Regioregular-block-Regiorandom Poly(3-hexylthiophene) Copolymers for Mechanically Robust and High-Performance Thin-Film Transistors

Design of Microstructure-Engineered Polymers for Energy and Environmental Conservation

With the ever-growing demand for sustainability, designing polymeric materials using readily accessible feedstocks provides potential solutions to address the challenges in energy and environmental conservation. Complementing the prevailing strategy of varying chemical composition, engineering microstructures of polymer chains by precisely controlling their chain length distribution, main chain regio-/stereoregularity, monomer or segment sequence, and architecture creates a powerful toolbox to rapidly access diversified material properties. In this Perspective, we lay out recent advances in utilizing appropriately designed polymers in a wide range of applications such as plastic recycling, water purification, and solar energy storage and conversion. With decoupled structural parameters, these studies have established various microstructure-function relationships. Given the progress outlined here, we envision that the microstructure-engineering strategy will accelerate the design and optimization of polymeric materials to meet sustainability criteria.

Interfacial and confined molecular-assembly of poly(3-hexylthiophene) and its application in organic electronic devices

Poly(3-hexylthiophene) (P3HT) is a typical conducting polymer widely used in organic thin-film transistors, polymer solar cells, etc., due to good processability and remarkable charging carrier and hole mobility. It is known that the ordered structure assembled by π-conjugated P3HT chains could promote the performance of electronic devices. Interfacial and confined molecular-assembly is one effective way to generate an ordered structure by tuning surface geometry and substrate interaction. Great efforts have been made to investigate the molecular chain assembly of P3HT on a curved surface that is confined to different geometry. In this report, we review the recent advances of the interfacial chain assembly of P3HT in a flat or curved confined space and its application to organic electronic devices. In principle, this interfacial assembly of P3HT at a nanoscale could improve the electronic properties, such as the current transport, power conversion efficiency, etc. Therefore, this review on interfacial and confined assembly of P3HT could give general implications for designing high-performance organic electronic devices.

Material Design and Device Fabrication Strategies for Stretchable Organic Solar Cells

Recent advances in the power conversion efficiency (PCE) of organic solar cells (OSCs) have greatly enhanced their commercial viability. Considering the technical standards (e.g., mechanical robustness) required for wearable electronics, which are promising application platforms for OSCs, the development of fully stretchable OSCs (f-SOSCs) should be accelerated. Here, a comprehensive overview of f-SOSCs, which are aimed to reliably operate under various forms of mechanical stress, including bending and multidirectional stretching, is provided. First, the mechanical requirements of f-SOSCs, in terms of tensile and cohesion/adhesion properties, are summarized along with the experimental methods to evaluate those properties. Second, essential studies to make each layer of f-SOSCs stretchable and efficient are discussed, emphasizing strategies to simultaneously enhance the photovoltaic and mechanical properties of the active layer, ranging from material design to fabrication control. Key improvements to the other components/layers (i.e., substrate, electrodes, and interlayers) are also covered. Lastly, considering that f-SOSC research is in its infancy, the current challenges and future prospects are explored.

Polyurethane-Based Stretchable Semiconductor Nanofilms with High Intrinsic Recovery Similar to Conventional Elastomers

Polymer semiconductors with large elastic recovery (ER) under high strain in thin film state are highly desirable for stretchable electronics. Here we report a type of stretchable semiconductor PU(DPP)x, by copolymerization of oligodiketopyrrolopyrrole-based conjugated block and hydrogenated polybutadiene flexible block via urethane linkage for intermolecular hydrogen bonding. By regulating block ratio, PU(DPP)35 with 35 wt % conjugated block exhibits high intrinsic ER > 80% under 175% strain (ε) in pseudo free-standing thin film state, comparable with commercial elastomers, and crack onset strain (COS) > 300% along with maximum hole mobility of 0.19 cm2 V-1 s-1 in organic thin film transistors to bring it to the best performing block copolymer-type stretchable semiconductors. Enhanced mobility is achieved using PU(DPP)35 as the binder for conjugated polymer PDPPT3. The 25 wt %-PDPPT3 blend displays mobility up to 1.28 cm2 V-1 s-1 along with COS ∼120%, and 10 wt %-PDPPT3 blend exhibits ER of 78% at ε = 150%, COS of ∼230%, modulus of 36.5 MPa, maximum mobility of 0.62 cm2 V-1 s-1 and no obvious degradation of mobility at ε = 150% after 100 cycles of strain. Moreover, the structural similarity enables the blend film uniform and stable microstructure against mechanical and thermal deformation. Notably, PU(DPP)35 and the blend are characterized by high mechanical performance similar to that of commercial elastomers in thin film state, and demonstrate their potential for high performance stretchable electronics.

