Towards Sinter-free Printing of Photovoltaic Cell Interconnects

Lead Research Organisation: Glyndwr University
Department Name: Science Research Centre

Abstract

Metal thin films are used in a wide variety of technologies, such as solar cells and printed circuit boards for electronics. Inkjet printing has emerged as a practical and low-cost route for manufacturing electrical contacts in these applications. However existing manufacturing technologies use inks that often require a final heat treatment to consolidate or 'sinter' the film. If this last step can be eliminated, by depositing fully dense films, then the inkjet manufacturing process could be applied to temperature sensitive substrates like plastics or vulnerable semiconductor materials.

The purpose of this project is to develop 'sinter-free' inkjet manufacturing processes, by taking ink precursors developed for other thin film processes, and exploiting them to use the significant benefits of inkjet process technology e.g. the direct writing of interconnects or wires. If successful, the project will represent a step-change in the manufacturing methods for this type of film.

Publications

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Kartopu G (2016) Progression of metalorganic chemical vapour-deposited CdTe thin-film PV devices towards modules in Progress in Photovoltaics: Research and Applications

 
Description Metallic films/components can be printed at low cost using a solution based ink via the inkjet process. However, these precursor film often requires a post-deposition 'curing' treatment (by a laser or annealing in an oven) to become useful as a conductor. The 'sinter-free inkjet project' aimed at avoiding the post-treatment by developing new metalorganic inks which can be inkjet-printed at relatively modest temperatures, i.e. without causing damage to the printer, and displaying high conductivity without additional treatment. Printing conducting components in a single step and using low cost methods can be attractive for optoelectronic devices, such as solar cells/modules.
During the 30 months of this project, mainly metalorganic silver inks were formulated at Liverpool University and by the industrial partner SAFC Hitech. These were then tested across partner labs at Glyndwr University (GU) and The University of Nottingham. Work at GU assessed the inkjet printing behaviour of these inks on glass and cadmium telluride thin film solar cell surfaces. To obtain compact and sufficiently thick silver films with good conductivity, the substrates were heated to above 150 oC and several overlayers printed by multi-passing the injector. A further and surprising beneficial effect was that the new inks were 'non-selective' on the substrate type, as opposed to the conventional inks and could therefore be easily printed on all relevant solar device surfaces, providing a huge benefit for PV module design. In contrast, commercial (colloidal) inks required special substrate treatment (such as plasma cleaning, acid etch, etc.) for most substrate materials as well as sintering at 150 Celcius for at least one hour to attain good coverage and conductivity. Thus the new sinter-free process considerably reducing the thermal cycle and the processing time. Solar cell performances were found to be comparable when using the newly developed and conventional silver inks. As the final step towards sinter-free inkjet-printing of PV sub-cell interconnects (for PV modules), electrical resistance of sinter-free silver films deposited over 'P2 scribes' to assess the vital connection between the front and back contact of successive cells. This was assessed using novel test structures designed to mimic a portion of a thin film module. Good ohmic contacts, were obtained for all the test samples with the electrical resistance reducing proportionally with increasing film thickness, demonstrating excellent control of the electrical conductivity.
In the future, it would be desirable to work on the ink formulation, to further reduce the required substrate temperature (towards 100 Celcius and below) and increase the metal load/concentration, as well as fully demonstrate large-scale optoelectronic devices, e.g. solar modules, with high performance in order for the sinter-free inkjet process to become more attractive for industrial uptake.
Exploitation Route This project has opened up a new approach to printing conducting tracks onto electronic devices. The conformality of the coating over different surfaces could have benefit to a range of printed electronic devices from displays through to sensors. A further reduction in the printing temperature would expand this range of applications and, in the case of solar PV devices, enable novel device architectures for more integrated PV devices. More research will be required to fully unlock this enormous potential.
Sectors Aerospace, Defence and Marine,Chemicals,Digital/Communication/Information Technologies (including Software),Electronics,Energy,Environment,Manufacturing, including Industrial Biotechology

URL http://cser.org.uk/research/
 
Description During the course of this project, the main supplier of the organometallic inks, SAFC Hitech, withdrew from manufacturing at the Bromborough facility where a considerable amount of research had taken place over the years into organometallic precursors for compound semiconductor synthesis. Although this was disappointing the strength of the relationship with the Liverpool and Glyndwr partners has enabled key staff from SAFC Hitech to set-up a new organometallic supply business based in Saltney, Flintshire. This new business, Pegasus Chemicals, is now in a position to supply a range of organometallic precursors to both industry and academic researchers. This will also provide a route for commercialisation of the sinter-free precursors once they have become suitably developed.
First Year Of Impact 2015
Sector Chemicals,Electronics,Energy,Manufacturing, including Industrial Biotechology
Impact Types Economic

 
Title Ink Jet printer (PixPro LP50) 
Description The PixPro LP50 was commissioned and modified on the Sinter Free research project to produce highly conducting metal contacts for thin film photovoltaic (PV) cells. The information learnt on the research project is now being applied to innovative designs of monolithic modules and in particular thin film PV modules on ultra-thin glass (UTG). The use of UTG is extremely challenging when fabricating monolithic modules that could be the basis for practical ultra-light-weight PV arrays. The information gained on the Sinter-free inkjet project has enabled a new avenue of research to minimize strain that will be included in a new research proposal to EPSRC on fundamental aspects of strain in functional devices on UTG. 
Type Of Material Improvements to research infrastructure 
Year Produced 2017 
Provided To Others? Yes  
Impact Too early to assess impact but ongoing discussions with UTG glass manufacturers and research collaborators at University of Surrey on the strain proposal. 
URL http://www.cser.org.uk