Photocurrent again

I covered the photocurrent already before, for instance here. Market Place in Funchal, Madeira I pointed out that from the light intensity dependence of the short circuit current, it is impossible for many typical conditions to unambiguously determine the dominant loss mechanism or even the recombination order (1st (often called monomolecular, but not my favourite term;-) or 2nd order of decay).

If, however, you know (or guess) that the recombination order is two, you can use the above mentioned j_{sc} vs. P_L data to determine which fraction of charges is lost to bimolecular recombination, \eta_{br}. This was shown recently by [Koster 2011]. For j_{sc} \propto P_L^\alpha, they found \eta_{br} = \alpha^{-1}-1. Although I was not able to follow the exact derivation ([Update 5.4.2011] it can be derived by solving a simple differential equation, \alpha=(1-dR/dG)/(1-R/G)=(1-\eta_{br}')/(1-\eta_{br})), it seems to work. Easy method, although make sure not to have too much space charge in your device – even at the contacts, induced by low (ohmic) injection barriers (we compared it to our device simulation, and then you get significant deviations)! In my opinion, the latter point is not stressed enough in the paper, despite the nice approach. Continue reading “Photocurrent again”

This and that

Kid is growing, lack of sleep makes euphoric, but less time is less time;-)

The 2010 impact factors were just released by Thomson Reuters, Sunset at Ammerseeas most of you will know due to the mails sent by almost all publishers to tell about recent boosts of impact for their journals. A sober post was written by Jörg Heber, editor of Nature materials. A brief quote

So what use is the impact factor number? Well, being cynical one could say it is a quick measure for those that don’t read the journals but still want to know how good they are on average. The danger is of course that this is then used as a kind of metric to assess the quality of research or to decide on the career of researchers.

Continue reading “This and that”

Blue suits him better than pink

Children change the life, how very true. Not that I am less interested in Science in general, kleine HändeI do enjoy it! Nevertheless, somehow work seems less important these days – which maybe I should not admit openly ;-)

I received this statement,

Blue suits the lecturer better than pink

as one of the results of the lecture evaluation (Atom Physics for “Teachers to be”). Yes, I also received some other comments, most positive, some negative, all useful (including that one?;-)

Just to say that I am still amongst the living, here some bits and pieces I found during the last weeks, when time allowed. Continue reading “Blue suits him better than pink”

2011

A happy and successful new year to you! It is almost three years since I started this blog, this being the 69th post. A lot happened in this time, also for me: both personally (as some of the long term readers now;-) and professionally (despite still being in Würzburg;-). Golden Pavilion (Kinkaku-ji) So, let me thank you, valued reader – and comments contributor, an active participation which I highly appreciate!

Many things I want to write about I have not had time to handle in the recent months. For now, let me start with just briefly revisiting what I have written. Hints of what I will add in the coming weeks and months are to come soon (soon meaning: worst case mid February, as one proposal is submitted by then, lecture is finished and project meeting / seminar talk marathon “finished”;-).

Find the overview below. Continue reading “2011”

Two notes

A few weeks ago, Heliatek managed to take the lead for organic solar cell efficiencies, achieving 8.3% confirmed power conversion efficiency on 1.1cm2 active area with vacuum deposited small molecules. Madeira Rainbow in AutumnThe device was a tandem. Thomas Körner, VP of Sales, marketing and Business Development at Heliatek, added

The first products should be coming onto the market at the start of 2012.

Good!

Second, you may remember my post on photocurrent in organic solar cells back in July. It was inspired by a comment I wrote on a paper by Street et al, who proposed monomolecular recombination to dominate the loss of free charges in organic bulk heterojunction solar cells. My comment and Bob Street’s reply to it are now online at Phys Rev B. I’ll not comment this interesting exchange any further (unless requested by you;-), so read and think for yourself!

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Hot CT complexes and Geminate Recombination

Lately, the notion that geminate recombination in organic solar cells is a major loss mechanism is more and more under fire. Foothill MountainsStreet et al present an “experimental test” for geminate recombination [Street 2010a]. They investigate P3HT:PC60BM nor PCDTBT:PC70BM bulkheterojunction solar cells with a transient current technique at 200K and 300K between -1 and 1V external voltage bias. The authors conclude that neither exhibit significant geminate recombination, while pointing out that

Since the relative importance of geminate or nongeminate recombination depends on the specific materials comprising the cell and possibly on the method of preparation, other cells may or may not have a larger geminate recombination contribution.

Continue reading “Hot CT complexes and Geminate Recombination”

Efficiencies and other notes

I mentioned the record bulkheterojunction solar cell from Solarmer recently:On top of the others 8.13%, although on a small area of 0.1cm2. The evporated small molecule solar cells had almost 6% on a ~10 times larger area. On the SPIE Optics&Photonics conference in August in San Diego I heard inofficially that Heliatek achieved more than 6%, but now on foil. Even better: more than 7% (active area efficiency; about one percent-point less for the complete area) on a module with more than 70cm2! This one is not flexible, I believe. Amazing if you consider that the evaporation is by point sources. If these modules are encapsulated, they are said to have an extrapolated lifetime exceeding 10 years.

Continue reading “Efficiencies and other notes”

Brief Ad: Organic Solar Cell Review Online [Update]

Sunset in UmbriaIf interested, find it here (Reports on Progress in Physics 73, 096401 (2010)). Included: how do bulk heterojunctions and bilayers work, how to improve the performance, how to mass print, and a brief section on the cost. I am happy it is finally “on air”:) Free from IOP for the first 30 days, if you register. Otherwise, choose the arXiv version or drop me a line. As I am on vacation, expect some delay…

[Update 30.8.2010 ] Back from vacation for already a week: was very relaxing:) In order to avoid another “ad post”, I just extend this one a bit: the progress report on charge transfer complexes (submitted to Advanced Materials already in February) is now published online. You will not find this one on arXiv, so if you cannot access it, ask me to send you the preprint. As always, I am interested in your opinion and/or criticism!

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New Record for Organic Solar Cells and other stuff

Solarmer did it again: 8.13% power conversion efficiency, certified by NREL, were anounced at the currently running SPIE Optics and Photonics conference in San Diego.

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.

Foothill Mountains - Russian RidgeIn 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.

Continue reading “New Record for Organic Solar Cells and other stuff”

Photocurrent in organic solar cells – Part 2 [Update]

Almost a year ago, I already discussed the photocurrent in organic bulk heterojunction solar cells. Also, recently I posted about the difficulties to determine the dominant loss mechanism from the short circuit current density dependence on the light intensity. PhotocurrentToday, I would like to extend these statements to the photocurrent in somewhat more general terms.

The figure to the right contains the simulated photocurrent for a bulk heterojunction solar cell of 100nm thickness at room temperature. Parameters were chosen according to typical experimentally determined values for P3HT:PCBM solar cells: Bimolecular Langevin recombination with a reduction factor of 0.1 and electron and hole mobility of 10-4m2/Vs were assumed (is it possible I never discussed this reduction really? Seems so, just mentioned it with references here). The top graph shows the photocurrent, in the lower graph the photocurrent was divided by the illumination density in terms of suns (thus, the current densities given on the y-axis are only correct for 1 sun). Consequently, if the photocurrent scales linearly with the light intensity, all curves should coincide. Let me remind you that this was interpreted by different groups (Street et al. among them, but not the first to follow this explanation) as a sign of first order recombination.
Continue reading “Photocurrent in organic solar cells – Part 2 [Update]”