Conjugated Polymer Thin Films Research Articles

A Molecular Dynamics Study of Mechanical and Conformational Properties of Conjugated Polymer Thin Films.

Understanding and predicting the mechanical and conformational properties of conjugated polymer (CP) thin films are a central focus in flexible electronic device research. Employing molecular dynamics simulations with an architecture-transferable chemistry-specific coarse-grained (CG) model of poly(3-alkylthiophene)s (P3ATs), developed by using an energy renormalization approach, we investigate the mechanical and conformational behavior of P3AT thin films during deformation. The density profiles and measures of local mobility identify a softer interfacial layer for all films, the thickness of which does not depend on M w or side-chain length. Remarkably, Young's modulus measured via nanoindentation is more sensitive to M w than for tensile tests, which we attribute to distinct deformation mechanisms. High-M w thin films show increased toughness, whereas longer side-chain lengths of P3AT resulted in lower Young's modulus. Fractures in low-M w thin films occur through chain pullout due to insufficient chain entanglement and crazing in the plastic region. Importantly, stretching promoted both chain alignment and longer conjugation lengths of P3AT, potentially enhancing its electronic properties. For instance, at room temperature, stretching P3HT thin films to 150% increases the conjugated length of P3HT thin films from 2.7 nm to 4.7 nm, aligning with previous experimental findings and all-atom simulation results. Furthermore, high-M w thin films display elevated friction forces due to the chain accumulation on the indenter, with negligible variations in the friction coefficient across all thin film systems. These findings offer valuable insights that enhance our understanding and guide the rational design of CP thin films in flexible electronics.

2D Conjugated Polymer Thin Films for Organic Electronics: Opportunities and Challenges.

2D conjugated polymers (2DCPs) possess extended in-plane π-conjugated lattice and out-of-plane π-π stacking, which results in enhanced electronic performance and potentially unique band structures. These properties, along with predesignability, well-defined channels, easy postmodification, and order structure attract extensive attention from material science to organic electronics. In this review, the recent advance in the interfacial synthesis and conductivity tuning strategies of 2DCP thin films, as well as their application in organic electronics is summarized. Furthermore, it is shown that, by combining topology structure design and targeted conductivity adjustment, researchers have fabricated 2DCP thin films with predesigned active groups, highly ordered structures, and enhanced conductivity. These films exhibit great potential for various thin-film organic electronics, such as organic transistors, memristors, electrochromism, chemiresistors, and photodetectors. Finally, the future research directions and perspectives of 2DCPs are discussed in terms of the interfacial synthetic design and structure engineering for the fabrication of fully conjugated 2DCP thin films, as well as the functional manipulation of conductivity to advance their applications in future organic electronics.

Meticulous Molecular Engineering of Crystal Orientation and Morphology in Conjugated Polymer Thin Films for Field-Effect Transistors.

The ability to precisely tailor molecular packing and film morphology in conjugated polymers offers a robust means to control their optoelectronic properties. This, however, remains a grand challenge. Herein, we report the dependency of molecular packing of an important conjugated polymer, poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT), on a set of intrinsic parameters and unveil the correlation between their crystalline structures and charge transport characteristics. Specifically, a family of PBTTT with varying side chains (i.e., hexyl, octyl, decyl, dodecyl, tetradecyl, and hexadecyl referred to as C6, C8, C10, C12, C14, and C16, respectively) and molecular weights (MWs) with a focus on C14 are judiciously designed and synthesized. Various crystalline structures are yielded by tuning the alkyl chain and MW of PBTTT together with thermal annealing. It reveals that extending the alkyl chain length of PBTTT to C14, along with a larger MW and heating at 180 °C, promotes the formation of edge-on crystallites with significantly improved orientation and ordering. Furthermore, these distinct crystalline structures greatly impact their charge mobilities. This study sheds light on the tailored design of crystalline structures in PBTTT through a synergetic approach, which paves the way for potential applications of PBTTT and other conjugated polymers in optoelectronic devices with enhanced performance.

