M.C. Brito

Extending solar potential analysis in buildings to vertical facades

The assessment of local photovoltaic (PV) potential plays a critical role in the development of planning policies and financing schemes for the successful deployment of PV systems in cities. Considering the significant area available on facades in modern urban landscapes, the solar potential analysis at the local or municipal scale should be extended to take into account the irradiation on vertical walls. Starting from three different sets of information of a site, solar radiation model for roofs, ground and facades, 3D buildings model and a DSM derived from airborne LiDAR data, a methodology was developed in a GIS environment in order to fuse the datasets and allow for spatio-temporal analysis for solar energy quests. The resultant 3D database enables spatial visualization of the answers to the most interesting questions regarding new solar energy systems on buildings. The GIS database can be used as a tool to support municipal/local decision planners since statistics for the solar potential of each building can be easily inspected. In order to optimize solar collection, this system is particularly suited to identify which parts (roofs and/or facades) of a building are more favorable for the installation of solar arrays as well as the area available for the installation. Methodology for spatio-temporal analysis for solar energy on buildings (roofs and facades).Solar GIS database for support of municipal/local decision planners.Integration of LiDAR data with 3D buildings model.3D spatial analysis.High resolution solar potential analysis.

Measurement of residual stress in multicrystalline silicon ribbons by a self-calibrating infrared photoelastic method

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 determination of the photoelastic coefficient of monocrystalline (100) silicon wafers and by comparison with strain measurements using asymmetrical x-ray diffraction. The distribution of residual stress in multicrystalline silicon ribbons was also measured. The results showed strong evidence for tensile stress in the central region and compressive stress near the edges of the ribbons. Both the measured residual stress and the photoelastic coefficient distributions are correlated to grain boundaries.

Residual stress and dislocations density in silicon ribbons grown via optical zone melting

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) modeling. Residual stresses up to 20 MPa were measured using infrared birefringence imaging. The effect of crystallization speed on dislocation density and residual stress is discussed from the context of thermal stresses during growth. More broadly, we demonstrate the usefulness of combining spatially resolved stress and microstructure measurements with CFD simulations toward optimizing kerfless silicon wafer quality.

The silicon on dust substrate path to make solar cells directly from a gaseous feedstock

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 intrinsic ribbons were microcrystalline and porous. To make these ribbon films suitable for photovoltaic applications, a novel recrystallization with an in situ doping step was developed. To this purpose, the ribbons were sprayed with boric acid and then recrystallized by float zone melting. Simple solar cells were prepared by employing: aluminium back contacts, Ti/Pd/Ag front grid contacts, with no anti-reflective coating, doping optimization, passivation or gettering. The 1-sun I–V characteristics of the cells were: Voc ∼ 530 mV and Jsc ∼ 24 mA cm −2 .T he minority carrier diffusion length obtained from a spectral response at long wavelengths gave values of Ln ∼ 70 μm. (Some figures in this article are in colour only in the electronic version)

A differential mechanical profilometer for thickness measurement

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.

Linear electric molten zone in semiconductors

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 interest for crystal growth and semiconductor recrystallization.

Modeling the Effects of Inhomogeneous Irradiation and Temperature Profile on CPV Solar Cell Behavior

Solar cells that integrate concentration photovoltaic systems are usually exposed to inhomogeneous illumination and temperature profiles which influence their performance. Under such conditions, the solar cell behavior is only accurately modeled if the diode and resistive losses are considered to be distributed across the solar cell instead of being gathered, as in the conventional lumped model. This paper presents a distributed diode model and its experimental validation, which was carried out for standard test conditions and a range of temperature and concentration levels going from 25 to 70 °C and 1 to 30 suns, respectively, for both homogeneous and a set of inhomogeneous profiles. Modeled and experimental results showed good agreement, thus validating the model. The results of traditional and distributed model approaches are compared with homogeneous and inhomogeneous profiles of irradiation and temperature. Finally, we analyze the effect of different profiles on cell performance.

New Stress Activation Method for Kerfless Silicon Wafering Using Ag/Al and Epoxy Stress-Inducing Layers

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 layer; and a chemical cleaning to remove the stress-inducing layer. This paper describes a new stress activation method using Ag/Al and epoxy stress-inducing layers. The crack propagation is controlled along the sample length in order to avoid unwanted additional crack formation and interaction with other crack fronts. Silicon foils with thickness ranging between 50 and 130 μm were obtained with effective lifetimes between 1 and 81 μs.

Silicon tubes by a closed molten zone: a characterisation study

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 formed. The average diffusion length of minority carriers was found, from spectral response, to be around 100 μm. Low-resolution LBIC measurements showed lower diffusion lengths (around 40 μm) in the regions close to the tube edges. This behaviour was correlated to measurements of residual thermal stresses using infrared photoelasticity. Measurements of the changes of spectral response with increasing bias light intensity reveal an increase in the effective diffusion length, a known effect that is interpreted in terms of a density of trapping states.

Electric molten zone crystallization of silicon wafers

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 foreign materials and therefore is essentially contamination free. Experimental results show that the crystallized samples feature relatively low minority carrier lifetimes which are correlated to relatively high dislocation densities, associated with the sample temperature profile.