Undoped Polymer Research Articles

Effects of Planarization of the Triphenylamine Unit on the Electronic and Transport Properties of Triarylamine-Fluorene Copolymers in Both Doped and Undoped Forms.

Triarylamine-alt-fluorene (TAF) copolymers are widely used for hole injection and transport in organic electronics. Despite suggestions to planarize the triphenylamine moiety, little research has been conducted. Here, we report a comprehensive investigation of the effects of planarization on the electronic and transport properties of a model TAF polymer semiconductor core. We compared the conventional twisted-propeller N-4-methoxyphenyl-N,N-diphenylamine-4',4″-diyl (TA) unit and its planarized bridged analogue (bTA) where adjacent o,o'-positions are linked by 1,1-dimethylmethylene. We studied both polyelectrolyte and non-polyelectrolyte forms of this core in both doped and undoped states. We found that planarization leads to an unprecedented trap-free transport of holes, and a pronounced enhancement of their mobility in the undoped state though less so in the doped state. Planarization also induces a slight reduction in the ionization energy of the undoped polymer, consequently lowering the work function of the doped polymer. This is accompanied by small spectral shifts: a red shift in the first absorption band of the undoped polymer and a blue shift in the first absorption band of the polaron. Furthermore, this study unveils new fundamental features of TAF polymers: (i) Doping induces the formation of three polaron bands within the subgap. (ii) Absorption of both neutral and polaron segments exhibit a linear intensity relationship with doping level. (iii) Electrical conductivity reaches a maximum at the half-doped state, varying as σ ∼ (x (1 - x))3 for 0.1 ≲ x ≲ 0.9, where x is the doping level. Finally, we demonstrate the successful integration of these self-compensated hole-doped TAF polymers as efficient hole injection layers in organic semiconductor diodes.

Counterion docking: a general approach to reducing energetic disorder in doped polymeric semiconductors

Molecular doping plays an important role in controlling the carrier concentration of organic semiconductors. However, the introduction of dopant counterions often results in increased energetic disorder and traps due to the molecular packing disruption and Coulomb potential wells. To date, no general strategy has been proposed to reduce the counterion-induced structural and energetic disorder. Here, we demonstrate the critical role of non-covalent interactions (NCIs) between counterions and polymers. Employing a computer-aided approach, we identified the optimal counterions and discovered that NCIs determine their docking positions, which significantly affect the counterion-induced energetic disorder. With the optimal counterions, we successfully reduced the energetic disorder to levels even lower than that of the undoped polymer. As a result, we achieved a high n-doped electrical conductivity of over 200 S cm−1 and an eight-fold increase in the thermoelectric power factor. We found that the NCIs have substantial effects on doping efficiency, polymer backbone planarity, and Coulomb potential landscape. Our work not only provides a general strategy for identifying the most suitable counterions but also deepens our understanding of the counterion effects on doped polymeric semiconductors.

Diketopyrrolopyrrole‐Dioxo‐Benzodithiophene‐Based Multifunctional Conjugated Polymers for Organic Field‐Effect Transistors and Perovskite Solar Cells

