Abstract
The initial charge separation process of conjugated polymers is one of the key factors for understanding their conductivity. The structure of photogenerated transients in conjugated polymers can be observed by resonance Raman spectroscopy in the near-IR region because they exhibit characteristic low-energy transitions. Here, we investigate the structure and dynamics of photogenerated transients in a regioregular poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend film, as well as in a pristine P3HT film, using femtosecond time-resolved resonance inverse Raman spectroscopy in the near-IR region. The transient inverse Raman spectrum of the pristine P3HT film at 50 ps suggests coexistence of neutral and charged excitations, whereas that of the P3HT:PCBM blend film at 50 ps suggests formation of positive polarons with a different structure from those in an FeCl3-doped P3HT film. Time-resolved near-IR inverse Raman spectra of the blend film clearly show the absence of charge separation between P3HT and PCBM within the instrument response time of our spectrometer, while they indicate two independent pathways of the polaron formation with time constants of 0.3 and 10 ps.
Highlights
The mechanism of conductivity in conjugated polymers has been an important problem in fundamental physical chemistry and material science since conductive polyacetylene was first synthesized [1]
It has been widely accepted that the initial charge separation in P3AT is one of the key factors that determines the efficiency of the photoconductivity
A similar trend is observed for the P3HT:phenyl-C61-butyric acid methyl ester (PCBM) blend film with a smaller downshift than that in the pristine P3HT film (Figure 3c)
Summary
The mechanism of conductivity in conjugated polymers has been an important problem in fundamental physical chemistry and material science since conductive polyacetylene was first synthesized [1]. Poly(3-alkylthiophene) (P3AT) is a fundamental conjugated polymer that exhibits high conductivity with chemical doping or with photoirradiation [4]. Ultrafast time-resolved absorption and emission studies have been performed in the visible and near-IR regions to observe the dynamics of neutral and charged excitations formed in pristine P3AT films [5,6,7,8,9,10,11,12]. Pristine films of P3AT mainly show the self-localization dynamics of excitations initially created by a pump pulse in tens to hundreds of femtoseconds [7,8,10,11,12]
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