We investigate the delocalization of holes in the semicrystalline conjugated
polymer poly(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene) (PBTTT) by
directly measuring the hyperfine coupling between photogenerated polarons and
bound nuclear spins using electron nuclear double resonance spectroscopy. An
extrapolation of the corresponding oligomer spectra reveals that charges tend
to delocalize over 4.0–4.8 nm with delocalization strongly dependent on
molecular order and crystallinity of the PBTTT polymer thin films. Density
functional theory calculations of hyperfine couplings confirm that long-range
corrected functionals appropriately describe the change in coupling strength
with increasing oligomer size and agree well with the experimentally measured
polymer limit. Our discussion presents general guidelines illustrating the
various pitfalls and opportunities when deducing polaron localization lengths
from hyperfine coupling spectra of conjugated polymers
