Doping conjugated polymers is an effective way to tune their electronic properties for thin-film electronics applications. Chemical doping of semiconducting polymers involves the introduction of a strong electron acceptor or donor molecule that can undergo charge transfer (CT) with the polymer. The CT reaction creates electrical carriers on the polymer chain while the dopant molecules remain in the film as counterions. To dope polymer films, we employ a sequential process (SqP) in which a pure polymer layer is deposited first, followed by infiltration of the dopant in a second step using a semi-orthogonal solvent. SqP overcomes the problems typically incurred by blend-doping, where the polymer and dopant are mixed in solution, which results in aggregation at high concentration. The exceptional film quality achievable with our SqP doping method allows us to employ electrical measurements over macroscopic length scales, such as Van der Pauw conductivity measurements as well as AC Hall effect and impedance measurements of carrier mobility.
This work focuses on the use of substituted icosahedral dodecaborane (DDB) clusters of the form B12(OR)12 as a new class of dopant molecules, where R is a substituted benzyl group. The redox potentials of DDBs can be rationally tuned via modification of the R-group substituents without a significant change to the size or shape of the dopant molecule. These tunable dopants provide a unique handle on the energetic offset that governs the driving force for doping via integer CT. Here, we disentangle the effects of energetic offset on the production of free and trapped carriers in DDB-doped poly-3-hexylthiophene (P3HT) films.
In DDB-doped P3HT films, in general, we find that conductivity and polaron absorption amplitude increase with increasing reduction potential, yielding conductivities on the order of 12 S/cm. Since DDBs tend to localize electron density on their core, we have shown that the bulky corona of substituents on the clusters provide spatial isolation of the counterion. The polaron is therefore Coulombically shielded from the counterion, thereby reducing electrostatic interactions, resulting in highly delocalized and mobile carriers with mobilities upt 0.1 cm2/Vs and effective conductivities up to 32 S/cm even in very non-crystalline polymer films. In these films, nearly all carriers are free, whereas it has been shown that small molecule dopants like F4TCNQ trap 95% of carriers, due to their smaller size and inevitable proximity of the counterion. Given the high conductivity but distinct loss off crystallinity observed, we explore the properties of these DDB-doped films as potential thermoelectric materials. The low crystallinities should lead to low thermal conductivities, even while the electrical conductivity is maintained, allowing optimization of the thermoelectric figure of merit.