Flexible printed solar cells – a new manufacturing frontier?
At the University of Newcastle, a new industrial revolution is being ushered in. It’s based on organic photovoltaic (OPV) solar cells – flexible coatings that can be printed on plastic film or sheets in great quantity at low cost. As such, OPV printed solar cells promise to further revolutionize renewables by making solar cells sufficiently transparent and flexible that they can be coated on buildings, vehicles and daily consumer items. This is the revolution promised a decade ago by the US firm Konarka – except that Konarka never managed to bring the costs down sufficiently, and went bankrupt in 2012. Now the UoN team led by Professor Paul Dastoor is zeroing in on the costs and looks set to take the revolution forward – and in the process is creating a vast new manufacturing opportunity for Australian firms.
The first generation solar photovoltaic revolution has been based on polycrystalline silicon solar cells – which are hard-wired and framed as the original ‘solar cells’. The cells can be connected together into a ‘panel’ and then the panels can be connected into a solar module, which might have a power output of a few kW. When linked together they can produce ‘solar farms’ operating at MW and even GW-scale. The technology underpinning such 1G cells is the p-n junction transistor; the panels are manufactured in complex semiconductor processes that call for high temperatures and close manufacturing control. A second generation of solar PV utilized various ‘thin film’ processes where a thin film of photosensitive material is deposited on glass, such as gallium arsenide (GaAs) or CIGS (cadmium, indium, gallium and selenide). These 2G solar cells can extract more energy from the sunlight depending on the number of films utilized, to produce hybrid or tandem cells – but they are subject to the same 1G constraint of needing to be deposited on glass, a major cost and flexibility constraint.
Now a range of 3G solar cells are able to overcome this constraint by utilizing non-silicon based photosensitive materials and being printed on polymer sheets (plastic) which makes them light, flexible and low-cost. The outstanding contenders in this emerging 3G solar cell field are firstly the inorganic cells made from perovskite materials (abundant, cheap, but not yet sufficiently stable for mass utilization) and secondly organic polymer cells, where the photosensitive material is based on carbon molecules, such as those utilizing the fullerene lattice. These organic solar cells have great promise because the photosensitive material can be formed into an ‘ink’ and deposited at low temperature over a large area of cheap plastic (polymer) through a conventional printing process. It is these printed, flexible organic solar cells that are being developed by Professor Dastoor and his team at the University of Newcastle, in the Priority Research Centre for Organic Electronics and in the neighbouring Newcastle Institute for Energy and Resources (NIER). A key feature of the OPV cells is that they have no toxic ingredients, as compared with some of the alternative inorganic solar cells being developed.
The difference between what is being done at Newcastle and what was accomplished by Konarka a decade ago is the new focus on costs, to keep the cost of the various fabricated photosensitive materials as low as possible. Professor Dastoor hosted a visit from GGS last week, and explained how the new focus on costs marks a giant step forward for 3G solar cells. This will partly be a function of scale of production, but partly also the judicious choice of materials involved. The whole process does not have to utilize semiconductor fabrication techniques that has made the process of producing 1G and 2G solar cells so complex.
The Newcastle team at this stage is concerned to create a workable prototype of a functioning solar PV array. This has been accomplished, in a world-first 200 square-meter array erected on the roof of an industrial complex near Newcastle, by the bulk liquids and containers firm CHEP.
How do 3G OPV solar cells perform? First, the costs. Professor Dastoor and his team reckon that they can print these OPV solar cells for less than $10 per square meter – because the process is not complex, uses abundant and low-cost materials, and can be accomplished with standard printing technology. Scaling up to commercial quantities, of the square-kilometre range, the costs would be less than $10 million per square kilometre. That would be enough to cover several blocks of city buildings with their own power generators. How much power could such a city-based building-integrated OPV solar plant generate?
Let us scale the CHEP prototype up through calculation to see what could be accomplished. Working from the insolation received at the earth’s surface of 1000 W/m2, and assuming an unrealistically low level of conversion efficiency of 1% would mean that the OPVs would produce power at a rate of 10 W/m2. Scaling this up (and assuming effectively no limit to the availability of polymer substrate for the OPVs) would mean that a system could produce 10 MW from a sheet 1 km2 in area. Professor Dastoor considers the ‘sweet spot’ for this technology to be around 20 to 30 MW per km2, where (according to his group’s calculations) the OPV would deliver electrical energy at a levelized cost of 20 cents per kWh. So efficiency in sunlight conversion would only need to be improved 2 to 3 times to achieve such a ‘sweet spot’ – something which is actively being investigated by the Newcastle group. Professor Dastoor calculates that if a large area of OPV could produce power at the rate of 50 to 60 MW/km2 (raising the conversion efficiency 5 to 6 times) then power could be produced at a levelized cost even lower, at around 10 to 11 cents per kWh – highly competitive with present costs of power produced by burning fossil fuels.
