Nanoscale mobility mapping in semiconducting polymer films

Three novel polysulfide-based conjugated polymers and characterization of their optoelectronic properties

The aim of this study was to investigate the effect of side chain size on the optical and charge transport properties of thin films prepared from novel conjugated polysulfide-based polymers. Three polymers, labeled P1, P2, and P3, were derived from polysulfide derivatives and had different arylene groups (5,5'- biphenylene, 4,4'-biphenylene, and 2,6-pyridylene). Optical analysis was performed using photoluminescence (PL) and UV-visible absorption spectroscopy, revealing an energy band gap of 2.41-3.02 eV; P1 emitted yellow, P2 blue-green, and P3 green. Cyclic voltammetry measurements of the electrochemical band gap and HOMO and LUMO energy levels revealed that the polymer exhibited p-type semiconductor activity; the electrical properties of diodes based on the ITO/polysulfide derivative/Al structure were explored through analysis of current-voltage characteristics. The current space charge limitation (SCLC) mechanism was used to model the behavior of these diodes; the P2 thin film layer exhibited higher mobility than the other layers. The relationship between the geometry of the polymer thin films and their optical and electrical properties was thoroughly investigated.

Reproducibility in Time and Space-The Molecular Weight Effects of Polymeric Materials in Organic Photovoltaic Devices

The reproducibility issue is one of the major challenges for the commercialization of large-area organic electronic devices. It involves both the device-to-device variation and opto-electronic properties in different positions of a single thin film. Herein, the molecular weight effects in polymeric semiconductors with three widely used photovoltaic donor materials P3HT, PBDB-T, and PM6 are systematically investigated. A simple but effective method is proposed to evaluate the uniformity of large-area devices by adopting the micron-level grid electrodes in organic thin films. An interesting phenomenon is observed that the device is gradually improved uniformly with the M_w range lower than 100 kg mol^-1 . In neat films, both the mobility and energetic disorder values of hole carriers exhibit relatively lower coefficient of variation (c_v ) in high molecular-weight systems. After blending with the electron-accepting materials, their bulk heterojunction films also enjoy more uniform hole transfer rates, fluorescence lifetimes, and power conversion efficiencies in single and different devices. This work not only proposes a facile approach to evaluate the electrical properties of large-area organic thin films, but also demonstrates the relationship between molecular weight and device reproducibility in polymer solar cells. This contribution provides a new insight into the commercial large-scale production of organic electronics.

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.