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Papers by Irene Berardone

Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science

Accelerated ageing tests of photovoltaic laminates are requested by the IEC standards for quality... more Accelerated ageing tests of photovoltaic laminates are requested by the IEC standards for quality control, which require the assessment of the electrical power losses after a prescribed amount of temperature and/or humidity cycles inside a climatic chamber (thermal cycling, humidity freeze, and damp heat tests). Since electric damage is measured only at the end of such tests, its kinetics induced by thermo-elastic stresses and the related degradation phenomena are reported in few cases. The aim of this study is to investigate the progress of damage, reporting the results of an unprecedented experimental campaign on a photovoltaic mini-module composed of nine multi-crystalline silicon solar cells, one of them containing a cracked cell, subject to a revised thermal cycling test including moisture. Every 40 cycles, and up to 460 (corresponding to 1840 h of testing), the progress of electric damage is assessed by monitoring the evolution of the overall electric resistance of the module....

Energy Procedia, 2016

Kerfless wafering techniques have the potential to reduce the cost of crystalline silicon (Si) in... more Kerfless wafering techniques have the potential to reduce the cost of crystalline silicon (Si) in photovoltaics by saving material and production costs. Thermomechanical spallation from thick wafers with aluminium (Al) in the double function as stressor layer and contacting layer in the final solar cell is one attractive option. In principle this technique might allow to produce multiple thin film solar cells via thermo-mechanical exfoliation, which is essential to be ascertaining for the technique in an industrial context. The aim of this study concerns the feasibility to reuse the parental Si substrate in the presence of non-planar Si-Al interface originated by the first exfoliation. A numerical method based on the Finite Element Method (FEM) and Linear Elastic Fracture Mechanics (LEFM) is used to simulate the evolution of the non-planar Si-Al interface after successive exfoliations. We find a partial reduction in the amplitude of the transferred groove on the Si-Al interface for grooves having typical realistic dimension. The numerical results are confirmed by the reduction of the amplitude of the groove observed in the experimental tests. These results are promising for the repeated exfoliation from one substrate, inasmuch as the possible roughness on the parental substrate after the first use does not significantly affect the subsequent exfoliations.

$Research paper thumbnail of A coupled thermo-electro-mechanical model for fracture in solar cells$

Energy Procedia, 2015

In order to reduce the silicon consumption in the production of crystalline silicon solar cells, ... more In order to reduce the silicon consumption in the production of crystalline silicon solar cells, the improvement of sawing techniques or the use of a kerf-less process are possible solutions. This study focuses on a particular kerf-less technique based on thermally-induced spalling of thin silicon layers joined to aluminum. Via a controlled temperature variation we demonstrate that it is possible to drive an initially sharp crack, introduced by laser, into the silicon substrate and obtain the detachment of ultra-thin silicon layers. A numerical approach based on the finite element method (FEM) and Linear Elastic Fracture Mechanics (LEFM) is herein proposed to compute the Stress Intensity Factors (SIFs) that characterize the stress field at the crack tip and predict crack propagation of an initial notch, depending on the geometry of the specimen and on the boundary conditions. We propose a parametric study to evaluate the dependence of the crack path on the following parameters: (i) the distance between the notch and the aluminum-silicon interface, (ii) the thickness of the stressor (aluminum) layer, and (iii) the applied load. The results for the cooling process here analyzed show that T >43 K and a ratio =0.65 between the thickness of the stressor layer and the distance of the initial notch from the interface are suitable values to achieve a steady-state propagation in case of a ratio 0 =0.115 between the in plane thickness of the silicon substrate and the aluminum thickness, a value typically used in applications.

An experimental study based on the electroluminescence technique is herein proposed to demonstrat... more An experimental study based on the electroluminescence technique is herein proposed to demonstrate the existence of coupling between mechanical deformations and the intensity of the electric field due to cracks in monocrystalline Silicon cells embedded in photovoltaic modules. In spite of the very brittle nature of Silicon, due to the action of the encapsulating polymer and residual compressive stresses resulting from the lamination stage, cracks experience crack closure and contact during mechanical unloading, partially recovering their original electric response. Crack propagation in case of cyclic loading, as, e.g., in case of vibrations due to transportation and use, have also been reported for the very first time. The research results pinpoint the need of improving electric predictions based on the estimation of inactive cell areas, since worst case scenarios not accounting for electro-mechanical coupling are too conservative.

Solar Energy, 2015

Notice: this is the pre-print version of a work that was accepted for publication in Solar Energy... more Notice: this is the pre-print version of a work that was accepted for publication in Solar Energy. Changes resulting from the publishing process, such as minor modifications based on reviewers' comments, editing, structural formatting, and other quality control mechanisms may not be reflected in this document. A definitive version is available in: Solar Energy,

Energy Procedia, 2014

Damage, micro-cracks, grain boundaries and other defects in solar cells are impacting on the elec... more Damage, micro-cracks, grain boundaries and other defects in solar cells are impacting on the electric power-loss of photovoltaic modules, their actual solar conversion efficiency and also their lifetime. In the present contribution, a one-dimensional model for simulating the electric current distribution in solar cells accounting for a distributed series resistance is generalized to the presence of partially conductive cracks. The proposed model is used to perform a quantitative analysis of electroluminescence (EL) images of cracked monocrystalline silicon solar cells. A further generalization in a stochastic direction is also proposed in order to take into account randomly distributed defects typical of polycrystalline silicon.

