Author: deibel

Personal news, and a happy new year 2009

Dear valued reader, Anette Hammer)' on Flickr.complease allow me to share some personal news:

Just married !

On the 20th of December 2008, Anja and I celebrated our civil wedding, our “way into happiness” as we call it, in my home town Wuppertal! Afterwards we spent two very relaxing weeks on vacation, enjoying togetherness, thus regaining vigour and inspiration after a laborious year.

That news given, you will certainly believe me that for us this will be an excellent new year 2009, with many more as good ones to follow. Please accept my best wishes for a healthy, funny, splendid year to come.

All the best,

  Carsten

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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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Trimolecular Recombination … really?

As you might already have guessed, I am interested in loss mechanisms in organic photovoltaics. Despite considering the impact of recombination on the solar cell performance, also the physical origins are challenging… and many open questions remain.

Just a view days ago, there was another publication about recombination of free polarons (free carriers) – also called nongeminate recombination *1 – more specifically, trimolecular recombination. Abendstimmung im Vogelschutzgebiet GarstadtYou might remember that, a while ago, I already mentioned third order recombination, including a reference to private communications with Prof. Juska and another recent paper by the Durrant group [Shuttle 2008]) as well as a potential candidate for its origin. The new paper [Juska 2008] uses three different experimental methods, including photo-CELIV, to measure the temperature dependence of the trimolecular recombination rate in polymer:fullerene solar cell. The authors mention very briefly a possible mechanism responsible for the third order recombination, Auger processes. Shuttle et al. argue in their paper that a bimolecular recombination with a carrier concentration dependent prefactor could be the origin, in particular as they observe a decay law proportional to n2.5-n3.5, depending on the sample. We are also in the game, an accepted APL awaiting its publication (preprint here) Update 20.10.2008: now published online [Deibel 2008b]. We rather tend to believe the explanation by Shuttle, but that’s just an assumption at the present stage: the generally low recombination rate could also be due to a rather improbable process.

Continue reading “Trimolecular Recombination … really?”

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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iPhone Scientific Apps – and Apps for Science;-)

I have been quite quiet for a while, and now I am only briefly back with a somewhat off topic note: Science Apps for the iPhone. Macresearch already found a few of these, as described in their two blogposts. I quickly skimmed through the list of apps today, and found indeed some interesting stuff. One which will be particularly useful for me is VoiceNotes, which makes the iPhone a voice recorder (it also has some commercial but affordable competitors which I have not tested yet). Very useful to me, up to now I used to speak on my answering machine – which forwarded the messages as emails to my inbox – when some idea comes to me during driving to or from work… which happens rather frequently (both, the commuting and the voice recording;-) I am looking forward to see more applications soon.

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Picture Story – How Do Organic Solar Cells Function?

After the introductory posts about organic solar cells – split in parts zero, one and two, – I would like to present a somewhat more intuitive picture today… well, picture indeed says it all;-)

Step 1: Light Absorption => Exciton Generation

osc bhj morphology scheme - 1.jpg absorption bands polymer vs cis.jpg
  • light is absorbed in the donor material, e.g., a conjugated polymer
  • excitons are thus created, strongly bound electron-hole pairs on the polymer chain
  • very high absorption coefficient, device thickness on ~100nm scale, as compared to the inorganic polycrystalline semiconductor CuInSe2 (~1 micron) and crystalline Silicon (~100 micron)
  • but: only narrow absorption bands, as shown for two conjugated polymers P3HT and PCPDTBT in comparison to CuInSe2. This drawback could be circumvented by synthesis of novel materials, or multijunction concepts (tandem solar cells).

Continue reading “Picture Story – How Do Organic Solar Cells Function?”