Abstract
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.
Highlights
Doping is the process of introducing a small amount of dopants to a semiconductor in order to tune its electrical properties, such as Fermi level position, charge carrier density, and carrier mobility
The spectra of B(C6F5)3-doped P3HT in Figure 2a show the emergence of the P1 and the two P2 transitions of positive P3HT polarons,[56] which were reported from experiments in the range of 0.4–0.5 eV (P1) and 1.3–1.6 eV (P2), respectively.[22,24,30,31,57,58]
Up to 50% dopant concentration, the two polaron features P2a and P2b are well resolved at 1.3 and 1.6 eV, respectively, and grow in intensity along with the P1 transition, whose maximum cannot be seen in this plot, but is at ≈0.4 eV
Summary
Doping is the process of introducing a small amount of dopants (atoms or molecules) to a semiconductor in order to tune its electrical properties, such as Fermi level position, charge carrier density, and carrier mobility. While many studies favored the occurrence of mainly positive polarons (cation segment on a polymer chain), others suggested the formation of mainly positive bipolarons (dication segment on a polymer chain).[17,18,19,20,21,22,23] Today, it is commonly accepted that molecular doping of polymers, in general, leads to the formation of polarons as charge carriers, predominantly.[1,2,24,25] Positive bipolaron formation in P3HT has been evidenced only upon electrochemical doping and doping with FeCl3 (a small inorganic Lewis acid dopant),[26,27,28,29] but not with the molecular dopants employed nowadays, such as 2,3,5,6-tetrafluoro7,7,8,8-tetracyanoquinodimethane (F4TCNQ),[4,30,31,32] 1,3,4,5,7,8hexafluoro-11,11,12,12-tetracyanonaphtho-2,6-quinodimethane (F6TCNNQ),[33] hexacyano-trimethylene-cyclopropane (CN6CP),[34] and molybdenum tris[1,2-bis(trifluoromethyl)ethane1,2-dithiolene] (Mo(tfd)3).[35,36] While bipolaron formation was considered to occur in some of these studies,[22,37] no compelling evidence was provided, as no clear differentiation of polaron and bipolaron abundance in dependence of dopant concentration was possible This raises the question whether these dopants are not sufficiently strong to support bipolaron formation, or whether other factors apparently inhibit this process, which for electrochemical doping and doping with FeCl3 seems possible. The doping mechanism is identified to comprise electron transfer from the semiconductor polymer to Mes2B+, which leaves as volatile compound during film fabrication, and only the p-doped polymer with [B(C6F5)4]− as charge stabilizing agent for polarons/bipolarons remain in the films, according to polymer0
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