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.

Another recent paper investigates the role of hot charge transfer complexes for the separation yield of photogenerated polaron pairs – an alternative explanation for the high charge generation efficiency in organic bulk. Lee et al. consider the subgap quantum efficiency and other measures of P3HT:PCBM and PPV:PCBM solar cells [Lee 2010]. Their conclusion:

By varying the excitation wavelengths through the CT band we show that it is the thermally relaxed CT states, not hot CT states, which mediate the conversion between excitons and free charge carriers.

The latter finding contradicts the experiments of Imperial College [e.g., Clarke 2009], in which an exponential dependence of the charge generation yield on the excess energy of CT complexes after singlet exciton dissociation was reported (for optical thin films: no voltage bias).

As you may remember, we suggested last year – based on Monte Carlo simulations – that the driving force for charge separation is mainly due to delocalisation of charges along the conjugated segments of polymer chains (or, possibly, PCBM nanocrystals) [Deibel 2009]. We also did some photocurrent experiments [Limpinsel 2010]. In both cases, we found that the field dependence of the polaron pair dissociation is very weak in the working regime of organic solar cells at room temperature. The latter two points are important, and another one might be added: for good solar cells. We found that at open circuit, the CT complex dissociation probabilty for a given parameter set was about 50%, whereas it was 60% at short circuit. Clearly, a weak field dependence, but nevertheless: 40% “field-independent” loss due to geminate recombination within the narrow field range determined by the fourth quadrant of the current-voltage characteristics.

Our results thus confirm the findings of Street et al, but also (in my opinion) put them in a somewhat larger perspective. Concerning the 2nd paper on hot excitons, I am sure that more work will necessary to determine if (as the work from Imperial college implies) either Voc or jsc can be optimised for a given material, but not both. I am more positive about it (for instance, see [Deibel 2010 CT-review]), nevertheless: experiments to come on the field dependence of charge generation, also at low temperature and also for “bad” solar cells will, be interesting for seeking a more general understanding of polaron pair dissociation and, thus, geminate recombination.

Thanks to Thomas K for pointing me to the Lee paper.

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6 thoughts on “Hot CT complexes and Geminate Recombination”

  1. Hey Carsten,

    Thanks for the post. If you haven’t seen it already, you may be interested in the following paper: J. Materials Chemistry (2009), 19, 4609.

    They deposit P3HT onto ZnO and SAM-modified ZnO and find that the crystallinity of P3HT in proximity to the ZnO layer is changed dramatically, and they argue that the recombination dynamics are modified accordingly. I only just glanced at it, but it seems these findings support the conclusions of your earlier simulations. Hope all is well.

    -AA

  2. I’m having the hardest time with something.

    (1) In polymer (P3HT) : PCBM devices, the geminate recombination of the CT state is the dominant loss mechanism, correct?

    (2) Is that recombination radiative or non-radiative? Nobody seems to talk about the possibility of it being non-radiative.

    1. (1) In annealed rr-P3HT:PCBM, geminate recombination is neither the nor a dominant loss mechanism. Experimentally, the photogeneration is almost independent of electric field and temperature (in contrast to e.g. PPV:PCBM). See for instance [Kniepert 2011] and [Howard 2010] (Update 7.2.2012: should have been this link, [Howard 2010], thanks to Jens for pointing this out). Dominant is nongeminate recombination, as the I(V) reconstruction can be done solely based on bimolecular electron-hole recombination measured by TPV/CE [Shuttle 2010].
      (2) There is not that much information on radiative vs nonradiative recombination around in literature. Usually recombination of free charge carriers is radiative, nonradiative recombination involves traps. Some information is in [Vandewal 2010] and [Kirchartz 2011].

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