While dual‐acceptor‐type conjugated polymers have witnessed a great success in organic field‐effect transistors (OFETs), their potential multifunctionality in other optoelectronic devices has been overlooked. Herein, three conjugated polymers (DPPF‐BDD, DPPT‐BDD, and DPPSe‐BDD) comprising furan/thiophene/selenophene‐flanked diketopyrrolopyrrole (DPP) and dioxo‐benzodithiophene (BDD) as repeating units are designed, synthesized, and characterized. Modulating the chalcogen on DPP flank shows an impact on dual‐acceptor polymer optoelectronic properties. Subsequently, the potential of these polymers in both OFETs and perovskite solar cells (PSCs) either as semiconductors or as passivation materials, respectively, is investigated. Interestingly, DPPF‐BDD, DPPT‐BDD, and DPPSe‐BDD show ambipolar behavior in vacuum with hole (μh) and electron (μe) mobilities of 0.0263/0.0223, 0.0187/0.0123, and 0.0070/0.0051 cm2 V−1 s−1, respectively. Upon doping tetrabutylammonium iodide into DPPF‐BDD, DPPT‐BDD, and DPPSe‐BDD polymers, the respective OFETs show relatively higher μh and μe (0.0389/0.0503; 0.0289/0.0259; 0.0058/0.0156 cm2 V−1 s−1) than the undoped polymer OFETs. Furthermore, DPPF‐BDD‐, DPPT‐BDD‐, and DPPSe‐BDD‐incorporated (in the antisolvent treatment and PCBM electron transport layer) PSCs display maximum power conversion efficiency of 23.48%, 22.85%, and 23.35%, respectively, surpassing the control device (22.83%), which is benefited from the perovskite surface passivation and the charge extraction improvement. Overall, a new class of multifunctional DPP‐based dual‐acceptor‐type polymers that are highly compatible with OFETs and high‐performance PSCs is presented.

Long lifetimes white afterglow in slightly crosslinked polymer systems

Intrinsic polymer room-temperature phosphorescence (IPRTP) materials have attracted considerable attention for application in flexible electronics, information encryption, lighting displays, and other fields due to their excellent processabilities and luminescence properties. However, achieving multicolor long-lived luminescence, particularly white afterglow, in undoped polymers is challenging. Herein, we propose a strategy of covalently coupling different conjugated chromophores with poly(acrylic acid (AA)-AA-N-succinimide ester) (PAA-NHS) by a simple and rapid one-pot reaction to obtain pure polymers with long-lived RTPs of various colors. Among these polymers, the highest phosphorescence quantum yield of PAPHE reaches 14.7%. Furthermore, the afterglow colors of polymers can be modulated from blue to red by introducing three chromophores into them. Importantly, the acquired polymer TPAP-514 exhibits a white afterglow at room temperature with the chromaticity coordinates (0.33, 0.33) when the ratio of chromophores reaches a suitable value owing to the three-primary-color mechanism. Systematic studies prove that the emission comes from the superposition of different triplet excited states of the three components. Moreover, the potential applications of the obtained polymers in light-emitting diodes and dynamic anti-counterfeiting are explored. The proposed strategy provides a new idea for constructing intrinsic polymers with diverse white-light emission RTPs.

Optical and Dielectric Characteristics of Poly (Methyl Methacrylate)/Polyethylene Oxide/Carbon Nanoparticle Composites Loaded with Nano NiFe2O4

The current study proposes a simple, low-cost, and functional method of doping a pure matrix of poly (methyl methacrylate)/polyethylene oxide/carbon nanoparticles (PMMA/PEO/CNPs) with nano nickel ferrite (NiFe2O4) as a filler. To create NiFe2O4 nanoparticles, a straightforward chemical growth, sol-gel auto-combustion system was used. Next, environmentally friendly and cost-effective, undoped and blended polymers doped with NiFe2O4 using a solution-casting method were synthesized. Rietveld refinement analysis was used to investigate the structure and microstructure of the NiFe2O4. Transmittance electron microscopy was used to confirm the nano nature of the filler. X-ray diffraction was used to verify the crystallite nature of the undoped and doped PMMA/PEO/CNPs blends with NiFe2O4. The linear optical characteristics of the proposed blends were obtained using a diffuse reflectance spectrophotometer. The measured optical parameters included transmittance, absorbance, reflectance, direct and indirect optical bandgaps, absorption coefficient, linear refractive index, extinction coefficient, dielectric constant, energy loss functions, in addition to optical and electrical conductivities. Also, nonlinear optical constants for the synthesized blends, such as the linear and third nonlinear susceptibilities, nonlinear refraction coefficient and nonlinear absorption coefficient, were calculated. The direct and indirect optical band gap energies of the PMMA/PEO/CNPs blends were 4.97 and 4.8 eV, respectively. As the NiFe2O4 doping level increased in the blend to 2.5 wt%, the optical band gap energy values decreased to 4.54 and 4.64 eV, respectively. The fluorescence spectra of the undoped and doped PMMA/PEO/CNPs blended polymers with nano NiFe2O4 were also explored. The effect of NiFe2O4 doping amount on the energy density, electric dielectric constant, ac conductivity and electric moduli of the host blend was also studied. In conclusion, the considered NiFe2O4-doped PMMA/PEO/CNPs blends have an effective and promising role in optoelectronic applications.