Scientific Reports, 2014

Cracking in Silicon solar cells is an important factor for the electrical power-loss of photovolt... more Cracking in Silicon solar cells is an important factor for the electrical power-loss of photovoltaic modules. Simple geometrical criteria identifying the amount of inactive cell areas depending on the position of cracks with respect to the main electric conductors have been proposed in the literature to predict worst case scenarios. Here we present an experimental study based on the electroluminescence (EL) technique showing that crack propagation in monocrystalline Silicon cells embedded in photovoltaic (PV) modules is a much more complex phenomenon. In spite of the very brittle nature of Silicon, due to the action of the encapsulating polymer and residual thermo-elastic stresses, cracked regions can recover the electric conductivity during mechanical unloading due to crack closure. During cyclic bending, fatigue degradation is reported. This pinpoints the importance of reducing cyclic stresses caused by vibrations due to transportation and use, in order to limit the effect of cracking in Silicon cells. P V modules are supposed to have a lifetime longer than 20 years under the exposure to environmental conditions. Thermo-mechanical loads induce stresses into the components of the module, especially into the crystalline Silicon (Si) solar cells, which are affected by cracking 1-9. The Institute for Solar Energy Research Hamelin and TÜ V Rheinland provided detailed reports on PV modules quality during the period 2008-2011 10. The percentage of cracked cells in modules as delivered before their installation in the field was 6%, presumably related to vibrations and impacts during transportation. Then, ageing effects due to environmental conditions such as snow, wind gusts, hail and rapid temperature variations are responsible for further propagation of cracks 11 , although it is nearly impossible to assess the individual contribution of each factor. Analysis and statistics of degradation mechanisms in Silicon modules observed in the field have reported various sources of failure of PV modules, namely: laminate internal electric circuit failure, glass breakage, junction box or cables failure, encapsulant decoloration or backsheet debonding, cell failures due to cracking. Among them, cell failure is considered to be responsible for 10% of the totally observed PV module failures, with an occurrence analogous to that of junction box or cables failure and to encapsulant decoloration or backsheet debonding 12. Cracks on the millimetre or centimetre size are mostly invisible by naked eye but they can be localized according to the EL technique 13. Such cracks can lead to electrically inactive cell areas thus reducing the power output of the module and the fill factor 14. This takes place via the following mechanisms, i.e.: a linear decreasing of the short circuit current by increasing the inactive cell area 3,15,16 , and an increase in the series resistance of the cell due to cracking 3,5. For instance, experimental results 5 have shown an increase in the series resistance of the cell of about 7% due to cracking with a corresponding power-loss of 4% and a fill factor reduction of 3%. Other experimental investigations 3 have shown that cracks inserted in solar cells by the application of a uniform pressure to simulate snow can lead up to 1.5% of power loss. After the subsequent application of 200 humidity freeze cycles according to standard specifications 17 , such cracks propagate, the electrically disconnected areas increase in size and up to 10% of power loss has been reported. Potentially, if a crack crossing an Aluminium conductor (called finger) is sufficiently open, then the finger may fail and the electric flow to the busbar in case of normal operating condition, or from the busbar in case of forward bias condition as in the EL testing, would be interrupted. Therefore, portions of Si cells can be potentially deactivated by cracks and their impact on power-loss reasonably depends on their inclination and position with respect to the busbars, see Figs. 1(a) and (b). For instance, a crack parallel to the busbar on the upper side of the cell could lead up to 25% of electrically inactive area (Fig. 1(b)). According to this pure geometrical criterion which does not take into account neither physical

Acta Physica Polonica A, 2013

We propose a 3D branched ZnO nanostructure for the fabrication of highly ecient dye-sensitized so... more We propose a 3D branched ZnO nanostructure for the fabrication of highly ecient dye-sensitized solar cell photoanodes. A coral-shaped structured Zn layer was deposited by radio frequency magnetron sputtering at room temperature onto uorine-doped tin oxide/glass sheets and then thermally oxidized in ambient atmosphere, obtaining a high-density branched ZnO lm. The porous structure provides a large surface area, and, as a consequence, a high number of adsorption sites, and the size and spacing of the nanostructures (on the order of the exciton diusion length) are optimal for good electron collection eciency. The proposed synthesis technique is simple and scalable and the reproducibility of the growth results was tested. The crystalline phase of the lm was investigated, evidencing the complete oxidation and the formation of a pure wurtzite crystalline structure. ZnO-based solar harvesters were fabricated in a microuidic architecture, using conventional sensitizer and electrolyte. The dependence of the cell eciency on dye incubation time and lm thickness was studied with IV electrical characterization and electrochemical impedance spectroscopy. The obtained conversion eciency values, with a maximum value of 4.83%, conrm the highly promising properties of this material for the implementation in dye-sensitized solar cell photoanodes.