Counterion Control and the Spectral Signatures of Polarons, Coupled Polarons, and Bipolarons in Doped P3HT Films

Abstract

When an electron is removed from a conjugated polymer, such as poly(3‐hexylthiophene‐2,5‐diyl) (P3HT), the remaining hole and associated change in the polymer backbone structure from aromatic to quinoidal are referred to as a polaron. Bipolarons are created by removing the unpaired electron from an already‐oxidized polymer segment. In electrochemically‐doped P3HT films, polarons, and bipolarons are readily observed, but in chemically‐doped P3HT films, bipolarons rarely form. This is explained by studying the effects of 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 calculations. The counterion positions control whether two polarons spin‐pair to form a bipolaron or whether they strongly couple without spin‐pairing are found. 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. These results explain the observed formation order of single polarons, coupled single polarons, and singlet bipolarons in electrochemically‐ and chemically‐doped conjugated polymers.

Crystallinity Determines Ion Injection Kinetics and Local Ion Density in Organic Mixed Conductors

“…In Figure , we present spectroelectrochemistry and GIWAXS measurements for 25:75, 50:50, and 75:25 RR/RRa P3HT blends. The steady-state spectroelectrochemistry measurements exhibit a decrease in the height of the π–π* absorption peak and the appearance and subsequent increase in the height of the polaron absorption peak as the voltage increases . For each blend, the red side of the π–π* peak decreases before the blue side of the π–π* peak, indicating that polarons form in the more crystalline, smaller band gap regions of the film before forming in the more amorphous, larger band gap regions of the film (Figure A).…”

“…The steady-state spectroelectrochemistry measurements exhibit a decrease in the height of the π−π* absorption peak and the appearance and subsequent increase in the height of the polaron absorption peak as the voltage increases. 46 For each blend, the red side of the π−π* peak decreases before the blue side of the π−π* peak, indicating that polarons form in the more crystalline, smaller band gap regions of the film before forming in the more amorphous, larger band gap regions of the film (Figure 2A). If there were no shift observed for the π−π* peak, we would hypothesize that the dopants enter the crystalline and amorphous regions simultaneously, but that is not the case with the P3HT blends.…”

Does the Traditional Band Picture Describe the Electronic Structure of Doped Conjugated Polymers? TD-DFT and Natural Transition Orbital Study of Doped P3HT

“…43−45 For bipolaronic P3HT, point-charge counterions were placed on opposite sides of the center of the chain at a distance of 5 Å, as required for bipolarons to be bound in this system. 21 For geometry optimization with point charge(s), the terminal hydrogen atoms at the ends of the chain and one carbon atom at the middle of the chain (one of the carbon atoms that bridges monomer 5 and monomer 6) are fixed during geometry optimization. By fixing the positions of these three atoms, all the bonds are still able to fully relax, which was checked via frequency calculations.…”

“…The precise chain and counterion geometries are the same as those explored in our previous work. 21 To benchmark our use of TD-DFT as a quantum chemistry method to determine the electronic structure of doped conjugated polymers, in Figure 1 we compare the calculated and experimental absorption spectra of neutral and (bi)polaronic P3HT. As expected for any DFT-based method, 47−49 the calculated spectra are slightly blue-shifted from experiment, but remarkably, the two sets of spectra are quite similar.…”

“…Indeed, we recently argued that the new peaks that appear to the red of P1 at high doping concentrations should be assigned to coupled polarons and not to bipolarons. 21 Despite the fact that basic observations of the P1, P2, and BP1 transitions seem to fit well with the traditional band picture, several groups have argued on the basis of quantum chemistry calculations that borrowing the traditional band picture of conjugated polymer doping from inorganic semiconductors is incorrect. 25−31 These groups used density-functional theory (DFT)-based calculations, and upon examination of the density of states of the Kohn−Sham orbitals yielded by such calculations, concluded that polaron creation causes only one unoccupied state to move into the band gap and that states in the conduction and valence bands associated with the unpaired electron bend downward in energy.…”