Structural, Optical and Electrical Studies on Poly(methyl methacrylate) Polymer Loaded with CuCo2O4/CuS Mixtures

Polymethyl methacrylate (PMMA)/(1-x)CuCo2O4/xCuS (x = 0, 0.1, 0.25, 0.5) nanocomposites polymers were formed using hydrothermal, solid state and solvent casting procedures. The structure of the filler and polymer nano-composite samples were examined using an X-ray diffraction technique. The impacts of the (1-x)CuCo2O4/xCuS mixture on the diffuse absorption, transmittance, reflectance, extinction coefficient, refractive index, optical dielectric constant, energy loss functions and optical conductivity were investigated. The absorbance of PMMA/(1-x)CuCo2O4/xCuS changed depending on the percentage of CuS in the nanocomposite. The optical band gaps (Eg ) for PMMA were 4.52 and 4.46 eV, directly and indirectly, respectively. The sample with x = 0.25 produced the lowest direct/indirect Eg values, 4.34 and 3.79 eV. The fluorescence spectra of the undoped and doped PMMA polymers were explored. Depending on the filler composition in the PMMA/(1-x)CuCo2O4/xCuS samples, the emitted colors could be modulated. The effects of the (1-x)CuCo2O4/xCuS mixture as a function of the dielectric constant, dielectric loss, ac conductivity and electric modulus of the host polymer were examined.

Influence of Single-Wall Carbon Nanotube Suspension on the Mechanical Properties of Polymeric Films and Electrospun Scaffolds.

Carbon nanotubes (CNTs) are used in applications ranging from electrical engineering to medical device manufacturing. It is well known that the addition of nanotubes can influence the mechanical properties of various industrial materials, including plastics. Electrospinning is a popular method for fabricating nanomaterials, widely suggested for polymer scaffold manufacturing. In this study, we aimed to describe the influence of single-walled carbon nanotube (SWCNT) suspensions on polymeric poured films and electrospun scaffolds and to investigate their structural and mechanical properties obtained from various compositions. To obtain films and electrospun scaffolds of 8 mm diameter, we used poly-ε-caprolactone (PCL) and poly(cyclohexene carbonate) (PCHC) solutions containing several mass fractions of SWCNT. The samples were characterized using tensile tests, atomic force and scanning electronic microscopy (AFM and SEM). All the studied SWCNT concentrations were shown to decrease the extensibility and strength of electrospun scaffolds, so SWCNT use was considered unsuitable for this technique. The 0.01% mass fraction of SWCNT in PCL films increased the polymer strength, while fractions of 0.03% and more significantly decreased the polymer strength and extensibility compared to the undoped polymer. The PHCH polymeric films showed a similar behavior with an extremum at 0.02% concentration for strength at break.

Impact of ZnS/Mn on the Structure, Optical, and Electric Properties of PVC Polymer.

The most efficient way to create novel materials that may be used in a variety of optoelectronic applications is thought to be doped mixed polymers with appropriate fillers. Undoped and doped PVC polymers with ZnS/Mn formed at different temperatures were fabricated using the casting method. The Rietveld method was used to discover the structure and microstructure of Zn0.95Mn0.05S prepared at T = 300, 400, and 500 °C. The distribution and existence of the nanofiller over the PVC matrix were determined via XRD, FTIR, EDS, and SEM techniques. The effect of the preparation temperatures of the ZnS/Mn nanofiller on the absorption, transmittance, reflectance, refractive index, extinction coefficient, dielectric constant, AC conductivity, electrical modulus, and DC conductivity activation energy data of the PVC polymer was studied using the diffused reflectance technique. Doping PVC with ZnS/Mn (prepared at 300 °C) lowered the direct and indirect optical band gaps from 5.4 and 4.52 eV to minimum values of 4.55 and 3.63 eV. The fluorescence intensity of pure PVC is greatly enhanced upon loading with ZnS/Mn. The PVC exhibited two near UV peaks, one violet and one blue color, while, in addition, the doped polymers exhibited green and orange colors. The corresponding CIE diagram for all the samples was also determined.

