Joris Libal - Academia.edu (original) (raw)
Papers by Joris Libal
Progress in Photovoltaics, Mar 11, 2020
One of the prerequisites for a reliable energy yield prediction of bifacial photovoltaic (PV) sys... more One of the prerequisites for a reliable energy yield prediction of bifacial photovoltaic (PV) systems is the capability of modeling the backside irradiance of those systems with high accuracy. Currently, the most important optical models used to quantify the reflected irradiance on the backside of a bifacial solar panel are view factor and ray tracing. The MoBiDiG simulation tool has been developed at ISC Konstanz uses the view factor (VF) concept to model the rear irradiance. In addition to the VF concept, ray tracing (RT) has been adopted to determine the backside irradiance of bifacial modules by using the open-source tool bifacial_radiance that has been developed by the National Renewable Energy Laboratory (NREL). A customized monocrystalline silicon solar panel has been built in order to evaluate the accuracy of the existing optical models by locally resolved rear irradiance measurement. The performance of rear irradiance has been investigated along the rows of the customized PV module during sunny and cloudy days with typical back irradiance values of ≈50 and ≈ 150 W∕m 2. The comparison of measured and modeled data has been carried out on hourly, daily, and monthly basis, and the results show lower deviations for solar cells located in the center of the PV module than on the edge. Moreover, the concept of decisive solar cells has been introduced and applied to both measured and modeled data, solar cells located in the center rows were found to act as the most decisive solar cells. Finally, considering the installation configuration studied here, ie, bifacial mounting with low clearance height (below 0.2 m), both hourly RT and VF approaches are able to model long-term cumulative irradiance received by decisive solar cells with a very high accuracy ranging from ±0.5% to ±2%.
The surface passivation of an industrial applicable (e.g. screen printing) 60 Omegasq/ p+ boron e... more The surface passivation of an industrial applicable (e.g. screen printing) 60 Omegasq/ p+ boron emitter on Cz n-type c-Si wafers by amorphous SiCx films is investigated. A partial optimization of the deposition conditions of the SiCx films was performed resulting in an improved passivation quality of the SiCx films which also serves as anti-reflection coating. Passivation quality is determined by
The shortage of the p-type silicon (Si) feedstock and the high minority carrier lifetimes in mult... more The shortage of the p-type silicon (Si) feedstock and the high minority carrier lifetimes in multicrystalline (mc) n-type Si reported by different authors ([1]-[3]) make n-type mc-Si solar cell fabrication more and more interesting. Given the high electronic quality of the material-that is confirmed in our studies again-the task remains to develop an adapted solar cell process. A key feature of the concept presented here is the BBr3diffused emitter on the front side and the surface passivation of this emitter. We show that BBr3 emitter-diffusion is possible without degradation of the high initial carrier lifetimes in the n-type mc-Si material-on contrary the diffusion even improves the average lifetime to a large extend. SiO2 provides an excellent surface passivation of the p +-Si surface. Application of PECVD SiNx resulted in a decrease of the (implied) Voc measured on lifetime teststructures as well as on solar cell level. As an alternative, a low temperature surface passivation process by PECVD SiCx is investigated. First trials resulted in a very promising value for the emitter saturation current Joe: 180 fA/cm 2 for a 90 Ω/sq emitter. N-type Si solar cells with SiO2-passivated BBr3-emitter were processed in laboratory scale (area of 4 cm 2) with an efficiency of 15.2% on mc and 16.4% on Cz-Si. With an industrial screen printing process 14.1% and 14.8% were obtained on an area of 12.5 x 12.5 cm 2 on n-type mc-Si and Cz-Si respectively.
PROCEEDINGS OF THE 10TH WORKSHOP ON METALLIZATION AND INTERCONNECTION FOR CRYSTALLINE SILICON SOLAR CELLS
In this work we metallize the busbars of n-type Zebra IBC cell with a low temperature curable cop... more In this work we metallize the busbars of n-type Zebra IBC cell with a low temperature curable copper based metallization paste. We show that the properties of Cu busbars such as line resistance are similar to comparable Ag based products, and that Zebra IBC cells with Cu busbars feature the same efficiencies as the Ag metallized reference cells. Furthermore, and most importantly, initial climate chamber tests indicate that the silicon bulk material is not contaminated by Cu diffusing from the cells surface even after TC600 and DH3000. However, the paste still shows insufficient adhesion on the substrate surface. This issue is visible in peel force tests and also in climate chamber tests, mainly temperature cycling tests.
