Aaltodoc Repository :: Browsing by Author "Vahlman, Henri"

Browsing by Author "Vahlman, Henri"

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Black silicon back contact module with wide light acceptance angle
(Wiley, 2020-03-01) Ortega, Pablo; Garin, Moises; von Gastrow, Guillaume; Savisalo, Tuukka; Tolvanen, Antti; Vahlman, Henri; Vähänissi, Ville; Pasanen, Toni; Carrio, David; Savin, Hele; Alcubilla, Ramon; Department of Electronics and Nanoengineering; Hele Savin Group; Polytechnic University of Catalonia; Valoe Oyj; Endeas Oy
In this work a photovoltaic mini-module combining interdigitated back contacted solar cells with black silicon in the front was implemented as a proof of concept. The module consists of nine solar cells connected in series with an active area of 86.5 cm2. Both the assembly and panel encapsulation were made using industrial back contact module technology. Noticeable photovoltaic efficiencies of 18.1% and 19.9% of the whole module and the best individual cell of the module respectively, demonstrate that fragile nanostructures can withstand standard module fabrication stages. Open circuit voltage and fill factor values of 5.76 V and 81.6% respectively reveal that series interconnection between cells works as expected, confirming a good ohmic contact between cell busbars and the conductive back sheet. Additionally, the excellent quasi-omnidirectional anti-reflection properties of black silicon surfaces prevails at module-level, as it is corroborated by light incidence angle dependence measurements of the short circuit current parameter.
Combining a dye-sensitized solar cell and an electric double layer capacitor: the photocapacitor
(2012-08-31) Landström, Mariko; Vahlman, Henri; ; Perustieteiden korkeakoulu; Lund, Peter
Effect of low-temperature annealing on defect causing copper-related light-induced degradation in p-type silicon
(Elsevier, 2017) Vahlman, Henri; Haarahiltunen, Antti; Kwapil, Wolfram; Schön, Jonas; Yli-Koski, Marko; Inglese, Alessandro; Modanese, Chiara; Savin, Hele; Department of Electronics and Nanoengineering; Hele Savin Group; Fraunhofer Institute for Solar Energy Systems
Copper is a common impurity in photovoltaic silicon. While reported to precipitate instantly in n-type Si, copper causes light-induced degradation (Cu-LID) in p-type Si. Recently, partial recovery of Cu-LID was observed after only few minutes of dark annealing at 200°C. In this contribution, we investigate the effects of the dark anneal on Cu-LID-limited minority carrier lifetime both experimentally and by simulations. Surprisingly, after initial recovery, the dark anneal results in further degradation corresponding to a many-fold increase in recombination activity compared to the degraded state after illumination. This anneal-induced degradation can potentially cause additional losses in accidentally Cu-contaminated devices when exposed to elevated temperatures, for example during recovery and regeneration treatments of solar cells. Transient ion drift measurements confirmed that the anneal-induced degradation cannot be attributed to residual interstitial Cu after illumination. After hundreds of hours of annealing, the samples showed another recovery. To analyze these experimental results, a comparison to simulations is performed at the end of the paper.
Industrial Applicability of Antireflection-Coating-Free Black Silicon on PERC Solar Cells and Modules
(2018) Pasanen, Toni; Vähänissi, Ville; Wolny, Franziska; Oehlke, Alexander; Wagner, Matthias; Juntunen, Mikko A.; Heikkinen, Ismo T. S.; Salmi, Emma; Sneck, Sami; Vahlman, Henri; Tolvanen, Antti; Hyvärinen, Jaakko; Savin, Hele; Hele Savin Group; Aalto Nanofab; SolarWorld Industries GmbH; SolarWorld Innovations GmbH; Naps Solar Systems Inc.; Beneq Oy; Department of Electronics and Nanoengineering; Endeas Oy
Black silicon (b-Si) is of particular interest within the photovoltaic community due to its ability to texture diamond wire-sawn multicrystalline silicon wafers. In this work, we apply a deep dry-etched black silicon nanotexture, which possesses less than 1 % solar-weighted reflectance with no antireflection coating (ARC), to industrial Passivated Emitter and Rear Cells (PERC). Additionally, the cells are processed further into small solar modules, which are characterized by their electrical and optical performance. The fragile nanostructures remain intact during industrial cell and module fabrication, maintaining the excellent optical properties until the final product. We show that b-Si modules with a typical cover glass retain their performance until incident angles larger than 60°, whereas the heavily increased reflectance of acidic-textured modules decreases their efficiency already after a 30° tilt. Furthermore, at an incidence angle of 70°, the efficiency of b-Si modules has reduced only 7 %, while that of the acidic-textured equivalents has decreased more than 25 %. Hence, the excellent optical properties of ARC-free b-Si are maintained also at module level. The results demonstrate that deep dry-etched b-Si nanostructures are fully applicable to current industrial PERC production facilities.
