(750b) 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.