‘‘Hot or cold’’: how do charge transfer states at the donor–acceptor interface of an organic solar cell dissociate?

erstellt am: 12.10.2015 | von: frank | Kategorie(n):

Electron transfer from an excited donor to an acceptor in an organic solar cell (OSC) is an exothermic
process, determined by the difference in the electronegativities of donor and acceptor. It has been
suggested that the associated excess energy facilitates the escape of the initially generated electron–
hole pair from their mutual coulomb well. Recent photocurrent excitation spectroscopy on conjugated
polymer/PCBM cells challenged this view. In this perspective we shall briefly outline the strengths and
weaknesses of relevant experimental approaches and concepts. We shall enforce the notion that the
charge separating state is a vibrationally cold charge transfer (CT) state. It can easily dissociate provided
that (i) there is electrostatic screening at the interface and (ii) the charge carriers are delocalized, e.g. if
the donor is a well ordered conjugated polymer. Both effects diminish the coulomb attraction and
assure that the in-built electric field existing in the OSC under short current condition is already
sufficient to separate most the CT states. The remaining CT excitations relax towards tail states of the
disorder controlled density of states distribution, such as excimer forming states, that are more tightly
bound and have longer lifetimes.

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