Single-walled carbon nanotubes (SWNTs) have potential as electron acceptors in organic photovoltaics (OPVs), but the currently low-power conversion efficiencies of devices remain largely unexplained. We demonstrate effective redispersion of isolated, highly enriched semiconducting and metallic SWNTs into poly(3-hexylthiophene) (P3HT). We use these enriched blends to provide the first experimental evidence of the negative impact of metallic nanotubes. Time-resolved microwave conductivity reveals that the long-lived carrier population can be significantly increased by incorporating highly enriched semiconducting SWNTs into semiconducting polymer composites.

We used a variety of optoelectronic techniques such as broadband fs transient and cw photomodulation spectroscopies, electroabsorption, and short-circuit photocurrent in bulk heterojunctions organic solar cells for studying the photophysics in pi-conjugated polymer-fullerene blends with below-gap excitation. In contrast to the traditional view, we found that below-gap excitation, which is incapable of generating intrachain excitons, nevertheless efficiently generates polarons on the polymer chains and fullerene molecules. The polaron action spectrum extends deep inside the gap as a result of a charge-transfer complex state formed between the polymer chain and fullerene molecule. With appropriate design engineering the long-lived polarons might be harvested in solar cell devices.

In this work, we investigate the impact of the solvation environment on single-walled carbon nanotube (SWCNT) photoluminescence quantum yield and optical transition energies (E(ii)) using a highly charged aryleneethynylene polymer. This novel surfactant produces dispersions in a variety of polar solvents having a wide range of dielectric constants (methanol, dimethyl sulfoxide, aqueous dimethylformamide, and deuterium oxide). Because a common surfactant can be used while maintaining a constant SWCNT-surfactant morphology, we are able to straightforwardly evaluate the impact of the solvation environment upon SWCNT optical properties. We find that (i) the SWCNT quantum yield is strongly dependent on both the polarity and electrophilicity of the solvent and (ii) solvatochromic shifts correlate with the extent of SWCNT solvation. These findings provide a deeper understanding of the environmental dependence of SWCNT excitonic properties and underscore that the solvent provides a tool with which to modulate SWCNT electronic and optical properties.

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A detailed knowledge of the manifold of both bright and dark excitons in single-walled carbon nanotubes (SWCNTs) is critical to understanding radiative and nonradiative recombination processes. Exciton-phonon coupling opens up additional absorption and emission channels, some of which may "brighten" the sidebands of optically forbidden (dark) excitonic transitions in optical spectra. In this report, we compare (12)C and (13)C-labeled SWCNTs that are highly enriched in the (6,5) species to identify both absorptive and emissive vibronic transitions. We find two vibronic sidebands near the bright (1)E(11) singlet exciton, one absorptive sideband ~200 meV above, and one emissive sideband ~140 meV below, the bright singlet exciton. Both sidebands demonstrate a ~50 cm(-1) isotope-induced shift, which is commensurate with exciton-phonon coupling involving phonons of A[Formula: see text] symmetry (D band, ω ~ 1330 cm(-1)). Independent analysis of each sideband indicates that both sidebands arise from the same dark exciton level, which lies at an energy approximately 25 meV above the bright singlet exciton. Our observations support the recent prediction of, and mounting experimental evidence for, the dark K-momentum singlet exciton lying ~25 meV (for the (6,5) SWCNT) above the bright Γ-momentum singlet. This study represents the first use of (13)C-labeled SWCNTs highly enriched in a single nanotube species to unequivocally confirm these sidebands as vibronic sidebands of the dark K-momentum singlet exciton.