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Oliver Eibl - One of the best experts on this subject based on the ideXlab platform.
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High-efficiency, single-crystalline, p- and n-type Si solar cells: Microstructure and chemical analysis of the Glass Layer
Solar Energy Materials and Solar Cells, 2018Co-Authors: Praveen Kumar, Zainul Aabdin, Michael Pfeffer, Oliver EiblAbstract:Abstract Recent investigations of the front side metallization of high-efficiency mono-crystalline Si solar cells proved that the Glass Layer formed at the Si/metallization interface during the screen printing plays an essential role for the charge transport, both in n- and p-type cells. High-efficiency cells (~ 18.0% for p-type and ~ 20% for n-type cells) show similar microstructure of the Glass Layer and similar temperature dependence of the series resistance. From this it is concluded that the microstructure of the Glass Layer determines the series and contact resistance of the front side metallization. The Glass Layers of high-efficiency cells contain a high density of nano-Ag colloids and other precipitates which reduce the contact resistance. A percolation model was proposed and is best suited to describe the charge transport in a dirty semiconductor containing metallic precipitates. Quantitative chemical composition of the Glass Layer of p- and n-type cells was investigated by Energy Dispersive X-ray (EDX) microanalysis in SEM and TEM. The chemical composition of the Glass Layer showed (SiO x )Pb, as main constitutes and Zn, Ti, Al, Ag, P and B as minor constituents with mole fractions above the detection limit of EDX. The Glass Layer is therefore considered to be a dirty semiconductor rather than a perfect insulator. The mole fractions of Zn (~ 1 at%) and Al (~ 1 at%) were quantitatively analyzed. Such analyses are important to correlate microstructural features with electrical properties of the front side metallization. We could prove that in p-type cells the efficiency of the cells correlated with the chemical composition of the Glass Layer. In n-type cells, with Al-containing pastes EDX spectroscopy yielded Al beyond the detection limit in the Glass Layer, whereas for Al free pastes the Al mole fraction was below the detection limit of EDX and yielded reduced efficiencies. In p-type cells pastes with enhanced Zn mole fraction yielding zinc oxide phases in the bulk Ag finger and Zn mole fractions up to 5 at% in the Glass Layer.
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Front Side Metallization of n- and p-Type, High-Efficiency, Single-Crystalline Si Solar Cells: Assessing the Temperature-Dependent Series Resistance
Journal of Electronic Materials, 2016Co-Authors: Benjamin Willsch, Praveen Kumar, Oliver EiblAbstract:The series resistance of high-quality, single crystalline p -type and n -type solar cells was measured in a temperature range between 80 K and room temperature. Among one cell type ( n or p ), cells were processed identically. Only the processing of the front side metallization was varied by using different processing conditions and screen printing pastes. High-efficiency n - ( η = 20.0%) and p -type ( η = 18.0%) cells yielded similar contact and series resistance and common features of the microstructure of the front side contact, i.e. a Glass Layer containing Ag colloids with typical diameters of 5–200 nm. Temperature-dependent current voltage curves ( I – V curves) were acquired and evaluated with respect to the series resistance by using two different methods yielding different results. On average the series resistance follows the trends of the contact resistance of the front side metallization determined at room temperature. Optimally processed cells yielded series resistances of less than 25 mΩ cm^2 (method #1) both for n - and p -type cells. It could be shown that the series resistance reflected the processing conditions and paste properties and yielded similar temperature dependence for p - and n -type cells with small contact resistance. Therefore, the relevant current paths of high-efficiency n - and p -type cells appear to be similar in the front side metallization and include the Glass Layer which contains a high density of Ag colloids.
