(Invited) Ultrafast X-Ray Studies of Interfacial Energy- and Charge-Transfer Dynamics

Abstract

The success of many emerging molecular electronics concepts hinges on an atomistic understanding of the underlying electronic dynamics. Processes evolving on spatial and temporal scales spanning orders of magnitude have to be connected in order to gain a comprehensive understanding of the fundamental dynamics and scaling laws that enable molecular, interfacial, and macroscopic charge and energy transport. Time-domain X-ray spectroscopy techniques have the potential to provide a deeper understanding of electronic dynamics in complex, heterogeneous systems owing to their elemental site specificity and sensitivity to local valence electron configurations. We present femtosecond to picosecond time-resolved X-ray photoelectron spectroscopy (TRXPS) studies of photoinduced charge transfer dynamics in nanoporous films of N3 dye-sensitized ZnO and bilayer heterojunctions consisting of copper phthalocyanine (CuPc) electron donors and C60 acceptors. For the N3/ZnO system, differential TRXPS line shifts provide site-specific access to the locations of intermittently trapped electrons, interfacial dipoles, as well as charge delocalization and recombination dynamics. For the CuPc/C60 heterojunction, a deeper understanding of the predominant energy transport and charge generation mechanisms is achieved. Contrary to common belief, fast intersystem crossing from initially excited singlet excitons to triplet excitons within the bulk of the donor domain is not a loss channel but the triplet excitons contribute to a significantly larger extent to the time-integrated interfacial charge generation than the initially excited interfacial singlet excitons. The TRXPS data provide direct access to the diffusivity of the triplet excitons in the CuPc donor domain DCuPc = (1.8 ±1.2) × 10−5 cm2/s and their diffusion length Ldiff = (8 ± 3) nm. Preliminary TRXPS results will be presented for a model system for plasmon-enabled photocatalytic nano-assemblies consisting of a nanoporous TiO2 layer sensitized with gold nanoparticles (Au NPs). Clear differences between the temporal responses of the Au4f and Ti2p photolines are observed, indicating both the interfacial site-specificity of photoinduced electronic dynamics and the capacity of TRXPS to distinguish them. Figure 1