2020 47th IEEE Photovoltaic Specialists Conference (PVSC)
This work presents measurements from 40 individual 125 mm x 125 mm crystalline silicon (c-Si) cel... more This work presents measurements from 40 individual 125 mm x 125 mm crystalline silicon (c-Si) cells placed on the backside of a horizontal single axis tracker (HSAT) located in Roskilde, Denmark (55.6°N, 12.1°E). The measurements are used to validate a general set of conclusions gathered from recent literature, to compare to simulated backside irradiance results from view factor and ray-trace based methods, and to estimate the electrical losses caused by nonuniform illumination at the module and array level. In this work, all simulations are performed using the open source tools bifacialvf, bifacial_radiance, and pvmismatch. The tracker studied is 45 m long with 60-cell bifacial photovoltaic (PV) modules mounted "two-in-portrait"-a configuration commonly implemented in utility scale PV parks. Our measurements corroborate the conclusions from several simulation-based studies made by other authors. The measurements and simulations indicate that the irradiationnonuniformity-induced electrical mismatch of the bifacial array is not higher than 0.25% when mounted above grass (albedo 0.22) on a clear sky day. But the array-level mismatch can go up to 3% when the PV park is uniformly covered by a white polymeric material (albedo 0.60). During a cloudy day, the mismatch of the bifacial system over grass is as high as 1%, but is lower than 0.25% around solar noon. Above the white ground cover on a cloudy day, the mismatch is around 1-2%, even at solar noon.
The purpose of this work is to demonstrate an alternative process to a standard Al-full side devi... more The purpose of this work is to demonstrate an alternative process to a standard Al-full side device which can be used for thin wafers. One way to do so is to process a passivated boron diffused rear side with an open rear contact. The cell processed in this way has a better passivation on the rear and in addition is bi-facial allowing light penetrating through the rear, increasing the power output. Most of the process steps have been optimized. With this process sequence we established a homogeneous set of bifacial solar cells on Cz-Si material with similar characteristics and efficiencies up to 17.3% under front side illumination. In order to assemble and build high power output modules it is important to have similar short circuit currents in each bifacial solar cell. From a fifty cells batch we obtained an average short circuit current of 35.4 [mA/cm], constructing with these a 4x4 cells mini module for testing. Test results show that the reflecting light can increase the power o...
In this study we quantify encapsulation losses for 6 inch IBC solar cells considering different c... more In this study we quantify encapsulation losses for 6 inch IBC solar cells considering different colored module backsheet foils and different cell contacting methods. The solar cells investigated were developed and fabricated at ISC Konstanz. They are bifacial n-type IBC solar cells, so called "Zebra" cells [1, 2, 3]. Cell processing relies on standard industrial processing equipment like tube furnace diffusion, laser ablation of a masking layer and screen printed metallization. The cells investigated are contacted either by conductive adhesive (CA) gluing of bare Cu ribbon or by soldering of structured stress relieve ribbon. Subsequently, mini-modules are assembled using a standard module stack employing white, black or transparent backsheet on the rear side. Our analysis shows that the main loss mechanism from cell to encapsulated cell is the optical loss. Comparing the short circuit current of bare cells with the one of encapsulated cells average relative losses between 4.8% (white backsheet) and 6.5% (black backsheet) are measured. Electrical losses depend on the contacting scheme and the amount of metal provided for current transport but also on the measurement method. A minimum fill factor loss of 1.1% relative is achieved for a CA connected IBC cell while attaching a voltage probe in about 1 mm distance to the cell during IV measurement.
We present here results on 60-cell modules assembled with 6 inch n-type interdigitated back conta... more We present here results on 60-cell modules assembled with 6 inch n-type interdigitated back contact (IBC) solar cells which were accomplished in close collaboration between ISC Konstanz, who developed an industrial process to fabricate IBC cells which we call Zebra cells [1] and Eurotron BV, who supplies equipment for back contact module assembly and processed Zebra cells into 60-cells modules. Interconnection is realized by electrically conductive adhesive (ECA) gluing of cells to a conductive copper backsheet. The module sandwich is laminated with commercially available encapsulation material in a standard laminator. We assembled three 60-cells modules so far reaching 294 W, 298 W and 303 W peak power at 1 sun respectively. The cell to module (CTM) power losses range between 1.1 to 1.8% relative which is an excellent result taking into account the absence of ISC gain due to finger reflection which eases the CTM loss for modules assembled with front contacted solar cells.
In this report, high-efficiency solar cell concepts for n-type multicrystalline silicon (mc-Si) a... more In this report, high-efficiency solar cell concepts for n-type multicrystalline silicon (mc-Si) are presented. Cells with a totally diffused and metallised Back Surface Field (BSF) reached an efficiency of 15.1% with an antireflection coating. For a further improvement of the cell efficiency the application of the PERC [1] cell design on n-type mc-Si is proposed. First PERC cells have been processed and an efficiency of 10.2% has been obtained without antireflection coating. Optimisation of this process and application of a Double Layer Antireflection Coating (DLARC) is expected to lead to efficiencies exceeding 16%.
