Semiconducting Polymer Films Research Articles

Strain-dependent charge trapping and its impact on the operational stability of polymer field-effect transistors

Despite recent dramatic improvements in the electronic characteristics of stretchable organic field-effect transistors (FETs), their low operational stability remains a bottleneck for their use in practical applications. Here, the operational stability, especially the bias-stress stability, of semiconducting polymer-based FETs under various tensile strains is investigated. Analyses on the structure of stretched semiconducting polymer films and spectroscopic quantification of trapped charges within them reveal the major cause of the strain-dependent bias-stress instability of the FETs. Devices with larger strains exhibit lower stability than those with smaller strains because of the increased water content, which is accompanied by the formation of cracks and nanoscale cavities in the semiconducting polymer film as results of the applied strain. The strain-dependence of bias-stress stability of stretchable OFETs can be eliminated by passivating the devices to avoid penetration of water molecules. This work provides new insights for the development of bias-stable stretchable OFETs.

Liquid nitrogen post-treatment for improved aggregation and electrical properties in organic semiconductors

Organic semiconductors are promising candidates as active layers in flexible and biocompatible electronics owing to their solution processability and molecular design flexibility. However, it remains necessary to establish a green processing approach to acquire desirable electrical properties for scalable industrial applications. Here, a highly efficient and environmentally friendly post-treatment method using liquid nitrogen as a cooling bath is developed to optimize the aggregation structure and electrical performance of organic semiconductors. The carrier mobility has increased by nearly 60 % with this treatment, achieving a performance boost comparable to that of traditional annealing methods. This performance improvement is attributable to the denser aggregation structure and enhanced molecular ordering compared with those of as-cast semiconducting polymer films. Impressively, the entire process can be completed within a few minutes without additional vacuum or high-temperature conditions, offering an economical and efficient alternative to traditional methods. Furthermore, the enhancement effect and long-term stability of this treatment are validated across a wide range of organic semiconductors, positioning this green and versatile approach as a promising substitute for conventional post-treatment, thereby facilitating the development of next-generation sustainable electronics.

Polymeric Semiconductor in Field-Effect Transistors Utilizing Flexible and High-Surface Area Expanded Poly(tetrafluoroethylene) Membrane Gate Dielectrics.

Organic field-effect transistors (OFETs) were fabricated using three high-surface area and flexible expanded-poly(tetrafluoroethylene) (ePTFE) membranes in gate dielectrics, along with the semiconducting polymer poly[2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-3,6-diyl)-alt-(2,2':5',2″:5″,2‴-quaterthiophen-5,5‴-diyl)] (PDPP4T). The transistor behavior of these devices was investigated following annealing at 50, 100, 150, and 200 °C, all sustained for 1 h. For annealing temperatures above 50 °C, the OFETs displayed improved transistor behavior and a significant increase in output current while maintaining similar magnitudes of Vth shifts when subjected to static voltage compared to those kept at ambient temperature. We also tested the response to NO2 gas for further characterization and for possible applications. The ePTFE-PDPP4T interface of each membrane was characterized via scanning electron microscopy for all four annealing temperatures to derive a model for the hole mobility of the ePTFE-PDPP4T OFETs that accounts for the microporous structure of the ePTFE and consequently adjusts the channel width of the OFET. Using this model, a maximum hole mobility of 1.8 ± 1.0 cm2/V s was calculated for the polymer in an ePTFE-PDPP4T OFET annealed at 200 °C, whereas a PDPP4T OFET using only the native silicon wafer oxide as a gate dielectric exhibited a hole mobility of just 0.09 ± 0.03 cm2/V s at the same annealing condition. This work demonstrates that responsive semiconducting polymer films can be deposited on nominally nonwetting and extremely bendable membranes, and the charge carrier mobility can be significantly increased compared to their as-prepared state by using thermally durable polymer membranes with unique microstructures as gate dielectrics.

Effect of SbCl5 Doping on the Electrical and Optical Properties of Polyamide Nylon 6 Films

