Macroscopic alignment of poly(3-hexylthiophene) for enhanced long-range collection of photogenerated carriers

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 of μ_out-of-plane and μ_in-plane are 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 about μ_in-plane and its optimization for devices, the current challenges are associated with μ_out-of-plane and its optimization for device performance. Therefore, this review is devoted to strategies for improving μ_out-of-plane in 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 improving μ_out-of-plane include solvent/field processing methods, chemical modification, thickness confinement, chemical additives, and different post-annealing strategies, including annealing with supercritical fluids. This review leverages the most recent fundamental understanding of mechanisms of charge transport and connections to morphology, identifying robust design strategies for targeted improvements of μ_out-of-plane.

Controlled polymer crystal/two-dimensional material heterostructures for high-performance photoelectronic applications

The control of atomically thin two-dimensional (2D) crystal-based heterostructures wherein the interfaces of 2D nanomaterials are vertically stacked with other thin functional materials via van der Waals interactions is highly important for not only optimizing the excellent properties of 2D nanomaterials, but also for utilizing the functionality of the contact materials. In particular, when 2D nanomaterials are combined with soft polymeric components, the resulting photoelectronic devices are potentially scalable and mechanically flexible, allowing the development of a variety of prototype soft-electronic devices, such as solar cells, displays, photodetectors, and non-volatile memory devices. Diverse polymer/2D heterostructures are frequently employed, but the performance of the devices with heterostructures is limited, mainly because of the difficulty in controlling the molecular structures of the polymers on the 2D surface. Thus, understanding the crystal interactions of polymers on atomically flat and dangling-bond-free surfaces of 2D materials is essential for ensuring high performance. In this study, the recent progress made in the development of thin polymer films fabricated on the surfaces of various 2D nanomaterials for high-performance photoelectronic devices is comprehensively reviewed, with an emphasis on the control of the molecular and crystalline structures of the polymers on the 2D surface.

Uniaxial orientation of P3HT film prepared by soft friction transfer method

The realization of room-temperature processes is an important factor in the development of flexible electronic devices composed of organic materials. In addition, a simple and cost-effective process is essential to produce stable working devices and to enhance the performance of a smart material for flexible, wearable, or stretchable-skin devices. Here, we present a soft friction transfer method for producing aligned polymer films; a glass substrate was mechanically brushed with a velvet fabric and poly(3-hexylthiophene) (P3HT) solution was then spin-coated on the substrate. A P3HT film with a uniaxial orientation was obtained in air at room temperature. The orientation factor was 17 times higher than that of a film prepared using a conventional friction transfer technique at a high temperature of 120 °C. In addition, an oriented film with a thickness of 40 nm was easily picked up and transferred to another substrate. The mechanism for orientation of the film was investigated using six experimental methods and theoretical calculation, and was thereby attributed to a chemical process, i.e., cellulose molecules attach to the substrate and act as a template for molecular alignment.