Author: deibel

Nongeminate recombination in organic solar cells – slower than expected

In a “recent” post (just 3 posts but 10 months ago;-) I wrote once again on the derivation of the Langevin recombination rate for nongeminate recombination. Tiger (Zoo Wuppertal)The question is, is Langevin recombination really what governs the charge carrier loss rate in organic solar cells?

Recombination of electrons with holes is usually a 2nd order decay. As electrons n and holes p are photogenerated pair wise, the respective excess charge carrier concentrations are symmetric, n=p. Then a recombination rate R is

R = k n p = k n^2,

where the recombination prefactor k could be a Langevin prefactor – more on that later. In a transient experiment with a photogenerating, short laser pulse at t=0, the continuity equation for charge carriers (here, e.g. electrons) under open circuti conditions (no external current flow, for instance if the experiment is done on a thin film without electrodes)

\frac{dn}{dt} = G-R

becomes

\frac{dn}{dt} = -R

for t>0 (as the generation was only at t=0).

If all electrons and hole are available for recombination (i.e., can reach all other charge carriers and can be reached by them), then the recombination rate and the continuity equation for t>0 yield

\frac{dn}{dt} = k n^2 Continue reading “Nongeminate recombination in organic solar cells – slower than expected”

New life – again

On 11th of February (my birthday, incidentally), our 2nd child was born: Nicolas Jacob. We are so happy, as is our 2 year old daughter Chiara.

Even with only one kid, it was already rather quiet on this blog in the last months (well, 2 years…). Still, I do have some hope to be able to dedicate one or the other quiet minute to write some urgently needed updates to previous posts. All the best, Carsten

Belectric aquires German Konarka Daughter

Related to this post on Konarka’s bankruptcy: According to a range of news sites, Bear Lake in the Rocky Mountainsincluding pv-tech.org, the german company Belectric has acquired Konarka Technologies. Find the press release here (pdf).

The system integrator Belectric is situated in Lower Franconia, less than 50km from Würzburg and less than 10km from where I live. Let’s keep our fingers crossed!

Nongeminate Recombination: Langevin (again) and beyond (later;-)

Nongeminate recombination is the major loss mechanism for state-of-the-art organic solar cells. In an early blog post, I showed how the Langevin recombination was derived. Summer in Lower FranconiaAlthough there is more to nongeminate recombination than just this mechanism, it is still instructive and also relevant to trap-assisted recombination mechanisms, due to its mobility-containing prefactor.

[Nenashev 2010] pointed out that in the derivation of the Langevin recombination,

since the electric field scales as r−2 and the surface area of the sphere scales as 2, the value of r chosen is unimportant, leading to a simple solution with constant electron den- sity, thus justifying the neglect of diffusion.

Continue reading “Nongeminate Recombination: Langevin (again) and beyond (later;-)”

Konarka bankrupt

Via Juan Bisquert’s post: solar cell company Konarka filed for bankrupty yesterday according to Businessweek. It is always hard to be the first… Konarka received its first venture capital in mid 2001.

Howard Berke, CEO of Konarka:

This is a tragedy for Konarka’s shareholders and employees and for the development of alternative energy in the U.S.

Let’s hope that our friends from Konarka find other suitable positions, and that other companies such as Heliatek take up the lead!

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Again Links Only

Although I hope I can post something more substantial next week, but do not want to promise what I may not be able to keep…

Organic solar cells:

Science Management and Marketing:

Other Stuff:

German physical society (DPG) spring meeting in Berlin is still ongoing, although I had to leave already. 6000 (mostly german) physicists on the TU Berlin Campus, nice place to be!

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Links

and nothing else.

Photovoltaics:

Global warming:

  • Climate sceptics on the go, via /.: Don’t Worry About Global Warming, Say 16 Scientists in the WSJ; I am no climate scientist, but what I read usually points in the other direction… at least judging from most scientists with peer reviewed publications in contrast to non-peer reviewed “scientists”. Nevertheless, the scientists cited above seem to be real ones, although (mostly?) not with scientific background related to the global climate
  • we have a similar discussion here in Germany, with RWE manager Fritz Vahrenholt writing a book trying to confute evidence of global warming, relating any temperature change to the solar activity: summary by Die Zeit (german, google translate) and an article (again Die Zeit) by Toralf Staud, refuting the seven main theses of Vahrenholt ( german, google translate).

Other stuff:

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Links only

Hi, a link list just to keep you occupied;-)

Next week a round of referees will come to Würzburg to decide on another set of grant proposals, so I’ll go back to my preparations now…

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Pseudosymmetry of the photocurrent physically relevant?

Two days ago, a paper considering the role of the “quasiflat band” case in bulk heterojunction solar cells by device simulations was published online [Petersen 2012]. It is critical of the pseudosymmetric photocurrent found and interpreted by [Ooi 2008] and later also ourselves [Limpinsel 2010]. In order to focus on the physical relevance of the (non)symmetry of the photocurrent, the paper by Petersen et al neglects a field dependent photogeneration. As some good points are raised, read the new paper if you are interested in the photocurrent.

[Update 2.4.2012] Another paper showing that band bending is not needed to explain the particular shape of the photocurrent: [Wehenkel 2012].

I will come back to field dependent photogeneration later, it is still intruiging: also here, the photocurrent should (and will be) complemented by pulsed measurements such as time delayed collection field, see e.g. [Kniepert 2011].

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