Speaker
Description
Understanding and controlling ultrafast vibronic dynamics of conjugated donor polymers is crucial for further advancing exciton transport and charge separation in organic solar cells (OSCs). In this contribution, we demonstrate the first application of the chirp-dependent time-resolved (CDTR) photoluminescence double-pump technique to relevant donor materials: PM6, PBDB-T, and PBDB-T thin films with added Chloronaphthalene (CN) [1].
By employing programmable pulse shaping with a pair of phase-locked femtosecond pulses with one transform-limited (TL) and one linearly chirped pulse, we are able to record two-dimensional CDTR photoluminescence maps under near-resonant excitation of the (intra-molecular) charge-transfer state. This approach enables us to selectively prepare and investigate vibronic coherences within the excited state of the material under investigation.
In all materials investigated here, pronounced delay-dependent oscillations with periods of 60–80 fs are found to be related to low-frequency vibrational modes of 0.05–0.08 eV associated with torsional and collective motions of the polymer backbone. The oscillation amplitude and phase are found to be strongly dependent on the chirp value and sign. This indicates that the temporal order of spectral components determines the generation of the initial wavepacket. In contrast to this finding, the contribution of high-frequency C=C stretching modes to the CDTR signal is found to be negligible despite being prominent features of the linear absorption spectrum. This underlines the intrinsic selectivity of this technique for low-frequency modes.
In addition to these common features of the investigated materials, material-dependent differences are found for PM6 with a more pronounced and longer-lived coherence related to its higher structural order compared to PBDB-T. Pristine PBDB-T shows faster dephasing of the oscillation, which improves upon CN doping with enhanced contrast and coherence lifetime due to improved π–π stacking and reduced disorder [2].
The results are supported by simulations of the CDTR signal with a two-level excitonic model coupled to an effective vibrational coordinate and clearly show the interference between the chirped pulse phases and the generation of the wavepacket to be responsible for the population of the excited state. This contribution establishes chirp-dependent double-pump spectroscopy as a powerful tool for tailoring low-frequency vibronic coherence in state-of-the-art OSC donor polymers.
References
[1] Xu, Xin-peng, Richard Hildner, and Elisa Palacino-González. "Selective manipulation of low-frequency modes in conjugated polymers via chirped pulses." The Journal of Chemical Physics 163.17 (2025).
[2] Kroh, Daniel, et al. "Identifying the signatures of intermolecular interactions in blends of PM6 with Y6 and N4 using absorption spectroscopy." Advanced Functional Materials 32.44 (2022): 2205711.
[3] Wang, Xinkang, et al. "High‐efficiency (16.93%) pseudo‐planar heterojunction organic solar cells enabled by binary additives strategy." Advanced Functional Materials 31.33 (2021): 2102291.