Characterization of nanoscale morphology and mechanical properties of conjugated polymer thin films dynamically exposed to a secondary solvent

Solution processing techniques are often used to enhance intra and interchain order in semiconducting conjugated polymer thin films used in the active layer of organic optoelectronic devices. While there has been extensive research into the impact of solution processing on the local structural and electronic properties of these films, the nanomechanical properties of the films are less well understood and are challenging to characterize. We investigate the nanomechanical properties of conjugated polymer thin films arising from solution processing techniques. Our study relies on dynamic secondary solvent dripping to induce subtle changes to the film morphology. We examine thin films of P3HT, PCDTBT, PTB7, and PBDB-T-SF conjugated polymers, commonly used in organic photovoltaics, with the bimodal amplitude modulation-frequency modulation (AM-FM) imaging mode of the atomic force microscopy (AFM) viscoelastic method to probe their local physical and mechanical properties. We find that Young's Modulus data measured by AM-FM AFM can detect additional changes in film properties not discernible by other commonly used bulk thin-film characterization techniques. For PBDB-T-SF, we detect an increase in molecular order that is not noticable by UV–visible absorption spectroscopy, X-ray diffraction or nanoindentation. PCDTBT, the most amorphous of the polymers studied, shows no changes in absorption or X-ray diffraction data, yet clear changes in Young's Modulus were detected by AFM. On the other hand, increases in P3HT order that occurred due to dynamic secondary solvent dripping are detected in both the bulk and AFM measurements. Our study demonstrates the applicability of nanomechanical measurements for characterizing local structural variations in conjugated polymer films. This work is relevant to ongoing efforts to control and understand the complex structure-property-processing relationships of conjugated polymer thin films.

Dopant Enhanced Conjugated Polymer Thin Film for Low-Power, Flexible and Wearable DMMP Sensor.

Conjugated polymer has the potential to be applied on flexible devices as an active layer, but further investigation is still hindered by poor conductivity and mechanical stability. Here, this work demonstrates a dopant-enhanced conductive polymer thin film and its application in dimethyl methylphosphonate (DMMP) sensor. Among five comparable polymers this work employs, poly(bisdodecylthioquaterthiophene) (PQTS12) achieves the highest doping efficiency after doped by FeCl3, with the conductivity increasing by about five orders of magnitude. The changes in Young's modulus are also considered to optimize the conductivity and flexibility of this thin film, and finally the decay of conductivity is only 9.2% after 3000 times of mechanical bending. This work applies this thin film as the active layer of the DMMP gas sensor, which could be operated under 1mV driving voltage and 28 nW power consumption, with a sustainable durability against bending and compression. In addition, this sensor is provided with alarm capability while exposed to the DMMP atmospheres at different hazard levels. This work expects that this general approach could offer solutions for the fabrication of low-power and flexible gas sensors, and provide guidance for next-generation wearable devices with broader applications.

Multicolor Conjugated Polymer Thin Films with Tunable Responsivity to Oxidative and Reductive Environments.

Dynamic colors that respond to environmental changes are of great interest for diverse areas of science and technology ranging from chemical and biological sensors to smart information display. Here, we demonstrate a multitude of responsive colors from a conjugated polymer film arising from a thin-film interference. This mechanism provides an excellent control over the thin-film color by varying the film thickness, type of substrate, and degree of polaron population and is generally applicable to various conjugated polymers for further color variation. Furthermore, multiple sets of responsive colors are achieved from a single polymer layer by patterning the underlying substrate to spatially modify the interference conditions. Using this system, we demonstrate the reversible color changes induced by an oxidative or reductive environment with color responsivity controllable with the nature of the polaron state.

Improved NO2 Gas-Sensing Performance of an Organic Field-Effect Transistor Based on Reduced Graphene Oxide-Incorporated Nanoporous Conjugated Polymer Thin Films

Improved NO₂ Gas-Sensing Performance of an Organic Field-Effect Transistor Based on Reduced Graphene Oxide-Incorporated Nanoporous Conjugated Polymer Thin Films

Improving Carrier Mobility of Near-Amorphous Donor–Acceptor Conjugated Polymer Thin Films via Promoting Intensive and Continuous Polymer Aggregations

Improving Carrier Mobility of Near-Amorphous Donor–Acceptor Conjugated Polymer Thin Films via Promoting Intensive and Continuous Polymer Aggregations

Direct Observation of Two-Step, Stratified Crystallization and Morphology in Conjugated Polymer Thin Films

