Distance Dependence of Exciton Dissociation at a Phthalocyanine-C60 Interface

Exciton dissociation at donor-acceptor (DA) interfaces is critical for the operation of organic photovoltaic (OPV) devices, yet a detailed physical understanding of this process is lacking. This work examines an important aspect of this process, namely the dependence of the exciton dissociation rate on distance from the DA interface. Time-resolved two-photon photoemission (TR-2PPE) measurements were performed on bilayer H2Pc\C-60 heterojunctions fabricated using organic molecular beam epitaxy (MBE) with varying H2Pc thickness. In the measurements, the dynamics of the H2Pc S-1 exciton population created with a 1.55 eV pump pulse were monitored via photoemission with a delayed UV probe pulse. The depth sensitivity of TR-2PPE, due to the short electron attenuation length, provides the means to follow excited state dynamics as a function of H2Pc thickness. Analysis of the S-1 population decay as a function of H2Pc thickness revealed that the electron transfer rate for the first H2Pc layer, adjacent to C-60, is k(CT) = (2.3 +/- 0.4) X 10(12) s(-1). Exciton dissociation is reduced by a factor of at least 10 for the second H2Pc layer and beyond.