Polyamide nylon 6 (PA6) films were fabricated using a solution casting method and the films were doped with SbCl5. The electrical conductivity increased with the dopant concentration due to the increase in formation of radical cations through the charge transfer from the isolated double bonds (–C=O–) in the polymer chains. The DC electrical conductivity reached a maximum of 0.01 S/cm for 0.5 ml SbCl5 doped films. Structural and optical characteristics of the pure and doped films were examined using Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR), X-Ray Diffraction (XRD), and Ultraviolet-Visible (UV-Vis) absorption spectra. The doped films exhibited discernible shifts in the optical band gap energy. The direct band gap values of PA6 films were reduced from 4.5 eV to 1.7 eV when doped with 0.5 ml SbCl5. ATR-FTIR spectral studies, and optical studies indicated the formation of charge transfer complexes with the amide group of nylon 6, resulting in the electrical conduction. The XRD studies disclosed the reduction in the crystallinity of the doped films. The thermal stability was studied using differential scanning calorimetry (DSC) and thermogravimetric analysis-derivative thermogravimetric (TGA-DTG) studies. The melting and crystallization temperatures were recorded from the DSC thermograms. The peak melting temperature of PA6 reduced from 217 °C to 186 °C for 0.5 ml SbCl5 doped films. The peak of crystallization observed at 182 °C for pure films was absent, for the doped samples. The peak degradation temperatures for both pure and doped samples were above 400 °C, revealing the thermal stability of both materials. The thermally stable, semi-crystalline and semiconducting polymer films, we suggest, would be suitable for optoelectronic applications.

Measuring the anisotropic conductivity of rub-aligned doped semiconducting polymer films: The role of electrode geometry

Measuring the anisotropic conductivity of rub-aligned doped semiconducting polymer films: The role of electrode geometry

Improving the hole mobility of conjugated semiconducting polymer films by fast backbone aggregation during the film formation process

Forming high-performance IDTBT film with large aggregations with tight π–π stacking via fast aggregation between backbones during film formation by adjusting the solubility difference Ra (b–s) of the backbone and side chain in the solvent.

Films of linear conjugated polymer as photoanodes for oxidation reactions.

Photoelectrochemical (PEC) devices hold huge potential to convert solar energy into chemical energy. However, the high cost of raw materials and film processing has hindered its practical use. In this study, we attempt to tackle this issue by fabricating straightforward semiconducting polymer films. These films function as photoanodes for various oxidation reactions, including water oxidation and oxidative organosynthesis. The structures of the polymer were assessed by incorporating electron-rich and electron-deficient co-monomers into dibenzo[b,d]thiophene sulfone materials. Furthermore, to gain comprehensive insight into the performance, we conducted both steady-state and in operando investigations, revealing that the active site on the polymer surface determines the rate of the conversion process. This study marks a significant stride towards leveraging economically viable semiconductors in PEC systems for efficient solar-to-chemical conversions. It addresses the challenges of high material costs and complex film processing, paving the way for the scaled-up application of this burgeoning technology.

The effects of humidity on the electrical properties and carrier mobility of semiconducting polymers anion-exchange doped with hygroscopic salts

To improve their electrical conductivity for various applications, semiconducting polymer films are often chemically doped to increase their equilibrium charge carrier density. Recently, a novel doping method involving anion exchange has provided control over the identity of the counterions that reside in such films, leading to increased stability under ambient conditions. In this work, however, we show that by ion-exchanging 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane-doped poly(3-hexylthiophene-2,5-diyl) films with hygroscopic salts like bis(trifluoromethane)sulfonimide lithium or LiPF6, the doped film's electrical conductivity drops significantly when exposed to ambient humidity. The change in electrical conductivity depends directly on the degree of hygroscopicity of the counterion and can be over 50% with relatively modest changes in relative humidity (RH), and up to a factor of four between ambient and completely dry conditions. The film's humidity response is entirely reversible when adsorbed water is removed, potentially allowing the doped semiconducting polymer films to function as humidity sensors. Hall effect measurements show that the cause of the drop in conductivity with increasing RH is due to a decrease in carrier mobility and not due to de-doping. Our results emphasize that it is important to control the sample environment when making electrical measurements on anion-exchange doped semiconducting polymer films.

Effect of drying behavior-induced self-organization process on the morphology and electronic properties of conjugated polymer films

In this study, the morphology and electronic properties of semiconducting polymer films were investigated in relation to a different drying behavior induced by the spinning time variation. Morphology changes related to the decreased spinning time were evaluated for p-type (P3HT, PQT-12) and n-type (N2200, PDBPyBT) polymer films fabricated at different preparation conditions. Based on the improved morphology, OFET devices were fabricated showing a 2.5 times carrier mobility increase (0.044 ± 0.012 cm2V−1s−1 for 3 s spin to 0.018 ± 0.011 cm2V−1s−1 for 30 s spin) for P3HT films, and 3 times improvement (0.088 ± 0.023 cm2V−1s−1 for 50 s to 0.28 ± 0.017 cm2V−1s−1 for 5 s) for N2200 films. For short spinning times, the slow drying of residual solvent allowed for the self-organization of polymer chains improving the crystallinity of polymer films. The presented approach synergizes the benefits of the spin coating method, like highly controlled film uniformity and thickness, with improved film morphology provided by other methods, like drop casting, or dip-coating.

