Comments? I just changed the comment settings: up to now, a WordPress account was needed in order to be able to comment on my posts. Believe me or not, I did not even know about this setting till today. Now, you only have to give your (or some;-) name and an email address (yours? I do not know). Maybe this lowers the barrier for some of you to ask questions or provide further insights and critical views.

Tomorrow is abstract deadline for the SPIE Optics and Photonics 2011, including the session Organic Photovoltaics XII. See you there:-)

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Comment on Primary Photoexcitation in Polymer:Fullerene Blends

It seems that one prominent discussion in organic photovoltaics has officially ended, the one about the primary photoexcitation in disordered organic solar cells being excitons (with a binding energy clearly above 100meV) or free charges (with excitons having binding energies in the range of the thermal energy, i.e. <<100meV). Hwang, Moses and Heeger, have just published a paper on polymer:fullerene blends [Hwang 2008] where they describe the charge generation as

Mobile carriers are generated via a two-step process: initial ultrafast charge separation to an intermediate charge transfer (CT) bound state, followed by the transfer of carriers onto the bicontinuous networks.

They explicitly mention

[…] indicating ultrafast dissociation of the singlet excitons at the polymer-PCBM interface and the build-up of the initial CT state.

The paper is nice but in itself not that remarkable, except that previously, Moses and Heeger always claimed the primary photoexcitation to be free charges instead of bound excitons. Their measurements yielded exciton binding energies in the range of the thermal energy, i.e., no donor acceptor interface being necessary for charge separation. To quote an older paper [Moses 2000],

Thus, carriers are photoexcited directly and not generated via a secondary process from exciton annihilation.

Now I have to mention that in the new paper they use P3HT:PCBM, and in the old one MEH-PPV:PCBM. But as they do not mention this in the new paper, I assume that either I missed something, or they changed their point of view concerning the primary photoexcitation.

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Comments on Estimates on the Efficiencies of Organic Tandem Solar Cells

Ideally, tandem solar cell made of a series connection of two subcells work as follows. tandem.png Both sub cells generate their own photocurrent by absorbing light and generating charges (as described for single layer cells in here), and have their own open-circuit voltage. Of course, as the two cells are connected in series, they influence each other. The photogenerated holes of cell 1 are extracted by the ITO, but where to the electrons go? They have to recombine with photogenerated holes from cell 2: that is what the intermediate recombination layer is for. If the photocurrent of sub cells 1 and 2 is initially unbalanced, the electric field is redistributed, such that the photocurrent becomes balanced… at a lower value, approximately determined by the worse of the two cells. The open-circuit voltage is aproximately the sum of both sub cells’ open circuit voltages. Of course, in cases of field redistribution, that does not quite hold true. So, what approximately happens in a tandem solar cell of subcells 1 and 2:

  • open circuit voltage Voc = Voc1 + Voc2
  • short circuit current Isc = min(Isc1, Isc2)
  • fill factor is more difficult, but as a rough guide lets stick to the minimum of both as well

So for an ideal tandem solar cell, complementary absorption ranges, and balanced photocurrents are needed.

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