Subnanosecond charge recombination dynamics in P3HT/PC61BM films

Area of Science:

• Organic Electronics
• Materials Science
• Photophysics

Background:

• P3HT/PC(61)BM blend films are crucial for organic solar cells.
• Charge recombination (CR) dynamics significantly impact device performance.
• Film morphology and excitation energy influence charge carrier behavior.

Purpose of the Study:

• Investigate the influence of film morphology and excitation photon energy on CR dynamics.
• Analyze CR dynamics on the nanosecond timescale.
• Understand the role of photogenerated free charges in early timescales.

Main Methods:

• Ultrafast near-infrared absorption spectroscopy.
• Fabrication of P3HT/PC(61)BM blend films using CS(2)-cast and solvent vapor annealing (SVA) methods.
• Measurement of hole mobility and bimolecular CR rates at different timescales (70 ps and 1 ns).

Main Results:

• SVA films exhibit 2–3-fold higher hole mobility and a >5-fold reduction in polymer-localized trap states compared to CS(2)-cast films.
• SVA films showed hole mobility of 8.7 × 10(−4) cm2 × s(−1) × V(−1) and a bimolecular CR rate of 4.5 × 10(−10) cm3 × s(−1) at 70 ps, decreasing to 8.7 × 10(−5) cm2 × s(−1) × V(−1) and 4.6 × 10(−11) cm3 × s(−1) at 1 ns.
• Hole mobility increased faster with increasing hole concentration under above-gap photoexcitation than under band-gap photoexcitation, regardless of film morphology.

Conclusions:

• Solvent vapor annealing significantly optimizes film morphology for improved charge transport and reduced recombination.
• Early timescale dynamics of photogenerated free charges are critical for efficient charge utilization in P3HT/PC(61)BM blends.
• Excitation photon energy plays a key role in modulating charge carrier dynamics and mobility.