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Contact formation of front side metallization in p-type, single crystalline Si solar cells: Microstructure, temperature dependent series resistance and percolation model
Solar Energy Materials and Solar Cells, 2016Co-Authors: Praveen Kumar, Michael Pfeffer, Benjamin Willsch, Oliver EiblAbstract:Abstract Screen printed front side contacts were investigated in single-crystalline (planar and textured) Si solar cells with n-type emitters, yielding maximum efficiencies of 18.0%. The crystallographic orientation of the Si surface and the paste strongly affect the contact formation as well as the contact resistance of the cells. For textured cells a continuous Glass Layer together with the formation of Ag colloids yielded a small contact resistance. Planar 〈111〉oriented Si yielded specifically lower contact resistance (〈5 mΩ cm 2 ) as compared to planar 〈100〉 orientation (〉10–40 mΩ cm 2 ) for different pastes. Pyramidal Ag crystals are formed only on 〈100〉 oriented Si, whereas lens shaped Ag crystals are grown on 〈111〉surfaces. From this it was concluded that the shape of the Ag nanocrystals determines the contact resistance, pyramidal Ag crystals formed on 〈100〉 planar surfaces yielded cells with large contact resistance and are, therefore, not considered to be necessary for a low contact resistance. Temperature dependent series resistance measurements yielded metallic behavior for cells with the lowest contact resistance bound to a certain paste. For other pastes and processing conditions a semiconducting behavior of the series resistance was found. However, cells with significant density of colloids in the Glass Layer yielded a small series and contact resistance. By considering the above arguments, a percolation model has been introduced in which metallic Ag colloids generate current filaments across the Glass Layer. This reduces the resistivity of the Glass Layer and thereby introduces a percolative nature of the current via Ag nanocolloids. The percolation limit for the 2d case was calculated for periodically arranged colloids with equal size and yields a minimum volume fraction of 15% for the Ag colloids in the Glass Layer.
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Combined Microstructural and Electrical Characterization of Metallization Layers in Industrial Solar Cells
Energy Procedia, 2015Co-Authors: Praveen Kumar, Florian Clement, Zainul Aabdin, Benjamin Willsch, M. Dürrschnabel, Rene Hoenig, N. Peranio, Daniel Biro, Oliver EiblAbstract:Abstract Screen printed front side contacts of textured, mono-crystalline p-type silicon solar cells with n-type emitters were investigated. The different pastes (FSP1 and 2) and the different crystallographic orientations of the Si surfaces studied strongly affected the contact resistance. The microstructure of the contacts was analyzed in plan-view and cross-section by combined scanning and analytical transmission electron microscopy. A controlled grinding process rather than a chemical etching process was applied for the plan-view sample preparation. For textured cells processed with different pastes, pronounced differences were seen in the contact resistances (FSP1: efficiency 16.9% and contact resistance 20 mΩ cm 2 , FSP2: 17.8% and 2 ). A discontinuous Glass Layer was found for FSP1 but a continuous Glass Layer was found for FSP2, yielding a smaller contact resistance. Glass Layers contained (Si 2 Pb)O x as a main constituent but different mole fractions of Zn, Ti, P, and B as minor constituents, varying for the different pastes. Glass Layers were up to 500 nm thick and revealed inhomogeneously distributed spherical Ag colloids 5-200 nm in size. Planar cells were also studied and served as model systems: planar 〈111〉 oriented Si surfaces yielded specifically lower contact resistance as compared to planar 〈100〉 orientation. Pyramidal Ag crystallites were only observed for 〈100〉 oriented Si surfaces not for 〈111〉 surfaces. Therefore, it is concluded that pyramidal Ag crystallites are not necessary for contacts yielding low contact resistance. Instead, lens shaped Ag precipitates together with a high density of Ag colloids in the Glass Layer yield low contact resistance, as found for oriented Si surfaces. A percolative current path including charge transport via Ag colloids in the Glass Layer is proposed. For textured cells, in accordance with these results, pyramidal Ag crystallites were only observed at step edges of {111} faces or at the edges of the Si pyramids.
Praveen Kumar - One of the best experts on this subject based on the ideXlab platform.
