  Earlier reports demonstrated that ternary organic solar cells (OSC) made of
donor polymers (D) blended with different mixtures of fullerene acceptors (A:A)
performed very similarly. This finding is surprising, as the corresponding
fullerene LUMO levels are slightly different, which might result in decisive
differences in the charge transfer step. We investigate ternary OSC (D:A:A)
made of the donor polymer P3HT with stoichiometric mixtures of different
fullerene derivatives, PC60BM:PC70BM and PC70BM:IC60BA, respectively. Using
quantitative electron paramagnetic resonance (EPR) we can distinguish between
positive and negative polarons, localized on the specific molecules. We found
that after the initial charge transfer step, the electrons are re-distributed
over two nearby acceptors in agreement with their stoichiometry and their
relative LUMO energy difference. Remarkably, the measured delta LUMO
differences in fullerene mixtures are reduced by an order of magnitude compared
to that of the pristine materials, i.e., below 1 meV for PC60BM:PC70BM and (20
+/- 5) meV for PC70BM:IC60BA. Furthermore, we found that this reduced delta
LUMO explains the shift in open circuit voltage for D:A:A organic solar cells.
We attribute these findings to hybridization, leading to an effective fullerene
LUMO. Consequently, multi-acceptor blends are indeed a viable option for
photodetectors and solar cells, as they combine the best electron acceptor and
light absorbing properties.
