Optical Gain in Semiconducting Polymer Nano and Mesoparticles

Area of Science:

• Organic electronics
• Photophysics
• Materials science

Background:

• Conjugated polymers like PCDTBT are crucial for organic electronic devices.
• Understanding excited-state dynamics and charge separation is key to optimizing device performance.
• Nanoparticle and mesoparticle film morphologies can significantly alter photophysical properties.

Purpose of the Study:

• To investigate the photophysical behavior of poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) in nanoparticle and mesoparticle films.
• To explore the influence of film morphology on excited-state dynamics and charge generation.
• To assess the potential for stimulated emission in different PCDTBT film structures.

Main Methods:

• Femtosecond transient absorption spectroscopy (UV-Vis pump, Vis/NIR probe).
• Fabrication of spin-coated films of PCDTBT nanoparticles and mesoparticles.
• Analysis of excited-state species and charge-separated species formation kinetics.

Main Results:

• Optical gain was observed in mesoparticle films upon excitation at both 400 nm and 610 nm.
• Charge generation was delayed in mesoparticles (around 50 ps) after UV excitation, with minimal generation after visible excitation.
• Nanoparticle films showed efficient charge formation with UV excitation, but visible excitation led to optical gain and reduced charge formation.
• Differences in intermolecular interactions and molecular order between nanoparticle and mesoparticle films explain the distinct photophysical behaviors.

Conclusions:

• Film morphology significantly impacts the photophysical properties of PCDTBT.
• Nanoparticle and mesoparticle structures offer distinct pathways for light-matter interactions.
• A novel route to stimulated emission in conjugated polymers is demonstrated with simple film preparation.