Molecular p-doping induced dielectric constant increase of polythiophene films determined by impedance spectroscopy

The dielectric constant (εr) is a fundamental material parameter that governs charge transfer processes in organic semiconductors, yet its value is often assumed rather than measured. Here, we use impedance spectroscopy to determine εr in regioregular poly(3-hexylthiophen-2,5-diyl) (P3HT) thin films p-doped with the molecular dopants hexafluoro-tetracyanonaphthoquinodimethane and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). We fit the impedance spectra using a single RC circuit model to determine the frequency-dependent capacitance and extract εr. The value of the dielectric constant increases by around two-thirds from 2.9 ± 0.1 (undoped polymer) to 4.9 ± 0.6 on the addition of one F4TCNQ molecule per 500 P3HT monomer units. In contrast, the addition of the weak dopant 7,7,8,8-tetracyanoquinodimethane (TCNQ), which does not undergo ground state charge transfer with P3HT, has no effect on the dielectric constant. Our results support the hypothesis that molecular doping has a considerable impact on the materials dielectric constant via polarizable host-dopant complexes.

Self-Doping Naphthalene Diimide Conjugated Polymers for Flexible Unipolar n-Type OTFTs.

The development of high-performance organic thin-film transistor (OTFT) materials is vital for flexible electronics. Numerous OTFTs are so far reported but obtaining high-performance and reliable OTFTs simultaneously for flexible electronics is still challenging. Herein, it is reported that self-doping in conjugated polymer enables high unipolar n-type charge mobility in flexible OTFTs, as well as good operational/ambient stability and bending resistance. New naphthalene diimide (NDI)-conjugated polymers PNDI2T-NM17 and PNDI2T-NM50 with different contents of self-doping groups on their side chains are designed and synthesized. The effects of self-doping on the electronic properties of resulting flexible OTFTs are investigated. The results reveal that the flexible OTFTs based on self-doped PNDI2T-NM17 exhibit unipolar n-type charge-carrier properties and good operational/ambient stability thanks to the appropriate doping level and intermolecular interactions. The charge mobility and on/off ratio are fourfold and four orders of magnitude higher than those of undoped model polymer, respectively. Overall, the proposed self-doping strategy is useful for rationally designing OTFT materials with high semiconducting performance and reliability.

Anomalous dips in reflection spectra of optical polymers deposited on plasmonic metals

Abstract We have studied reflection spectra of dye-doped and undoped polymers deposited onto Ag and Au substrates and found anomalous dips in the UV spectral range. On top of Ag substrates, the λ ∼ 375 nm dips were observed in undoped PMMA, PVP, and PS polymers as well as PMMA doped with Rh590 and HITC laser dyes. In silver-based samples, the spectral positions of the observed reflection dips were close to singularities in the refractive indexes of surface plasmon polaritons (SPPs) propagating at the interface between Ag and polymer. The latter singularities can tentatively explain the λ ∼ 375 nm reflection dips, if the scattering of Ag and polymeric films is large enough to launch SPP without any prism or grating. The dips observed in reflection of Rh590:PMMA and HITC:PMMA on top of Au, were more pronounced than those on Ag, broader, shifted to shorter wavelengths, and their spectral positions had large standard deviations. Furthermore, no anomalous dips in gold-based samples were observed in the reflection spectra of undoped PMMA, PVP, and PS polymers, and a modest singularity in the SPP refractive index, predicted theoretically at λ ∼ 500 nm, cannot explain the dips in the UV reflection spectra observed experimentally. It appears likely that the origin of the reflection dips on top of Au substrates is different from that on top of Ag substrates.