Users may download and print one copy of any publication from the public portal for the purpose... more Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
In this paper we present relevant characteristics of our Zebra cells and one-cell modules. The Ze... more In this paper we present relevant characteristics of our Zebra cells and one-cell modules. The Zebra cell is an interdigitated back contact (IBC) solar cell produced at ISC Konstanz using only industrially proven technologies and standard industrial size 156x156mm2 n-type Cz wafers. Wafers of different suppliers have been evaluated. The best efficiencies achieved so far on a batch of 21 wafers are 21% average (champion cell: 21.3%). Reverse bias electroluminescence (ReBEL) measurements reveal the unique breakdown behavior of the Zebra cell. Due to its alternating pn regions on the rear, a homogenous breakdown along the parallel junctions is observed which does not lead to local shunting. The Zebra cell can be stringed using standard soldering or bonding with conductive adhesives. Cell to module losses are comparable to standard p-type Al BSF cells. Due to its open rear side structure, the Zebra cell is bifacial. When testing one-cell modules with transparent and black backsheet outd...
Energies, 2021
In this paper we summarize the status of bifacial photovoltaics (PV) and explain why the move to ... more In this paper we summarize the status of bifacial photovoltaics (PV) and explain why the move to bifaciality is unavoidable when it comes to e.g., lowest electricity generation costs or agricultural PV (AgriPV). Bifacial modules—those that are sensitive to light incident from both sides—are finally available at the same price per watt peak as their standard monofacial equivalents. The reason for this is that bifacial solar cells are the result of an evolution of crystalline Si PV cell technology and, at the same time, module producers are increasingly switching to double glass modules anyway due to the improved module lifetimes, which allows them to offer longer product warrantees. We describe the general properties of the state-of-the-art bifacial module, review the different bifacial solar cells and module technologies available on the market, and summarize their average costs. Adding complexity to a module comes with the increase of possible degradation mechanisms, requiring more...
2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC) (A Joint Conference of 45th IEEE PVSC, 28th PVSEC & 34th EU PVSEC), 2018
Soiling is currently one of the major issues for decreasing power production of PV plants in Chil... more Soiling is currently one of the major issues for decreasing power production of PV plants in Chile. In order to prevent significant power losses cleaning frequencies of more than one time per month are scheduled. In the present study, we investigate the impact of soiling on energy yield (YA) of bifacial solar modules with different material combinations. We fabricated half-size and full-size one cell modules employing different glass thicknesses, encapsulants, transparent backsheets and glass-glass module configurations. Based on indoor current-voltage (IV) measurements which were carried out on front and rear side of each bifacial module type at different irradiance levels, a mathematical YA model was applied. Front and rear measurements were taken separately and simultaneously assuming the same bifaciality factor at a given irradiation. YA estimations show that module groups with half-cell design achieve the highest YA values compared to full-cell design. In addition, vertically mounted bifacial modules (VMBM) produce higher YA than tilted mounted bifacial modules (TMBM) if the power loss rate due to soiling is higher than 0.10% per day. To validate the YA model, real outdoor measurements were carried out in the Atacama Desert in Chile for TMBM and for VMBM.
Applied Sciences, 2020
The size and number of utility-scale bifacial photovoltaic (PV) installations has proliferated in... more The size and number of utility-scale bifacial photovoltaic (PV) installations has proliferated in recent years but concerns over modeling accuracy remain. The aim of this work is to provide the PV community with a validation study of eight tools used to simulate bifacial PV performance. We simulate real 26 kilowatt-peak (kWp) bifacial arrays within a 420-kWp site located in northern Europe (55.6° N, 12.1° E). The substructures investigated include horizontal single-axis trackers (HSATs) and fixed tilt racks that have dimensions analogous to those found in utility-scale PV installations. Each bifacial system has a monofacial reference system with similar front side power. We use on-site solar radiation (global, diffuse, and beam) and albedo measurements from spectrally flat class A sensors as inputs to the simulation tools, and compare the modeled values to field measurements of string level power, rear and front plane of array irradiance, and module temperature. Our results show tha...
Progress in Photovoltaics: Research and Applications, 2020
One of the prerequisites for a reliable energy yield prediction of bifacial photovoltaic (PV) sys... more One of the prerequisites for a reliable energy yield prediction of bifacial photovoltaic (PV) systems is the capability of modeling the backside irradiance of those systems with high accuracy. Currently, the most important optical models used to quantify the reflected irradiance on the backside of a bifacial solar panel are view factor and ray tracing. The MoBiDiG simulation tool has been developed at ISC Konstanz uses the view factor (VF) concept to model the rear irradiance. In addition to the VF concept, ray tracing (RT) has been adopted to determine the backside irradiance of bifacial modules by using the open‐source tool bifacial_radiance that has been developed by the National Renewable Energy Laboratory (NREL). A customized monocrystalline silicon solar panel has been built in order to evaluate the accuracy of the existing optical models by locally resolved rear irradiance measurement. The performance of rear irradiance has been investigated along the rows of the customized P...
