Uncovering the Mechanism by Which Wheland-Type Complexes Act as P-Dopants to Improve the Performance of Organic Semiconducting Polymers

Organic semiconducting polymers are being considered for a wide range of commercially relevant applications. From thermoelectric materials (imagine a medical monitor that does not need a battery) to flexible, large-area solar cells (a benefit of solution processability), these common-element materials could contribute to a greener, more sustainable future. Doping these materials with small molecules increases their charge carrier density and thus their conductivity, increasing their utility. This presentation will demonstrate the feasibility of p-doping organic semiconducting polymers with novel types of complexes, namely zwitterionic Wheland-type adducts, as models for mechanistic intermediates in already established doping processes and as dopants in their own right.<br/>We will discuss Wheland adducts comprised of tris(pentafluorophenyl)borane (TPFB), a Lewis acid, and small-molecule conjugated systems. We obtained experimental results revealing the possibility of p¬-doping commercially available polymers (PBTTT, PNTz4T, and PTAA) with Wheland-type complexes. While TPFB alone can, indeed, dope these polymers itself and lead to high conductivities, we found that the Seebeck coefficients (voltage differences per degree of temperature difference) of Wheland adduct-doped polymers were higher than those doped with TPFB alone, by up to an order of magnitude. Moreover, conductivities obtained by adding intact adducts depended on the structures of the adducts, and were different from values obtained by adding the adduct components sequentially.<br/>We used an ab initio computational chemistry approach to probe the nature, structure, and properties of these Wheland-type dopants and their effects on the polymers. This grounded their experimental findings in theory-driven calculations that helped elucidate the adduct dopants’ structural and electronic properties. We used the ORCA density functional theory (DFT) code with a def2-TZVP basis set, a B97-D3 functional, and an implicitly modeled acetonitrile solvent, choices that were based on our previous work and as recommended by Grimme (Phys. Chem. Chem. Phys., 2011). We will show that Wheland-type adducts are energetically favored to form for all the TPFB:conjugated molecule pairs we studied. We will also show that the electron affinities of these adducts nearly exactly matches that of TPFB alone, providing insight into why these complexes can act as p-dopants. Finally, we will demonstrate that some charge transfer is plausible between polymers and dopants through a comparative study of their respective ionization energies and Coulombic effects. This study strongly supports the hypothesis that Wheland-type adducts act as p-dopants for organic semiconducting polymers.