Predicting the yield of ion pair formation in molecular electrical doping: redox-potentials versus ionization energy / electron affinity
Efficient electrical doping of organic semiconductors relies on identifying appropriate molecular dopants that are capable of ionizing semiconductor molecules with a high yield, thereby creating mobile charges. We explore the suitability of two different material parameters to predict ion pair formation for different sets of semiconductor-dopant combinations: (i) redox-potentials measured by cyclic voltammetry in solution and (ii) ionization energy (IE) / electron affinity (EA) measured on thin films by ultraviolet / inverse photoelectron spectroscopy. Our study suggests, at least for molecular semiconductors and dopants, that redox-potentials are better suited to identify matching material pairs and their ion pair formation yield than IE/EA values. This is ascribed to the dependence of IE/EA values on molecular orientation and film structure on and above the meso-scale. In contrast, cyclic voltammetry measurements, although performed on solution rather than on thin films, capture dopant-semiconductor energy levels on the molecular scale, which is more relevant for doping even in the case of solid thin films.