Periodic Report Summary 1 - CU-PV (Cradle to cradle sustainable pv modules)
Project Context and Objectives:
Even though solar power is pollution-free during use, production of solar (PV) modules consumes energy and natural resources and hence, has environmental impact. Recycling is so far hardly considered during module design and production, and therefore cumbersome and inefficient. The fast growth of the PV-industry entails similarly fast growth in resource consumption with growing production capacity: currently modest amounts of use can become very high. Hence, Cu-PV aims to maximise resource productivity and reduce pollution in PV production, through minimising use of critical resources like energy (by reducing silicon consumption and improving the conversion efficiency), silver and lead, while simultaneously maximising recycling possibilities, by introducing design for recycling in this sector, and collaborating over the value chain for improvements in recycling.
Current PV module manufacturing puts limitations on reduction of costs and environmental footprint: screen-printed Ag-based metallisation prohibits progress towards phasing out lead and silver consumption, and reducing silicon and energy consumption, while module materials and assembly technology prohibit efficient recycling of modules. Cu-PV will develop new metallisation methods based on, in particular, physical vapour deposition of non-Ag layers in WP1 or inkjetting of Ag seed layers in WP2, which are plated afterwards with Ni and Cu in WP3, which results in at least 99% reduction of Ag consumption compared to current standard solar cells, enables replacement of silver, and as non-contact (i.e. low impact) metallisation methods enable the use of thin wafers.
The solar cell process before metallisation needs to be adjusted (WP1) to be compatible with this metallisation: back contact solar cell design will allow thin wafers and high efficiencies, resulting in 50% Si and about 30% PV system energy consumption reduction. The module assembly and interconnection of cells will need to be optimised for the new cell metallisation. Back-contact interconnection will allow completely abandoning the use of lead (WP4), and will be designed, developed, and tested, for recycling (WP5). WP5 will in particular develop and demonstrate alternatives for the current practice of destruction of PV modules at end of life.
Project Results:
In WP1 reference cell processes have been established for n-type silicon metal wrap-through (MWT) cells and interdigitated back contact (IBC) cells. The best n-MWT cell efficiency has been 20.5% on full size wafers, still based on traditional paste metallisation (large Ag consumption). The smallest cell thickness was 120 m, and the efficiency loss associated with thickness reduction was characterised to be about 0.2%. The best IBC cell efficiency on a size of 2x2 cm2 has been 23.3% with optical lithography, and 22.7% with laser-based lithography. WP1 provided half-fabricates of n-MWT cells as well as industry standard p- and n-type cells for inkjet and thin screen print metallisation followed by Ni/Cu plating. WP1 has also investigated in detail the possible seed layers for IBC cells for Ni/Cu plating. In the latter case the focus has been on Al/barrier/seed stacks deposited by physical vapour deposition.
In WP2 inkjet seed metallisation of n-type cells was established with a consumption of approximately 10 mg of Ag for the front of a cell and 2020 mg2020mg of Ag for the rear of a cell. This results in about 4.5 mg of Ag consumption per watt-peak (Wp), a factor 10 reduction from the present typical 50 mg/Wp. After Ni/Cu plating the efficiency results of the seeded cells were at the same level as the reference cells with conventional metallisation. Finger width on the front of the cell, after plating, was slightly below 55 m.
In WP3 Ni/Cu plating of p- and n-type regular and n-type back contact cells down to 150 m thickness was established. The plating tool was optimised in particular for use on bifacial cells. By choice, front, rear, or both sides can be plated. As described under WP2, after plating the efficiency results of the seeded cells were typically at the same level as the reference cells with conventional metallisation. After some optimisation in the future, efficiency gain is expected. Plated back contact cells were incorporated in modules and tested in accelerated degradation tests in WP4, with thus far good results.