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High-efficiency, single-crystalline, p- and n-type Si solar cells: Microstructure and chemical analysis of the Glass Layer
Solar Energy Materials and Solar Cells, 2018Co-Authors: Praveen Kumar, Zainul Aabdin, Michael Pfeffer, Oliver EiblAbstract:Abstract Recent investigations of the front side metallization of high-efficiency mono-crystalline Si solar cells proved that the Glass Layer formed at the Si/metallization interface during the screen printing plays an essential role for the charge transport, both in n- and p-type cells. High-efficiency cells (~ 18.0% for p-type and ~ 20% for n-type cells) show similar microstructure of the Glass Layer and similar temperature dependence of the series resistance. From this it is concluded that the microstructure of the Glass Layer determines the series and contact resistance of the front side metallization. The Glass Layers of high-efficiency cells contain a high density of nano-Ag colloids and other precipitates which reduce the contact resistance. A percolation model was proposed and is best suited to describe the charge transport in a dirty semiconductor containing metallic precipitates. Quantitative chemical composition of the Glass Layer of p- and n-type cells was investigated by Energy Dispersive X-ray (EDX) microanalysis in SEM and TEM. The chemical composition of the Glass Layer showed (SiO x )Pb, as main constitutes and Zn, Ti, Al, Ag, P and B as minor constituents with mole fractions above the detection limit of EDX. The Glass Layer is therefore considered to be a dirty semiconductor rather than a perfect insulator. The mole fractions of Zn (~ 1 at%) and Al (~ 1 at%) were quantitatively analyzed. Such analyses are important to correlate microstructural features with electrical properties of the front side metallization. We could prove that in p-type cells the efficiency of the cells correlated with the chemical composition of the Glass Layer. In n-type cells, with Al-containing pastes EDX spectroscopy yielded Al beyond the detection limit in the Glass Layer, whereas for Al free pastes the Al mole fraction was below the detection limit of EDX and yielded reduced efficiencies. In p-type cells pastes with enhanced Zn mole fraction yielding zinc oxide phases in the bulk Ag finger and Zn mole fractions up to 5 at% in the Glass Layer.
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high efficiency crystalline si solar cells with screen printed front side metallization a percolation model to explainthe current path
Journal of Electronic Materials, 2016Co-Authors: Michael Pfeffer, Praveen Kumar, O EiblAbstract:Resistive losses corresponding to the front-side metallization limit the efficiency of Si solar cells. At the front-side contact, the Si emitter is covered by a Glass Layer that is less than 1 μm thick embedded with Ag colloids to volume fraction >20%. Bulk Ag fingers are arranged on top of the Glass Layer. A similar microstructure is found for both n-type and p-type cells showing high efficiency. The Ag colloids constitute current filaments with reduced resistance in the Glass Layer, thereby introducing a percolative current which is the basis of the proposed model. This model is new and differs from the classical percolation model in its direct reliance on the macroscopic resistance of these filaments, and in considering the matrix as semiconducting rather than insulating. For periodically arranged Ag colloids of fixed diameter, the percolative limit of 13% in two dimensions (2D) and 15% in three dimensions (3D) depends only on the volume fraction of colloids but not their size. The resistance of randomly arranged and sized Ag colloids confirms the analytical results. The model explains quantitatively, consistent with microstructural analyses, why low contact resistances are found in solar cells with high colloid density. The introduced percolation model is also relevant for other systems in which metallic precipitates are found in a semiconducting matrix.
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Front Side Metallization of n- and p-Type, High-Efficiency, Single-Crystalline Si Solar Cells: Assessing the Temperature-Dependent Series Resistance
Journal of Electronic Materials, 2016Co-Authors: Benjamin Willsch, Praveen Kumar, Oliver EiblAbstract:The series resistance of high-quality, single crystalline p -type and n -type solar cells was measured in a temperature range between 80 K and room temperature. Among one cell type ( n or p ), cells were processed identically. Only the processing of the front side metallization was varied by using different processing conditions and screen printing pastes. High-efficiency n - ( η = 20.0%) and p -type ( η = 18.0%) cells yielded similar contact and series resistance and common features of the microstructure of the front side contact, i.e. a Glass Layer containing Ag colloids with typical diameters of 5–200 nm. Temperature-dependent current voltage curves ( I – V curves) were acquired and evaluated with respect to the series resistance by using two different methods yielding different results. On average the series resistance follows the trends of the contact resistance of the front side metallization determined at room temperature. Optimally processed cells yielded series resistances of less than 25 mΩ cm^2 (method #1) both for n - and p -type cells. It could be shown that the series resistance reflected the processing conditions and paste properties and yielded similar temperature dependence for p - and n -type cells with small contact resistance. Therefore, the relevant current paths of high-efficiency n - and p -type cells appear to be similar in the front side metallization and include the Glass Layer which contains a high density of Ag colloids.