A two-step stratified crystallization process has been directly observed during cooling of poly(3-hexylthiophene) (P3HT) thin films of thickness h = 20–250 nm: a thin (<20 nm) layer at the free surface crystallizes ∼25 °C higher than the bulk crystallization temperature (TC,bulk), whereas the rest of the film (i.e., the underlayer) crystallizes near TC,bulk. In situ measurements of films with and without a free surface unequivocally ascribe the high-temperature crystallization to a surface-induced process, which correlates with the formation of large birefringent structures and highly oriented edge-on crystallites at the air–polymer interface. In contrast, crystallization of the bulk-like underlayer leads to mostly edge-on-oriented crystallites in thinner films and becomes increasingly isotropic in thicker films. For h < 20 nm, free-surface effects dominate and only high-temperature crystallization is observed. These results highlight the potential of melt crystallization to tailor morphology and orientation across thin film thickness for specific electronic applications.

Effect of solvent on the emulsion and morphology of polyfluorene films: all-atom molecular dynamics approach.

The morphology of conjugated polymer thin films deposited by the resonant infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) process is related to the emulsion characteristics. However, a fundamental understanding of how and why the emulsion characteristics control the film properties and device performance is yet unclear. We performed all-atom molecular dynamics simulations of emulsions containing a mixture of polyfluorene (PFO) polymer, various primary solvents, secondary solvent, and water. The emulsion properties were then examined as a function of variable primary solvent and correlated with the morphology of deposited PFO thin films. The examination of the explicit interactions between all components of the emulsion indicated that using a primary solvent with a lower solubility-in-water and a higher non-bonded interaction energy ratio, between the solvent, polymer, and water in the emulsion recipe, produced the best result with smoother and denser films. Additionally, our simulation results are consistent with the AFM experimental results, indicating that interactions driven by trichlorobenzene (TCB) primary solvent within the emulsion are responsible for high-quality, smooth, and continuous thin film surfaces. Overall, this study can support the choice of a suitable primary solvent and provides the computational framework for predictions of new recipes for polymeric emulsion systems.

Heterometallic Porphyrin Conjugated Polymer Thin Films—A Gas‐Phase Approach for the Engineering of New Fused Porphyrin Systems

AbstractFor the first time, the simultaneous synthesis and deposition of heterometallic porphyrin conjugated polymer thin films from a simple and scalable gas‐phase approach are reported. The oxidative chemical vapor deposition (oCVD) reaction of 5,15‐(diaryl) porphyrins chelated with different metal cations (M = Co(II), Ni(II), Zn(II), Cu(II), Pd(II)) readily yields the formation of new hetero‐metalated fused porphyrin tapes, such as evidenced by in‐depth high‐resolution mass spectrometry studies. The impact of the coreactants on the regioselectivity of the intermolecular dehydrogenative coupling reaction and the formation of double or triple bonds between the porphyrin units is demonstrated. The oCVD reaction of multiple porphyrins brings the possibility to easily engineer the chemical features and the electronic and optoelectronic properties of these highly conjugated porphyrin polymers. Particularly, it is demonstrated that porphyrin conjugated polymers having different chelated metal cations can achieve higher electrical conductivities and promote narrower bandgaps. The gas‐phase approach presented herein overcomes the main limitations of the conventional solution‐based approaches and opens the path to the engineering of a novel class donor–acceptor heterometallic polymers with high interest in several fields including multimetallic electrocatalysis, photocatalysis, sensing, and nonlinear optics applications.

Optimizing the Intercrystallite Connection of a Donor–Acceptor Conjugated Semiconductor Polymer by Controlling the Crystallization Rate via Temperature

Back Cover: In article number 2200084 by Hua Yang, Yu Chen, Yanchun Han and co-workers, the tie-chain connection behavior of conjugated polymer thin film is optimized by decreasing the crystal region length (lc) and increasing the crystallization rate. The film prepared at 55 °C exhibits a smaller crystal region length (lc, 7.6 nm), the highest tie-chains content, and the best device mobility.

