Power 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.

The synthesis of four non-fullerene acceptors (NFAs) with a "Aπ-D-π-A" structure, in which the electron-donating core is extended, was achieved. The molecules differed by the nature of the solubilizing groups on the π-spacer and/or the presence of fluorine atoms on the peripheral electron-accepting units. The optoelectronic properties of the molecules were characterized in solution, in thin film, and in photovoltaic devices. The nature of the solubilizing groups had a minor influence on the optoelectronic properties but affected the organization in the solid state. On the other hand, the fluorine atoms influenced the optoelectronics properties and increased the photo-stability of the molecules in thin films. Compared to reference ITIC, the extended molecules showed a wider absorption across the visible range and higher lowest unoccupied molecular orbital energy levels. The photovoltaic performances of the four NFAs were assessed in binary blends using PM6 (PBDB-T-2F) as the donating polymer and in ternary blends with ITIC-4F. Solar cells (active area 0.27 cm 2 ) showed power conversion efficiencies of up to 11.1 % when ternary blends were processed from nonhalogenated solvents, without any thermal post-treatment or use of halogenated additives, making this process compatible with industrial requirements.

The nanoscale morphology of polymer blends is a key parameter to reach high efficiency in bulk heterojunction solar cells. Thereby, research typically focuses on optimal blend morphologies while studying non-optimized blends may give insight into blend design that can be more robust against morphology defects. Here we focus on the direct correlation of morphology and device performance of PTB7:PC71BM bulk heterojunction (BHJ) blends processed without additive in different donor:acceptor weight ratios. We show that while blends of a 1:1.5 ratio are composed of large donor enriched and fullerene domains beyond exciton diffusion length, reducing the ratio below 1:0.5 leads to blends composed purely of polymer enriched domains. Importantly photocurrent density in such blends can reach values between 45 to 60% of those reached of fully optimized blend using additives. We provide here a direct visual evidence that fullerenes in the donor enriched domains are not distributed homogeneously but fluctuate locally. To this end, we performed compositional nanoscale morphology analysis of the blend using spectroscopic imaging of low energy-loss electrons in the transmission electron microscope. Charge transport measurement in combination with molecular dynamics simulations show that the fullerene sub-structures inside the polymer phase generate efficient electron transport in the polymer enriched phase. Furthermore, we show that the formation of densely 3 packed regions of fullerene inside the polymer phase is driven by the PTB7:PC71BM enthalpy of mixing. The occurrence of such a nanoscale network of fullerene clusters leads to a reduction of electron trap states and thus efficient extraction of photocurrent inside the polymer domain. Suitable tuning of the polymer acceptor interaction can thus introduce acceptor sub-networks in polymer enriched phases improving the tolerance for high efficiency BHJ towards morphological defects such as donor enriched domains exceeding exciton diffusion length.