15th International Conference on Concentrator Photovoltaic Systems (CPV-15), 2019
In this work, a high voltage, both sides screen-printed n-type passivated emitter and rear totall... more In this work, a high voltage, both sides screen-printed n-type passivated emitter and rear totally diffused rear junction (n-PERT-RJ) solar cell concept with Al point contacts on its rear side is proposed. A dash pattern for the firing through Ag contact is applied instead of performing the selective doping on the front surface field (FSF), that effectively reduced the area-weighted metal-induced recombination current J0, Met (Ag). The impact of different n-bulk properties on the n-PERT-RJ solar cell performance is revealed by Quokka3 simulation. An innovative "point-line" concept is then introduced which is composed of dot-shaped laser contact opening (LCO) and Al metal grid, aims to possess bifacial gain in addition to the preservation of high VOC in order to further reduce the levelized cost of energy (LCOE). This industrial type n-PERT-RJ solar cell demonstrates the same performance level as the advanced passivated contact solar cell concepts, the cost of ownership (COO) is close to the p-PERC solar cells, last but not least, shows the performance stability after light and elevated temperature induced degradation (LeTID) tests.
IEEE Journal of Photovoltaics, 2019
Compared with standard monofacial photovoltaic (PV) systems, the simulation of the energy yield o... more Compared with standard monofacial photovoltaic (PV) systems, the simulation of the energy yield of bifacial PV systems is more challenging since the impact of factors such as the installation height, the ground albedo, shadowing of neighbored rows, the diffuse irradiance fraction, and the PV module design is more pronounced. Therefore, many academic institutions as well as companies are currently working on the development of suitable modeling tools that allow an accurate energy yield prediction of bifacial systems. In this article, we present the results of energy-yield simulations of bifacial PV systems with fixed tilt and horizontal single-axis tracking (HSAT) in comparison to their monofacial counterparts using a tool that has been developed at ISC Konstanz. In addition, the simulated data are compared with measured results. The energy yield of fixed tilt bifacial systems is simulated as a function of the number of rows and number of modules in a row as well as a function of the installation height. The simulated data have been compared with measured data obtained using a PV system with continuously changing tilt angels. The accuracy of the simulated data is shown to be from +/−0.1% to +/−4% depending on the tilt angle of the bifacial modules. In addition, the energy yield of bifacial HSAT PV systems have been simulated and compared with measured data for a bifacial HSAT system in Chile. In this comparison, the use of ray tracing instead of the view factor (VF) concept for modeling of the rear irradiance reduced the deviation between simulated and measured gain significantly. Therefore, the approach of using VF-based calculations for the front irradiance and ray tracing for the rear irradiance was then used to evaluate different bifacial system configurations in comparison to monofacial ones. Especially, the influence of a variation of the ground coverage ratio on the energy yield for various monofacial, bifacial fixed tilt, and HSAT systems was studied.
Energy Procedia, 2016
Bifacial solar cells enable the absorption of light also by the cell's rear side, hence increasin... more Bifacial solar cells enable the absorption of light also by the cell's rear side, hence increasing the energy yield of a bifacial module, , compared to the energy yield of a monofacial module installed under the same conditions, , by , the bifacial gain. This contribution presents a simulation model for the prediction of the of bifacial PV modules (standalone and integrated in a PV field). The model has been implemented as a software tool and the results obtained by applying the tool to various relevant system configurations (ground albedo, geographical locations, module height and tilt, diffuse irradiation fraction) are shown. These results allow to determine the optimum installation parameters for highest for a given installation site. Finally, the tool is validated by comparing the simulated results with the actual monitored on bifacial modules during several months on an outdoor testing site.
2006 IEEE 4th World Conference on Photovoltaic Energy Conference, 2006
In this paper we present n-type Si solar cells on large area mc-Si wafers with a boron diffused e... more In this paper we present n-type Si solar cells on large area mc-Si wafers with a boron diffused emitter at the front side. The focus of our studies is mainly related to the front surface of the solar cell. We have optimised BBr3-diffusion and in-situ oxidation with respect to the homogeneity of the sheet resistance and substrate degradation. After diffusion even a slight improvement of the minority charge carrier lifetime was measured, which can be related to Bgettering. The emitter is contacted by AgAl-paste and passivated by thermal SiO2. The development and optimisation of all processes led to solar cells with efficiencies of 14.7% on mc-Si and 17.1% on Cz-Si substrates. In addition to this we present an innovative interconnection of modules using our developed cell (patent pending). We show an alternate serial interconnection of p-and n-type solar cells resulting in easier module processing.
