Wednesday, July 21, 2010

Deconstructing claims about multiple carrier generation in quantum dots

Over the past few years I have heard claims being made that semiconductor quantum dots could be used to make efficient solar cells because they allow multiple carrier generation. The basic idea is that a high energy photon produces multiple excitons (as many as 7 had been claimed) and each exciton then decays into an electron and hole. Thus the quantum yield of charge carriers could be more than hundred per cent and significantly enhance the photocurrent in a solar cell. A review is here.
Sounds exciting. Well .....

I learnt from Eran Rabani yesterday that it turns out that these claims of large quantum yields are based on a mis-interpretation of experimental data. Skimming the literature I found this recent paper which notes:
uncontrolled photocharging of the nanocrystal core can lead to exaggeration of the Auger decay component and, as a result, significant deviations of the apparent [carrier multiplication] CM efficiencies from their true values. Specifically, we observe that for the same sample, apparent multiexciton yields can differ by a factor of 3 depending on whether the nanocrystal solution is static or stirred.
I guess it goes back to "extra-ordinary claims require extra-ordinary evidence". It cannot be emphasized enough the importance of checking results and on considering alternative explanations for data, rather than uncritically embracing the interpretation we (or others) hope to be true.

1 comment:

  1. Thanks for the tip of that new article, very interesting.

    Like the blog also,
    keep it up!