Driving Force and Optical Signatures of Bipolaron Formation in Chemically Doped Conjugated Polymers

Matthew G Voss; J Reddy Challa; D Tyler Scholes; Patrick Y Yee; Eric C Wu; Xiao Liu; Sanghyun J Park; Omar León Ruiz; Selvam Subramaniyan; Mengdan Chen; Samson A Jenekhe; Xiaolin Wang; Sarah H Tolbert; Benjamin J Schwartz

doi:10.1002/adma.202000228

Driving Force and Optical Signatures of Bipolaron Formation in Chemically Doped Conjugated Polymers

Adv Mater. 2021 Jan;33(3):e2000228. doi: 10.1002/adma.202000228. Epub 2020 Dec 9.

Authors

Matthew G Voss¹, J Reddy Challa¹, D Tyler Scholes¹, Patrick Y Yee¹, Eric C Wu¹, Xiao Liu¹, Sanghyun J Park¹, Omar León Ruiz¹, Selvam Subramaniyan², Mengdan Chen³, Samson A Jenekhe², Xiaolin Wang^{1

3}, Sarah H Tolbert^{1

4

5}, Benjamin J Schwartz^{1

4}

Affiliations

¹ Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, CA, 90095-1569, USA.
² Department of Chemical Engineering and Department of Chemistry, University of Washington, Seattle, WA, 98195-1750, USA.
³ Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, Key Laboratory of Cluster Science of Ministry of Education, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing, 100081, China.
⁴ California NanoSystems Institute, University of California, Los Angeles, Los Angeles, CA, 90095-8352, USA.
⁵ Department of Materials Science and Engineering, University of California, Los Angeles, Los Angeles, CA, 90095-1595, USA.

PMID: 33296113
DOI: 10.1002/adma.202000228

Abstract

Molecular dopants are often added to semiconducting polymers to improve electrical conductivity. However, the use of such dopants does not always produce mobile charge carriers. In this work, ultrafast spectroscopy is used to explore the nature of the carriers created following doping of conjugated push-pull polymers with both F₄ TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) and FeCl₃ . It is shown that for one particular push-pull material, the charge carriers created by doping are entirely non-conductive bipolarons and not single polarons, and that transient absorption spectroscopy following excitation in the infrared can readily distinguish the two types of charge carriers. Based on density functional theory calculations and experiments on multiple push-pull conjugated polymers, it is argued that the size of the donor push units determines the relative stabilities of polarons and bipolarons, with larger donor units stabilizing the bipolarons by providing more area for two charges to co-reside.

Keywords: bipolarons; donor-acceptor copolymers; molecular dopants; semiconducting polymers; transient absorption spectroscopy.