IMEC's Poortmans: Organic PV solar cells more than "niche" - Photovoltaics World
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IMEC's Poortmans: Organic PV solar cells more than "niche"

October 13, 2009 - IMEC recently demonstrated a fully solution-processes organic solar cell with a spray-coated active layer and metal top contact spray-coated on top, with power conversion efficiency (>3%) comparable to cells made with a spin-coated organic layer and vacuum-evaporated top contact metal. For the metal top contact, IMEC spray-coated a solution with silver nanoparticles at 150°C, a temperature compatible with processing on flexible substrates. The challenges are to do this process without dissolving the underlying layer and without damaging it by the temperature needed to sinter the silver nanoparticles, but IMEC says that it demonstrated that spray-coating greatly reduces the damage to underlying layers compared to other techniques.

[IMEC also demonstrated results from its work with BP Solar: silicon solar cells with 18% conversion efficiency made with the company's Mono2 silicon, billed as a low-cost alternative to Czochralski silicon substrates.]

The research group believes that the spin-coating work is an important step toward producing organic solar cells with less expensive and large-area processes. Jef Poortmans, IMEC's program director for photovoltaics, sat down with PV World during the research center's annual media event in Leuven, Belgium, (Oct. 5-6) to discuss his outlook for organic PV technology.

Poortmans listed several main reasons IMEC has chosen to work on organic solar cells: sufficient efficiency potential for the technology, sustainable technology, and synergy with other activities at IMEC. The world record efficiency for organic PVs is 6.7%, but he believes that this is not the technology's limit. "If you think about the specific properties of organic materials, they have a relatively narrow absorption band," he said. This property means that these materials can be used in an organic multijunction structure, or stack of layers, each of them absorbing a different part of the spectrum. "So you would expect much higher efficiencies with this sort of approach -- you'd be absorbing each part of the spectrum with the applicable organic material."

The polymer solar cell with a spray coated Ag top contact: (a) schematic build-up, (b) SEM, and (c) FIB/TEM cross-sections. (Source: IMEC)

In such a multijunction organic structure, for example, light would come into the first layer of material that would absorb the high-energy photons; the light not absorbed in the first cell goes to the second, and so forth. With this sort of structure an efficiency of 15% is conceivable, Poortmans thinks. Much lower manufacturing costs would be incurred than with other types of cell technologies, though. Organic solar cells can be deposited in non-vacuum conditions using simple techniques such as printing, "so it could be a very low cost technique where you could have bottom up patterning -- you can immediately print a thin layer with a pattern," said Poortmans.

Admittedly, organic PV cells initially will be limited to niche markets, acknowledges Poortmans, "but once we improve efficiency and stability simultaneously, we could have a situation where 10-15 years from now, we could use organic PV cells as an option in grid-connected high-power applications." [Ref. the roadmap by orgapvnet concerning market penetration and grid-connected high-power applications.]

The payoff for developing organic PV cells is in its promise of low-cost production and high throughput. However, this can only become true if all the layers of the cells can be deposited by solution-based, in-line compatible methods, according to IMEC. Its research shows that spray-coating is a suitable deposition technique, and that it can be used to deposit all layers, including the metal top contact. Spray-coating is a high-rate, large-area deposition technique that ensures an ideal coating on a variety of surfaces with different morphologies and topographies.


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