Progress in Photovoltaics, Mar 11, 2020
One of the prerequisites for a reliable energy yield prediction of bifacial photovoltaic (PV) sys... more One of the prerequisites for a reliable energy yield prediction of bifacial photovoltaic (PV) systems is the capability of modeling the backside irradiance of those systems with high accuracy. Currently, the most important optical models used to quantify the reflected irradiance on the backside of a bifacial solar panel are view factor and ray tracing. The MoBiDiG simulation tool has been developed at ISC Konstanz uses the view factor (VF) concept to model the rear irradiance. In addition to the VF concept, ray tracing (RT) has been adopted to determine the backside irradiance of bifacial modules by using the open-source tool bifacial_radiance that has been developed by the National Renewable Energy Laboratory (NREL). A customized monocrystalline silicon solar panel has been built in order to evaluate the accuracy of the existing optical models by locally resolved rear irradiance measurement. The performance of rear irradiance has been investigated along the rows of the customized PV module during sunny and cloudy days with typical back irradiance values of ≈50 and ≈ 150 W∕m 2. The comparison of measured and modeled data has been carried out on hourly, daily, and monthly basis, and the results show lower deviations for solar cells located in the center of the PV module than on the edge. Moreover, the concept of decisive solar cells has been introduced and applied to both measured and modeled data, solar cells located in the center rows were found to act as the most decisive solar cells. Finally, considering the installation configuration studied here, ie, bifacial mounting with low clearance height (below 0.2 m), both hourly RT and VF approaches are able to model long-term cumulative irradiance received by decisive solar cells with a very high accuracy ranging from ±0.5% to ±2%.
The surface passivation of an industrial applicable (e.g. screen printing) 60 Omegasq/ p+ boron e... more The surface passivation of an industrial applicable (e.g. screen printing) 60 Omegasq/ p+ boron emitter on Cz n-type c-Si wafers by amorphous SiCx films is investigated. A partial optimization of the deposition conditions of the SiCx films was performed resulting in an improved passivation quality of the SiCx films which also serves as anti-reflection coating. Passivation quality is determined by
The shortage of the p-type silicon (Si) feedstock and the high minority carrier lifetimes in mult... more The shortage of the p-type silicon (Si) feedstock and the high minority carrier lifetimes in multicrystalline (mc) n-type Si reported by different authors ([1]-[3]) make n-type mc-Si solar cell fabrication more and more interesting. Given the high electronic quality of the material-that is confirmed in our studies again-the task remains to develop an adapted solar cell process. A key feature of the concept presented here is the BBr3diffused emitter on the front side and the surface passivation of this emitter. We show that BBr3 emitter-diffusion is possible without degradation of the high initial carrier lifetimes in the n-type mc-Si material-on contrary the diffusion even improves the average lifetime to a large extend. SiO2 provides an excellent surface passivation of the p +-Si surface. Application of PECVD SiNx resulted in a decrease of the (implied) Voc measured on lifetime teststructures as well as on solar cell level. As an alternative, a low temperature surface passivation process by PECVD SiCx is investigated. First trials resulted in a very promising value for the emitter saturation current Joe: 180 fA/cm 2 for a 90 Ω/sq emitter. N-type Si solar cells with SiO2-passivated BBr3-emitter were processed in laboratory scale (area of 4 cm 2) with an efficiency of 15.2% on mc and 16.4% on Cz-Si. With an industrial screen printing process 14.1% and 14.8% were obtained on an area of 12.5 x 12.5 cm 2 on n-type mc-Si and Cz-Si respectively.
PROCEEDINGS OF THE 10TH WORKSHOP ON METALLIZATION AND INTERCONNECTION FOR CRYSTALLINE SILICON SOLAR CELLS
In this work we metallize the busbars of n-type Zebra IBC cell with a low temperature curable cop... more In this work we metallize the busbars of n-type Zebra IBC cell with a low temperature curable copper based metallization paste. We show that the properties of Cu busbars such as line resistance are similar to comparable Ag based products, and that Zebra IBC cells with Cu busbars feature the same efficiencies as the Ag metallized reference cells. Furthermore, and most importantly, initial climate chamber tests indicate that the silicon bulk material is not contaminated by Cu diffusing from the cells surface even after TC600 and DH3000. However, the paste still shows insufficient adhesion on the substrate surface. This issue is visible in peel force tests and also in climate chamber tests, mainly temperature cycling tests.
