Direct Observation of Confinement Effects of Semiconducting Polymers in Polymer Blend Electronic Systems

Organic Artificial Nerves: Neuromorphic Robotics and Bioelectronics

Neuromorphic electronics are inspired by the human brain's compact, energy-efficient nature and its parallel-processing capabilities. Beyond the brain, the entire human nervous system, with its hierarchical structure, efficiently preprocesses complex sensory information to support high-level neural functions such as perception and memory. Emulating these biological processes, artificial nerve electronics have been developed to replicate the energy-efficient preprocessing observed in human nerves. These systems integrate sensors, artificial neurons, artificial synapses, and actuators to mimic sensory and motor functions, surpassing conventional circuits in sensor-integrated electronics. Organic synaptic transistors (OSTs) are key components in constructing artificial nerves, offering tunable synaptic plasticity for complex sensory processing and the mechanical flexibility required for applications in soft robotics and bioelectronics. Compared to traditional sensor-integrated electronics, early implementations of organic artificial nerves (OANs) incorporating OSTs have demonstrated a higher signal-to-noise ratio, lower power consumption, and simpler circuit designs along with on-device processing capabilities and precise control of actuators and biological limbs, driving progress in neuromorphic robotics and bioelectronics. This paper reviews the materials, device engineering, and system integration of the OAN design, highlights recent advancements in neuromorphic robotics and bioelectronics utilizing the OANs, and discusses current challenges and future research directions.

Two Is Better than One: How the Addition of Multiple Biodegradable Polymers Can Improve Organic Thin-Film Transistor Performance

Developing sustainable electronics requires using materials that are either recyclable or biodegradable, without compromising on electrical performance. Here, we introduce a solution-processed biodegradable polymer blend consisting of a diketopyrrolopyrrole-based semiconducting polymer (DPP2T) and different mixtures of two biodegradable polymers, polycaprolactone (PCL) and polylactic acid (PLA). We find that controlling the ratio of components enables a reduction in semiconductor polymer loading (∼70:80% reduction) while maintaining or improving field-effect transistor performance. At a ratio of 30 wt % DPP2T versus 70 wt % PLA and PCL (56:14 ratio), DPP2T self-assembled and crystallized inside the biodegradable polymer matrix, exhibiting a high field-effect mobility (0.18 cm2 V-1 s-1), a reduced threshold voltage (∼17 V), and a high on/off ratio (∼106). This study demonstrates that performance does not need to be sacrificed for biodegradability.

Harnessing Compositional Gradients to Elucidate Phase Behaviors toward High Performance Polymer Semiconductor Blends

Polymer semiconductor/insulator blends offer a promising avenue to achieve desired mechanical properties, environmental stability, and high device performance in organic field-effect transistors. A comprehensive understanding of process-structure-property relationships necessitates a thorough exploration of the composition space to identify transitions in performance, morphology, and phase behavior. Hence, this study employs a high-throughput gradient thin film library, enabling rapid and continuous screening of composition-morphology-device performance relationships in conjugated polymer blends. Applied to a donor-acceptor copolymer blend, this technique efficiently surveys a broad composition range, capturing trends in device performance across the gradient. Furthermore, characterizing the gradient library using microscopy and depth profiling techniques pinpointed composition-dependent transitions in morphology. To validate the results and gain deeper insights, uniform-composition experiments were conducted on select compositions within and outside the gradient range. Depth profiling experiments on the constant composition films unveil the presence of the semiconducting polymer at the air interface, with apparent enrichment of the semiconductor at the substrate interface at low ratios of the semiconducting component, transitioning to a more even distribution within the bulk of the film at higher ratios. The generalizability of the gradient approach was further confirmed by its application to a homopolymer under different solution processing conditions.

