Just a quick addition to Mobility and Efficiency of Polymer Solar Cells. You might remember that with increasing mobility, the
open circuit voltage Voc, however, decreases steadily. Actually, the slope steepness is maximum due to our implicit assumption of ideal charge extraction ; for a realistic charge extraction (= finite surface recombination), the Voc slope with mobility is weaker… or even constant for zero surface recombination. The fill factor is maximum at intermediate charge carrier mobilities, not far from the experimentally found values!
As we were finally able to calculate the open circuit voltage with a surface recombination less than infinity (thanks to Alexander Wagenpfahl),
I can show you how it looks. ([Update 3rd March 2010] For details, have a look here: [Wagenpfahl 2010, arxiv])
In the figure, the power conversion efficiency is again plotted vs. charge carrier mobility. Here, in we made the assumption that the majorities (=electrons at the electron injecting contact, or holes at the hole injecting contact) maintain an infinite surface recombination velocity, whereas the minorities (=electrons at the hole injecting contact, and …) have a surface recombination velocity Smin of 1050m/s (=infinity in terms of the simulation) or 10-4m/s. As you can see, the assumption of infinite surface recombination for electrons and holes leads to the reduction of the power conversion efficiency. This effect comes almost exclusively from a reduction of the open circuit voltage. If the minority surface recombination velocity is lowered drastically, than the open circuit voltage does not break down for high mobilities, and the power conversion efficiency remains high… but does not increase much any more.
There is almost no work an surface recombination in organic solar cells, essentially only the model by Scott and Malliaras [Scott 1999]. For typical mobilities, the surface recombination velocity after their considerations comes out at 10-2m/s or so. Nevertheless, experimental work is lacking so far. Taking into account that conjugated polymers and such do not have dangling bonds, however, a low surface recombination velocity is certainly more probable than a high one. Thus, the publications predicting a lowering of the efficiency at high mobility [Mandoc 2007, Deibel 2008] should be reconsidered (I’d like to point out that we already mentioned the effect, but were not able to calculate it at that time… ;-). Recently, a finite surface recombination influencing the efficiency was considered by [Kirchartz 2008].