Ionic Liquid Electrolytes and Their Quasi-solidification with Carbon Nanoparticles for Dye Solar Cells
(2011) Vahlman, Henri; Halme, Janne; Teknillisen fysiikan laitos; Perustieteiden korkeakoulu; School of Science; Lund, Peter
Dye solar cells (DSC) are a potentially viable future source of low-cost renewable energy, provided that certain critical problems related to their stability and suitability for mass production are solved. The main problems preventing wide-spread utilization of DSCs include high cost of transparent conducting substrates, degrading dyes and volatile organic solvent-based electrolytes. In this work we concentrate on the electrolyte volatility issue, or to be more precise, replacement of the organic solvents with nonvolatile ionic liquids. Non-volatility of ionic liquids is accompanied by their high viscosity, which poses a problem in view of electrolyte charge transfer. In this thesis, means of alleviating the charge transfer problem through quasi-solidification of ionic liquid electrolytes with carbon nanoparticles are studied first of all through a literature survey, and secondly through experimental work. The experimental section consists of two parts, including firstly an analysis of performance limiting factors in DSCs with an ionic liquid electrolyte, and secondly attempts to remove or relieve these restrictions through the dispersion of carbon nanoparticles into the electrolyte. Experimental results are consequently paralleled with a diffusion model in order to shed light on the principles behind the observed phenomena. In this work, a maximum average efficiency of 1.1 % was achieved by using a two-component electrolyte composed of a polyaniline-loaded carbon black (PACB) dispersed in 1-propyl-3-methylimidazolium iodide (PMII) ionic liquid without additive iodine. Although the above efficiency lags behind that of both an organic solvent based electrolyte DSC (5.3 %) and a reference ionic liquid electrolyte DSC with additive iodine (1.5 %), the non-volatility, simple consistence and screen-printability, added with the fact that no separate platinum catalyst was required at the counter electrode due to catalytic activity of PACB, speak for the two-component ionic liquid-carbon nanoparticle composite electrolyte. New information about the working principle and impedance behavior of the ionic liquid-carbon nanoparticle composite electrolyte DSC was obtained in this work. The fact that we managed to improve the efficiency of the composite electrolyte DSC through introducing an insulating zirconium dioxide spacer layer between the photoelectrode and the electrolyte layer opens up new research pathways on the topic.
Light-induced degradation due to Cu precipitation in crystalline silicon: Modeling and impact on PERC solar cells
(Aalto University, 2018) Vahlman, Henri; Savin, Hele, Prof., Aalto University, Department of Electronics and Nanoengineering, Finland; Haarahiltunen, Antti, Dr., Aalto University, Department of Micro and Nanosciences, Finland; Elektroniikan ja nanotekniikan laitos; Department of Electronics and Nanoengineering; Electron Physics Group; Sähkötekniikan korkeakoulu; School of Electrical Engineering; Savin, Hele, Prof., Aalto University, Department of Electronics and Nanoengineering, Finland
Copper is a common element in the environment and hence a difficult contaminant to control on silicon device manufacture lines. In p-type Si devices (e.g. solar cells), even very low copper concentrations can lead to copper-related light-induced degradation (Cu-LID), that is, degradation of the bulk minority carrier lifetime under excess carrier injection. As copper concentrations even below the detection limits of analytical methods can cause Cu-LID, it is necessary to identify its presence directly from device-level effects, which can be perturbed by the simultaneous occurrence of other light-induced degradation (LID) mechanisms. Hence, the first aim of this work is to clarify the properties of Cu-LID that enable its distinction from other LID mechanisms at the solar cell level. Thus, strong LID observed in industrial passivated emitter and rear contact (PERC) solar cells was characterized, and complemented with an analysis of the effects of corona charging on lifetime sample wafers; a method that has earlier been used in the detection of Cu-LID. The results reveal that Cu-LID in solar cells can be recognized based on its relatively fast degradation rate and laterally heavy degradation patterns on extended defects. On the other hand, Cu-LID and another LID mechanism called Sponge-LID showed mutually similar properties, and further investigations possibly involving lifetime spectroscopic methods are necessary to clarify their relationship. The second objective of the thesis is to deepen the theoretical understanding of Cu-LID. Hence, a physical model based on a theory of electrostatically limited copper precipitation was derived, which together with a previously published Schottky junction model of metal precipitates enables the modeling of Cu-LID directly at the minority carrier lifetime level. Agreement between the model and experiments was obtained in most of the investigated cases. Consequently, the different material properties and environmental conditions that affect the strength of Cu-LID were identified. Theoretically, the strength of Cu-LID is mostly affected by the Cu and the doping concentrations, and the density of heterogeneous nucleation sites, all of which influence the final precipitate size that was found to reside between few to few tens of nanometers in radius. These results provide confirmation for the precipitation theory of Cu-LID, and provide insights for mitigating its effects in silicon devices.