2020 47th IEEE Photovoltaic Specialists Conference (PVSC)
This work presents measurements from 40 individual 125 mm x 125 mm crystalline silicon (c-Si) cel... more This work presents measurements from 40 individual 125 mm x 125 mm crystalline silicon (c-Si) cells placed on the backside of a horizontal single axis tracker (HSAT) located in Roskilde, Denmark (55.6°N, 12.1°E). The measurements are used to validate a general set of conclusions gathered from recent literature, to compare to simulated backside irradiance results from view factor and ray-trace based methods, and to estimate the electrical losses caused by nonuniform illumination at the module and array level. In this work, all simulations are performed using the open source tools bifacialvf, bifacial_radiance, and pvmismatch. The tracker studied is 45 m long with 60-cell bifacial photovoltaic (PV) modules mounted "two-in-portrait"-a configuration commonly implemented in utility scale PV parks. Our measurements corroborate the conclusions from several simulation-based studies made by other authors. The measurements and simulations indicate that the irradiationnonuniformity-induced electrical mismatch of the bifacial array is not higher than 0.25% when mounted above grass (albedo 0.22) on a clear sky day. But the array-level mismatch can go up to 3% when the PV park is uniformly covered by a white polymeric material (albedo 0.60). During a cloudy day, the mismatch of the bifacial system over grass is as high as 1%, but is lower than 0.25% around solar noon. Above the white ground cover on a cloudy day, the mismatch is around 1-2%, even at solar noon.
The purpose of this work is to demonstrate an alternative process to a standard Al-full side devi... more The purpose of this work is to demonstrate an alternative process to a standard Al-full side device which can be used for thin wafers. One way to do so is to process a passivated boron diffused rear side with an open rear contact. The cell processed in this way has a better passivation on the rear and in addition is bi-facial allowing light penetrating through the rear, increasing the power output. Most of the process steps have been optimized. With this process sequence we established a homogeneous set of bifacial solar cells on Cz-Si material with similar characteristics and efficiencies up to 17.3% under front side illumination. In order to assemble and build high power output modules it is important to have similar short circuit currents in each bifacial solar cell. From a fifty cells batch we obtained an average short circuit current of 35.4 [mA/cm], constructing with these a 4x4 cells mini module for testing. Test results show that the reflecting light can increase the power o...
In this study we quantify encapsulation losses for 6 inch IBC solar cells considering different c... more In this study we quantify encapsulation losses for 6 inch IBC solar cells considering different colored module backsheet foils and different cell contacting methods. The solar cells investigated were developed and fabricated at ISC Konstanz. They are bifacial n-type IBC solar cells, so called "Zebra" cells [1, 2, 3]. Cell processing relies on standard industrial processing equipment like tube furnace diffusion, laser ablation of a masking layer and screen printed metallization. The cells investigated are contacted either by conductive adhesive (CA) gluing of bare Cu ribbon or by soldering of structured stress relieve ribbon. Subsequently, mini-modules are assembled using a standard module stack employing white, black or transparent backsheet on the rear side. Our analysis shows that the main loss mechanism from cell to encapsulated cell is the optical loss. Comparing the short circuit current of bare cells with the one of encapsulated cells average relative losses between 4.8% (white backsheet) and 6.5% (black backsheet) are measured. Electrical losses depend on the contacting scheme and the amount of metal provided for current transport but also on the measurement method. A minimum fill factor loss of 1.1% relative is achieved for a CA connected IBC cell while attaching a voltage probe in about 1 mm distance to the cell during IV measurement.
We present here results on 60-cell modules assembled with 6 inch n-type interdigitated back conta... more We present here results on 60-cell modules assembled with 6 inch n-type interdigitated back contact (IBC) solar cells which were accomplished in close collaboration between ISC Konstanz, who developed an industrial process to fabricate IBC cells which we call Zebra cells [1] and Eurotron BV, who supplies equipment for back contact module assembly and processed Zebra cells into 60-cells modules. Interconnection is realized by electrically conductive adhesive (ECA) gluing of cells to a conductive copper backsheet. The module sandwich is laminated with commercially available encapsulation material in a standard laminator. We assembled three 60-cells modules so far reaching 294 W, 298 W and 303 W peak power at 1 sun respectively. The cell to module (CTM) power losses range between 1.1 to 1.8% relative which is an excellent result taking into account the absence of ISC gain due to finger reflection which eases the CTM loss for modules assembled with front contacted solar cells.
In this report, high-efficiency solar cell concepts for n-type multicrystalline silicon (mc-Si) a... more In this report, high-efficiency solar cell concepts for n-type multicrystalline silicon (mc-Si) are presented. Cells with a totally diffused and metallised Back Surface Field (BSF) reached an efficiency of 15.1% with an antireflection coating. For a further improvement of the cell efficiency the application of the PERC [1] cell design on n-type mc-Si is proposed. First PERC cells have been processed and an efficiency of 10.2% has been obtained without antireflection coating. Optimisation of this process and application of a Double Layer Antireflection Coating (DLARC) is expected to lead to efficiencies exceeding 16%.
