The Role of Polymer Crystallization and Counterions in the Formation of Polarons and Bipolarons in Chemically -Doped Semiconducting Polymers

It is well established that chemical doping not only introduces polarons in conjugated polymers but also induces ordering, particularly if the polymer is initially amorphous. Recent work has shown that the formation of bipolarons creates disordering, even in initially highly crystalline polymer materials. However, the connections between crystallization energy and the doping-induced creation of polarons and bipolarons is still unclear. In this talk, we unravel the details of how doping and (bi)polaron formation are connected to crystallinity by studying chemically-doped dihexyl-substituted poly(3,4-propylenedioxythiophene) (PProDOT-Hx₂). PProDOT-Hx₂ is an amorphous polymer in its pristine state, but its structure changes when doped with molecular dopants such as F₄TCNQ, TCNQ, FeCl₃ and large dopants based on dodecaborane (DDB) clusters. Electrochemically, PProDOT-Hx₂ is ~200 mV easier to oxidize than P3HT due to the presence of the electron-donating O atoms in conjugation with the backbone. We find, however, that PProDOT-Hx₂ can be doped by dopant solutions with concentrations orders of magnitude smaller than those used to dope P3HT. For example, UV-vis-NIR spectroscopy shows that the bandgap absorption of PProDOT-Hx₂ is entirely bleached even when very modest concentrations of F4TCNQ are applied, achieving conductivities up to 18 S/cm. GIWAXS diffraction shows that PProDOT-Hx₂ readily undergoes doping-induced crystallization. We hypothesize that the induced crystallization energy helps drive doping reactions in PProDOT-Hx₂, making it effectively hundreds of mV easier to dope. We also see that when doped with TCNQ, a relatively weak dopant, PProDOT-Hx₂ creates crystalline bipolarons, a result in sharp contrast to what is observed with other conjugated polymers like P3HT. Our conclusion is that when bipolarons are created directly and are stabilized by a doubly-charged counterion, they can be formed without introducing disorder. Bipolarons that form by the merger of two polarons need to become disordered to accommodate two singly-charged polymers in close proximity.