Low-temperature dark anneal as pre-treatment for LeTID in multicrystalline silicon
(2019-04-01) Yli-Koski, Marko; Serué, Michael; Modanese, Chiara; Vahlman, Henri; Savin, Hele; Department of Electronics and Nanoengineering; Hele Savin Group
Light and elevated temperature induced degradation (LeTID) is currently a severe issue in crystalline silicon photovoltaics, which has led to numerous efforts to both understand the mechanism and to mitigate it. Here we show that a low-temperature dark anneal performed as the last step in typical solar cell processing influences greatly LeTID characteristics, both the strength of the degradation and the degradation kinetics. While a relatively short anneal in the temperature range of 200–240 °C can be detrimental to LeTID by doubling the degradation intensity, an optimized anneal at 300 °C shows the opposite trend providing an efficient means to eliminate LeTID. Furthermore, we show that the simulated recombination activity of metal precipitation and dissolution during the dark anneal correlates with the experiments, suggesting a possible explanation for the LeTID mechanism.
(oral talk) Vertically integrated modeling of light-induced defects: Process modeling, degradation kinetics and device impact
(2018) Laine, Hannu S.; Vahlman, Henri; Haarahiltunen, Antti; Jensen, Mallory A.; Modanese, Chiara; Wagner, Matthias; Wolny, Franziska; Buonassisi, Tonio; Savin, Hele; Department of Electronics and Nanoengineering; Hele Savin Group; Massachusetts Institute of Technology; SolarWorld Industries GmbH
Recombination activity of light-activated copper defects in p-type silicon studied by injection- and temperature-dependent lifetime spectroscopy
(2016-09-26) Inglese, Alessandro; Lindroos, Jeanette; Vahlman, Henri; Savin, Hele; Department of Micro and Nanosciences; Karlstad University
The presence of copper contamination is known to cause strong light-induced degradation (Cu-LID) in silicon. In this paper, we parametrize the recombination activity of light-activated copper defects in terms of Shockley—Read—Hall recombination statistics through injection- and temperature dependent lifetime spectroscopy (TDLS) performed on deliberately contaminated float zone silicon wafers. We obtain an accurate fit of the experimental data via two non-interacting energy levels, i.e., a deep recombination center featuring an energy level at Ec−Et=0.48−0.62 eVEc−Et=0.48−0.62 eV with a moderate donor-like capture asymmetry (k=1.7−2.6) k=1.7−2.6) and an additional shallow energy state located at Ec−Et=0.1−0.2 eVEc−Et=0.1−0.2 eV, which mostly affects the carrier lifetime only at high-injection conditions. Besides confirming these defect parameters, TDLS measurements also indicate a power-law temperature dependence of the capture cross sections associated with the deep energy state. Eventually, we compare theseresults with the available literature data, and we find that the formation of copper precipitates is the probable root cause behind Cu-LID.
Vertically integrated modeling of light-induced defects
(2018-08-10) Laine, Hannu S.; Vahlman, Henri; Haarahiltunen, Antti; Jensen, Mallory A.; Modanese, Chiara; Wagner, Matthias; Wolny, Franziska; Buonassisi, Tonio; Savin, Hele; Department of Electronics and Nanoengineering; Massachusetts Institute of Technology MIT; SolarWorld Industries GmbH
As photovoltaic (PV) device architectures advance, they turn more sensitive to bulk minority charge carrier lifetime. The conflicting needs to develop ever advancing cell architectures on ever cheapening silicon substrates ensure that various impurity-related light-induced degradation (LID) mechanisms will remain an active research area in the silicon PV community. Here, we propose vertically integrated defect modeling as a framework to accelerate the identification and mitigation of different light induced defects. More specifically, we propose using modeled LID-kinetics to identify the dominant LID mechanism or mechanisms within complete PV devices. Coupling the LID-kinetics model into a process model allows development of process guidelines to mitigate the identified LID-mechanism within the same vertically integrated simulation tool. We use copper as an example of a well-characterized light-induced defect: we model the evolution of copper during solar cell processing and light soaking, and then map the deleterious lifetime effect of Cu-LID onto device performance. We validate our model using intentionally Cu-contaminated material processed on an industrial PERC-line and find that our model reproduces the LID-behavior of the manufactured solar cells. We further show via simulations that Cu-LID can be mitigated by reducing the contact co-firing peak temperature, or the cooling rate after the firing process.

Browsing by Author "Vahlman, Henri"

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