Quasi-Instantaneous Doping Mechanism of Organic Semiconductors by a Strong Lewis Acid

The doping of organic semiconductors (OSCs) is of crucial importance for applications in electronic devices, where it is used to alter the opto-electronic properties of OSCs. P-type doping is generally achieved via the admixture of strong electron acceptors, i.e. molecules that feature a high electron affinity, in order to remove a full electron from the OSC. Because of their typically conjugated nature such molecules tend to form supramolecular complexes with the organic semiconductor instead of promoting electron transfer, which quenches the efficiency of the dopant. Furthermore, control over structure in doped systems, which is paramount to performances in applications, is difficult to achieve because these molecules do not interact covalently.<br/><br/>In this light, trivalent group XIII Lewis acids (boron, aluminum centers) have been used as dopants, paired with OSCs that contain hard nitrogen atoms to which they can bind covalently. This resulted, for example, in the possibility to tune the bandgap of OSCs by employing acids of different strengths, but not in full electron transfer. In other systems where OSCs do <i>not</i> feature hard basic sites to which LAs can bind covalently, however, full electron transfers were reported, notably in the poly(3-hexylthiophene) (P3HT) - tris(pentafluorophenyl)borane (BCF) system. This came as a surprise as the electron affinity of BCF is thought to be insufficient to induce such electron transfers. As such, it was more recently proposed that this doping proceeds via a proton transfer that can be introduced by water traces even in inert environments, through a mechanism that was calculated to be highly endergonic.<br/><br/>Considering all these findings, we will report on the surprising observation that BCF can readily dope three different thiophene-containing OSCs of relatively low ionization energy in a quasi-instantaneous manner that is at odds with a highly endergonic mechanism. We investigated more closely the doping behavior of several Lewis acids analogous to BCF with P3HT in order to understand the factors determining the ability of Lewis acids to produce such doping. We performed supporting DFT calculations to assess the energetics involved in different possible mechanisms. Altogether, our data strongly suggest that this quasi-instantaneous doping is not mediated by a proton transfer alone, which we will discuss in the framework of the previously suggested doping processes known to be at work for organic semiconductors.