From two-dimensional electron gas to localized charge: Dynamics of polaron formation in organic semiconductors

Abstract

Electronic transport in organic semiconductors is mediated by localized polarons. However, the dynamics on how delocalized electrons collapse into polarons through electron-nuclear interaction is not well-known. In this work, we use time and angle resolved photoemission spectroscopy (TR-ARPES) to study polaron formation in titanyl phthalocyanine deposited on Au(111) surfaces. Electrons are optically excited from the metal to the organic layer via the image potential state, which evolves from a dispersive to a non-dispersive state after photoexcitation. The spatial size of the electrons is determined from the band-structure using a tight-binding model. It is observed that the two-dimensional electron wave collapses into a wave packet of size ~ 3 nm within 100 fs after photoexcitation.

Excitation-wavelength-dependent small polaron trapping of photoexcited carriers in α-Fe2O3

“…2d. The fit gives an electron-phonon scattering rate of 20-30 ps −1 (30-60 fs) and an initial small polaron bimolecular formation rate of 11 ± 0.5 ps −1 (90 ± 5 fs), similar to previous reports for polaron formation [22][23][24] . The competition between electron-phonon scattering to create phonons, then subsequent removal of electrons and phonons in the generation of polarons, leads to the fast rise in small polaron population followed by a slowing down of polaron formation at ∼3 ps in Fig.…”

“…For transport to occur, phonon motion must bring two Fe centres in proximity, giving the charge carrier a finite probability to hop between atoms. Similar to a mid-gap or surface trap state, the small polaron acts to localize the carrier to a specific site until sufficient thermal energy is present for conduction [22][23][24][25] . Given that polaron formation is accelerated at defect sites, small polaron formation also acts to trap mobile carriers at recombination centres 21 .…”

Multistep and multiscale electron transfer and localization dynamics at a model electrolyte/metal interface

“…Previous ultrafast surface science investigations have shown that the dynamics of an electronic state in a molecular film can differ based on the molecular properties of the film, the electronic state localization, influence of the metal substrate, and molecular film thickness. 8,10,[27][28][29][30][31][32][33][34][35] The contrasting dynamics of electron localization in nonpolar n-heptane versus polar water and ammonia highlight the influence of the molecular properties of the film. In ultra-thin non-polar n-heptane layers on a Ag(111) surface, electron injection leads to the formation of small polarons that are self-trapped via the in-phase methylene rocking of n-heptane on an approximately 360 fs timescale.…”

Effect of Interlayer Coupling on Ultrafast Charge Transfer from Semiconducting Molecules to Mono- and Bilayer Graphene

“…The IPS is distinct from other peaks in the time-resolved spectrum, since it has a negative lifetime (it is pumped by the UV pulse and probed by the visible pulse). The IPS has been studied previously [44,45] and is outside the scope of this work. In this work, we will focus on the S 1 state.…”