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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 solutions 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. 

Abstract Doping the electron‐transport polymer poly{[N,N′‐bis(2‐octyldodecyl)naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)} [P(NDI2OD‐T₂)] with the bulky, strongly reducing metallocene 1,2,3,4,1′,2′,3′,4′‐octaphenylrhodocene (OPR) leads to an increased bulk conductivity and a decreased contact resistance. While the former arises from low‐level n‐doping of the intrinsic polymer and increased carrier mobility due to trap‐filling, the latter arises from a pronounced accumulation of dopant molecules at an indium tin oxide (ITO) substrate. Electron transfer from OPR to ITO leads to a work function reduction, which pins the Fermi level at the P(NDI2OD‐T₂) conduction band and thus minimizes the electron injection barrier and the contact resistance. The results demonstrate that disentangling the effects of electrode modification by the dopant and bulk doping is essential to comprehensively understand doped organic semiconductors.