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Contact formation of front side metallization in p-type, single crystalline Si solar cells: Microstructure, temperature dependent series resistance and percolation model
Solar Energy Materials and Solar Cells, 2016Co-Authors: Praveen Kumar, Michael Pfeffer, Benjamin Willsch, Oliver EiblAbstract:Abstract Screen printed front side contacts were investigated in single-crystalline (planar and textured) Si solar cells with n-type emitters, yielding maximum efficiencies of 18.0%. The crystallographic orientation of the Si surface and the paste strongly affect the contact formation as well as the contact resistance of the cells. For textured cells a continuous Glass Layer together with the formation of Ag colloids yielded a small contact resistance. Planar 〈111〉oriented Si yielded specifically lower contact resistance (〈5 mΩ cm 2 ) as compared to planar 〈100〉 orientation (〉10–40 mΩ cm 2 ) for different pastes. Pyramidal Ag crystals are formed only on 〈100〉 oriented Si, whereas lens shaped Ag crystals are grown on 〈111〉surfaces. From this it was concluded that the shape of the Ag nanocrystals determines the contact resistance, pyramidal Ag crystals formed on 〈100〉 planar surfaces yielded cells with large contact resistance and are, therefore, not considered to be necessary for a low contact resistance. Temperature dependent series resistance measurements yielded metallic behavior for cells with the lowest contact resistance bound to a certain paste. For other pastes and processing conditions a semiconducting behavior of the series resistance was found. However, cells with significant density of colloids in the Glass Layer yielded a small series and contact resistance. By considering the above arguments, a percolation model has been introduced in which metallic Ag colloids generate current filaments across the Glass Layer. This reduces the resistivity of the Glass Layer and thereby introduces a percolative nature of the current via Ag nanocolloids. The percolation limit for the 2d case was calculated for periodically arranged colloids with equal size and yields a minimum volume fraction of 15% for the Ag colloids in the Glass Layer.
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Combined Microstructural and Electrical Characterization of Metallization Layers in Industrial Solar Cells
Energy Procedia, 2015Co-Authors: Praveen Kumar, Florian Clement, Zainul Aabdin, Benjamin Willsch, M. Dürrschnabel, Rene Hoenig, N. Peranio, Daniel Biro, Oliver EiblAbstract:Abstract Screen printed front side contacts of textured, mono-crystalline p-type silicon solar cells with n-type emitters were investigated. The different pastes (FSP1 and 2) and the different crystallographic orientations of the Si surfaces studied strongly affected the contact resistance. The microstructure of the contacts was analyzed in plan-view and cross-section by combined scanning and analytical transmission electron microscopy. A controlled grinding process rather than a chemical etching process was applied for the plan-view sample preparation. For textured cells processed with different pastes, pronounced differences were seen in the contact resistances (FSP1: efficiency 16.9% and contact resistance 20 mΩ cm 2 , FSP2: 17.8% and 2 ). A discontinuous Glass Layer was found for FSP1 but a continuous Glass Layer was found for FSP2, yielding a smaller contact resistance. Glass Layers contained (Si 2 Pb)O x as a main constituent but different mole fractions of Zn, Ti, P, and B as minor constituents, varying for the different pastes. Glass Layers were up to 500 nm thick and revealed inhomogeneously distributed spherical Ag colloids 5-200 nm in size. Planar cells were also studied and served as model systems: planar 〈111〉 oriented Si surfaces yielded specifically lower contact resistance as compared to planar 〈100〉 orientation. Pyramidal Ag crystallites were only observed for 〈100〉 oriented Si surfaces not for 〈111〉 surfaces. Therefore, it is concluded that pyramidal Ag crystallites are not necessary for contacts yielding low contact resistance. Instead, lens shaped Ag precipitates together with a high density of Ag colloids in the Glass Layer yield low contact resistance, as found for oriented Si surfaces. A percolative current path including charge transport via Ag colloids in the Glass Layer is proposed. For textured cells, in accordance with these results, pyramidal Ag crystallites were only observed at step edges of {111} faces or at the edges of the Si pyramids.
Gunnar Schubert - One of the best experts on this subject based on the ideXlab platform.
