AbstractPower conversion efficiency of organic solar cells increases continuously due the emergence of novel nonfullerene acceptors (NFAs). As solar cell efficiency is governed by photogeneration followed by charge carrier transport toward the electrode, understanding of univocal electron transport properties of NFAs is of great importance. Acceptors from the indacenodithienothiophene‐based family such as ITIC, ITIC‐Th, and ITIC‐4F have been intensively studied in solar cells. Importantly, ITIC‐ and ITIC‐4F‐based films evolve with increasing annealing temperature from 100 °C up to 250 °C into several polymorphs that may impact the electronic transport strongly. Here, for the first time, the effects of temperature‐dependent polymorphism on the charge transport properties of ITIC, ITIC‐Th, and ITIC‐4F are studied. A unipolar, high performance, and high‐temperature stable thin‐film transistor structure is developed first using divinyltetramethyldisiloxane‐bis(benzocyclobutene) as dielectric passivation material to explore the intrinsic charge transport up to 240 °C. It is shown that electron mobility in the three ITIC‐based films is strongly influenced by the molecule specific polymorphism at optimal temperatures. This leads to a strong increase in electron mobility compared to the as‐cast films, which is correlated to changes in molecule aggregation, domain crystallinity, and orientation as well as intermolecular electronic coupling.