To be improved: Lifetime of Organic Solar Cells

I just came across this press release from the before-mentioned organic solar cell company Konarka. Boston Evening OneI mention it particularly, as our research group participates in this BMBF project to improve the stability of organic solar cells.

A somewhat older press release (see here and here) by the belgian research institute IMEC shows how they managed to improve the stability of the donor material, a conjugated polymer. The improvement is apparent from electrical characteristics and TEM images.

Not being quite as fancy as efficiency improvements, the lifespan of organic solar cells is probably more important for a ssuccessful commercialisation. As you know now that we are “officially” involved, stay tuned: this topics interests me from a fundamental research perspective.

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Mass Production of Plastic Solar Cells

LookTechnology Review has a piece on the first commercial fab for organic solar cells.

In a significant milestone in the deployment of flexible, printed photovoltaics, Konarka, a solar-cell startup based in Lowell, MA, has opened a commercial-scale factory, with the capacity to produce enough organic solar cells every year to generate one gigawatt of electricity, the equivalent of a large nuclear reactor.

Read it here, or the corresponding Konarka press release.

Thanks to Henning for the link.

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Brief Headline: Organic Bulk Heterojunction Solar Cell Efficiency

Continuing my recent history of only brief notes (sorry, busy…) here a short headline from the SPIE Optics and Photonics Conference in San Diego.

Today I heard a talk by Darin Laird, Plextronics. Green Lizards in the Loire valleyUsing an undisclosed organic donor material (well, they call their product Plexcore OS 2000 [Update below], as opposed to their P3HT OS 1000 or so) blended with the usual suspect PCBM, they managed to process an NREL certified lab scale (0.1cm2) solar cell with 5.94% power conversion efficiency! Fill factor was almost 72%, I believe, with the major improvement as compared to the reference material P3HT coming from an increased open-circuit voltage.

The corresponding solar cell module, 15×15 cm2 large, has an efficiency of 1.1% (or 2.3% active area efficiency, if you consider that only 46% of the module are active area). These numbers are brand new, but generally, uptodate solar cell efficiencies can be found in the efficiency tables (V32) by Martin Green.

So, who’s next to boost the organic solar cell efficiencies? ;-)  

P.S. As there sadly was a history of overestimated efficiencies published, followed by letters to the editors by watchful scientists and statements, a solar cell characterised by a certified institute is important to regain the trust.

P.P.S. Of course, not every university group can afford to spend 1000 bucks on a certified solar cell measurement. Still, at least some effort can be put into doing the current-voltage characterisations carefully. In January, Jan Kroon gave an interesting talk about measuring organic solar cells properly; find the video here.

Update (5.9.2008): The donor Plexcore OS 2100 available at Sigma Aldrich is not the one with which the 5.9% efficiency where achieved. The undisclosed donor material used is not yet available commercially, it seems.

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Optimisation Routes for Organic Solar Cells – Absorption

In order to improve the power conversion efficiency of organic solar cells, novel donor and acceptor materials will have to be synthesised. energy-levels-in-bilayer-solar-cell.png Properties looked for are the ability to self-organise – enhancing order and thus charge transport – and an absorption spectrum as wide as possible, being one of the major limiting factors as of yet. Nowadays, in most cases only the donor material absorbs light efficiently; an absorbing acceptor has a large potential for increasing the photocurrent. Additionally, by a variation of the relative energy levels of donor and acceptor material, the energy loss due to the electron transfer can be minimised: For light absorption in the donor, it is hoped that if the energy offset between donor LUMO (lowest unoccupied molecular orbital) and acceptor LUMO is a tiny bit larger than the exciton binding energy, a positive impact on the open-circuit voltage will be seen.

In the figure, the schematic energy level diagram of a bilayer solar cell is shown. The anode is made of TCO (transparent conductive oxide), then follow donor and acceptor, and finally the metal cathode. Church inside top The exciton is photogenerated in the donor, which can diffuse to and dissociate at the interface to the acceptor. The resulting polaron pair then is energetically separated by the effective band gap of the organic solar cell, Eg.The smaller the LUMO-LUMO offset & – which still has to be larger than the exciton binding energy – the larger Eg: the open circuit voltage is maximised, as it equals Eg minus band bending BB and the injection barriers phi [Cheyns 2008].

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Why Disordered Materials?

So why disordered materials? Arguing from an application based (=engineer) point of view, disordered materials are usually easier and cheaper to be manufactured than (single or poly) crystalline ones. Looking at organic semiconductors, such as conjugated polymers or fullerene derivatives (“bucky balls”): they are soluble and can thus be deposited from the liquid phase – e.g., by printing (offset, inkjet, you name it). Device configuration of a Disordered Organic Solar Cell made of conjugated polymers (red chains) blended with fullerene molecules (bucky balls) The vision for the so called plastic electronics is to print circuits and devices on flexible substrates. This can be done at room temperature (low energy) and ideally with roll-to-roll processes (high throughput). Sounds good, eh?

Well, there are some drawbacks in terms of the application… though not for researchers;-) Printing semiconductors usually leads to rather disordered films, which have very low charge carrier mobilities as compared to Silicon and other inorganic semiconductors, and are thus not suitable for high-frequency applications. However, for photovoltaic devices, the low mobilities are not that much of a drawback. That said, organic solar cells are still at below 7% power conversion efficiencies… for very small areas. Single crystalline Silicon, on the other hand, sees already above 20% for modules.

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