Ultrafast Charge and Triplet State Formation in Diketopyrrolopyrrole Low Band Gap Polymer/Fullerene Blends: Influence of Nanoscale Morphology of Organic Photovoltaic Materials on Charge Recombination to the Triplet State

Figure 7

Representation of the effect of morphology on charge recombination to the triplet state. Free-charge generation (FCG) occurs in 0.2 ps. In a disordered morphology (COARSE), the charge transfer state is characterized by a larger J (exchange integral) and SOCT-ISC can play a more important role. In the well-ordered almost crystalline morphology, the charge transfer state has a smaller J. The contribution of the slower H-HFI interactions will become more important as J diminishes in size. The contribution of the faster SOCT-ISC becomes smaller upon going to the right side of the figure. For free-charge carriers (FCC), J equals zero. Charge recombination from ³CT to T₁ is spin allowed. Note that the effect of optimized morphology (increased organization) on polaron energetics is neglected. More organization can lead to lowering of the energies of the positive polaron, the negative polaron, and the free-charge carriers (FCC) (relative to the coulomb bound charge transfer state) because of delocalization and entropy effects. In principle, photo-induced electron transfer (PET) can also populate the CT states over long ranges (LR) for the optimized morphology.