Users may download and print one copy of any publication from the public portal for the purpose... more Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
In this paper we present relevant characteristics of our Zebra cells and one-cell modules. The Ze... more In this paper we present relevant characteristics of our Zebra cells and one-cell modules. The Zebra cell is an interdigitated back contact (IBC) solar cell produced at ISC Konstanz using only industrially proven technologies and standard industrial size 156x156mm2 n-type Cz wafers. Wafers of different suppliers have been evaluated. The best efficiencies achieved so far on a batch of 21 wafers are 21% average (champion cell: 21.3%). Reverse bias electroluminescence (ReBEL) measurements reveal the unique breakdown behavior of the Zebra cell. Due to its alternating pn regions on the rear, a homogenous breakdown along the parallel junctions is observed which does not lead to local shunting. The Zebra cell can be stringed using standard soldering or bonding with conductive adhesives. Cell to module losses are comparable to standard p-type Al BSF cells. Due to its open rear side structure, the Zebra cell is bifacial. When testing one-cell modules with transparent and black backsheet outd...
Energies, 2021
In this paper we summarize the status of bifacial photovoltaics (PV) and explain why the move to ... more In this paper we summarize the status of bifacial photovoltaics (PV) and explain why the move to bifaciality is unavoidable when it comes to e.g., lowest electricity generation costs or agricultural PV (AgriPV). Bifacial modules—those that are sensitive to light incident from both sides—are finally available at the same price per watt peak as their standard monofacial equivalents. The reason for this is that bifacial solar cells are the result of an evolution of crystalline Si PV cell technology and, at the same time, module producers are increasingly switching to double glass modules anyway due to the improved module lifetimes, which allows them to offer longer product warrantees. We describe the general properties of the state-of-the-art bifacial module, review the different bifacial solar cells and module technologies available on the market, and summarize their average costs. Adding complexity to a module comes with the increase of possible degradation mechanisms, requiring more...
2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC) (A Joint Conference of 45th IEEE PVSC, 28th PVSEC & 34th EU PVSEC), 2018
Soiling is currently one of the major issues for decreasing power production of PV plants in Chil... more Soiling is currently one of the major issues for decreasing power production of PV plants in Chile. In order to prevent significant power losses cleaning frequencies of more than one time per month are scheduled. In the present study, we investigate the impact of soiling on energy yield (YA) of bifacial solar modules with different material combinations. We fabricated half-size and full-size one cell modules employing different glass thicknesses, encapsulants, transparent backsheets and glass-glass module configurations. Based on indoor current-voltage (IV) measurements which were carried out on front and rear side of each bifacial module type at different irradiance levels, a mathematical YA model was applied. Front and rear measurements were taken separately and simultaneously assuming the same bifaciality factor at a given irradiation. YA estimations show that module groups with half-cell design achieve the highest YA values compared to full-cell design. In addition, vertically mounted bifacial modules (VMBM) produce higher YA than tilted mounted bifacial modules (TMBM) if the power loss rate due to soiling is higher than 0.10% per day. To validate the YA model, real outdoor measurements were carried out in the Atacama Desert in Chile for TMBM and for VMBM.
Applied Sciences, 2020
The size and number of utility-scale bifacial photovoltaic (PV) installations has proliferated in... more The size and number of utility-scale bifacial photovoltaic (PV) installations has proliferated in recent years but concerns over modeling accuracy remain. The aim of this work is to provide the PV community with a validation study of eight tools used to simulate bifacial PV performance. We simulate real 26 kilowatt-peak (kWp) bifacial arrays within a 420-kWp site located in northern Europe (55.6° N, 12.1° E). The substructures investigated include horizontal single-axis trackers (HSATs) and fixed tilt racks that have dimensions analogous to those found in utility-scale PV installations. Each bifacial system has a monofacial reference system with similar front side power. We use on-site solar radiation (global, diffuse, and beam) and albedo measurements from spectrally flat class A sensors as inputs to the simulation tools, and compare the modeled values to field measurements of string level power, rear and front plane of array irradiance, and module temperature. Our results show tha...
Progress in Photovoltaics: Research and Applications, 2020
One of the prerequisites for a reliable energy yield prediction of bifacial photovoltaic (PV) sys... more One of the prerequisites for a reliable energy yield prediction of bifacial photovoltaic (PV) systems is the capability of modeling the backside irradiance of those systems with high accuracy. Currently, the most important optical models used to quantify the reflected irradiance on the backside of a bifacial solar panel are view factor and ray tracing. The MoBiDiG simulation tool has been developed at ISC Konstanz uses the view factor (VF) concept to model the rear irradiance. In addition to the VF concept, ray tracing (RT) has been adopted to determine the backside irradiance of bifacial modules by using the open‐source tool bifacial_radiance that has been developed by the National Renewable Energy Laboratory (NREL). A customized monocrystalline silicon solar panel has been built in order to evaluate the accuracy of the existing optical models by locally resolved rear irradiance measurement. The performance of rear irradiance has been investigated along the rows of the customized P...
