Bipolar transport and Nernst effect measurements in polymer at 300 K. (a) Schematic illustration of general bipolar charge transport in doped polymer (middle panel) , in which the greyish-green lines represent polymer chains, the gray-blue shaded areas represent crystalline regions, and the bluish-violet circles represent the dopant counterions; schematic of bipolar Seebeck effect with longitudinal temperature gradient and the transverse Nernst effect with an additional perpendicular magnetic field in polymer devices (left panel); schematic plot of thermopower S (dark green line) polarity transition versus the difference between transport level and Fermi level E_T-E_F in polymers (right panel), where the S = 0 transition point is indicated by the dashed circle, while the dashed blue and red curves are hole and electron partial thermopower, respectively. (b) Molecular structure of polymer PDPP4T and dopant FeCl₃. (c) Ultraviolet-visible-near-infrared (UV-vis-NIR) absorption spectra, (d) thermopower and electrical conductivity, and (e) power factor of PDPP4T films doped with different FeCl₃ concentrations. (f) Transverse Nernst effect voltage signal versus magnetic field under different temperature gradient in 20 mmol L⁻¹ FeCl₃-doped PDPP4T device. The colored lines correspond to linear fits. (g) PDPP4T (20 mmol L⁻¹) Nernst coefficients versus the angle θ between the temperature gradient (∆T = 4 K) and the magnetic field direction when B = 0 and +9 T. The dotted black curve plots the sine function. The device was rotated in the xz plane while the magnetic field was fixed in the z direction.