Fundamental Mechanisms of Lewis Acid Doping of Organic Semiconductors
Authors: Berteau-Rainville Michael , Muchall Heidi , Salzmann Ingo 
Published: May 2020

One key challenge in the field of organic semiconductors (OSCs) and their application in organic electronics is controlling the electronic properties of the OSC by p-/n-doping in a predictable manner. It has been established that upon dopant admixture either integer or fractional charge transfer can occur, where the parameters deciding one over the other are still under debate. In numerous studies integer charge transfer was found for conjugated polymers, while small molecular OSCs show a tendency towards fractional charge transfer forming ground state charge transfer complexes (1). The typical approach to assure integer charge transfer is using molecular dopants with a high electron affinity, exceeding the ionization energy of the OSC (for p-doping), as the dopant then exhibits an empty orbital lying lower in energy than the occupied frontier molecular orbital of the semiconductor. Recently, however, a transition between the two scenarios has been reported for polythiophene and the strong molecular p-dopant F4TCNQ, where aging of the samples appeared to promote the formation of charge-transfer complexes showing fractional charge transfer (2). Another study highlighted the key role of the microstructure for the doping process, where different polymorphs show either fractional or integer charge transfer (3).
This lack of predictability of traditional doping employing strong donors/acceptors recently led to intense research efforts to find alternative approaches. There, using Lewis acids (LAs) as molecular p-dopants for OSCs has been found to be particularly promising, as these compounds react selectively with Lewis bases to form robust adducts. This makes LAs, in principle, chemically selective dopants that attack OSCs only at the sites where electron lone pairs are located. Interestingly, despite an electron affinity far below the ionization energy of polythiophene, it has been found that the LA BCF can readily ionize this material, which is incompatible with the conventional picture of integer charge transfer as detailed above (4). Very recently, it has been suggested that LA doping does not follow from the formation of an adduct, but instead, that the LA simply act as a protonating agent after forming a complex with water and, hence, becoming a Br∅nsted acid (5).
In this presentation, I will briefly review these ideas and discuss how the concepts of electron affinity, ionization energy, integer, and partial charge transfer as used in the field relate to the doping using LAs. By correlating experimental data from optical and infrared spectroscopy with DFT calculations, I will provide alternative descriptors that allow to clarify the discussion and, more importantly, that have predictive power towards the use of Lewis acids as dopants for OSCs. Finally, I will show that the use of these descriptors, in the case of polythiophene doped with BCF discussed above (4), lift the apparent theoretical inconsistency, and as such, will be of high value for a comprehensive larger-scale exploration of OSCs doping using LAs.
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