Can plastic solar cells deliver?

The promise of polymer solar cells is that they will be cheap enough and produced on a large enough scale to transform our energy economy, unlocking the sun’s potential to meet all our energy needs in a sustainable way. But there’s a long way to go from a device in a laboratory, or even a company’s demonstrator product, to an economically viable product that can be made at scale. How big is that gap, are there insuperable obstacles standing in the way, and if not, how long might it take us to get there? Some answers to these questions are now beginning to emerge, and I’m cautiously optimistic. Although most attention is focused on efficiency, the biggest outstanding technical issue is to prolong the lifetime of the solar cells. But before plastic solar cells can be introduced on a mass scale, it’s going to be necessary to find a substitute for indium tin oxide as a transparent electrode. But if we can do this, the way is open for a real transformation of our energy system.

The obstacles are both technical and economic – but of course it doesn’t make sense to consider these separately, since it is technical improvements that will make the economics look better. A recent study starts to break down the likely costs and identify where we need to find improvements. The paper – Economic assessment of solar electricity production from organic-based photovoltaic modules in a domestic environment, by Brian Azzopardi, from Manchester University, with coworkers from Imperial College, Cartagena, and Riso (Energy and Environmental Science 4 p3741, 2011) – breaks down an estimate of the cost of power generated by a polymer photovoltaic fabricated on a plastic substrate by a manufacturing process already at the prototype stage. This process uses the most common combination of materials – the polymer P3HT together with the fullerene derivative PCBM. The so-called “levelised power cost” – i.e. the cost per unit of electricity, including all capital costs, averaged over the lifetime of the plant, comes in between €0.19 and €0.50 per kWh for 7% efficient solar cells with a lifetime of 5 years, assuming southern European sunshine. This is, of course, too expensive both compared to alternatives like fossil fuel or nuclear energy, and to conventional solar cells, though the gap with conventional solar isn’t massive. But the technology is still immature, so what improvements in performance and reductions in cost is it reasonable to expect?

The two key technical parameters are efficiency and lifetime. Most research effort so far has concentrated on improving efficiencies – values greater than 4% are now routine for the P3HT/PCBM system; a newer system, involving a different fullerene derivative, PC70BM blended with the polymer PCDTBT (I find even the acronym difficult to remember, but for the record the full name is poly[9’-hepta-decanyl-2,7- carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)]), achieves efficiencies greater than 6%. These values will improve, through further tweaking of the materials and processes. Azzopardi’s analysis suggests that efficiencies in the range 7-10% may already be looking viable… as long as the cells last long enough. This is potentially a problem – it’s been understood for a while that the lifetime of polymer solar cells may well prove to be their undoing. The active materials in polymer solar cells – conjugated polymer semiconductors – are essentially overgrown dyes, and we all know that dyes tend to bleach in the sun. Five years seems to be a minimum lifetime to make this a viable technology, but up to now many laboratory devices have struggled to last more than a few days. Another recent paper, however, gives grounds for more optimism. This paper – High Efficiency Polymer Solar Cells with Long Operating Lifetimes, Advanced Energy Materials 1 p491, 2011), from the Stanford group of Michael McGehee – demonstrates a PCDTBT/PC70BM solar cell with a lifetime of nearly seven years. This doesn’t mean all our problems are solved, though – this device was encapsulated in glass, rather than printed on a flexible plastic sheet. Glass is much better than plastics at keeping harmful oxygen away from the active materials; to reproduce this lifetime in an all-plastic device will need more work to improve the oxygen barrier properties of the module.

How does the cost of a plastic solar cell break down, and what reductions is it realistic to expect? The analysis by Azzopardi and coworkers shows that the cost of the system is dominated by the cost of the modules, and the cost of the modules is dominated by the cost of the materials. The other elements of the system cost will probably continue to decrease anyway, as much of this is shared in common with other types of solar cells. What we don’t know yet is the extent to which the special advantages of plastic solar cells over conventional ones – their lightness and flexibility – can reduce the installation costs. As we’ve been expecting, the cheapness of processing plastic solar cells means that manufacturing costs – including the capital costs of the equipment to make them – are small compared to the cost of materials. The cost of these materials make up 60-80% of the cost of the modules. Part of this is simply the cost of the semiconducting polymers; these will certainly reduce with time as experience grows at making them at scale. But the surprise for me is the importance of the cost of the substrate, or more accurately the cost of the thin, transparent conducting electrode which coats the substrate – this represents up to half of the total cost of materials. This is going to be a real barrier to the large scale uptake of this technology.

The transparent electrode currently used is a thin layer of indium tin oxide – ITO. This is a very widely used material in touch screens and liquid crystal displays, and it currently represents the major use of the metal indium, which is rare and expensive. So unless a replacement for ITO can be found, it’s the cost and availability of this material that’s going to limit the use of plastic solar cells. Transparency and electrical conductivity don’t usually go together, so it’s not straightforward to find a substitute. Carbon nanotubes, and more recently graphene, have been suggested, but currently they’re neither good enough by themselves, nor is there a process to make them cheaply at scale (a good summary of the current contenders can be found in Rational Design of Hybrid Graphene Films for High-Performance Transparent Electrodes by Zhu et al, ACS Nano 5 p6472, 2011). So, to make this technology work, much more effort needs to be put into finding a substitute for ITO.

8 Responses to “Can plastic solar cells deliver?”

  1. It would be good if your optimism pans out! In the meantime, what do you make of the recent publication from Heeger’s group using a small molecule rather than a polymer with the PC70BM (http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat3160.html)? They are claiming rather high efficiencies of 6.7% and that the small molecules will be more reproducible than polymers, although they seem to have a fairly complex (and of course empirical) sample preparation route involving solvent additives in order to reduce domain size. That polymers tend to have significant batch-to-batch variations is undeniable so I can see the attraction of a small molecule in this case. But nevertheless high efficiencies have been quoted in the past for solar cells which don’t subsequently translate into a successful large scale production route.