15th International Conference on Concentrator Photovoltaic Systems (CPV-15), 2019
In this work, a high voltage, both sides screen-printed n-type passivated emitter and rear totall... more In this work, a high voltage, both sides screen-printed n-type passivated emitter and rear totally diffused rear junction (n-PERT-RJ) solar cell concept with Al point contacts on its rear side is proposed. A dash pattern for the firing through Ag contact is applied instead of performing the selective doping on the front surface field (FSF), that effectively reduced the area-weighted metal-induced recombination current J0, Met (Ag). The impact of different n-bulk properties on the n-PERT-RJ solar cell performance is revealed by Quokka3 simulation. An innovative "point-line" concept is then introduced which is composed of dot-shaped laser contact opening (LCO) and Al metal grid, aims to possess bifacial gain in addition to the preservation of high VOC in order to further reduce the levelized cost of energy (LCOE). This industrial type n-PERT-RJ solar cell demonstrates the same performance level as the advanced passivated contact solar cell concepts, the cost of ownership (COO) is close to the p-PERC solar cells, last but not least, shows the performance stability after light and elevated temperature induced degradation (LeTID) tests.
IEEE Journal of Photovoltaics, 2019
Compared with standard monofacial photovoltaic (PV) systems, the simulation of the energy yield o... more Compared with standard monofacial photovoltaic (PV) systems, the simulation of the energy yield of bifacial PV systems is more challenging since the impact of factors such as the installation height, the ground albedo, shadowing of neighbored rows, the diffuse irradiance fraction, and the PV module design is more pronounced. Therefore, many academic institutions as well as companies are currently working on the development of suitable modeling tools that allow an accurate energy yield prediction of bifacial systems. In this article, we present the results of energy-yield simulations of bifacial PV systems with fixed tilt and horizontal single-axis tracking (HSAT) in comparison to their monofacial counterparts using a tool that has been developed at ISC Konstanz. In addition, the simulated data are compared with measured results. The energy yield of fixed tilt bifacial systems is simulated as a function of the number of rows and number of modules in a row as well as a function of the installation height. The simulated data have been compared with measured data obtained using a PV system with continuously changing tilt angels. The accuracy of the simulated data is shown to be from +/−0.1% to +/−4% depending on the tilt angle of the bifacial modules. In addition, the energy yield of bifacial HSAT PV systems have been simulated and compared with measured data for a bifacial HSAT system in Chile. In this comparison, the use of ray tracing instead of the view factor (VF) concept for modeling of the rear irradiance reduced the deviation between simulated and measured gain significantly. Therefore, the approach of using VF-based calculations for the front irradiance and ray tracing for the rear irradiance was then used to evaluate different bifacial system configurations in comparison to monofacial ones. Especially, the influence of a variation of the ground coverage ratio on the energy yield for various monofacial, bifacial fixed tilt, and HSAT systems was studied.
Energy Procedia, 2016
Bifacial solar cells enable the absorption of light also by the cell's rear side, hence increasin... more Bifacial solar cells enable the absorption of light also by the cell's rear side, hence increasing the energy yield of a bifacial module, , compared to the energy yield of a monofacial module installed under the same conditions, , by , the bifacial gain. This contribution presents a simulation model for the prediction of the of bifacial PV modules (standalone and integrated in a PV field). The model has been implemented as a software tool and the results obtained by applying the tool to various relevant system configurations (ground albedo, geographical locations, module height and tilt, diffuse irradiation fraction) are shown. These results allow to determine the optimum installation parameters for highest for a given installation site. Finally, the tool is validated by comparing the simulated results with the actual monitored on bifacial modules during several months on an outdoor testing site.
2006 IEEE 4th World Conference on Photovoltaic Energy Conference, 2006
In this paper we present n-type Si solar cells on large area mc-Si wafers with a boron diffused e... more In this paper we present n-type Si solar cells on large area mc-Si wafers with a boron diffused emitter at the front side. The focus of our studies is mainly related to the front surface of the solar cell. We have optimised BBr3-diffusion and in-situ oxidation with respect to the homogeneity of the sheet resistance and substrate degradation. After diffusion even a slight improvement of the minority charge carrier lifetime was measured, which can be related to Bgettering. The emitter is contacted by AgAl-paste and passivated by thermal SiO2. The development and optimisation of all processes led to solar cells with efficiencies of 14.7% on mc-Si and 17.1% on Cz-Si substrates. In addition to this we present an innovative interconnection of modules using our developed cell (patent pending). We show an alternate serial interconnection of p-and n-type solar cells resulting in easier module processing.