• Maimaris, M.; Pettipher, A.J.; Azzouzi, M.; Walke, D.J.; Zheng, X.; Gorodetsky, A.; Dong, Y.; Tuladhar, P.S.; Crespo, H.; Nelson, J.; Tisch, J.W.G.; Bakulin, A.A.: Sub-10-fs observation of bound exciton formation in organic optoelectronic devices. Nature Communications 13 (2022), p. 4949/1-10

Open Access Version

Fundamental mechanisms underlying exciton formation in organic semiconductors are complex and elusive as it occurs on ultrashort sub-100-fs timescales. Some fundamental aspects of this process, such as the evolution of exciton binding energy, have not been resolved in time experimentally. Here, we apply a combination of sub-10-fs Pump-Push-Photocurrent, Pump-Push-Photoluminescence, and Pump-Probe spectroscopies to polyfluorene devices to track the ultrafast formation of excitons. While Pump-Probe is sensitive to the total concentration of excited states, Pump-Push-Photocurrent and Pump-Push-Photoluminescence are sensitive to bound states only, providing access to exciton binding dynamics. We find that excitons created by near-absorption-edge photons are intrinsically bound states, or become such within 10 fs after excitation. Meanwhile, excitons with a modest >0.3 eV excess energy can dissociate spontaneously within 50 fs before acquiring bound character. These conclusions are supported by excited-state molecular dynamics simulations and a global kinetic model which quantitatively reproduce experimental data.