Morphological Design Strategies to Tailor Out-of-Plane Charge Transport in Conjugated Polymer Systems for Device Applications

The transport of charge carriers throughout an active conjugated polymer (CP) host, characterized by a heterogeneous morphology of locally varying degrees of order and disorder, profoundly influences the performance of CP-based electronic devices, including diodes, photovoltaics, sensors, and supercapacitors. Out-of-plane charge carrier mobilities (μ_out-of-plane) across the bulk of the active material host and in-plane mobilities (μ_in-plane) parallel to a substrate are highly sensitive to local morphological features along their migration pathways. In general, the magnitudes ofμ_out-of-planeandμ_in-planeare very different, in part because these carriers experience different morphological environments along their migration pathways. Suppressing the impact of variations in the morphological order/disorder on carrier migration remains an important challenge. While much is known aboutμ_in-planeand its optimization for devices, the current challenges are associated withμ_out-of-planeand its optimization for device performance. Therefore, this review is devoted to strategies for improvingμ_out-of-planein neat CP films and the implications for more complex systems, such as D:A blends which are relevant to OPV devices. The specific strategies discussed for improvingμ_out-of-planeinclude solvent/field processing methods, chemical modification, thickness confinement, chemical additives, and different post-annealing strategies, including annealing with supercritical fluids.