Enhanced Electrical Conductivity and Mechanical Properties of Stretchable Thermoelectric Generators Formed by Doped Semiconducting Polymer/Elastomer Blends

Recent research efforts have concentrated on the development of flexible and stretchable thermoelectric (TE) materials. However, significant challenges have emerged, including increased resistance and reduced electrical conductivity when subjected to strain. To address these issues, rigid semiconducting polymers and elastic insulating polymers have been incorporated and nanoconfinement effects have been exploited to enhance the charge mobility. Herein, a feasible approach is presented for fabricating stretchable TE materials by using a doped semiconducting polymer blend consisting of either poly(3-hexylthiophene) (P3HT) or poly(3,6-dithiophen-2-yl-2,5-di(2-decyltetradecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-thienylenevinylene-2,5-yl) (PDVT-10) as the rigid polymer with styrene-ethylene-butylene-styrene (SEBS) as the elastic polymer. In particular, the blend composition is optimized to achieve a continuous network structure with SEBS, thereby improving the stretchability. The optimized polymer films exhibit well-ordered microstructural aggregates, indicative of good miscibility with FeCl3 and enhanced doping efficiency. Notably, a lower activation energy and higher charge-carrier concentration contribute to an improved electrical conductivity under high tensile strain, with a maximum output power of 1.39 nW at a ΔT of 22.4 K. These findings offer valuable insights and serve as guidelines for the development of stretchable p-n junction thermoelectric generators based on doped semiconducting polymer blends with potential applications in wearable electronics and energy harvesting.

Molecular Design of Stretchable Polymer Semiconductors: Current Progress and Future Directions

Stretchable polymer semiconductors have advanced rapidly in the past decade as materials required to realize conformable and soft skin-like electronics become available. Through rational molecular-level design, stretchable polymer semiconductor films are now able to retain their electrical functionalities even when subjected to repeated mechanical deformations. Furthermore, their charge-carrier mobilities are on par with the best flexible polymer semiconductors, with some even exceeding that of amorphous silicon. The key advancements are molecular-design concepts that allow multiple strain energy-dissipation mechanisms, while maintaining efficient charge-transport pathways over multiple length scales. In this perspective article, we review recent approaches to confer stretchability to polymer semiconductors while maintaining high charge carrier mobilities, with emphasis on the control of both polymer-chain dynamics and thin-film morphology. Additionally, we present molecular design considerations toward intrinsically elastic semiconductors that are needed for reliable device operation under reversible and repeated deformation. A general approach involving inducing polymer semiconductor nanoconfinement allows for incorporation of several other desired functionalities, such as biodegradability, self-healing, and photopatternability, while enhancing the charge transport. Lastly, we point out future directions, including advancing the fundamental understanding of morphology evolution and its correlation with the change of charge transport under strain, and needs for strain-resilient polymer semiconductors with high mobility retention.

Improving Charge Injection at Gold/Conjugated Polymer Contacts by Polymer Insulator-Assisted Annealing for Transistors

The poor chemical miscibility between metal and organic materials usually leads to both structural and energetic mismatches at gold/organic interfaces, and thereby, high contact resistance of organic electronic devices. This study shows that the contact resistance of organic field-effect transistors is significantly reduced by one order of magnitude, by reforming the contact interface between gold electrodes and conjugated polymers upon a polymer insulator-assisted thermal annealing. Upon an optimized solution process, the conjugated polymer is homogenously distributed within the amorphous polymer insulator matrix with relatively low glass transition temperature, and thus, even a moderate annealing temperature can induce sufficient motion of conjugated polymer chains to simultaneously adjust the polymer orientation and improve the packing of gold atoms. Consequently, gold/conjugated polymer contact is reorganized after annealing, which improves both charge transport from bulk gold to interface and charge injection from gold into conjugated polymers. This method, with appropriate insulator matrix, is effective for improving the injection of both holes and electrons, and widely applicable for many unipolar and ambipolar conjugated polymers to optimize the device performance and simultaneously increase the optical transparency (over 80%). A frequency doubler and a phase modulator are demonstrated, respectively, using the ambipolar transistors with optimized charge injection properties.