We report the site-specific probing of charge-transfer dynamics in a
prototype system for organic photovoltaics (OPV) by picosecond time-resolved
X-ray photoelectron spectroscopy. A layered system consisting of approximately
two monolayers of C60​ deposited on top of a thin film of
Copper-Phthalocyanine (CuPC) is excited by an optical pump pulse and the
induced electronic dynamics are probed with 590 eV X-ray pulses. Charge
transfer from the electron donor (CuPC) to the acceptor (C60​) and
subsequent charge carrier dynamics are monitored by recording the
time-dependent C 1s core level photoemission spectrum of the system. The
arrival of electrons in the C60​ layer is readily observed as a completely
reversible, transient shift of the C60​ associated C 1s core level, while
the C 1s level of the CuPC remains unchanged. The capability to probe charge
transfer and recombination dynamics in OPV assemblies directly in the time
domain and from the perspective of well-defined domains is expected to open
additional pathways to better understand and optimize the performance of this
emerging technology