Polymorphism in Non‐Fullerene Acceptors Based on Indacenodithienothiophene

Abstract

Organic solar cells incorporating non-fullerene acceptors (NFAs) have reached remarkable power conversion efficiencies of over 18%. Unlike fullerene derivatives, NFAs tend to crystallize from solutions, resulting in bulk heterojunctions that include a crystalline acceptor phase. This must be considered in any morphology-function models. Here, it is confirmed that high-performing solution-processed indacenodithienothiophene-based NFAs, i.e., ITIC and its derivatives ITIC-M, ITIC-2F, and ITIC-Th, exhibit at least two crystalline forms. In addition to highly ordered polymorphs that form at high temperatures, NFAs arrange into a low-temperature metastable phase that is readily promoted via solution processing and leads to the highest device efficiencies. Intriguingly, the low-temperature forms seem to feature a continuous network that favors charge transport despite of a poorly order along the π-π stacking direction. As the optical absorption of the structurally more disordered low-temperature phase can surpass that of the more ordered polymorphs while displaying comparable-or even higher-charge transport properties, it is argued that such a packing structure is an important feature for reaching highest device efficiencies, thus, providing guidelines for future materials design and crystal engineering activities.

Reconciling the Driving Force and the Barrier to Charge Separation in Donor–Nonfullerene Acceptor Films

“…These moderate μ e estimates (ca. 10 –4 cm 2 V –1 s –1 ) for NFA phases are consistent with reports of significant polymorphism and disordered packing in similar NFAs based on indacenodithienothiophene (IDTT) . Significant yield enhancements for BTID-2F blends are consistent with the more pronounced dependence of ϕΣμ on laser fluence shown in Figure S9f.…”

Strong Intermolecular Interactions Induced by High Quadrupole Moments Enable Excellent Photostability of Non‐Fullerene Acceptors for Organic Photovoltaics

“…[40] These stronger interactions also result in a stronger propensity of ITIC-2F to pack as evidenced by the stronger diffraction peaks observed for ITIC-2F compared to ITIC in as-cast films (using the same conditions used here). [41] This increased crystallinity in ITIC-2F films is also likely to reduce the amount of triplet formation, [42,43] closing off the triplet mediated isomerization. [16] The improved packing of ITIC-2F would also inhibit any conformational or isomeric changes due to the close packing of molecules restricting molecular motion and improving ITIC-2F photostability in the solid state relative to ITIC and ITIC-DM.…”

Delicate crystallinity control enables high-efficiency P3HT organic photovoltaic cells

“…Different from fullerene derivatives, nonfullerene acceptors exhibited much more complex aggregation characteristics and often crystallized in various crystalline forms. 45–48 However, our attempts to capture crystalline signals by growing single crystals of ZY-4Cl or differential scanning calorimetry (DSC) measurements failed, which prevented implementing a more detailed analysis of its polymorphism. As shown in Fig.…”