IMEC Tech Forum: Spray-coating for organic cells, cheaper c-Si

Two announcements coming out of IMEC’s annual IMEC’s annual Technology Forum this week were a new spray-coating technique for solution-processed organic solar cells, and work with BP Solar on making solar cells with a lower-cost alternative to Czochralski silicon.

October 7, 2009  – IMEC has produced 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 comparable to that of spin-coated devices — “an important step,” they say, toward making organic solar cells with less expensive and large-area processes.

Polymer-based (organic) solar cells have potential for low-cost production and high throughput, but only if all the layers can be deposited by solution-based in-line compatible methods, IMEC notes. Spray-coating, in which ink is atomized at the nozzle by pressure or ultrasound and directed by a gas toward the substrate — a process already frequently used in industrial coating and in-line deposition processes — is a suitable deposition technique for this, and can be used to deposit all layers including the metal top contact, they claim.

In IMEC’s work, researchers spray-coating deposited an active-layer solution of poly-3-hexylthiophene (P3HT) and PCBM (a fullerene derivative of the C-60 buckyball), and for the top metal contact spray-coated a solution with silver nanoparticles. The resulting cell show power conversion efficiency of >3%, comparable to cells made with spin-coated organic layer and vacuum-evaporated top contact metal.

Moreover, IMEC says, spin-coating “greatly reduces the damage to underlying layers,” either dissolving the underlying layer, or damaging it with required temperatures to sinter the silver. The group said it figured out how to sinter the silver nanoparticles at 150°C, compatible with processing on flexible substrates.

Click to Enlarge
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)

Cheaper silicon for c-Si cells

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 silicon, developed in 2006 with support by the US DoE, involves casting ingots with a proprietary growth nucleation method into single bricks and wafers to achieve “preferred crystallographic orientations” — i.e., minimizing dislocations where metallic impurities cluster. The result is silicon with “extremely low defect densities and high conversion efficiencies,” the company notes. (BP Solar wrote white paper comparing the performance of its Mono2 vs. other multicrystalline silicon modules.)

Using Mono2, IMEC has made solar cells (156mm × 156mm) with 130μm thickness — “a dramatic reduction” — including dielectric passivation and localized back surface field, and an 18% conversion efficiency which is “in the range of crystalline solar cells,” the group said in a statement. The switch to Mono2 silicon adds three process steps to the standard process of full aluminum back surface field, so it is a feasible as an industrial production process, they note.

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