In WP4 module materials and fabrication methods and tools have been investigated, designed and built for the combined objectives of 1) reliable, low-cost, and high performance back contact modules with low or zero consumption of Ag and zero consumption of Pb, and 2) improved possibilities for recycling (improved recovery of valuable or environmentally costly components, reduced cost of recycling). Suitable candidates for these combined objectives were identified. Cell-module interconnection methods were investigated and compared, and interconnection by conductive adhesive was identified as most promising, with interconnection with low-temperature solder as an alternative with some technical issues with respect to reliability. For both techniques accelerated degradation tests and investigations of new materials and technologies, for better performance and environmental footprint, are continuously going on.
In WP5 surveys and life cycle analyses (LCA) were done to analyse the bottlenecks in present module recycling, and the possibilities and needs for improvements in recycling, and thus improvement of environmental impact of modules. First results were published at the EUPVSEC 2013. The LCA shows that major environmental impact is from the Si wafer, Ag, Cu used for module interconnect, aluminium frame, and glass. For back contact modules, which may use more Cu than conventional modules, this may imply that recycling of Cu will become more important than for present modules. From environmental perspective it is valuable to start to recover (additionally to present recycling) in particular the silicon, and additionally to start to recover and recycle in purer format other module materials (in particular fluorinated backsheet, silver, glass, and for as far as not yet done, copper). The surveys to module manufacturers and recycling companies indicated the bottlenecks with respect to recycling. Results are analysed in an evaluation report with recommendations. Cost considerations dominate the feedback, but in general there is interest and willingness to investigate new routes for easier or increased material recovery. Frameless modules would be, for the present state-of-the-art recycling, very unfavourable because the aluminium recovered from the module frames presently provides a large part of the economic business case for recycling.
Also in WP5, development of recycling technology was done. Equipment was set up for recycling of current modules and tests were performed for semi-automated frame removal. Laminate recycling methods based on pyrolysis or chemical dissolution of encapsulant were investigated, as well as further novel methods. In addition, first recycling tests of modules with modified materials and construction aimed at better recycling possibilities were done and showed that glass sheets, cells, and backsheet can be recovered in a simple process with low environmental impact. Further work on the most promising routes is going on.
Potential Impact:
In the final half of the project, in WP1 a choice will be made for the type of cell (n-type MWT or IBC) and seeding technology to be used for the further development of cell processes and recyclable module technology in the project. The target is to obtain a cell technology for cells of <120 m thickness, using 2 mg or less of Ag per Wp power, and conversion efficiency of 22% or more. The cells will be processed between WP1, 2 and 3, and will be used for the module technology developments in WP4 and 5.
In WP2 the target for the seeding technology is to demonstrate potential for 1.25 mg or less of Ag consumption per Wp power, and demonstrate seeding for n-type MWT as well as IBC cells. The options and progress in seed technology will be continuously assessed.
In WP3 the thickness of the plated cells will be further reduced, to 120 m, with focus on industrially acceptable uniformity and yield, and in the second half of the project IBC cells will start to play a more important role next to n-type MWT cells.
In WP1-3 therefore cell technology will be demonstrated with potentially large impact on the use of silver and silicon in the PV industry. Reduction of the use of silver by 90-100% is targeted, and silicon consumption will be much reduced thanks to both increased efficiency and much smaller cell thickness. The technologies will be aimed at industrial take-up, and demonstration with tools representative for mass production. Our expectation is that at least several of the demonstrated technologies and concepts will be implemented in pilot production within 3-5 years.
In WP4 the focus will continue to be on developing and demonstrating module technology suitable for copper plated back contact cells and recycling. Continuous assessment of materials by accelerated degradation tests as well as recycling tests (in WP5) will take place. At the end of the project full-size modules will be made to demonstrate the integral Cu-PV technologies, and a targeted efficiency of over 19%. Again the technologies will be aimed at industrial take-up, and demonstration with tools representative for mass production. The impact of this work should be demonstration that back-contact technology, which is prominent in the PV roadmaps for the near future, can also result in progress in environmental profile of PV technology.
In WP5 the focus will continue to be on 3 topics: developing recycling technology for current PV modules; developing improved recycling methods for back-contact modules based on the technologies developed in WP4; and assessments of design rules, designs of automated tools, and life cycle analyses.
