Abstract
Donor-π-acceptor conjugated polymers form the material basis for high power conversion efficiencies in organic solar cells. Large dipole moment change upon photoexcitation via intramolecular charge transfer in donor-π-acceptor backbone is conjectured to facilitate efficient charge-carrier generation. However, the primary structural changes that drive ultrafast charge transfer step have remained elusive thereby limiting a rational structure-function correlation for such copolymers. Here we use structure-sensitive femtosecond stimulated Raman spectroscopy to demonstrate that π-bridge torsion forms the primary reaction coordinate for intramolecular charge transfer in donor-π-acceptor copolymers. Resonance-selective Raman snapshots of exciton relaxation reveal rich vibrational dynamics of the bridge modes associated with backbone planarization within 400 fs, leading to hot intramolecular charge transfer state formation while subsequent cooling dynamics of backbone-centric modes probe the charge transfer relaxation. Our work establishes a phenomenological gating role of bridge torsions in determining the fundamental timescale and energy of photogenerated carriers, and therefore opens up dynamics-based guidelines for fabricating energy-efficient organic photovoltaics.
Highlights
Donor-π-acceptor conjugated polymers form the material basis for high power conversion efficiencies in organic solar cells
It is imperative to understand the fundamental limits of charge transfer (CT) timescales and the energy associated with these CT states in the context of a given donor−acceptor backbone by tracking exciton dynamics
Recent work from various groups have suggested that in donor-π-acceptor copolymers where the donor and acceptor moieties are separated by the π-bridge, intramolecular charge transfer (ICT) character in the excited state with large dipole moment promotes the formation of polaron pairs, the precursor state for charge carriers[12, 13]
Summary
Donor-π-acceptor conjugated polymers form the material basis for high power conversion efficiencies in organic solar cells. Multiple studies have indicated that the instantaneously generated hot excitons in bulk heterojunction solar cells have large delocalization lengths which lead to coherent generation of charges at polymer:fullerene interfaces[15,16,17,18,19,20,21] These photogenerated hot excitons on pristine homopolymer backbone can go through various temporally segregated relaxation pathways: exciton self-trapping in 30−100 fs, local torsional relaxation within 200−500 fs, and excitonic energy transfer in one to few tens of picoseconds[22,23,24,25,26,27,28,29].
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
