Raman Snapshots of Side-Chain Dependent Polaron Dynamics in PolyThiophene Films

Area of Science:

• Materials Science
• Photovoltaics
• Polymer Chemistry

Background:

• Photogenerated polarons are crucial for charge generation in organic photovoltaic (OPV) devices.
• Efficient charge generation relies on understanding the link between film morphology and polaron formation.
• Tracking ultrafast polaron dynamics in films is essential but challenging.

Purpose of the Study:

• To investigate polaron generation dynamics in crystalline poly(3-hexylthiophene) (P3HT) and amorphous poly(3-(2-ethylhexyl)thiophene-2,5-diyl) (P3EHT) films.
• To elucidate the role of film morphology in ultrafast polaron dynamics.
• To evaluate molecular coordinates driving charge generation in OPVs.

Main Methods:

• Broadband femtosecond transient absorption spectroscopy.
• Resonance-selective femtosecond stimulated Raman spectroscopy (fs-SRS).
• Comparative study of crystalline P3HT and amorphous P3EHT films.

Main Results:

• Femtosecond stimulated Raman spectroscopy provided direct evidence of polaron delocalization via thiophene backbone planarization within ~100 fs.
• Morphology-dependent cooling dynamics of polarons were observed over a few picoseconds.
• Crystalline morphology was shown to be significant for hot-charge generation.

Conclusions:

• The study highlights the critical role of crystalline morphology in generating hot-charges.
• Side-chain engineering is emphasized as a key strategy for designing efficient conjugated polymer films for hot-carrier solar cells.
• Understanding ultrafast polaron dynamics is vital for advancing OPV technology.