Counterion Control and the Spectral Signatures of Polarons,Coupled Polarons,and Bipolarons in Doped P3HT Films |
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Authors: | Eric C. Wu Charlene Z. Salamat Omar León Ruiz Thomas Qu Alexis Kim Sarah H. Tolbert Benjamin J. Schwartz |
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Affiliation: | Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, CA, 90095-1569 USA |
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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. |
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Keywords: | bipolarons chemical doping counterions electrochemical doping polarons semiconducting polymers |
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