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!

Add to Connotea

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:

sciseekclaimtoken-4f7073f2d8ca8

Add to Connotea

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…

Add to Connotea

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].

Add to Connotea

Charge transport in disordered organic matter: hopping transport

As I won a proposal today, I feel up to contributing once again some physics to this blog… I know, it has been a long long wait. So today it is time to consider some fundamentals of charge transport, as this is not only important for the extraction of charge carriers from the device PV in Japan(see earlier posts on mobility and efficiency, surface recombination velocity and photocurrent) but also the nongeminate recombination (see e.g. photocurrent part 2 and 3).

In disordered systems without long range order – such as an organic semiconductor which is processed into a thin film by sin coating – in which charge carriers are localised on different molecular sites, charge transport occurs by a hopping process. Due to the disorder, you can imagine that adjacent molecules are differently aligned and have varying distances across the device. Then, the charge carriers can only move by a combination of tunneling to cover the distance, and thermal activation to jump up in energy. In the 1950s, Rudolph A. Marcus proposed a hopping rate (jumps per second), which is suitable to describe the local charge transport. By the way, he received the 1992 Nobel prize in chemistry for his contributions to this theory of electron transfer reactions in chemical systems. Continue reading “Charge transport in disordered organic matter: hopping transport”

2012

Hi there, I am late again, but nevertheless: a happy and successful year 2012!

I have collected a few links which might or might not interest you. Also, I plan to start with some scientific (background) posts again. Locust in ItalyLet’s see how this works out:-)

Press release of Heliatek: Heliatek achieves new world record for organic solar cells with certified 9.8 % cell efficiency. Evaporated small molecule tandem with area above 1cm2. Very good! Also, Mitsubishi Chemical has reached 10.1% efficiency on solution processed small molecules.

Nature looks back at the science year 2011: 365 days: Images of the year.

Interesting, although not related to physics: Syllabus for David Foster Wallace’s class “English 102-Literary Analysis: Prose Fiction Fall ’94”. Clear rules, yeah! Forgot who linked to it, sorry. Continue reading “2012”

SPIE Pickings

Already 8 weeks past, recently some Videos (well, stills of the slides plus audio) of the Solar and LED Session of the SPIE Optics and Photonics 2011, San Diego went online.

Happy Family: Apes in Khandala, MaharashtraHere are two or three which might interest you (well, they got my attention;-) but there is more to be found on the above mentioned web site – although I had to modify the settings of my ad blocker to be able to watch. No, there are no ads; still…

Before you scroll down, let me mention some other “findings” of potential interest:

But now to these SPIE presentations [Update: WordPress does not accept the embedded vidos, so here just the links to the videos].

James Durrant, Imperial: Charge photogeneration and recombination in organic solar cells

Continue reading “SPIE Pickings”

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”