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influence of surface topography on the Glass coverage in the contact formation of silver screen printed si solar cells
IEEE Journal of Photovoltaics, 2013Co-Authors: Enrique Cabrera, Sara Olibet, Dominik Rudolph, Radovan Kopecek, Daniel Reinke, Eckard Wefringhaus, Gunnar SchubertAbstract:For screen-printed Ag-paste metallization, the growth of Ag crystallites into Si is essential for the current conduction from the Si emitter to the silver finger. There are strong indications that for low contact resistances, some of these Ag crystallites need to be in direct contact with the silver finger without a separating Glass Layer in between. In this paper, we concentrate on the origin of Ag crystallites grown into Si in direct contact with the silver finger. On textured surfaces, we vary the Si pyramid sizes, round the pyramid tips to varying degrees, and fabricate flat smooth surfaces. We observe that the size of the pyramids does not play an important role in the achievement of low specific contact resistivity unless the pyramid heights become smaller than the thickness of the Glass Layer, but rounding of the pyramid tips with standard heights increases specific contact resistivity significantly. From our microscopic investigations, we conclude that the largest influence on the topography-dependent contact resistance comes from the Glass coverage governing the amount of Ag crystallites directly connected with the silver finger bulk. Furthermore, Ag crystallites in direct contact with the silver finger are also observed on c-Si without n+ emitter doping.
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Impact of Si surface topography on the Glass Layer resulting from screen printed Ag-paste solar cell contacts
2012 38th IEEE Photovoltaic Specialists Conference, 2012Co-Authors: Enrique Cabrera, Sara Olibet, Dominik Rudolph, Joachim Glatz-reichenbach, Radovan Kopecek, Daniel Reinke, Anne Götz, Gunnar SchubertAbstract:For the current transport mechanism between the n+ Si emitter and the screen printed silver contact, there is strong experimental evidence that the largest current contribution flows through Ag crystallites directly connected with the silver finger, which are preferably concentrated in some tips of the Si pyramid surface. For this purpose, we focused on the origin of these Ag crystallites and we analyzed the contact formation on different surface topographies such as pyramid height and rounding degree variations, and flat smooth surfaces, with and without phosphorus doped emitter and not only on mono- but also on multicrystalline Si material with its dislocations, grain boundaries and impurities. Combining contact resistance measurements with SEM investigations, we discovered that smaller pyramids are more capable of creating better contact than rounded pyramids, even though in our case they cover only a few percent of the area on the Si surfaces. This can be explained with the wetting behavior of the Glass, which leads to some pyramid tips being Glass-free and thus in direct contact with the Ag-bulk, as long as the pyramid heights exceed the thickness of the Glass Layer. In the case of surfaces which are smoothly polished or with strongly rounded pyramidal surfaces, a continuous Glass-Layer separates the Ag crystallites from the Ag-finger preventing good contact. Moreover, without a P doped emitter it is possible not only to create Ag crystallites underneath the Glass, but also Ag crystallites in direct contact with the silver finger.
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Physical understanding of printed thick-film front contacts of crystalline Si solar cells—Review of existing models and recent developments
Solar Energy Materials and Solar Cells, 2006Co-Authors: Gunnar Schubert, Frank Huster, Pete FathAbstract:Abstract Existing models of the silver thick-film contact to an emitter are summarised and recent developments focusing on contact formation and current transport mechanisms are presented. As a Glass Layer exists at the silicon-thick-film interface the current transport mechanisms are not obvious. The main hypotheses are: current transport via spike-like direct silver–silicon interconnections or via tunnelling through the chemically modified Glass Layer. Recent investigations showed that silver crystallites grow into the emitter from the Glass frit containing dissolved silver [G. Schubert, B. Fischer, P. Fath, in: Proceedings of photovoltaics PV in Europe Conference, Rome, 2002, pp. 343–346 [1] ]. These silver islands are covered by a thin Glass Layer [C. Ballif, D.M. Huljic, A. Hessler-Wyser, G. Willeke, in: Proceedings of the 29th IEEE PVSC, Glasgow, 2002, pp. 360–363 [2] ]. Further investigations are necessary to study the crystallite-growth mechanism as well as the current-transport mechanism from the crystallites to the finger.
Pete Fath - One of the best experts on this subject based on the ideXlab platform.
