Exciton dissociation and charge transfer processes occurring within 10's of nanometers of donor-acceptor interfaces are critical for the performance of organic photovoltaic (OPV) structures. We investigated fundamental issues of exciton dissociation near prototypical donor-acceptor interface using time-resolved two-photon photoemission (TR-2PPE). Phthalocyanine (Pc)-C60 interfaces, with structures known from STM, were formed using organic molecular beam epitaxy (MBE) and Pc ???* (Q-band) transitions were created in these structures by a sub-picosecond pump pulse, producing a population of singlet (S1) Pc excitons. The dynamics of this population were then probed via photoemission by a time-delayed UV pulse. For the donor-acceptor pair of CuPc and C60, the dynamics for excitons created far from the interface could be modeled using a combination of vibrational or intraband relaxation plus intersystem crossing (ISC) to triplet levels on timescales of ~ 1-2 ps. Relaxation in the bulk leads to a triplet (T1) exciton population at significantly reduced energy. Dynamics (decay) for the initial population of Pc S1 excitons excited in the CuPc layer adjacent to C60 were significantly faster than in the bulk due to exciton dissociation with a rate determined to be ~ 7 x 10 12 sec-1. In contrast, T1 excitons created by ISC at the interface were found to dissociate with a rate ~ 500 to 1000 times slower because of relaxation below interfacial charge transfer states. These results provide a picture of the energy dependence of exciton dissociation at this interface. We will also discuss more detailed Pc thickness dependent measurements of the dynamics at analogous H2Pc C60 interfaces that indicate exciton dissociation is limited essentially to the interfacial layer. Dissociation from the 2nd H2Pc layer is reduced by at least a factor of 10.

Exciton dissociation at phthalocyanine-C<sub>60</sub> interfaces

246th National Meeting (2013)

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Exciton dissociation at phthalocyanine-C60 interfaces

Exciton dissociation at phthalocyanine-C₆₀ interfaces