Photoinduced Absorption within Single-Walled Carbon Nanotube Systems

TL;DR

This study investigates the photoinduced absorption in single-walled carbon nanotubes within heterojunctions, revealing how charge dynamics influence optical properties relevant for organic photovoltaics.

Contribution

It combines experimental pump-probe spectroscopy with TDDFT-RPA calculations to elucidate the excited state dynamics and photoinduced absorption mechanisms in SWNT-based heterojunctions.

Findings

Photoinduced absorption peak relates to the width of the photobleach peak.

P3HT narrows the PB peak, revealing the PA peak in blends.

PA originates from band gap widening due to excited charge carriers.

Abstract

We study the photoabsorption properties of photoactive bulk polymer/ fullerene/nanotube heterojunctions in the near-infrared region. By combining pump-probe spectroscopy and linear response time-dependent density functional theory within the random phase approximation (TDDFT-RPA) we elucidate the excited state dynamics of the $E_{11}$ transition within (6,5) and (7,5) single-walled carbon nanotubes (SWNTs) and combined with poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C$_{61}$-butyric acid methyl ester (PCBM) in P3HT/PCBM/SWNT blended samples. We find the presence of a photoinduced absorption (PA) peak is related mainly to the width of the photobleach (PB) peak and the charge carrier density of the SWNT system. For mixed SWNT samples, the PB peak is too broad to observe the PA peak, whereas within P3HT/PCBM/SWNT blended samples P3HT acts as a hole acceptor, narrowing the PB peak by exciton delocalization, which reveals a PA peak. Our results suggest that the PA peak originates from a widening of the band gap in the presence of excited electrons and holes. These results have important implications for the development of new organic photovoltaic heterojunctions including SWNTs.