Photoexcited organic molecules en route to highly efficient autoionization

Abstract

The conversion of optical and electrical energies 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'-spirobifluorene (SP6) molecules adsorbed on ZnO(10-10) using femtosecond time-resolved two-photon photoelectron and optical spectroscopies. We find that, after initial ultrafast relaxation on femtosecond and picosecond time scales, an optically dark state is populated, likely the SP6 triplet (T) state, that undergoes Dexter-type energy transfer (r_Dex = 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 k_TTA = 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.