Direct observation of ultrafast exciton localization in an organic semiconductor with soft X-ray transient absorption spectroscopy

Abstract

The localization dynamics of excitons in organic semiconductors influence the efficiency of charge transfer and separation in these materials. Here we apply time-resolved X-ray absorption spectroscopy to track photoinduced dynamics of a paradigmatic crystalline conjugated polymer: poly(3-hexylthiophene) (P3HT) commonly used in solar cell devices. The π→π* transition, the first step of solar energy conversion, is pumped with a 15 fs optical pulse and the dynamics are probed by an attosecond soft X-ray pulse at the carbon K-edge. We observe X-ray spectroscopic signatures of the initially hot excitonic state, indicating that it is delocalized over multiple polymer chains. This undergoes a rapid evolution on a sub 50 fs timescale which can be directly associated with cooling and localization to form either a localized exciton or polaron pair.

An on-the-fly deep neural network for simulating time-resolved spectroscopy: predicting the ultrafast ring opening dynamics of 1,2-dithiane

“…Modern light sources and optics have led to a dramatic increase in sophisticated time-resolved experimental techniques that can reveal fine details about the excited-state dynamics of molecules and materials on the atomic scales of time (femtosecond) and length (Angstro ¨m). Methodologies such as ultrafast multidimensional spectroscopy, 1 femtosecond stimulated Raman spectroscopy, 2 ultrafast electron diffraction (UED), 3 and time-resolved X-ray absorption spectroscopy (TR-XAS) 4,5 offer complimentary insights into the competition between different photochemical and photophysical channels, couplings between key vibrational modes and electronic states, and the electronic and nuclear structural dynamics which underpin the functional properties of the molecules and materials under study. However, the complexity of these experiments and their data are such that extracting the dynamics from the experimentally-obtained observables can be extremely challenging and, in practice, often requires strong support from theory and computation.…”

Attosecond imaging of photo-induced dynamics in molecules using time-resolved photoelectron momentum microscopy

“…This barrier is now feasible to overcome due to the new capabilities to produce attosecond x-ray pulses at free-electron lasers [16][17][18] or attosecond XUV pulses using high-harmonics generation sources [19][20][21][22][23][24][25]. These sources have already been successfully employed to study electronic processes with few-femtosecond to attosecond time and atomic-scale space resolution [26][27][28][29][30][31][32][33][34][35][36], and new experimental schemes are being explored theoretically [37][38][39][40][41][42][43][44][45][46][47][48][49]. The interpretation of a signal from attosecond probe pulses is challenging, since they have a broad bandwidth in the energy domain, which smears out spectral lines in a signal.…”

Delivery of stable ultra-thin liquid sheets in vacuum for biochemical spectroscopy

“…With the proliferation of lab-scale high harmonic generation (HHG) sources capable of producing isolated sub-femtosecond X-ray pulses ( Goulielmakis et al, 2008 ) and the recent advent of X-ray free electron laser (XFEL) technology capable of producing the same ( Joseph et al, 2020 ), ultrafast X-ray spectroscopy is able to provide unparalleled access to the electronic states and dynamics of a system during photon initiated reactions with sub-femtosecond resolution. Recent work has elucidated ultrafast exciton localisation in organic semiconductors (and other solid examples) ( Garratt et al, 2022 ), and many experiments have been performed in the gas phase following ionisation or excitation and tracked the subsequent electronic and structural changes ( Attar et al, 2015 ; Schnorr et al, 2019 ).…”