Design strategies for optimized molecular p-dopants: decoupling electronic and geometric effects

Abstract The design of efficient molecular p-dopants for organic semiconductors relies on maximizing their electron affinity to generate holes in the semiconductor host. The formation of ground-state charge transfer complexes, detrimental to the doping efficiency, can be overcome by increasing the steric bulk of dopants. However, while electron affinity is typically increased by adding electron-withdrawing groups to the dopant core, enhancing its steric demand is done via the substitution of bulky side groups, where both strategies may conflict. Here, we systematically analyze a library of existing and proposed new molecular p-dopants based on cyclohexadiene and cyclopropane cores using density functional theory. The effect of direct bonding of electron-withdrawing groups to the core on the electron affinity is contrasted to their bridging via a phenyl moiety, thereby decoupling electronic from geometric effects. Double doping, that is, the transfer of two electrons per dopant, is also examined. The ensuing (non-)linearity of shifts in cyano vibrational modes, characteristic of the degree of charge transfer, is shown to be pronounced for several dopants. Our study offers guidelines for balancing high electron affinity with steric demands and predicts several three-dimensional dopants with record-high electron affinity. The agreement between quantitative predictions and mechanistic explanations allows for disentangling the roles of substituents, steric bulk, and molecular core. It ensures that the results can inform the development of future p-dopants.