  Organic/inorganic hybrid systems offer great potential for novel solar cell
design combining the tunability of organic chromophore absorption properties
with high charge carrier mobilities of inorganic semiconductors. However, often
such material combinations do not show the expected performance: while ZnO, for
example, basically exhibits all necessary properties for a successful
application in light-harvesting, it was clearly outpaced by TiO$_2$ in terms of
charge separation efficiency. The origin of this deficiency has long been
debated. This study employs femtosecond time-resolved photoelectron
spectroscopy and many-body ab initio calculations to identify and quantify all
elementary steps leading to the suppression of charge separation at an
exemplary organic/ZnO interface. We demonstrate that charge separation indeed
occurs efficiently on ultrafast (350 fs) timescales, but that electrons are
recaptured at the interface on a 100 ps timescale and subsequently trapped in a
strongly bound (0.7 eV) hybrid exciton state with a lifetime exceeding 5
$\mu$s. Thus, initially successful charge separation is followed by delayed
electron capture at the interface, leading to apparently low charge separation
efficiencies. This finding provides a sufficiently large timeframe for
counter-measures in device design to successfully implement specifically ZnO
and, moreover, invites material scientists to revisit charge separation in
various kinds of previously discarded hybrid systems.
