Polaron dynamics in two types of long oligothiophenes revealed byQ- andX-band ESR measurements

Abstract

The polaron dynamics has been investigated through the $X$- and $Q$-band ESR measurements for two types of iodine-doped long oligothiophenes, the 20-mer with octyl substituents $(o\text{\ensuremath{-}}20\mathrm{T})$ and the 16-mer with hexyl substituents $(h\text{\ensuremath{-}}16\mathrm{T})$. $o\text{\ensuremath{-}}20\mathrm{T}$, used as a model compound of conjugated polymers with crystalline grains, gives anisotropic ESR spectra attributed to $g$ anisotropy at low temperatures. The anisotropic spectra are found to be brought by polarons moving within the crystalline grains consisting of parallel chains. The anisotropy is shown to decrease with increasing temperature. This provides definite evidence that the polarons transfer among some grains by the assist of temperature. In contrast, $h\text{\ensuremath{-}}16\mathrm{T}$, used as a model of the polymers with amorphous morphology, gives almost isotropic ESR spectra even in the $Q$-band measurement. This feature of $h\text{\ensuremath{-}}16\mathrm{T}$ is explained to be caused by a rapid interchain transfer of polarons. Spectral simulations performed for obtained spectra reveal that the ESR linewidth in the $Q$-band measurement is larger than that in the $X$ band for both oligothiophenes. The difference of the linewidth is analyzed by a simplified motional narrowing model in order to draw the information of polaron dynamics. Analyses for $o\text{\ensuremath{-}}20\mathrm{T}$ show that the intergrain motion almost follows the variable range hopping model. The interchain motion in $h\text{\ensuremath{-}}16\mathrm{T}$ is found to have a much weaker temperature dependence than the intergrain motion in $o\text{\ensuremath{-}}20\mathrm{T}$. This result suggests that the interchain dynamics of $h\text{\ensuremath{-}}16\mathrm{T}$ revealed by the ESR technique includes a variety of processes of motion.