As promised, here a glimpse of why I believe that recombination in organic bulk heterojunction solar cells under short circuit conditions (and also at Voc) is not necessarily monomolecular.
Sometimes, the short circuit current density vs light intensity is measured, and from the linear scaling a dominant monomolecular recombination is concluded. In (partial) answer, we have performed some relevant device simulations (thanx to wapf). In short, we varied the generation of free charges over four orders of magnitude, assuming different polaron recombination mechanisms.
In the first figure, we assumed monomolecular recombination with different polaron lifetimes, so that the continuity equation for electrons reads . In all cases, even with negligible monomolecular recombination (longer lifetimes), is the scaling linear.
In the second figure we assumed bimolecular (Langevin type) recombination, using the continuity equation . Here, is the Langevin recombination prefactor, proportional to the charge carrier mobility, and is the experimentally found reduction factor [Deibel 2009]. The static part of the latter (for details read the paper) is about 0.1. In the graph we used a range of up till 100. This number (actually, everything above 1) is completely unphysical; we just used it here in order to boost the bimolecular recombination. The simulated open circuit voltage vs light intensity (expressed as generation rate) under standard conditions ( =0.1 (updated 15th May: 10 was typo) and illumination of 1 sun, i.e., G=1028m-3s-1) shows very much a slope 1 behaviour. However, there is no monomolecular recombination present in this simulation. Only at 100 suns you see a slight deviation from slope 1 for the typical value of . Only by boosting the bimolecular recombination unnaturally, can the slope 1/2 behaviour be observed at high illumination intensities.
By the way, experimentally the recombination mechanism in these devices is usually found to be bimolecular. See for instance [Shuttle 2008], [Foertig 2009] or as an overview the review [Deibel 2010] – the latter is now accepted by Reports on Progress in Physics with minor revisions :-)
My conclusion: under typical conditions and for the bulk heterojunction solar cells I know, is it impossible to distinguish from the short circuit current vs light intensity whether or not recombination is monomolecular or bimolecular, respectively. I am interested in your comments!
[Update 2.7.2010] I moved my previous comment from here to a new blog post, with a brief overview on whether or not the whole voltage-dependent photocurrent, as opposed to the short circuit current, gives information on the dominant type of loss in bulk heterojunction solar cells.