Evidence of Conjugation Enhancement in P3HT/SWNT Mixtures for Organic Photovoltaics

Classical molecular dynamics (MD) simulations in conjunction with optical absorption and AFM/nano-Raman experiments are employed to relate the molecular-scale arrangement and conjugation of poly-3-hexylthiophene (P3HT) adsorbed onto single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs). Taken together our results demonstrate the templating role of carbon nanotubes in increasing the π-conjugation length of the P3HT at the P3HT/carbon nanotube interface. The MD simulations show that SWNTs and MWNTs, due to their inherent 1-dimensional (1D) cylindrical shape and π-conjugation, planarize the P3HT molecules adsorbed at their surface and thus quench their torsional disorder, regardless of the P3HT conformation and nanotube chirality. This effect is more significant for higher SWNT weight fractions in the sample (since it is an interface effect). We investigated this effect experimentally by acquiring nano-Raman spectra in regions of high-MWNT/low-P3HT content in addition to optical absorption spectra of P3HT-SWNT composites with different SWNT concentrations. The increase in the P3HT conjugation is confirmed by a shift of a P3HT feature in the Raman spectrum when going from P3HT-rich to SWNT-rich areas in the mixture. The significance of this work for charge transfer at the P3HT-SWNT interface in bulk-heterojunction solar cells is discussed