List of Websites:
http://www.sustainablepv.eu/cu-pv
Even though solar power is pollution-free during use, production of solar (PV) modules consumes energy and natural resources and hence, has environmental impact. Recycling is so far hardly considered during module design and production, and therefore cumbersome and inefficient. The fast growth of the PV-industry entails similarly fast growth in resource consumption with growing production capacity: currently modest amounts of use can become very high. Hence, Cu-PV aims to maximise resource productivity and reduce pollution in PV production, through minimising use of critical resources like energy (by reducing silicon consumption and improving the conversion efficiency), silver and lead, while simultaneously maximising recycling possibilities, by introducing design for recycling in this sector, and collaborating over the value chain for improvements in recycling.
Current PV module manufacturing puts limitations on reduction of costs and environmental footprint: screen-printed Ag-based metallisation prohibits progress towards phasing out lead and silver consumption, and reducing silicon and energy consumption, while module materials and assembly technology prohibit efficient recycling of modules. Cu-PV will develop new metallisation methods based on, in particular, physical vapour deposition of non-Ag layers in WP1 or inkjetting of Ag seed layers in WP2, which are plated afterwards with Ni and Cu in WP3, which results in at least 99% reduction of Ag consumption compared to current standard solar cells, enables replacement of silver, and as non-contact (i.e. low impact) metallisation methods enable the use of thin wafers.
The solar cell process before metallisation needs to be adjusted (WP1) to be compatible with this metallisation: back contact solar cell design will allow thin wafers and high efficiencies, resulting in 50% Si and about 30% PV system energy consumption reduction. The module assembly and interconnection of cells will need to be optimised for the new cell metallisation. Back-contact interconnection will allow completely abandoning the use of lead (WP4), and will be designed, developed, and tested, for recycling (WP5). WP5 will in particular develop and demonstrate alternatives for the current practice of destruction of PV modules at end of life.
Project Results:
In WP1 reference cell processes have been established for n-type silicon metal wrap-through (MWT) cells and interdigitated back contact (IBC) cells. The best n-MWT cell efficiency has been 20.5% on full size wafers, still based on traditional paste metallisation (large Ag consumption). The smallest cell thickness was 120 m, and the efficiency loss associated with thickness reduction was characterised to be about 0.2%. The best IBC cell efficiency on a size of 2x2 cm2 has been 23.3% with optical lithography, and 22.7% with laser-based lithography. WP1 provided half-fabricates of n-MWT cells as well as industry standard p- and n-type cells for inkjet and thin screen print metallisation followed by Ni/Cu plating. WP1 has also investigated in detail the possible seed layers for IBC cells for Ni/Cu plating. In the latter case the focus has been on Al/barrier/seed stacks deposited by physical vapour deposition.
In WP2 inkjet seed metallisation of n-type cells was established with a consumption of approximately 10 mg of Ag for the front of a cell and 2020 mg2020mg of Ag for the rear of a cell. This results in about 4.5 mg of Ag consumption per watt-peak (Wp), a factor 10 reduction from the present typical 50 mg/Wp. After Ni/Cu plating the efficiency results of the seeded cells were at the same level as the reference cells with conventional metallisation. Finger width on the front of the cell, after plating, was slightly below 55 m.
In WP3 Ni/Cu plating of p- and n-type regular and n-type back contact cells down to 150 m thickness was established. The plating tool was optimised in particular for use on bifacial cells. By choice, front, rear, or both sides can be plated. As described under WP2, after plating the efficiency results of the seeded cells were typically at the same level as the reference cells with conventional metallisation. After some optimisation in the future, efficiency gain is expected. Plated back contact cells were incorporated in modules and tested in accelerated degradation tests in WP4, with thus far good results.
