Measuring the anisotropic conductivity of rub-aligned doped semiconducting polymer films: The role of electrode geometry

Doped semiconducting polymers are attractive for optoelectronic applications due to their solution processibility, low cost, flexibility, and potential for use in near-room-temperature thermoelectric generators. One way to improve charge transport in doped conjugated polymer films is to orient all the polymer chains in the same direction via rub-aligning, although aligned doped polymer films have an anisotropic electrical conductivity. Here, we compare the use of different electrode geometries to measure the anisotropic conductivity of rub-aligned doped poly(3-hexylthiophene-2,5,-diyl) (P3HT) films. We show that for P3HT films doped both conventionally with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) and via anion exchange with F4TCNQ and lithium bis(trifluoromethanesulfonyl)imide, a rectangular electrode geometry in combination with the modified Montgomery method can determine the anisotropic conductivity as accurately as the use of the Hall bar geometry. This rectangular geometry provides two significant advantages. First, the rectangular electrode geometry is much simpler to fabricate than the standard Hall bar geometry. Second, the rectangular geometry requires only a single sample to obtain both components of the anisotropic conductivity, unlike the Hall bar geometry for which two separate samples are required. This is particularly important for rub-aligned doped conjugated polymer films, which have large sample-to-sample variabilities. We also explore anisotropic Hall effect measurements of rub-aligned doped polymer films, and examine the use of a four-line electrode geometry, which has been popular in the literature for measuring the anisotropic conductivities of rub-aligned doped conjugated polymer films. We find that the four-line geometry overestimates the conductivity by approximately 40%.