I am also here, my talk will be on wednesday afternoon – but do not expect any breakthroughs in terms of performance from me:-) Maybe there will be more news here in the days to come.

In June and July, I was visiting scientist in the group of Mike McGehee at Stanford University for five very interesting weeks. Thanks again for hosting me, and for the interesting discussions we had! I also had a brief visit to PARC, the Palo Alto Research Center, for an interesting discussion with Robert Street about the photocurrent in organic solar cells. We finally agreed to disagree on some issues, but from my point of view, that’s absolutely fine.

During my Stanford visit, there was fortunately time enough for hiking in the Foothill Mountains as well! Highly recommended. Thanks to Andreas and Verena as well as Matthias for getting me started.

Another bit, highly unrelated, which you should skip if you are not interested in software related stuff: you may know that I am a Mac user, and since the MRS meeting back in April also a happy iPad owner. One of the favourite programs, exclusively for Mac, is Papers, a program to organise and read your scientific articles. It is also available for iPhone/iPad, and includes sync to your Mac (WLAN or via iTunes) and online backups (if you like). I mention it now as of last week, the iPhone/iPad version has the ability to add annotations and highlights to the pdfs. As far as I know, these annotations cannot yet be synced, but I am sure it is a top priority for the next version. Just one of the many useful programs assisting scientists to get their work done:-) If you wonder what other software I use, you will notice that I do not mind paying for good software, although a lot of very good freeware is around is well on Macs. My top ten is (disordered;-) TeXshop, Daylite, DevonThink Pro Office (or Journler), Textexpander, Cornerstone (or svnX), MarsEdit, Launchbar (or Quicksilver), Intaglio, Dropbox, and Textmate (or TextWrangler). The alternative option is brackets are the programs which are free or shareware programs, whereas I use commercial ones. The ones without alternatives have none (or are free or shareware;-)

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Is Organic PV the Future of Solar?.

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Heliatek will be utilizing the new funding primarily to build an initial production facility in Dresden. In this step and right through to mass production, the company will be using its proprietary tandem technology to efficiently produce, flexible and very lightweight PV modules on a film substrate. Their weight will be merely 500 grams per square meter, instead of today’s customary 20 kilograms per square meter. This will open up a forward-looking market for mobile applications, for architectural solutions and for independently supplying regions with weak infrastructures.

Indeed, interesting times for OPV – particularly in view of the commercial aspects! The science aspects are also getting more and more interesting, but unfortunately I thus have less and less time to write about them here…

P.S. Another company Solexant just starts the production of hybrid solar cells after the process developed by the group of Paul Alivisatos at Berkeley, as reported by Technology Review.

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Yesterday, a new report on the future prospects of the organic photovoltaics business was presented by the analyst firm Nanomarkets. It is said to include a roadmap for improvements in organic solar cell lifetimes and efficiencies, as well as forecast of volume and price of relevant materials over the course of eight years.

I cannot comment on the analysts’ expertise, although they are specialised on market research for organic and printable electronics – which has pros (they know what they are talking about) and cons (they might be pretty subjective), I reckon;-) See their press release here. All in all, a promising future is just what we need:-)

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on organic solar cell and organic light emitting diodes (OLED), specifically the conductive inks and process technologies that enable those and other similar applications.

I mentioned Plextronics already last year, as they presented the (up to now, I believe) highest certified power conversion efficiency for an organic solar cell.

Indeed, industry news again… for next time, I promise more fundamentals;-)

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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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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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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. 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].

As a side note, recently there was a report that claims that the polaron pair dissociation yield is exponentially proportional to the LUMO-LUMO offset [Ohkita 2008]. This implies that always a trade-off between short circuit current and open circuit voltage will have to be made. Please note, however, that the photoinduced absorption measurements by Shuttle et al. are done without the application of an external (or internal for that matter, the samples being optical thin films without electrodes) electric field; so there is still hope;-)

But back to absorption and its impact on the power conversion efficiency: you may know the detailed balance calculation of Shockley and Queisser for inorganic solar cells, power conversion efficiency vs band gap, which has been mentioned recently. It is shown in the left figure as black solid line. For organic solar cells, there is no complete analytic theory to describe all parameters proportional to the properties, therefore I made an estimate based on a couple of assumptions:

• quantum efficiency 100% within absorption band of 200 (blue) or 400nm (red) width
• fill factor 80%
• thickness 200nm
• open circuit voltage after [Koster 2005] with recombination strength gamma either 10^-15m³/s (as in the article, even if not explicitly mentioned) or a reduced recombination strength of 10^-18m³/s (green); the other parameters are as in the paper.
• Koster’s Eg is set to “Energy Gap” (as in “x axis”) minus exciton binding energy Eb of 0.3eV

Clearly, disordered organic solar cells have a lot of potential in view of manufacturing by roll-to-roll printing, but the maximum power conversion efficiencies which can be reached are lower as compared to inorganic solar cells… unless a clever chemist (actually, we need a genius here;-) manages to make suitable donor and acceptor materials with wider absorption ranges… Of course, higher intrinsic absorption is not the only route for higher organic photovoltaic performances: another possibility are (solution-processed!) multijunction solar cells, thus combining different absorption ranges of already existing materials.

Update 24.3.2008: I exchanged the wrong reference Shuttle 2008 by Ohkita 2008. Apologies!

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