  2. Abelard Lindsey says:

    Nice of you to mention the necessity of replacing ITO for the upper electrode. I have gone around (and around and around) on this issue with the various “new technology” solar materials start-ups. It is actually irritating that no one seems to be working on this issue. Any and all solar concepts are non-starters without an effective replacement for ITO. The industry seems to be in denial of this reality.

  3. Richard Jones says:

    Athene, that’s an interesting paper, but I note there’s no lifetime data quoted, nor is there any reason to believe small molecules will have any particular advantage over polymers on this front. Batch-to-batch variability of polymers is a problem now, but I wonder if it will remain one when their synthesis moves from an R&D to a production basis.

    Abelard, my perception is similar to yours in terms of the awareness of the problem. But since yesterday I’ve been told of a very major commercial effort to develop an ITO replacement which is now looking very promising, so perhaps there is a solution on the horizon.

  4. I was visiting the Institut Jean Rouxel in Nantes as part of a panel last year, and was interested to hear of their efforts in the very problem of life after ITO. Indeed, for the interested, there are even workshops being organised to that end:

    http://www.leti.org/fr/layout/set/print/layout/set/print/Prochain-evenement/2011/Transparent-electrode-what-else-after-ITO-State-of-the-art-and-future-prospects

    I think there is strong interest in the area; it is just that the major journals would be more interested in another fraction of a percentage point added to the efficiency and a few minutes added onto the lifetime rather than something as apparently dull as an electrode.

  5. Keystone garter says:

    CNT manufacture uses industrial equipment and the results of say, novel methane mixes or bake time are often annoyingly a trade secret. One of the biggest applications I thought was rural Africa farm worker and biz owner homes. But now I’m reading an often crappy book that spelled out in one brilliant page how 3rd world economies are developed. Also mentioned unions (middle class) form easiest in cities. 1st is local perishables (I never understood why Coke FEMSA was a decent investment, just trusted Buffet), then bulky expensive to ship products, concrete and lumber, then assembly and tool-and-die and my kind of unskilled jobs, then the components, then finally making the machines.
    This suggests cities and central power; Toronto is too big at 10M but okay at present if an environmental mayor.

    I’m not sure if the solution here is using cheaper experimental materials like titanium, nickel…in existing depostion processes, or if it means inventing or waiting for the invention of new processes. The upshot is light small products can be shipped abroad if cheap inputs. I’m bullish on plastic food spoil metres, esp given BPA. An expiry date is okay but having more accuracy even better; need to smell the can interior or sense poison in liquid.

  6. Abelard Lindsey says:

    Lifetime is another issue. In order to be useful, plastic solar cells must be conductive. Of course, if they are conductive, they will easily react with the Oxygen in the atmosphere and degrade over time. A catch 22 as we say. The conductive plastic material must be hermetically sealed from the atmosphere. The “new” PV technology start-ups I know are not dealing effectively with this challenge either.

  7. Abelard Lindsey says:

    Its good to see that they are finally working to develop a replacement to ITO. This is fundamental to the success of PV solar (in any form). Indium is inherently scarce material. It is produced as a by-product from Zinc refining. Also, 70% of the world’s supply of Indium comes from one company – The Indium Corporation (an appropriate name). Even today, much of the Indium used to make displays comes from recycled ITO sputtering targets. The supply of Indium will not allow for the scale-up of PV production to millions of square meters per year.

    Speaking of supply constraints, don’t even get me started on CIGS, given that both Indium and Gallium are inherently scarce. I think CIGS is delusional. The world’s supply of Ga is sufficient for making III-V compound devices, where you are using very small amounts of the Ga per wafer. The supply of Ga cannot scale up to the millions of square meters per year necessary for a real PV solar industry.

    The other problem with ITO (and AZO) is that it must be sputtered. I know of people who have tried many different methods (colloidal nano-particles, etc.) to make spray deposition of ITO possible. They have all failed. This is an economics issue because vacuum process technology is inherently expensive and inherently through-put limited. The promise of plastic solar cells to begin with is to eliminate the need for vacuum process in favor of spray painting and other cheap manufacturing process. This can hardly work if ITO must still be sputtered onto the PV stack to make the top electrode.

    There is NO WAY solar PV will scale without a suitable replacement for ITO for the top electrode, a replacement that can be deposited on using a low-temperature, non-vacuum deposition process. This is technical and economic reality.

    IF a replacement for ITO is developed AND the lifetime issues with plastic PV materials can be overcome, THEN I think plastic solar cells have the future. CIGS and CdTe are non-starters and Silicon, even in thin-film, is not feasible from a cost stand-point. Dye-sensitized TiO2 cells are also a possibility. However, these have major technical issues as well.

  8. Zelah says:

    To continue this debate…

    The real problem for organics is the extreme competition from numerous other forms of Solar PV

    The most exciting is Thin Films, in particular CZTS. CZTS has recently had a huge breakthrough via IBM, and this solves the indium problem posed above and in particular by the commenter called Abelard Lindsey.

    The real problem for solar is the high installation costs…

    I have high hopes of 1$ US per watt installed for Large scale installations…

    Certainly Solar will acheive 2$ US per watt installed in a couple of years.

    Please note that as European prices of electricity at 10 cents US per Kilowatt Baseline, 20 cents in the Home makes 2$ US a watt Solar competitive even with Gas in Southern Italy, Greece and Spain.
    Just the tonic for Europe in these Dark Times…

    Zelah