Photoinduced Absorption within Single-Walled Carbon Nanotube Systems

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 E11 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-C61-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.