Ultrafast Charge Generation in an Organic Bilayer Film

The dynamics of charge generation in a high performing molecular photovoltaic system, p-SIDT(FBTTh2)(2) (see Figure 1) is studied with transient absorption. The optimized bulk heterojunction material shows behavior observed in many other systems; the majority of charges are generated at short time scales (<150 fs), and a slower contribution from incoherently diffusing excitons is observed at low pump fluence. In a separate experiment, the role of bulk heterojunction material morphology on the process of ultrafast charge generation was investigated with bilayers made with solution processed donor molecules on a photopolymerized C-60 layer. The majority of carriers are again produced at short time scales, ruling out the idea that subpicosecond charge generation can be understood wholly in terms of localized excitons. We evaluate possible causes of this behavior and propose that the excited state is highly delocalized on short time scales, providing ample probability density at the charge generating interface.