Solution-processed ambipolar polymer semiconductor films for high-performance complementary-like printed and flexible electronic circuits

Solution-processed ambipolar polymer semiconductor films for high-performance complementary-like printed and flexible electronic circuits

Oxidative-Reductive Near-Infrared Electrochromic Switching Enabled by Porous Vertically Stacked Multilayer Devices.

Here, we demonstrate for the first time the ability of a porous π-conjugated semiconducting polymer film to enable facile electrolyte penetration through vertically stacked redox-active polymer layers, thereby enabling electrochromic switching between p-type and/or n-type polymers. The polymers P1 and P2, with structures diketopyrrolopyrrole (DPP)-πbridge-3,4,-ethylenedioxythiophene (EDOT)-πbridge [πbridge = 2,5-thienyl for P1 and πbridge = 2,5-thiazolyl for P2] are selected as the p-type polymers and N2200 (a known naphthalenediimide-dithiophene semiconductor) as the n-type polymer. Single-layer porous and dense (control) polymer films are fabricated and extensively characterized using optical microscopy, atomic force microscopy, scanning electron microscopy, and grazing incidence wide-angle X-ray scattering. The semiconducting films are then incorporated into single and multilayer electrochromic devices (ECDs). It is found that when a p-type (P2) porous top layer is used in a multilayer ECD, it enables electrolyte penetration to the bottom layer, enabling oxidative electrochromic switching of the P1 bottom layer at low potentials (+0.4 V versus +1.2 V with dense P2). Importantly, when using a porous P1 as the top layer with an n-type N2200 bottom layer, dynamic oxidative-reductive electrochromic switching is also realized. These results offer a proof of concept for development of new types of multilayer electrochromic devices where precise control of the semiconductor film morphology and polymer electronic structure is essential.

Field-effect bulk mobilities in polymer semiconductor films measured by sourcemeters.

Semiconducting polymers inherently exhibit polydispersity in terms of molecular structure and microscopic morphology, which often results in a broad distribution of energy levels for localized electronic states. Therefore, the bulk charge mobility strongly depends on the free charge density. In this study, we propose a method to measure the charge-density-dependent bulk mobility of conjugated polymer films with widely spread localized states using a conventional field-effect transistor configuration. The gate-induced variation of bulk charge density typically ranges within ±1018cm-3; however, this range depends significantly on the energetic dispersion width of localized states. The field-effect bulk mobility and field-effect mobility near the semiconductor-dielectric interface along with their dependence on charge density can be simultaneously extracted from the transistor characteristics using various gate voltage ranges.

Conductivity maxima in electrolyte-gated transistors with molecular-doped semiconducting polymer films

Conductivity maxima in electrolyte-gated transistors with molecular-doped semiconducting polymer films

The role of solvents in the microstructure and charge transport of semiconducting polymer films prepared at the air–liquid interface

Fabricating devices using floating films of semiconducting polymers offers many advantages. In floating films, a judicious selection of solvents is essential to achieve maximum device performance based on the nature of the semiconducting polymer.

Tough-interface-enabled stretchable electronics using non-stretchable polymer semiconductors and conductors.

Semiconducting polymer thin films are essential elements of soft electronics for both wearable and biomedical applications1-11. However, high-mobility semiconducting polymers are usually brittle and can be easily fractured under small strains (<10%)12-14. Recently, the improved intrinsic mechanical properties of semiconducting polymer films have been reported through molecular design15-18 and nanoconfinement19. Here we show that engineering the interfacial properties between a semiconducting thin film and a substrate can notably delay microcrack formation in the film. We present a universal design strategy that involves covalently bonding a dissipative interfacial polymer layer, consisting of dynamic non-covalent crosslinks, between a semiconducting thin film and a substrate. This enables high interfacial toughness between the layers, suppression of delamination and delocalization of strain. As a result, crack initiation and propagation are notably delayed to much higher strains. Specifically, the crack-onset strain of a high-mobility semiconducting polymer thin film improved from 30% to 110% strain without any noticeable microcracks. Despite the presence of a large mismatch in strain between the plastic semiconducting thin film and elastic substrate after unloading, the tough interface layer helped maintain bonding and exceptional cyclic durability and robustness. Furthermore, we found that our interfacial layer reduces the mismatch of thermal expansion coefficients between the different layers. This approach can improve the crack-onset strain of various semiconducting polymers, conducting polymers and even metal thin films.

