a, Temperature-dependent electron spin resonance (ESR) spectra obtained from PBTTT thin film anion-exchange-doped with EMIM-TFSI. The single Lorentzian ESR spectra are observed to follow the Curie law. Note that Curie susceptibility is attributed to localized spins either on F4TCNQ radical anions or on PBTTT radical cations. Hall effect measurements indicate that carriers in the highly doped PBTTT are likely to undergo delocalized transport, thus producing Pauli paramagnetic susceptibility that is negligible compared with the Curie effect. Although the g-factors of the PBTTT radical cation and F4TCNQ radical anion are identical, it is reasonable to assume that the observed Curie susceptibility originates from localized F4TCNQ radical anions. We found the experimentally determined spin concentration to be much less than the actual carrier concentration in the PBTTT thin film, as discussed in the main text. b, ESR spectrum for anion-exchange-doped PBTTT, acquired at 4.3 K with the external magnetic field perpendicular to the film plane. The result of single Lorentzian fitting is plotted as a black curve. c, The effect of temperature (T) on spin susceptibility, as determined by double integration of the ESR spectra.