Well-Ordered Poly(3-alkylthiophene) Diblock Copolymers for Organic Photovoltaic Applications

Conjugated, semiconducting macromolecules have risen to the fore as electron-donating materials in organic photovoltaics (OPVs). In fact, advances in the fabrication and post-processing of polymer–fullerene bulk heterojunction solar cells have allowed for devices with power conversion efficiencies approaching those of amorphous silicon devices to be realized. Better knowledge of how exciton (a bound electron-hole pair) dissociation and the internal morphology of the active layer affects device performance should facilitate processing optimization and, ultimately, lead to devices with higher power conversion efficiencies. Because block copolymers self-assemble on the same length scale as the exciton dissociation length in organic materials (~10 nm), diblock copolymers containing a rigid, semiconducting moiety (rod) covalently linked to a flexible block (coil) have attracted an ever-increasing amount of attention. In contrast to coil-coil diblock copolymers, conjugated rod-coil diblock copolymers self-assemble due to a balance of liquid crystalline (rod-rod) and enthalpic (rod-coil) interactions. Previous work has demonstrated that while coil-coil diblock copolymers self-assemble into a host of nanostructures with varying degrees of interfacial curvature, when rod-rod interactions dominate self-assembly in rod-coil block copolymers, lamellar structures with little curvature are preferred.

Here, it is demonstrated that non-lamellar, potentially more useful nanostructures can be formed when liquid crystalline and enthalpic interactions are more closely balanced through precise molecular design of the conjugated moiety. Specifically, hexagonally-packed cylinders of the polylactide (PLA) coil block embedded in a semiconducting poly(3-alkylthiophene) (P3AT) matrix are shown. Note that this particular semiconducting moiety, P3AT, is of great interest as polythiophenes are used commonly as active layer components in plastic electronic devices. Previous efforts to generate this phase in polythiophene-based block copolymers have failed due to the high driving force for P3AT crystallization at room temperature. By carefully designing the molecular architecture of the P3AT moiety, we are able to strike a balance between crystallization and microphase separation due to chemical dissimilarity between copolymer segments. In addition to hexagonally-packed cylinders, P3AT-PLA block copolymers form nanostructures (i.e., lamellae) with long-range order at almost all block copolymer compositions. Importantly, and despite the presence of large weight fraction of PLA present in the diblock copolymers (w_PLA > 0.50), the conjugated moiety of the P3AT-PLA block copolymers retains the crystalline packing structure and characteristic high time-of-flight (TOF) charge transport of the homopolymer polythiophene (μ_h ~10^-4 cm² V^-1 s^-1) in the confined block copolymer domains.