20. July 2009
In at least two previous posts (Picture Story and How do organic solar cells function – Part 1), I highlighted the field dependence of the photocurrent in organic solar cells, and its connection to the polaron pair dissociation. Actually, there is more to it.
The field dependence of the photocurrent is due to different contributions:
- polaron pair dissociation (bulk heterojunctions and bilayers)
- polaron recombination (mostly bulk heterojunctions)
- charge extraction (bulk heterojunctions and bilayers)
An experimental curve of the photocurrent of a P3HT:PCBM solar cell is shown in the figure (relative to the point of optimum symmetry, as described by [Ooi 2008]. The symbols show our experimental data, the green curve a fit with two of the contributions mentioned above: polaron pair dissociation (after [Braun 1984]) and charge extraction (after [Sokel 1982]). Both models are simplified, but more on that later. Polaron recombination has been covered before (here and here); 
it is pretty low in state-of-the-art bulk heterojunction solar cells, and has therefore been neglected. For now, lets concentrate on the contribution from polaron pair dissociation. For the sample shown in the figure, the separation yield approaches 60% at short circuit current (at about 0.6V on the rescaled voltage axis, 0V corresponding to the flatband case). The question is, why is it so high in polymer-fullerene solar cells, considering that a charge pair has a binding energy og almost half an electron Volt at 1 nm distance, and that recombinatoin is on the order of nanoseconds [Veldman 2008].
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physics | Tagged: organic solar cells, physics | 
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Posted by deibel
13. July 2009
… and the best news is, it is the same lady;-)
On the 11th of July 2009, Anja and I celebrated our church wedding. After the civil wedding last December in my home town Wuppertal, we now married in Anja’s home village Schwebheim in Lower Frankonia. It was a wonderful ceremony, and a very nice festivity afterwards (I believe… but I might be biased;-).
The only reason I am already back at my computer is that the beginning of our honey moon is somewhat delayed, as both of us have very interesting but also demanding jobs. Well, postponed is not abandoned!
All the best,
Carsten
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Posted by deibel
8. July 2009
Brief note: 5.9 % power conversion efficiency (german, translation here; [Update 28.11.2009] it now says 6.07%) from small molecule p-i-n tandem solar cell with 2 sqcm area, made by Heliatek in Dresden. Nice picture (also by Heliatek:-)
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The claim “new world record: efficiency of organic solar cell increased to 5.9%” should be preceded by “almost”, or succeeded by “based on small molecules”, because less than 2 months ago, Konarka had a press release about a certified efficiency of 6.4% for an organic bulk heterojunction solar cell. Although not mentioned in the press release, this one is probably not a tandem cell.
[Update 3.9.2009] After talking to Moritz Riede, a researcher from Dresden, I understood that the world record is unique in as far as the area is above one square-centimeter: 2 cm2, whereas the Konarka cell has only 0.76 cm2 – almost at, but not quite above “unity”. This distinction comes from the solar cell efficiency tables by Green et al. (see for instance [Green 2009]).
Thus, the 5.9% are best for small molecule based solar cells, and the best organic solar cells above one cm2: congratulations!
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Posted by deibel
