Understanding the physical and electronic structure of polarons and bipolarons on doped semiconducting polymers (Invited Paper)

We explore the effects of crystal structure and counterion position on the formation of polarons, strongly coupled polarons, and bipolarons using both spectroscopic and X-ray diffraction experiments and time-dependent density functional theory (TD-DFT) calculations. The counterion positions control whether two polarons spin-pair to form a bipolaron or whether they strongly couple without spin-pairing. When two counterions lie close to the same polymer segment, bipolarons can form, with an absorption spectrum that is blueshifted from that of a single polaron. Otherwise, polarons at high concentrations do not spin-pair, but instead J-couple, leading to a redshifted absorption spectrum. The counterion location needed for bipolaron formation is accompanied by a loss of polymer crystallinity, so that bipolarons can form only in disordered regions of conjugated polymer films. Our experiments and calculations also suggest that the ease with which charge carriers can be produced depends on the barrier to transforming the neutral polymer crystal structure into the doped structure that is able to incorporate the counterions.