Thermomechanical Behavior of Poly(3-hexylthiophene) Thin Films on the Water Surface

The thermomechanical behavior of a conjugated polymer (CP) in a thin film state has rarely been studied despite the importance of understanding the polymer morphologies and optimizing the thermal processes of organic semiconductors. Moreover, the seamless integration of multilayers without mechanical failures in CP-based electronic devices is crucial for determining their operational stability. Large differences in the coefficients of thermal expansion (CTEs) between the multilayers can cause serious degradation of devices under thermal stress. In this study, we measure the intrinsic thermomechanical properties of poly(3-hexylthiophene) (P3HT) thin films in a pseudo-freestanding state on the water surface. The as-cast P3HT thin films exhibited a large thermal shrinkage (-1001 ppm K^-1) during heating on the water surface. Morphological analyses revealed that the thermal shrinkage of the polymer films was caused by the rearrangement of the polymer chain networks accompanied by crystallization, thus indicating that preheating the polymer films is essential for estimating their intrinsic CTE values. Moreover, the rigidity of the substrate significantly influences the thermomechanical behavior of the polymer films. The polymer films that were preheated on the glass substrate showed nonlinear thermal expansion due to the substrate constraint inhibiting sufficient relaxation of the polymer chains. In comparison, a linear expansion behavior is observed after preheating the films on the water surface, exhibiting a consistent CTE value (185 ppm K^-1) regardless of the number of thermal strain measurements. Thus, this work provides a direct method for measuring in-plane CTE values and an in-depth understanding of the thermomechanical behaviors of CP thin films to design thermomechanically reliable organic semiconductors.

High-Performance, Flexible NO2 Chemiresistors Achieved by Design of Imine-Incorporated n-Type Conjugated Polymers

Flexible and mechanically robust gas sensors are the key technologies for wearable and implantable electronics. Herein, the authors demonstrate the high-performance, flexible nitrogen dioxide (NO2 ) chemiresistors using a series of n-type conjugated polymers (CPs: PNDIT2/IM-x) and a polymer dopant (poly(ethyleneimine), PEI). Imine double bonds (C = N) are incorporated into the backbones of the CPs with different imine contents (x) to facilitate strong and selective interactions with NO2 . The PEI provides doping stability, enhanced electrical conductivity, and flexibility. As a result, the NO2 sensors with PNDIT2/IM-0.1 and PEI (1:1 by weight ratio) exhibit outstanding sensing performances, such as excellent sensitivity (ΔR/Rb = 240% @ 1 ppm), ultralow detection limit (0.1 ppm), high selectivity (ΔR/Rb < 8% @ 1 ppm of interfering analytes), and high stability, thereby outperforming other state-of-the-art CP-based chemiresistors. Furthermore, the thin film of PNDIT2/IM-0.1 and PEI blend is stretchable and mechanically robust, providing excellent flexibility to the NO2 sensors. Our study contributes to the rational design of high-performance flexible gas sensors.

Intrinsically Elastic Organic Semiconductors (IEOSs)

Elastic semiconductors are becoming more and more important to the development of flexible wearable electronic devices, which can be prepared by structural engineering design, blending, and the intrinsic elastification of organic semiconductors (intrinsically elastic organic semiconductor, IEOS). Compared with the elastic semiconductors prepared by structural engineering and blending, the IEOS prepared by organic synthesis has attracted numerous attentions for its solution processability and highly tunable chemical structures. For IEOSs, reasonable designs of synthetic routes and methods are the basis for realizing good mechanical and electrical properties. This brief review begins with a concise introduction of elastic semiconductors, then follows with several synthetic methods of IEOSs, and concludes the characteristics of each method, which provides guidance for the synthesis of IEOSs in the future. Furthermore, the properties of IEOSs are involved from the aspects of electrical, mechanical properties, and the applications of the IEOSs in elastic electronic devices. Finally, the challenge and an outlook which IEOSs are facing are presented in conclusion.

Polythiophene-Doped Resorcinol-Formaldehyde Resin Photocatalysts for Solar-to-Hydrogen Peroxide Energy Conversion

The generation of hydrogen peroxide (H₂O₂) from water and dioxygen by sunlight-driven heterogeneous photocatalysis is a promising method for the artificial photosynthesis of a liquid solar fuel. We previously found that resorcinol-formaldehyde (RF) resin powders prepared by high-temperature hydrothermal synthesis act as highly active semiconductor photocatalysts for H₂O₂ generation. Herein, we report that RF resin powders doped with poly(3-hexylthiophene-2,5-diyl) (RF/P3HT) exhibit enhanced photocatalytic activities. The highly dispersed P3HT within the resin particles created charge transfer complexes with the conduction band of the resin via electron donation, facilitating efficient transfer of the photogenerated conduction band electrons through P3HT. This enhanced charge separation promoted efficient water oxidation and O₂ reduction. The solar-to-chemical conversion efficiency for H₂O₂ generation on the RF/P3HT resin in water under simulated sunlight irradiation with atmospheric pressure of O₂ was ∼1.0%, which is the highest efficiency reported for powder catalysts in artificial photosynthesis.

Electronic effects of nano-confinement in functional organic and inorganic materials for optoelectronics

This review provides a comprehensive overview of the electronic effects of nano-confinement (from 1D to 3D geometries) on optoelectronic materials and their applications.

