We developed the first time-resolved electric field induced sum frequency generation spectroscopy (EFI-SFG), which we used to investigate charge transfer dynamics at interfaces between organic semiconductor (P3HT) and metal (gold) (Fig.1a). We found direct charge transfer dynamics exist at interfaces. Using 800 nm pulse as the pump, time-dependent EFI-SFG signals were observed. (Fig.1b) The positive signal at large time delay is a result of electric field induced SFG signal, indicating charge separation occurs at interfaces. Control experiments on bare gold, and P3HT on SiO<sub>2</sub> show negligible dynamics, which demonstrates that there is no multi-photon absorption occurring at interfaces and the dynamics only exist when P3HT/Au interfaces are formed. Since one photon energy from 800 nm is not enough to directly excite P3HT, the 800 nm pump must excite a smaller band gap, i.e, the interfacial band-gap between P3Ht and gold, ~1.5 eV. By changing the polarization of 800 nm pump pulse from p to s, the time-resolved EFI-SFG signal disappeared. The difference between absorption cross sections of 800 nm light at p and s polarization is not large enough to explain this strong polarization dependence. Instead, since direct charge transfer from the Fermi level of gold to the conduction band of P3HT should interact strongly to electromagnetic fields perpendicular to the interfaces, whereas pulses parallel to the interfaces have little probability to promote charges from gold to P3HT directly, the strong dependence on 800 nm pump’s polarization indicates that a direct interfacial charge transfer pathway from Au to P3HT exists.<br/>To the best of our knowledge, this is the first time that direct charge transfer is observed at a solution-phase processed interface. This new charge transfer mechanism can open new ways of harvesting light, in particular at near IR spectral range.

Direct charge transfer at organic semiconductor/metal interfaces

253rd National Meeting (2017)

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