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physical understanding of printed thick film front contacts of crystalline si solar cells review of existing models and recent developments
Solar Energy Materials and Solar Cells, 2006Co-Authors: Gunna Schube, Frank Huste, Pete FathAbstract:Abstract Existing models of the silver thick-film contact to an emitter are summarised and recent developments focusing on contact formation and current transport mechanisms are presented. As a Glass Layer exists at the silicon-thick-film interface the current transport mechanisms are not obvious. The main hypotheses are: current transport via spike-like direct silver–silicon interconnections or via tunnelling through the chemically modified Glass Layer. Recent investigations showed that silver crystallites grow into the emitter from the Glass frit containing dissolved silver [G. Schubert, B. Fischer, P. Fath, in: Proceedings of photovoltaics PV in Europe Conference, Rome, 2002, pp. 343–346 [1] ]. These silver islands are covered by a thin Glass Layer [C. Ballif, D.M. Huljic, A. Hessler-Wyser, G. Willeke, in: Proceedings of the 29th IEEE PVSC, Glasgow, 2002, pp. 360–363 [2] ]. Further investigations are necessary to study the crystallite-growth mechanism as well as the current-transport mechanism from the crystallites to the finger.
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Physical understanding of printed thick-film front contacts of crystalline Si solar cells—Review of existing models and recent developments
Solar Energy Materials and Solar Cells, 2006Co-Authors: Gunnar Schubert, Frank Huster, Pete FathAbstract:Abstract Existing models of the silver thick-film contact to an emitter are summarised and recent developments focusing on contact formation and current transport mechanisms are presented. As a Glass Layer exists at the silicon-thick-film interface the current transport mechanisms are not obvious. The main hypotheses are: current transport via spike-like direct silver–silicon interconnections or via tunnelling through the chemically modified Glass Layer. Recent investigations showed that silver crystallites grow into the emitter from the Glass frit containing dissolved silver [G. Schubert, B. Fischer, P. Fath, in: Proceedings of photovoltaics PV in Europe Conference, Rome, 2002, pp. 343–346 [1] ]. These silver islands are covered by a thin Glass Layer [C. Ballif, D.M. Huljic, A. Hessler-Wyser, G. Willeke, in: Proceedings of the 29th IEEE PVSC, Glasgow, 2002, pp. 360–363 [2] ]. Further investigations are necessary to study the crystallite-growth mechanism as well as the current-transport mechanism from the crystallites to the finger.
Pierre Lefort - One of the best experts on this subject based on the ideXlab platform.
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Fabrication and characterization of ZrB2-SiC ceramic electrodes coated with a proton conducting, SiO2-rich Glass Layer
Electrochimica Acta, 2011Co-Authors: Quentin Lonne, Nicolas Glandut, Jean-claude Labbé, Pierre LefortAbstract:3.5 m thin Layers of dense, crack-free, proton conducting, SiO2-rich Glass have been developed on ZrB2-SiC ceramic composites, by thermal oxidation at 1400 ◦C for 30 min in air. A conductivity of 2 mS cm−1 at 25 ◦C was found, as measured by AC impedance and steady-state voltammetry, and was estimated at ca. 2 × 10−2 S cm−1 at 80 ◦C. A striking behaviour of the oxidized ZrB2-SiC composites is also pointed out: underneath the Glass Layer, there is a porous Layer rich in electronic conductive ZrB2, without well-defined interface between them, i.e., exhibiting a composition gradient in oxygen. In other words, protonic half-fuel cells could be fabricated under such conditions, for future use in hydrogen or direct alcohol fuel cells.
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Fabrication and characterization of ZrB2–SiC ceramic electrodes coated with a proton conducting, SiO2-rich Glass Layer
Electrochimica Acta, 2011Co-Authors: Quentin Lonne, Nicolas Glandut, Jean-claude Labbé, Pierre LefortAbstract:Abstract 3.5 μm thin Layers of dense, crack-free, proton conducting, SiO 2 -rich Glass have been developed on ZrB 2 –SiC ceramic composites, by thermal oxidation at 1400 °C for 30 min in air. A conductivity of 2 mS cm −1 at 25 °C was found, as measured by AC impedance and steady-state voltammetry, and was estimated at ca. 2 × 10 −2 S cm −1 at 80 °C. A striking behaviour of the oxidized ZrB 2 –SiC composites is also pointed out: underneath the Glass Layer, there is a porous Layer rich in electronic conductive ZrB 2 , without well-defined interface between them, i.e., exhibiting a composition gradient in oxygen. In other words, protonic half-fuel cells could be fabricated under such conditions, for future use in hydrogen or direct alcohol fuel cells.