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 copperphthalocyanine-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-CuPc= (1.8 +/- 1.2) x 10(-5) cm(2)/s (where CuPc is copper-phthalocyanine) and their diffusion length Lin(diff) = (8 +/- 3) nm.