranges of temperatures, where their components are not subjected to differential thermal expansions, recrystallizations, and grain size modifications. This paper reports...

During diffusion processes in thin solid layers, the usual assumption of a semi-infinite solid with constant bulk concentration is not valid. The well-known solution of the diffusion equation, based on complementary error functions, is therefore not applicable, and numerical solutions are normally used. In this work, we report an analytical solutio...

We report measurements of the room temperature optical absorption coefficient α(λ) of polycrystalline silicon ribbon material with high oxygen concentration (1.7 × 1018 at. cm−3) in the spectral range from 800 to 1200 nm. Between 800 and 1000 nm, absolute error is estimated as ≈2%. Sample to sample variations in this range are ±2%, and differences...

We report measurements of the room temperature optical absorption coefficient α(λ) of polycrystalline silicon ribbon material with high oxygen concentration (1.7 × 1018 at. cm-3) in the spectral range from 800 to 1200 nm. Between 800 and 1000 nm, absolute error is estimated as ≈ 2%. Sample to sample variations in this range are ± 2%, and difference...