The continuous fiber networks with a balanced bimodal orientation of P(NDI2OD-T2) by controlling solution nucleation and face-on and edge-on crystallization rates

The conjugated semiconducting polymer film morphology with long fiber and optimizing molecular packing is strongly correlated with charge transport properties. Herein, we report a method to obtain continuous fiber networks with a balanced bimodal orientation of poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} [P(NDI2OD-T2)] thin films by controlling solution nucleation and edge-on and face-on crystallization rates. This is enabled by adding ethylene glycol (EG) (the optimal content is 3 vol%) to P(NDI2OD-T2) in chloroform solution which improves the polarization of the NDI2OD units along the backbone direction and solution pre-aggregation. Therefore, the crystal nuclei are formed in solution state during the three-phase line receding as characterized by in situ polarized optical microscopy. Moreover, the peak intensity of the aggregated state increases with solvent evaporation, illustrating that the nucleation and crystal growth proceed simultaneously in the drying process as revealed by in situ ultraviolet–visible absorption spectra. The film morphology of continuous fibrous networks with an average fiber length of 531 nm is formed. In addition, the slopes of (100) peak area vs time in the out-of-plane and in-plane directions are similar, indicating that the edge-on and face-on crystallization rates are almost equal as obtained from in situ grazing incidence wide-angle X-ray scattering. Thus, the bimodal orientation with an edge-on and face-on mixing ratio is approximately 1. The average electron mobility of the field effect transistors of P(NDI2OD-T2) films with this continuous fiber network reaches to 0.2 cm2 V−1 s−1, an almost 3-fold improvement compared with devices without EG addition films.

Realizing Intrinsically Stretchable Semiconducting Polymer Films by Nontoxic Additives

Realizing Intrinsically Stretchable Semiconducting Polymer Films by Nontoxic Additives

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.

Enhanced Performance of Cyclopentadithiophene-Based Donor-Acceptor-Type Semiconducting Copolymer Transistors Obtained by a Wire Bar-Coating Method.

Herein, we report the fabrications of high-performance polymer field-effect transistors (PFETs) with wire bar-coated semiconducting polymer film as an active layer. For an active semiconducting material of the PFETs, we employed cyclopentadithiophene-alt-benzothiadiazole (CDT-BTZ) that is a D-A-type-conjugated copolymer consisting of a repeated electron-donating unit and an electron-accepting unit, and the other two CDT-based D-A-type copolymer analogues are cyclopentadithiophene-alt-fluorinated-benzothiadiazole (CDT-FBTZ) and cyclopentadithiophene-alt-thiadiazolopyridine (CDT-PTZ). The linear field-effect mobility values obtained from the transfer curve of the PFETs fabricated with the spin-coating were 0.04 cm2/Vs, 0.16 cm2/Vs, and 0.31 cm2/Vs, for CDT-BTZ, CDT-FBTZ, and CDT-PTZ, respectively, while the mobility values measured from the PFETs with the wire bar-coated CDT-BTZ film, CDT-FBTZ film, and CDT-PTZ film were 0.16 cm2/Vs, 0.28 cm2/Vs, and 0.95 cm2/Vs, respectively, which are about 2 to 4 times higher values than those of the PFETs with spin-coated films. These results revealed that the aligned molecular chain is beneficial for the D-A-type semiconducting copolymer even though the charge transport in the D-A-type semiconducting copolymer is known to be less critical to the degree of disorder in film.

From Chlorinated Solvents to Branched Polyethylene: Solvent‐Induced Phase Separation for the Greener Processing of Semiconducting Polymers

AbstractDespite having favorable optoelectronic and thermomechanical properties, the wide application of semiconducting polymers still suffers from limitations, particularly with regards to their processing in solution which necessitates toxic chlorinated solvents due to their intrinsic low solubility in common organic solvents. This work presents a novel greener approach to the fabrication of organic electronics without the use of toxic chlorinated solvents. Low‐molecular‐weight non‐toxic branched polyethylene (BPE) is used as a solvent to process diketopyrrolopyrrole‐based semiconducting polymers, then the solvent‐induced phase separation (SIPS) technique is adopted to produce films of semiconducting polymers from solution for the fabrication of organic field‐effect transistors (OFETs). The films of semiconducting polymers prepared from BPE using SIPS show a more porous granular morphology with preferential edge‐on crystalline orientation compared to the semiconducting polymer film processed from chloroform. OFETs based on the semiconducting films processed from BPE show similar device characteristics to those prepared from chloroform without thermal annealing, confirming the efficiency and suitability of BPE to replace traditional chlorinated solvents for green organic electronics. This new greener processing approach for semiconducting polymers is potentially compatible with different printing techniques and is particularly promising for the preparation of porous semiconducting layers and the fabrication of OFET‐based electronics.