Tuning non langevin recombination in an organic photovoltaic blend using a processing additive

The effect of altering the acceptor and exchanging a key atom in the polymer structure on the extent of non Langevin (suppressed) recombination has been examined using the polymer/fullerene photovoltaic blend PDTSiTTz:PC60BM. Time of flight data show that changing the acceptor from PC60BM to PC70BM maintains the non Langevin recombination. In contrast, altering the donor polymer by exchanging the silicon bridging atom for a carbon considerably reduces the non Langevin behavior. Importantly, the addition of a processing additive, diiodooctane (DIO), allows a partial recovery of this non Langevin recombination. The addition of DIO also decreases the ionization potential of the polymer, which not only explains the drop in open circuit voltage but may also contribute to the partial recovery of non Langevin behavior observed. It is proposed that localized, more crystalline areas of lower ionization potential (or higher electron affinity) within a mixed/amorphous phase may act as energy sinks for the holes (electrons), thus potentially inhibiting bimolecular recombination. Such a phenomenon could contribute to non Langevin behavior in organic photovoltaic blends.