An Organic Borate Salt with Superior p-Doping Capability for Organic Semiconductors

Molecular doping allows enhancement and precise control of electrical properties of organic semiconductors, and is thus of central technological relevance for organic (opto-) electronics. Beyond single-component molecular electron acceptors and donors, organic salts have recently emerged as a promising class of dopants. However, the pertinent fundamental understanding of doping mechanisms and doping capabilities is limited. Here, the unique capabilities of the salt consisting of a borinium cation (Mes₂B⁺; Mes: mesitylene) and the tetrakis(penta-fluorophenyl)borate anion [B(C₆F₅)₄]^- is demonstrated as p-type dopant for polymer semiconductors. With a range of experimental methods, the doping mechanism is identified to comprise electron transfer from the polymer to Mes₂B⁺, and the positive charge on the polymer is stabilized by [B(C₆F₅)₄]^-. Notably, the former salt cation leaves during processing and is not present in films. The anion [B(C₆F₅)₄]^- even enables the stabilization of polarons and bipolarons in poly(3-hexylthiophene), not yet achieved with other molecular dopants. From doping studies with high ionization energy polymer semiconductors, the effective electron affinity of Mes₂B⁺[B(C₆F₅)₄]^- is estimated to be an impressive 5.9 eV. This significantly extends the parameter space for doping of polymer semiconductors.