by Debra Vogler, senior technical editor, Solid State Technology

Sept. 10, 2008 – IMEC recently announced an agreement with Plextronics Inc. to develop a reproducible process for high-efficiency organic solar cells using Plextronics’ Plexcore materials and inks. The ultimate aim is to develop organic multi-junction solar cells with 10% efficiency by 2012, and then scale up the process for large-area industrial manufacturing with average 7% efficiency (+/- 0.5%) and a five-year lifetime.

Achieving that goal will require high-quality, highly reproducible commercial materials. The first phase of the joint work will investigate Plextronics’ Plexcore OS polymer (regioregular poly-3-hexylthiophene [P3HT]), which the company says has a high absorption coefficient close to the maximum photon flux in the solar spectrum and high mobility. The materials will be spin-coated and validated on film morphology, carrier mobility, and reproducibility; solar cells will be processed on different substrates using spin-coated films of the material.

Organic photovoltaic cell structure. (Source: Plextronics)

Describing the significance of using a regioregular polymer, Jef Poortmans, program director solar at IMEC, told SST that the molecules stack themselves to form a crystalline order in the material, which reduces series resistance in the solar cell.

A major challenge in achieving high-efficiency, large-area solar cell modules is controlling the nanomorphology of the active layer (i.e., the thin-film layer), which is influenced by the materials being mixed (i.e., organic donor and acceptor). Poortmans noted that a key advantage to working with Plextronics is that the materials supplier is able to provide sufficient quantities of the organic materials that also have the same properties — necessary to be able to separate process development issues from changes in the starting materials.

Besides control of the morphology of the nano-size domains using well-reproducible materials and inks, Poortmans explained that it is also crucial to control the energy levels in the donor/acceptor system. “This allows us to optimize exciton dissociation and to prevent voltage losses by a too large energy difference between the LUMO-levels [lowest unoccupied molecular orbital, the counterpart of the conduction band in inorganic semiconductors] of donor and acceptor material,” he said. “Our cooperation with Plextronics represents a longer-term opportunity with a first-class material supplier to make progress on this issue.”

It is also very important, he added, to have interconnect solutions (between solar cells) that are compatible with low-cost applications and that have a sufficiently high conductivity. A third challenge, particularly working with devices over a large area, is to not have performance degraded by the occurrence of too many shunts.

There is no roadmap or set milestone schedule for the collaboration at the present time, noted Poortmans. Future work will evaluate other Plexcore materials and inks using other deposition techniques such as screen and inkjet printing and spray coating on large-area substrates. — D.V.

IMEC organic solar cells made on glass, composed of materials with different absorption spectra. (Source: IMEC)