TR-2PPE Studies of Ultrafast Charge Separation at Organic Photovoltaic Interfaces

Dissociation of excitons in organic photovoltaic (OPV) devices occurs exclusively at interfaces between donor and acceptor molecular materials. To help understand critical charge separation processes, we have performed time-resolved two-photon photoemission (TR-2PPE) studies of sub-picosecond exciton dynamics at well-characterized organic donor-acceptor interfaces. Interfaces between phthalocyanines and C$_{60}$ were engineered using organic MBE and characterized using STM, STS, and UPS. Ultrafast TR-2PPE measurements were performed on CuPc$\backslash $C$_{60}$ structures by pumping the lowest optical $\pi \to \pi $* transitions (Q-band) to generate CuPc singlet (S$_{1})$ excitons and probing this population with a time-delayed UV pulse. For thick films, CuPc S$_{1}$ decay is dominated by vibrational relaxation (several 100's femtoseconds) and singlet-to-triplet conversion ($\sim $ 1 picosecond). Directly at the interface, however, charge transfer to C$_{60}$ dominates decay of S$_{1}$ exciton, ($\sim $ 100 femtoseconds) . We also find evidence for important recombination routes from the charge separated state back to lower-lying CuPc T$_{1}$ triplet excitons. To test the impact of intersystem crossing to triplet levels, we have performed analogous investigations for H$_{2}$Pc$\backslash $C$_{60}$ interfaces. Results for this interface will be compared and contrasted with the CuPc$\backslash $C$_{60}$ case.