Quantifying Charge Carrier Localization in PBTTT Using Thermoelectric and Spectroscopic Techniques

Chemically doped poly[2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) shows promise for many organic electronic
applications, but rationalizing its charge transport properties is
challenging because conjugated polymers are inhomogeneous, with convoluted
optical and solid-state transport properties. Herein, we use the semilocalized
transport (SLoT) model to quantify how the charge transport properties
of PBTTT change as a function of iron(III) chloride (FeCl3) doping level. We use the SLoT model to calculate fundamental transport
parameters, including the carrier density needed for metal-like electrical
conductivities and the position of the Fermi energy level with respect
to the transport edge. We then contextualize these parameters with
other polymer-dopant systems and previous PBTTT reports. Additionally,
we use grazing incidence wide-angle X-ray scattering and spectroscopic
ellipsometry techniques to better characterize inhomogeneity in PBTTT.
Our analyses indicate that PBTTT obtains high electrical conductivities
due to its quickly rising reduced Fermi energy level, and this rise
is afforded by its locally high carrier densities in highly ordered
microdomains. Ultimately, this report sets a benchmark for comparing
transport properties across polymer-dopant-processing systems.