So the key cost and power parameters are as follows. A printed OPV solar cell would cost less than $10 per m2, which would scale to $10 million per km2 – enough to cover several blocks of city buildings. Power would be generated at 10 W/m2, or scaled up at 10 MW/km2. So a city block of several buildings could be producing its own power at 10 MW at an installation cost of less than $10 million. A key target for research at UoN would be to validate these calculations with actual observations of power and energy generated by the experimental array installed at CHEP.
Producing fresh vegetables
Let us explore the potential applications of this emergent technology. Consider the case of a food production system producing fresh vegetables under glass, and utilizing only renewable energy for desalination of seawater and water circulation through the greenhouses – which is the model of food production perfected in Australia by Sundrop Farms. Now Sundrop Farms currently utilizes a Concentrated Solar Power (CSP) plant utilizing 23,100 heliostats and a 127-meter ‘power tower’ – all of which is working perfectly adequately. How would utilizing transparent printed OPV solar cells change things? The calculations above reveal that organic solar could deliver the same result, if scaled to a 1 km2 OPV array. The Sundrop greenhouses cover an area of 20 hectares (or 0.2 km2), so allowing for OPV to cover the sides and roof area of the greenhouses would call for photosensitive material covering an area of 0.3 km2. Three such farms, able to produce 45,000 tonnes of fresh tomatoes per year, would call for 1 square kilometre of OPV – which could easily be scaled up from the existing demonstration ‘strip’ of 200 m2.
Professor Dastoor considers this scaling up to be entirely feasible, given the ease of printing and producing the photosensitive materials. The power produced by such a 1 km2 OPV operating at 10 MW would be ample to power the desalination plant and pumping systems involved in running the farm. Here we have an obvious multi-million R&D project to propel Australia to the forefront in next-generation OPV printed solar cell technology.
It is the potential manufacturing possibilities raised by the OPV approach that are of great interest to the Newcastle team. There is a range of possibilities associated with OPV solar cells, utilizing different materials and different fabrication methods. There is as yet little international competition – but that could be expected to change as word gets out that OPV is on the brink of commercialization. China changed the global 1G solar cell industry in less than a decade, becoming world leader from a standing zero start, and is now addressing the 2G solar cells revolution (through companies such as Hanergy). There is enormous potential in China for a 3G solar cell industry – and great potential for the companies, and countries, that can supply the technology.
Australia missed out on the 1G and 2G solar cell industrial revolutions – not for want of trying. (Remember Pacific Power, with its efforts to commercialize 2G solar cells developed under Professor Martin Green’s leadership at UNSW?) Now there is another possibility presenting itself with 3G solar cells. To become a player Australia would have to move rapidly from the lab-based research currently being demonstrated at the University of Newcastle to industrial-scale demonstration – involving assessment and evaluation of kilometre-scale OPV material printed on commercial-scale printers adapted from the newsprint industry. Such a scaling-up needs to be fashioned by strong government leadership, with a clear focus on the value chain of materials supply that leads to the end product of the OPV solar cell and to its diffusion. Strong government leadership is of course what has been signally lacking in Australia as the country navigates the energy transition away from fossil fuels. Now here is a chance for a different outcome.
 See the article by Cara Mulligan and other UoN team members, Mulligan, C.J., Bilen, C., Zhou, X.J., Belcher, W.J. and Dastoor, P.C. 2015. Levelised cost of electricity for organic photovoltaics, Solar Energy Materials & Solar Cells, 133: 26-31, available at: https://www.sciencedirect.com/science/article/pii/S0927024814005741?via%3Dihub
 See the interview with me on Sundrop Farms published at RenewEconomy, at: https://reneweconomy.com.au/nectar-farms-sundrop-farms-blazing-trail-64882/
 See ‘The Eureka factory’ by Ceridwen Dovey, The Monthly, August 2016, at: https://www.themonthly.com.au/issue/2016/august/1469973600/ceridwen-dovey/eureka-factory