In WP4 module materials and fabrication methods and tools have been investigated, designed and built for the combined objectives of 1) reliable, low-cost, and high performance back contact modules with low or zero consumption of Ag and zero consumption of Pb, and 2) improved possibilities for recycling (improved recovery of valuable or environmentally costly components, reduced cost of recycling). Suitable candidates for these combined objectives were identified. Cell-module interconnection methods were investigated and compared, and interconnection by conductive adhesive was identified as most promising, with interconnection with low-temperature solder as an alternative with some technical issues with respect to reliability. For both techniques accelerated degradation tests and investigations of new materials and technologies, for better performance and environmental footprint, are continuously going on.
In WP5 surveys and life cycle analyses (LCA) were done to analyse the bottlenecks in present module recycling, and the possibilities and needs for improvements in recycling, and thus improvement of environmental impact of modules. First results were published at the EUPVSEC 2013. The LCA shows that major environmental impact is from the Si wafer, Ag, Cu used for module interconnect, aluminium frame, and glass. For back contact modules, which may use more Cu than conventional modules, this may imply that recycling of Cu will become more important than for present modules. From environmental perspective it is valuable to start to recover (additionally to present recycling) in particular the silicon, and additionally to start to recover and recycle in purer format other module materials (in particular fluorinated backsheet, silver, glass, and for as far as not yet done, copper). The surveys to module manufacturers and recycling companies indicated the bottlenecks with respect to recycling. Results are analysed in an evaluation report with recommendations. Cost considerations dominate the feedback, but in general there is interest and willingness to investigate new routes for easier or increased material recovery. Frameless modules would be, for the present state-of-the-art recycling, very unfavourable because the aluminium recovered from the module frames presently provides a large part of the economic business case for recycling.
Also in WP5, development of recycling technology was done. Equipment was set up for recycling of current modules and tests were performed for semi-automated frame removal. Laminate recycling methods based on pyrolysis or chemical dissolution of encapsulant were investigated, as well as further novel methods. In addition, first recycling tests of modules with modified materials and construction aimed at better recycling possibilities were done and showed that glass sheets, cells, and backsheet can be recovered in a simple process with low environmental impact. Further work on the most promising routes is going on.
Potential Impact:
In the final half of the project, in WP1 a choice will be made for the type of cell (n-type MWT or IBC) and seeding technology to be used for the further development of cell processes and recyclable module technology in the project. The target is to obtain a cell technology for cells of <120 m thickness, using 2 mg or less of Ag per Wp power, and conversion efficiency of 22% or more. The cells will be processed between WP1, 2 and 3, and will be used for the module technology developments in WP4 and 5.
In WP2 the target for the seeding technology is to demonstrate potential for 1.25 mg or less of Ag consumption per Wp power, and demonstrate seeding for n-type MWT as well as IBC cells. The options and progress in seed technology will be continuously assessed.
In WP3 the thickness of the plated cells will be further reduced, to 120 m, with focus on industrially acceptable uniformity and yield, and in the second half of the project IBC cells will start to play a more important role next to n-type MWT cells.
In WP1-3 therefore cell technology will be demonstrated with potentially large impact on the use of silver and silicon in the PV industry. Reduction of the use of silver by 90-100% is targeted, and silicon consumption will be much reduced thanks to both increased efficiency and much smaller cell thickness. The technologies will be aimed at industrial take-up, and demonstration with tools representative for mass production. Our expectation is that at least several of the demonstrated technologies and concepts will be implemented in pilot production within 3-5 years.
In WP4 the focus will continue to be on developing and demonstrating module technology suitable for copper plated back contact cells and recycling. Continuous assessment of materials by accelerated degradation tests as well as recycling tests (in WP5) will take place. At the end of the project full-size modules will be made to demonstrate the integral Cu-PV technologies, and a targeted efficiency of over 19%. Again the technologies will be aimed at industrial take-up, and demonstration with tools representative for mass production. The impact of this work should be demonstration that back-contact technology, which is prominent in the PV roadmaps for the near future, can also result in progress in environmental profile of PV technology.
In WP5 the focus will continue to be on 3 topics: developing recycling technology for current PV modules; developing improved recycling methods for back-contact modules based on the technologies developed in WP4; and assessments of design rules, designs of automated tools, and life cycle analyses.
List of Websites:
http://www.sustainablepv.eu/cu-pv