HI-PROSPECTS - HIgh resolution PRinting Of Solar Photovoltaic EleCTrode Structures

The research to be carried out by Swansea University is to understand the issues and develop solutions related to up
scaling the near transparent copper grid to larger sizes. There is a significant challenge in this in that increases in size
brings both scientific and engineering challenges. When scaling, issues such as substrate form (which can be ignored at
small scale) become important, substrate stressing due to differential temperature profiles can lead to catastrophic failure
and thus optimized curing / sintering at small scale will have to be refined as the substrate size increases. In addition the
tolerance to defects and variations in deposit due to materials or processing conditions (some of which cannot be
controlled) may be insignificant at small scale but become significant as the substrate size increases. As well as process
and material considerations, there are design consideration. As substrate size increases then there may be a need to alter
the nature of the deposited grid such that higher conductor density is required towards the centre of a cell / module as a
charge generated at the centre of the cell will have a extended resistive path length to reach an electrode. The optimization
of this patterning and its impact is to be investigated in the work. Understanding these scale effects such that their impact
can be mitigated is critical in developing an engineering solution for large area processing of electrodes. The research
activity will address the optimization of the line structure and geometry for large areas through modelling and in parallel
perform install the system for experimental development of the ESJET over large areas. SPECIFIC will also provide lifetime
testing of the PV cells / modules manufactured in order to establish whether the copper grid has any detrimental / beneficial
effect on PV performance.
In order to establish optimal grid patterning simulation software (such as PSPICE) will be used to model the geometries
and film thicknesses, their effect on the sheet resistance and its subsequent effect on performance of PV cells created
using the fine copper patterning. This will take material data from the lab trials and estimate the geometric design and
process windows.
The clean room facilities at SPECIFIC provide an ideal test environment where glass substrate > 1m2 are routinely printed
using conventional printing techniques. The second main research activity is design and install a larger scale ESJET
system on the glass processing line at SPECIFIC and demonstrate that the copper can be deposited to the substrate over
a large area and that this substrate can be used to create a large area PV cell. This will establish the design of the
installation based on operational / material tolerances, physical layout requirements and operational processing issues. To
compliment the deposition requirements of the ESJET copper, the sintering process which will need to be designed and
implemented. This provides additional challenges in terms of maintaining consistent energy distributions (thermal and
photonic) over a large area. Such a development is non trivial having to take account many complex interacting parameters
such as photonic absorbance, differential thermal expansions, thermal properties and real world intensity variations.
The performance and lifetime of the PV cells will be carried out in suite of PV characterization and lifetime testing facilities
at SPECIFIC. The standards used for these tests (illumination, RH and temperature) will be determined from the relevant
standards and in house best practice with perovskite and OPV cells. Control samples using conventional TCOs, Ag grid &
TCOs combinations will also be used to identify only those degradation routes which can be attributed to the presence of
the copper grid.

The research carried out by SPECIFIC as part of the project will have a direct short term impact on the partners in the
project.
1. PVI will have a demonstration of their unique ESJET technology at large scale. This will provide them with demonstrators
and proof that their deposition technology is scalable opening new markets and product areas for ESJET.
2. NSG will derive important information on the interaction between grid geometries, material properties and resultant sheet
conductivities. Identifying the means by which surface conductivity can be fine tuned the additional of additional liquid
coatings is known to be an important area of research for NSG.
3. Intrinsiq will derive benefit from the testing of their materials in full functioning third generation PV devices.
4. All supply chain partners will posses demonstrator samples of their technology being used to create novel technology for
decarbonisation of energy supply.
5. Intrinsiq will derive benefit from the use of alternative sintering technologies on their materials. Take up of copper ink
materials is handicapped by the necessity of a specialised sintering technology and its associated capital cost. As part of
the study other photonic sintering (NIR) will also be tested. Capital cost for NIR is approximately 20% of photonic or laser. If
lower cost sintering technology can be applied, then its impact will be to de-risk copper ink adoption and open the
possibility for large area low cost circuits
In addition to the short term direct impacts, the research will have longer term impacts to a wider industrial audience by
demonstrating an alternative additive metal pattering processes using a potentially low cost conductive material which is
capable of sub 10 micron features. This has been a barrier to many applications where small feature sizes are required e.g.
printed display backplanes, printable logic and "invisible electronics" where basic logic is laid out over a transparent
substrates. No scaleable printing process can readily achieve such features, photo lithography and subtractive processing
being the only mode of manufacture. If the technology is demonstrated on glass as rigid substrate then there is a natural
progression for it to extend to flexible substrates. The demonstration offers significant impact with increased production
rates, larger substrates and less wasteful manufacture leading to new products in displays, healthcare devices and IOT
devices.
If the technology is shown to be compatible with third generation PV devices (OPV and perovskite) then it will have
significant impact as it will remove a commercial barrier to adoption associated with the cost (and variation in cost) of silver.
This will become increasingly important as the perovskite research and fledgling commercial sector begins to move
towards module size devices where full circuit conductivity becomes increasingly dominant. Lower reliable sheet resistances may provide an additional impact by allowing simpler monolithic devices to be created in place of the more
complex series connected cells.