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

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.

Understanding how Lewis acids dope organic semiconductors: a “complex” story

“…Instead, adding BCF to a PCPDTBT based lm leads to an increase in electrical conductivity and to the formation of positive polarons, i.e., molecular pdoping. 16,24,48 As in the mechanism proposed by Doerrer and Green for oxidation of metallocenes, 14 Yurash et al suggested that the rst step of this p-doping was the protonation by the highly Brønsted acidic complex BCF(OH 2 ) of the CPDT moiety of the polymer backbone. 16 They further proposed that protonation would increase the EA sufficiently that a nearby neutral chain segment would be able to transfer an electron to the (positively charged) protonated segment (with the segments belonging either to the same or different physical polymer chains, if the process is intrachain or interchain, respectively).…”

Understanding the evolution of the Raman spectra of molecularly p-doped poly(3-hexylthiophene-2,5-diyl): signatures of polarons and bipolarons

“…12,13 The differentiation between polarons and bipolarons in doped polymers can be achieved using electron paramagnetic resonance spectroscopy (EPR), which directly probes charge carriers with S = ½. [14][15][16][17] The aforementioned local relaxation of the polymer geometry of a polaron/bipolaron is coupled with a shift of, likewise localized, electronic energy levels into the energy gap of the polymer. 13,[17][18][19] The energy of electronic transitions involving these intra-gap electronic states are characteristic of the type of the charge carriers, i.e.…”

“…The electronic transitions can be probed using optical absorption spectroscopy, which is often used to identify the type of charge carriers in doped polymers and to quantify the density of polarons and bipolarons, as well as the ionization efficiency of the doping process. 8,[15][16][17][20][21][22] Geometrical distortions on the charged polymer backbone have additionally offered means for probing polarons and bipolarons by investigating the associated changes in the charge-sensitive vibrational modes of doped polymers and oligomers using Raman spectroscopy and infrared absorption spectroscopy. [23][24][25][26][27] Raman spectroscopy is a non-destructive and facile method that has proven to be highly suitable for studying polymer backbone structure and conformation.…”

“…Recently, it was demonstrated that the organic salt dimesityl borinium tetrakis(pentafluorophenyl)borate (Mes 2 B + [B(C 6 F 5 ) 4 ] − ) (shown in Figure 1) is a potent p-dopant for P3HT, capable of forming both polarons and bipolarons depending on dopant concentration. 15 The underlying doping mechanism was identified to proceed through an electron transfer to the Mes 2 B + , which then leaves the sample, while the [B(C 6 F 5 ) 4 ] − acts as the counter-ion that stabilizes both polarons and bipolarons. With increasing dopant concentration, the majority charge carrier type was identified as polaron up to moderate dopant concentration, but beyond ca.…”

Correlating conductivity and Seebeck coefficient to doping within crystalline and amorphous domains in poly(3‐(methoxyethoxyethoxy)thiophene)

“…Note that EPR measurements capture the unpaired electrons that interacted with the applied magnetic field. A decrease in EPR spin concentration could be observed at high doping level when bipolarons are formed, which could lead to underestimation in the calculated carrier concentration 44 . However, the maximum carrier concentrations measured using other techniques (AC Hall effect, 45 XPS, 46,47 and UV–Vis spectroscopy 19,48 ) all lie in the order of 10 20 to 10 21 cm −3 , which is in agreement with our value calculated from EPR, indicating reasonable and commensurable carrier concentrations in this study.…”