Polaron and Exciton Delocalization in Oligomers of High-Performance Polymer PTB7

Journal Article · Fri Dec 20 00:00:00 EST 2019 · Journal of the American Chemical Society

DOI:https://doi.org/10.1021/jacs.9b10859· OSTI ID:1607374

Niklas, Jens ^[1]; Zheng, Tianyue ^[2]; Neshchadin, Andriy ^[2]; Mardis, Kristy L. ^[3]; Yu, Luping ^[2]; Poluektov, Oleg G. ^[1]

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1. Argonne National Lab. (ANL), Lemont, IL (United States)
2. Univ. of Chicago, IL (United States)
3. Chicago State Univ., IL (United States)

A key characteristic of organic photovoltaic cells is the efficient charge separation in the active layer. Sufficient delocalization of the positive polaron in organic photovoltaics is considered essential for the effective separation of the opposite charges and the suppression of recombination. Here we use light-induced EPR and ENDOR spectroscopy combined with DFT calculations to determine the electronic structure of the positive polaron in PTB7-type oligomers. Utilizing the superior spectral resolution of high-frequency (130 GHz) D-band EPR, the principal components of the g tensors were determined. Pulsed ENDOR spectroscopy at X-band allowed the measurement of H-1 hyperfine coupling constants. A comparison of g tensors and H-1 hyperfine coupling constants of the PTB7-type oligomers with the high-performance PTB7 polymer revealed a delocalization of the positive polaron in the polymer over about four monomeric units, corresponding to about 45 angstrom in length. Our current study thus not only determines the polaron delocalization length in PTB7 but also validates the approach combining EPR/ENDOR spectroscopy with DFT-calculated magnetic resonance parameters. This is of importance in those cases where oligomers of defined length are not easily obtained. In addition, the delocalization of the neutral triplet exciton was also determined in the oligomers and compared with polymer PTB7. The analysis revealed that the neutral triplet exciton is substantially more delocalized than the positive polaron, exceeding 10 monomeric units.

• Research Organization: Argonne National Laboratory (ANL), Argonne, IL (United States)
• Sponsoring Organization: Illinois Space Grant Consortium; National Institutes of Health (NIH); National Science Foundation (NSF); USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences & Biosciences Division
• Grant/Contract Number: AC02-06CH11357
• OSTI ID: 1607374
• Journal Information: Journal of the American Chemical Society, Journal Name: Journal of the American Chemical Society Journal Issue: 3 Vol. 142; ISSN 0002-7863
• Publisher: American Chemical Society (ACS)
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
ENDOR
PTB7 oligomer
density functional theory
electron paramagnetic eesonance
high-frequency EPR
polaron
triplet exciton