Oxidative Molecular Layer Deposition of Amine-Containing Conjugated Polymer Thin Films

Conjugated polymers such as polyethylenedioxythiophene (pEDOT), polypyrrole (pPy), and polyaniline (pAni) exhibit high electrochemical capacities, making them appealing as electrode materials for energy storage, electrochemical desalination, and chemical sensing. Recent work has established the growth of thin films of pEDOT using alternating gas-phase exposures of the EDOT monomer and a metal chloride (e.g., MoCl5) oxidant in a process termed oxidative molecular layer deposition (oMLD). Here, we describe the first demonstration of oMLD of amine-containing conjugated polymers. We find that pyrrole (Py) and MoCl5 undergo self-limiting surface reactions during oMLD exposures to form conformal pPy thin films, but oMLD using aniline (Ani) and p-phenylenediamine (PDA) monomers yields unexpected azo functionality. The formation of azo groups is attributed to an MoCl5-amine surface adduct that spatially constrains polymerization reactions near the amine group and produces azo groups when coupling two primary amines. pPy grown by oMLD exhibits a record-breaking 282 mAh/g capacity in an aqueous electrolyte, and PDA/MoCl5 oMLD yields azo polymers of interest as anode materials for alkali-ion batteries. Alternating between Py and PDA monomers during oMLD produces molecularly assembled copolymers with qualitatively different electrochemical responses from the isolated monomer structures. This work lays the foundation for the growth of conformal thin films of conjugated amine polymers with molecular-level control of composition and thickness.

Effect of Solvent on Optical Properties and Surface Topography of Nanocomposite Conjugated Polymer Thin Film

The addition of TiO2 nanoparticles (NPs) to a white-emission conjugated polymer (CP) blend of poly(9,9-dioctylfluorene-2,7-diyl) (PFO) and poly(2-methoxy-5(2-ethylhexyl)−1,4-phenylenevinylene (MEH-PPV) enhanced the optical properties. However, the agglomeration of the nanoparticles restricted enhancement. This drawback was successfully overcome in this study. The highly polar solvent chloroform was mixed with toluene and used to prepare thin films of the blends. Solution blending and spin-coating techniques were used to prepare thin films with different TiO2 nanoparticle contents. In addition, pure toluene and chloroform were investigated as solvents for the nanocomposite blend. These three cases were compared by studying the emission spectra, absorption spectra, Commission Internationale d’Eclairage coordinates (CIE), field emission scanning electron microscopy (FE-SEM) images and scanning probe microscope (SPM) images. The average roughness, quenching constant, and energy transfer probability were calculated. The optimum physical properties of the thin film were achieved using nanoparticles at 15wt% and applied to the binary blend with a mixture of equal amounts of toluene and chloroform. Although chloroform is better for nanoparticle distribution, toluene is mandatory for obtaining the highest yield of PFO.

Optimizing the Intercrystallite Connection of a Donor-Acceptor Conjugated Semiconductor Polymer by Controlling the Crystallization Rate via Temperature.

The charge carrier transport of conjugated polymer thin film is mainly decided by the crystalline domain and intercrystallite connection. High-density tie-chain can provide an effective bridge between crystalline domains. Herein, the tie-chain connection behavior is optimized by decreasing the crystal region length (lc ) and increasing the crystallization rate. Poly[4-(4,4-bis(2-octyldodecyl)-4H-cyclopenta[1,2-b:5,4-b']dithiophen-2-yl)-alt-[1,2,5]-thiadiazolo[3,4-c]pyridine] (PCDTPT-ODD) is dissolved in nonpolar solvent isooctane and high ordered rod-like aggregations are formed. As the temperature increases, the changes in solution state and crystallization behavior lead to three different chain arrangement morphologies in the films: 1) at 25 °C, large and separated crystal regions are formed; 2) at 55 °C, small and well-connected crystal regions are formed due to faster crystallization rate and smaller nucleus size; 3) at 90 °C, the amorphous film is formed. Further results show that the film prepared at 55 °C has a smaller crystal region length (lc , 7.6nm) and higher tie-chains content. Thus, the film exhibits the best device mobility of 2.3 × 10-3 cm2 V-1 s-1 . This result shows the great influence of crystallization kinetics on the microstructure of conjugated polymer films and provides an effective way for the optimization of the intercrystallite tie-chain.

