Reevaluating infrared spectroscopic signatures of polaron trapping in a chemically doped conducting polymer

Charge conductivity in conducting polymers is typically improved by increasing carrier density via chemical oxidation. However, the resulting electrostatic stabilization of the carriers by the dopant ions, combined with their nanostructural environment, are both known to crucially affect charge trapping. Although the effects of charge-ion electrostatic interactions on carrier trapping have been well-characterized using conventional infrared (IR) spectroscopy, the impacts of the polymer chain ordering and energetic environment are difficult to disentangle. In this study, we examine the limitations of conventional IR absorption spectroscopy and introduce a complementary spectroscopic approach capable of discerning polaron trapping more generally. To do so, we investigated films of poly(3-hexylthiophene-2,5-diyl) (P3HT) chemically doped using four different oxidants, of which each preferentially dopes the amorphous and crystalline (lamellar) phases to varying extents. Using this model system, we observed counterintuitive shifts in the polaron IR absorption band, indicating that IR spectroscopy is a clear reporter of trapping only when the carriers exclusively reside in the lamellar phase and in the absence of bipolarons or coupled polarons. Alternatively, we found that polaron excited state dynamics, probed using ultrafast near-infrared transient absorption spectroscopy, more clearly report on charge trapping. This study demonstrates near-infrared transient absorption spectroscopy as a complementary tool for probing charge trapping in conducting polymers when doping induces carriers in different nanostructural environments.