The Effect of Alkyl Chain Length on Well-Defined Fluoro-Arylated Polythiophenes for Temperature-Dependent Morphological Transitions

Understanding the relationship between the molecular structure and morphological behaviors of well-defined semiconducting polymers is essential for developing novel conjugated building blocks and determining the origin of the functional characteristics of semiconducting polymers. Here, we provide insights into the significant temperature-dependent morphological transitions of novel well-defined polythiophene derivatives with m-alkoxy-substituted fluoro-aryl side units: poly(3-(4-fluoro-3-(hexyloxy)phenyl)thiophene) (PHFPT) and poly(3-(4-fluoro-3-(dodecyloxy)phenyl)thiophene) (PDFPT). We found that these unique morphological transitions depend on the alkyl chain length of the substituted fluoro-aryl side units. In PHFPT with short alkyl chains, the thermal treatment promotes a crowded interdigitated packing structure, resulting in narrow lamellar spacings in its crystalline structure. In contrast, the long alkyl chain of PDFPT acts as a physical spacer and disturbs the crowded interdigitation. In addition, the thermal treatment induces the backbone planarization and an ordered packing morphology in PDFPT. These demonstrations provide a critical milestone for the phase transitions of semiconducting polymers with conjugated side units.

Configurationally Random Polythiophene for Improved Polymer Ordering and Charge-Transporting Ability

Random polythiophene polymers are characterized by the arbitrary sequences of monomeric units along polymer backbones. These untailored orientations generally result in the twisting of thiophene rings out of the conjugation planarity in addition to steric repulsions experienced among substituted alkyl chains. These tendencies have limited close polymer packing, which has been detrimental to charge transport in these moieties. To ameliorate charge transport in these classes of polymers, we make use of simple Stille coupling polymerization to synthesize highly random polythiophene polymers. We induced a positive microstructural change between polymer chains by attuning the ratio between alkyl-substituted and nonalkyl-substituted monomer units along the backbones. The optimized random polythiophene was found to have enhanced intermolecular interaction, increased size of crystallites, and stronger tendency to take edge orientation compared with both regiorandom and regioregular poly(3-hexylthiophene) polymers. Incorporation of the optimized random polythiophene as an active material in solid-state electrolyte-gated organic field-effect transistors exhibited better performance than the control device using regioregular poly(3-hexylthiophene), with a high hole mobility up to 4.52 cm² V^-1 s^-1 in ambient conditions.

Direct Synthesis of Chain-end-functionalized Poly(3-hexylthiophene) without Protecting Groups Using a Zincate Complex

Chain-end-functionalized poly(3-hexylthiophene)s (P3HTs) with benzyl alcohol (─PhCH₂ OH), phenol (─PhOH), and benzoic acid (─PhCOOH) groups are directly synthesized based on the Negishi catalyst-transfer polycondensation method utilizing the zincate complex of ^t Bu₄ ZnLi₂ . In this system, neither protection nor deprotection steps are required, and also providing a living polymerization system to control the molecular weight while maintaining a low molar mass dispersity (Ð_M ) of the obtained P3HT derivatives. Indeed, the chain-end-functionalized P3HTs can be synthesized along with controlled number-average molecular weights (M_n = 5100-20 000), low Ð_M (1.06-1.14), and high chain-end functionality (Fn = 46-86%). The Fn values for the alcohol and phenol groups are found to be high (86% for ─PhCH₂ OH and 71% for ─PhOH based on ¹ H NMR, respectively), as also confirmed by matrix-assisted laser desorption/ionization time of flight mass spectroscopy. The easily synthesizable chain-end-functionalized P3HTs will be applicable for the facile synthesis of block and branched polymers containing P3HT as well as its related semiconducting polymer segments.

Enhanced Vertical Charge Transport of Homo- and Blended Semiconducting Polymers by Nanoconfinement

The morphology of conjugated polymers has critical influences on electronic and optical properties of optoelectronic devices. Even though lots of techniques and methods are suggested to control the morphology of polymers, very few studies have been performed inducing high charge transport along out-of-plane direction. In this study, the self-assembly of homo- and blended conjugated polymers which are confined in nanostructures is utilized. The resulting structures lead to high charge mobility along vertical direction for both homo- and blended conjugated polymers. Both semicrystalline and amorphous polymers show highly increased population of face-on crystallite despite intrinsic crystallinity of polymers. They result in more than two orders of magnitude enhanced charge mobility along vertical direction revealed by nanoscale conductive scanning force microscopy and macroscale IV characteristic measurements. Moreover, blends of semicrystalline and amorphous polymers, which are known to show inferior optical and electrical properties due to their structural incompatibility, are formed into harmonious states by this approach. Assembly of blends of semicrystalline and amorphous polymers under nanoconfinement shows charge mobility in out-of-plane direction of 0.73 cm² V^-1 s^-1 with wide range of absorption wavelength from 300 to 750 nm demonstrating the synergistic effects of two different polymers.