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Going organic: Solar cells in a spray can may not be as farfetched as you think

30 November 2007 | By Tom Cheyney | Chip Shots

As I scarfed my pasta during the lunch break at the mid-October press gathering at IMEC, Paul Heremans told me about the center's SOLAR+ Program's work in the organic photovoltaics (OPV) arena, including an upcoming paper at the Materials Research Society meeting about a spray-on deposition method: Not sprayed on with a spin-coating tool but deposited with an inexpensive airbrush nozzle like those used to paint cars! After swallowing my mouthful, I confirmed what Paul had said and asked what kind of conversion efficiencies the method had achieved. When he told me the numbers were between 2% and 3%---comparable to conventional techniques---I was intrigued.

Paul and his team from IMEC and the neighboring Catholic University in Leuven presented their findings this week at the MRS confab in Boston in a paper titled "Polymer solar cells with a high-level of structural control fabricated using a novel spray-coating deposition technique." Here's some extracts from the abstract:

"Organic solar cells are either produced by solution processing of polymers or evaporation of small molecules. In the first case, spin coating is considered the most reliable and reproducible method, but is limited to small areas. To realize large-area coverage, various deposition techniques, such as ink-jet and screen or gravure printing, have been proposed and demonstrated. Here, we present a simple method for the deposition of large area devices based upon spray coating, and show that this method is a valid alternative to other techniques...."

"....To justify the usefulness of this technique, we compared a standard spin coated solar cell based on a mixture of poly(3-hexyl thiophene) (P3HT) and the C60-derivative (6,6)-phenyl C61-butyric acid methyl ester (PCBM) with a spray coated one, where the P3HT:PCBM blend was sprayed by a N2-powered airbrush. Spray-coated solar cells were found to have power conversion efficiencies above 2%, a performance which is comparable to that of the spin-coated devices.

"We also performed atomic force microscopy, ellipsometry and absorption measurements to compare the film quality of the two techniques. Significantly, we found that the spray coating technique allows for the demonstration of polymer solar cells with distinct layers, owing to the differences in the kinetics of the solvent evaporation process when compared to other solution-based techniques. The small femto-liter scale droplet size intrinsic to the spray-coating technique means that thin films can dry very rapidly, and subsequent films can be deposited despite a common solvent....

"We investigated different methods of spray coating, from single-pass to multiple-passes, and varied the concentration of the solutions, in order to optimize the technique, and looked into the effects of thermal annealing to the layers. Then we analyzed the relation between the performance of the devices and the characteristics of the films obtained from the two different depositions techniques to show that spray coating is an excellent alternative to spin coating for the fabrication of large area polymer-based devices."

The airbrush used in the study was a nitrogen gas-powered Badger 200 NH, which lists for well under a hundred bucks and is capable of solution flows between 0.1 and 1.0 ml per minute. The manufacturer says the device is used by the "advanced model painter or decorative stenciler" and is "also an excellent airbrush for painting posters, auto detailing, and ceramics." And now, apparently, OPV film applications.

As for those all-important conversion efficiency numbers, the single-pass method hit 2.17%, while 1.49% was achieved with a multiple-pass technique. Since the spray technique permits "stacking of subsequent layers," it can produce "more complex structures for better polymer solar cells."

Paul told me via email from Boston that "the presentation was received very well! This technique has the potential to provide more rapid processing than the earlier techniques, and there are enough degrees of freedom to optimize the morphology of the resulting film."

Paul and his colleagues will get a chance to "benchmark materials and accelerate technology development" in spray-on and other printed and evaporative OPV approaches when a planned organic solar-cell pilot-line---the O-line---is funded and built at IMEC. Sources at the research center are mum on any further details about progress on the investment front for the new OPV lab, although an announcement is likely within the next few months.

Widespread commercialization of organic PV is years away (although Konarka would argue with that), with significant challenges in the areas of cell and module lifetime, reliability, barrier/encapsulation, and volume manufacturing techniques. A goal of 10% conversion efficiency for organics seems attainable, though not without a lot of heavy lifting in the labs. When it does come online, IMEC's line will help OPV move faster down the industrial roadmap and eventually play a role both as another cheap (as in a lot less than a buck a watt) solar alternative to conventional power and a way to charge "ambient intelligence" devices.