Efficient charge generation from triplet excitons in metal-organic heterojunctions

The success of many emerging molecular electronics concepts hinges on an atomistic understanding of the underlying electronic dynamics. We employ picosecond time-resolved x-ray photoemission spectroscopy (tr-XPS) to elucidate the roles of singlet and triplet excitons for photoinduced charge generation at a copper-phthalocyanine--${\mathrm{C}}_{60}$ heterojunction. Contrary to common belief, fast intersystem crossing to triplet excitons after photoexcitation is not a loss channel but contributes to a significantly larger extent to the time-integrated interfacial charge generation than the initially excited singlet excitons. The tr-XPS data provide direct access to the diffusivity of the triplet excitons ${D}_{\mathrm{CuPc}}=(1.8\ifmmode\pm\else\textpm\fi{}1.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{s}$ (where CuPc is copper-phthalocyanine) and their diffusion length ${L}_{\mathrm{diff}}=(8\ifmmode\pm\else\textpm\fi{}3)\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$.