Evidence of Preformed Lewis Acid–Base and Wheland-Type Complexes Acting as Dopants for p-Type Conjugated Polymers

Efficient doping of polymer semiconductors is essential for their development as conductors. Although Lewis acids such as B(C6F5)3 have shown promise as dopants for polymers, their doping mechanism is not fully understood. We created 1:1 zwitterionic (including “Wheland-type”) complexes of B(C6F5)3 with conjugated molecules difluorobis(triethylsilylethynyl)anthradithiophene (diF-TES-ADT) and didodecylthienothiophene (DTT-12) and characterized them with 1H NMR, UV–vis spectroscopy, EPR spectroscopy, optical and scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM-EDS), and X-ray diffraction. We employed these complexes as p-dopants for three conjugated polymers and established their doping abilities by conductivity measurements, Seebeck studies, field effect transistor (FET), and remote-gate sensing (RG-FET) measurements. Conductivity changes were dependent on the conjugated molecule adduct component, consistent with the adduct itself serving as the oxidant. The adduct complexes were capable of inducing changes in the surface potential of spun polymer films similar to the behavior shown by conventional dopants. Charge carrier density calculations by remote gate sensing revealed that these adducts can generate holes. We also studied the effect of adding the B(C6F5)3 first, followed by addition of the neat conjugated molecules; the observation of behavior that was different from that using preformed adducts was consistent with the adducts remaining intact during doping. When B(C6F5)3 was added to the polymers, followed by uncomplexed conjugated molecules, the generated hole carrier density is lower than that generated by the B(C6F5)3 dopant but often greater than that generated by the Wheland complexes, indicating a high probability of adduct formation in this case. Density functional theory calculations show that adduct formation between boranes and the conjugated molecules and segments of the polymers is energetically favorable and that some charge transfer between adducts and neutral polymers is plausible if Coulombic and entropic effects are taken into consideration. Thus, such adducts can be considered as possible doping sites for conjugated polymers.