Sesha Vempati, Lea Bogner, Clemens Richter, Jan-Christoph Deinert, Laura Foglia, Lukas Gierster, and Julia Stähler:
J. Chem. Phys. 152 (7), 074715 (2020), pp.10;
arXiv:1911.04333 [cond-mat.mtrl-sci] (2019), pp. 22;
DOI: arXiv:1911.04333 [cond-mat.mtrl-sci]
The conversion of optical and electrical energy in novel materials is key to modern optoelectronic and light-harvesting applications. Here, we investigate the equilibration dynamics of photoexcited 2,7-bis(biphenyl-4-yl)-2,7-ditertbutyl-9,9-spirobiuorene (SP6) molecules adsorbed on ZnO(10-10) using femtosecond time-resolved two-photon photoelectron (2PPE) and optical spectroscopy. We find that, after initial ultrafast relaxation on fs and ps timescales, an optically dark state is populated, likely the SP6 triplet (T) state, that undergoes Dexter-type energy transfer (rDex=1.3 nm) and exhibits a long decay time of 0.1 s. Because of this long lifetime a photostationary state with average T-T distances below 2 nm is established at excitation densities in the 1020 cm-2 s-1 range. This large density enables decay by T-T annihilation (TTA) mediating autoionization despite an extremely low TTA rate of kTTA=4.5 10-26 m3 s-1. The large external quantum efficiency of the autoionization process (up to 15 %) and photocurrent densities in the mA~cm-2 range offer great potential for light-harvesting applications.
The original publication is available by link DOI: 10.1063/1.5136075