Probing ultrafast charge separation at organic donor/acceptor interfaces by a femtosecond electric field meter

Charge separation at organic donor/acceptor interfaces is a key step in the functioning of an organic photovoltaic cell, but experimental techniques that uniquely probe the interface on ultrafast time scales are absent. Here, we apply a technique sensitive to interfacial charge separation based on four-wave mixing to follow the transient electric field at the donor/acceptor interface. Examining the model system of copper phthalocyannine (CuPc) and fullerene (C-60) bilayers, we observe ultrafast electron transfer from photoexcited CuPc to C-60 on a 80 +/- 20 fs time scale. (C) 2011 American Institute of Physics. [doi:10.1063/1.3632062]