Abstract
Interfacial charge transfer from photoexcited ruthenium-based N3 dye molecules into ZnO thin films received controversial interpretations. To identify the physical origin for the delayed electron transfer in ZnO compared to TiO2, we probe directly the electronic structure at both dye-semiconductor interfaces by applying ultrafast XUV photoemission spectroscopy. In the range of pump-probe time delays between 0.5 to 1.0 ps, the transient signal of the intermediate states was compared, revealing a distinct difference in their electron binding energies of 0.4 eV. This finding strongly indicates the nature of the charge injection at the ZnO interface associated with the formation of an interfacial electron-cation complex. It further highlights that the energetic alignment between the dye donor and semiconductor acceptor states appears to be of minor importance for the injection kinetics and that the injection efficiency is dominated by the electronic coupling.
Highlights
The different electronic coupling of the ZnO/N3 interfacial state with the ground N3 molecular state leads to an increased recombination rate, resulting in a smaller electron-transfer efficiency. This is in accordance with the conclusion drawn by Sundström and co-workers[24] who found likewise the cause of a lowered device efficiency of ZnO-based solar cells in an increased recombination rate
Due to the high surface sensitivity, the PES provides a valuable insight into the charge transfer dynamics at dye-semiconductor interfaces
The ZnO and TiO2 films were prepared on FTO glass (TEC7 glass plates, Dyesol)
Summary
Mario Borgwardt[1,2], Martin Wilke[1,2], Thorsten Kampen[3], Sven Mähl[3], Manda Xiao[4], Leone Spiccia[4], Kathrin M. In the range of pump-probe time delays between 0.5 to 1.0 ps, the transient signal of the intermediate states was compared, revealing a distinct difference in their electron binding energies of 0.4 eV This finding strongly indicates the nature of the charge injection at the ZnO interface associated with the formation of an interfacial electron-cation complex. Ultrafast transient absorption spectroscopy has revealed that the injection kinetics in ZnO-based substrates occurs on a different, picosecond, timescale[21,22,23,24,25,26,27,28,29], whereas the charge separation at TiO2 interfaces is by order of magnitudes faster and takes place in a femtosecond time domain[30,31,32,33,34,35,36,37]. N3 molecules are excited by an optical pump pulse, close to the absorption maximum of the dye (λ ~ 530 nm), and the evolution of the excited system is followed by monitoring the transient changes in the electron kinetic energy spectra
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