Dec 11, 2020
Matthew Voss
A collaboration between UCLA, University of Washington, and Beijing Institute of Technology discovers a doped polymer system that exclusively forms bipolarons.
By Dr. Matthew Voss (Schwartz group)
The groups of Professor Ben Schwartz and Professor Sarah Tolbert, using a polymer synthesized by Professor Sam Jenekhe’s Group at the University of Washington and some computational help from Professor Xiaolin Wang’s Group at Beijing Institute of Technology, launched a collaborative effort to understand the chemical nature of bipolarons (dications) in chemically-doped semiconducting polymers. The paper, “Driving Force and Optical Signatures of Bipolaron Formation in Chemically Doped Conjugated Polymers”, was published online in Advanced Materials on December 9, 2020. 
Semiconducting polymers natively exist in a neutral state, but with the addition of small-molecule oxidizing agents (dopants) to the polymer film, radical cations, also called polarons, form on the semiconducting polymer backbone.  The standard picture in the literature was that polarons could pair up to form a dication, also called a bipolaron, only at very high concentrations of dopants in the polymer film.  However, this work demonstrates that for a particular conjugated polymer, PBTDTP (see figure below), doping leads exclusively to the formation of bipolarons instead of polarons, even at low doping concentrations.  We were able to verify the presence of bipolarons using a novel form of ultrafast transient absorption spectroscopy as well as synchrotron-based structural measurements.  This behavior of favoring bipolarons exclusively had never been seen before, nor had bipolarons ever been cleanly probed by ultrafast transient absorption spectroscopy.  
Left: The structure of the PBTDTP conjugated polymer, with the blue and yellow colors highlighting the  alternating electron-rich and electron-poor regions.  
To understand why bipolarons form preferentially in this material, we performed a series of density functional theory (DFT) calculations.  PBTDTP is technically a copolymer consisting of alternating electron-rich (blue shading, above) and electron-poor (yellow shading, above) regions along the conjugated backbone.  Our DFT calculations showed that the PBTDTP’s large electron-rich region was responsible for stabilizing the two positive charges of a bipolaron relative to two separate single polarons, thus encouraging bipolaron formation.  In contrast, most other conjugated polymers have smaller electron-rich regions, which would force bipolarons onto the energetically less-favorable electron-poor regions, disfavoring bipolaron formation.  The insights we gained provide design rules for encouraging or discouraging bipolaron formation as needed for new polymers that could be employed in organic electronic applications like thermoelectric devices, RFIDs, and LEDs.  
Matthew Voss, UCLA Department of Chemistry & Biochemistry,