Emulation of synaptic functions with low voltage organic memtransistor for hardware oriented neuromorphic computing

Here, various synaptic functions and neural network simulation based pattern-recognition using novel, solution-processed organic memtransistors (memTs) with an unconventional redox-gating mechanism are demonstrated. Our synaptic memT device using conjugated polymer thin-film and redox-active solid electrolyte as the gate dielectric can be routinely operated at gate voltages (VGS) below − 1.5 V, subthreshold-swings (S) smaller than 120 mV/dec, and ON/OFF current ratio larger than 108. Large hysteresis in transfer curves depicts the signature of non-volatile resistive switching (RS) property with ON/OFF ratio as high as 105. In addition, our memT device also shows many synaptic functions, including the availability of many conducting-states (> 500) that are used for efficient pattern recognition using the simplest neural network simulation model with training and test accuracy higher than 90%. Overall, the presented approach opens a new and promising way to fabricate high-performance artificial synapses and their arrays for the implementation of hardware-oriented neural network.

Semicrystalline Structural Correlations of Conductivity in Conjugated Polymer Thin Films Surface-Doped by the Vapor Phase Method

Semicrystalline Structural Correlations of Conductivity in Conjugated Polymer Thin Films Surface-Doped by the Vapor Phase Method

Dual-Mode Polymer-Based Temperature Sensor by Dedoping of Electrochemically Doped, Conjugated Polymer Thin Films

Polymer temperature sensors are important for applications in food packaging, air conditioning, wearable devices, and biomedicine. However, the sensing range of these sensors is narrow, and the mod...

Nanostructure-assisted solvent vapor annealing of conjugated polymer thin films for enhanced performance in volatile organic compound sensing

Controlling both the charge transport properties and morphologies of conjugated polymer (CP) films is critical for the practical use of CP-based organic field-effect transistor (OFET) sensors in diverse applications. Herein, we report a facile processing strategy for improving the sensing performance of OFET-based volatile organic compound (VOC) sensors, which significantly enhances charge transport and increases the surface-to-volume ratio of conjugated polymer thin films using solvent vapor annealing (SVA) in the presence of pre-formed nanostructures (PNs) on the films. In this study, two different PN-embedded poly(3-hexylthiophene) (P3HT) films, viz. PN1-P3HT and PN2-P3HT, were obtained from a sonication-treated P3HT solution with nanocrystals (NCs) and nanorods (NRs) and UV-light-irradiated P3HT solution with NCs and nanowires (NWs), respectively. The effect of SVA on the morphologies and charge transport of the PN-embedded P3HT films was systematically studied by atomic force microscopy, polarized optical microscopy, X-ray diffraction analysis, and charge-carrier mobility measurements. Gas sensing measurements were performed by exposing the sensors to VOCs in a concentration range of 1–100 ppm. After SVA, longer, one-dimensional P3HT nanostructures were formed in the PN-embedded films, while some small nanostructures appeared in the pristine P3HT films. Consequently, the PN2-P3HT OFETs exhibited a significant enhancement (2-fold) in charge transport and an increase (1.7-fold) in the surface-to-volume ratio after the SVA. Furthermore, the OFET sensors exhibited excellent responsivities (20–70%) and rapid response/recovery times (110–160 s) in the detection of VOCs.

Strain-Microstructure-Optoelectronic Inter-Relationship toward Engineering Mechano-Optoelectronic Conjugated Polymer Thin Films.

Mechano-optoelectronic (MO) behavior indicates changes in optoelectronic properties in response to the applied mechanical deformation. The MO behavior can be employed to monitor the mechanical deformation of a targeted system by tracing its optoelectronic properties. Poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (P3HT/PCBM) blend thin films exhibited changes in direct current under tensile strain. Although optoelectronic properties and photovoltaic performance of P3HT/PCBM blends have been studied extensively and intensively, research required for MO properties has a fundamental difference from previous research mostly for solar cells. In research for MO systems, a greater extent of changes in optoelectronic properties under mechanical deformation is favorable. Herein, previous research for optoelectronic properties and mechanical properties of conjugated polymers will be reviewed from a perspective on MO properties. The microstructure of a conjugated polymer thin film plays a pivotal role in its optoelectronic properties and mechanical properties. Key parameters involved in the microstructure of conjugated polymer thin films will be addressed. A scalable process is required to broaden applications of MO systems. Potential challenges in the fabrication of MO conjugated polymer thin films will be discussed. Finally, this review is envisioned to provide insight into the design and manufacturing of MO conjugated polymer thin films.