Effect of Halogenation on the Energetics of Pure and Mixed Phases in Model Organic Semiconductors Composed of Anthradithiophene Derivatives and C₆₀

Halogenation, particularly fluorination, is commonly used to manipulate the energetics, stability, and morphology of organic semiconductors. In the case of organic photovoltaics (OPVs), fluorination of electron donor molecules or polymers at appropriate positions can lead to improved performance. In this contribution, we use ultraviolet photoelectron spectroscopy, external quantum efficiency measurements of charge-transfer (CT) states, and density functional theory calculations to systematically investigate the effects of halogenation on the bulk solid-state energetics of model anthradithiophene (ADT) materials, their interfacial energetics with C₆₀, and the energetics of various ADT:C₆₀ blend compositions. In agreement with previous work, nonhalogenated ADT molecules show higher energy CT states in blends with C₆₀ and lower energy CT states in the ADT/C₆₀ bilayers. However, this trend is reversed in the halogenated ADT/C₆₀ systems, wherein the CT state energies of ADT:C₆₀ blends are lower than those in the bilayers. In bulk-heterojunction photovoltaics, the lower energy CT states present in the mixed phase with the halogenated ADT derivatives will likely decrease the probability of charge separation and increase charge recombination. The less favorable energy landscapes observed upon halogenation suggest that the benefits of fluorination observed in many OPV material systems may be more due to morphological factors.