Use of a Multiple Hydride Donor To Achieve an n-Doped Polymer with High Solvent Resistance.

The ability to insolubilize doped semiconducting polymer layers can help enable the fabrication of efficient multilayer solution-processed electronic and optoelectronic devices. Here, we present a promising approach to simultaneously n-dope and largely insolubilize conjugated polymer films using tetrakis[{4-(1,3-dimethyl-2,3-dihydro-1H-benzo[d]imidazol-2-yl)phenoxy}methyl]methane (tetrakis-O-DMBI-H), which consists of four 2,3-dihydro-1H-benzoimidazole (DMBI-H) n-dopant moieties covalently linked to one another. Doping a thiophene-fused benzodifurandione-based oligo(p-phenylenevinylene)-co-thiophene polymer (TBDOPV-T) with tetrakis-O-DMBI-H results in a highly n-doped film with bulk conductivity of 15 S cm-1. Optical absorption spectra provide evidence for film retention of ∼93% after immersion in o-dichlorobenzene for 5 min. The optical absorption signature of the charge carriers in the n-doped polymer decreases only slightly more than that of the neutral polymer under these conditions, indicating that the exposure to solvent also results in negligible dedoping of the film. Moreover, thermal treatment studies on a tetrakis-O-DMBI-H-doped TBDOPV-T film in contact with another undoped polymer film indicate immobilization of the molecular dopant in TBDOPV-T. This is attributed to the multiple electrostatic interactions between each dopant tetracation and up to four nearby anionic doped polymer segments.

In Situ Studies of the Swelling by an Electrolyte in Electrochemical Doping of Ethylene Glycol-Substituted Polythiophene.

Organic mixed ionic electronic conductors (OMIECs) have the potential to enable diverse new technologies, ranging from biosensors to flexible energy storage devices and neuromorphic computing platforms. However, a study of these materials in their operating state, which convolves both passive and potential-driven solvent, cation, and anion ingress, is extremely difficult, inhibiting rational material design. In this report, we present a novel approach to the in situ studies of the electrochemical switching of a prototypical OMIEC based on oligoethylene glycol (oEG) substitution of semicrystalline regioregular polythiophene via grazing-incidence X-ray scattering. By studying the crystal lattice both dry and in contact with the electrolyte while maintaining potential control, we can directly observe the evolution of the crystalline domains and their relationship to film performance in an electrochemically gated transistor. Despite the oEG side-chain enabling bulk electrolyte uptake, we find that the crystalline regions are relatively hydrophobic, exhibiting little (less than one water per thiophene) swelling of the undoped polymer, suggesting that the amorphous regions dominate the reported passive swelling behavior. With applied potential, we observe that the π-π separation in the crystals contracts while the lamella spacing increases in a balanced fashion, resulting in a negligible change in the crystal volume. The potential-induced changes in the crystal structure do not clearly correlate to the electrical performance of the film as an organic electrochemical transistor, suggesting that the transistor performance is strongly influenced by the amorphous regions of the film.

Electrochemical, in silico and time-resolved EPR behaviour of semiconductive π-conjugated poly(p-phenyleneethynylenevinylene)s (PPEV)

A series of poly(p-phenyleneethynylenevinylene)s (PPEV), synthesized by step-growth polymerization of 1,4-dialkoxybenzene and 9,9-bis(2’-ethylhexyl)fluorene-diethynyl monomers and featuring aryl-(E)-enyne repeating units were investigated by experimental and computational techniques with respect to their conjugative and conductive properties. HOMO-LUMO band gaps in the range 2.64–2.79 eV were determined by cyclic voltammetry and UV–vis spectroscopy. In agreement with these data, conductivity measurements performed on thin films drop cast from dichloromethane solutions exhibit values in the range 10-4–10-3 S cm-1 depending on the identity of the repeat units and on the length of alkyl chains. A p-type doping of the materials was achieved through exposure of the films to iodine vapor with a relevant increase of conductivity up to 7-folds compared to the undoped polymers. The cationic hole resulting by electron transfer from one PPEV to the electron acceptor PCBM was observed in situ by TREPR spectroscopy. In addition, DFT calculations performed on well-defined oligomeric models of the PPEVs provide HOMO-LUMO band gaps in agreement with the experimental values and disclose the dependence on both the length and type of conformers in the polymeric chain. Plots of the calculated band gaps versus 1/n, with n the number of monomer units, show a linear trend and predict that the maximum values of conductivity of the materials are reached with n as low as ≥ 6–7, which correspond to the average degrees of polymerization achievable by the synthetic procedure.

Synthesis and characterization of poly(3‐thiophene acetic acid) upon binding by cationic groups

AbstractStructure, surface properties and morphology of a regioirregular conjugated poly(3‐thiophene acetic acid) (PTAA) and four polyelectrolytes based on PTAA with various cations were studied by using FT‐IR, 1H‐NMR spectroscopy and SEM. The properties of PTAA and its polyelectrolytes depend on the unique combination of a hydrophobic backbone and hydrophilic ionic side groups. The thermal stability, fluorescence and conductivity properties were investigated. These polymers exhibited fluorescence emissions at about 565 nm characterized for the π‐conjugated polythiophene. Undoped polymers have a better conductivity than previous polymers.

Electro-optical properties of doped polymers with high transparency in the visible wavelength range

The electro-optical (EO) properties of poly(methyl methacrylate) and the photopolymer poly(vinyl cinnamate) doped with varying concentrations of the EO chromophore 2-Methyl-4-nitroaniline were measured. The EO polymers were embedded in Fabry-Pérot etalons for the simultaneous determination of the Pockels and Kerr coefficients from measurements of the fringe shift induced by an external electric field. It was found that the host polymer has a significant impact on the EO performance and that the undoped host polymers exhibit a significant Pockels effect. Moreover, the Kerr effect provides a substantial contribution of 27% to the total change of the refractive index at relatively high electric field strengths of E = 91.2 MV m−1.

Polymer dispersed liquid crystals doped with low concentration γ-Fe2O3 nanoparticles

ABSTRACT The polymer dispersed liquid crystal films doped with the low concentration γ-Fe2O3 nanoparticles coated with oleic acid were inverstigatited. The experimental results showed that the doped and un-doped polymer dispersed liquid crystal films were the similar surface morphologies, however their evolving curves of the capacitance with the applied voltage were different, as well as the transmission rate. The interpretations based on these differences were proposed, i.e. those came from the adsorbed free ions by the γ-Fe2O3 nanoparticles, instead of the electromagnetic properties of the γ-Fe2O3 nanoparticles. The effect of the free ions is equivalent to the negative dielectric constant and refractive index. Furthermore, the reduction of the free ion concentration due to the absorption in the doped polymer dispersed liquid crystal films can decrease the negative dielectric constant and refractive index, i.e. the effective dielectric constant and refractive index were increased. As a result, the contrast ratio increased significantly, and the driving and threshold voltages decreased distinctly. These results may be useful for electro-optics devices.

Effect of ZnO Nanoparticle Content on the Structural and Ionic Transport Parameters of Polyvinyl Alcohol Based Proton-Conducting Polymer Electrolyte Membranes.

Proton conducting nanocomposite solid polymer electrolytes (NSPEs) based on polyvinyl alcohol/ammonium nitrate (PVA/NH4NO3) and different contents of zinc oxide nanoparticles (ZnO-NPs) have been prepared using the casting solution method. The XRD analysis revealed that the sample with 2 wt.% ZnO-NPs has a high amorphous content. The ionic conductivity analysis for the prepared membranes has been carried out over a wide range of frequencies at varying temperatures. Impedance analysis shows that sample with 2 wt.% ZnO-NPs has a smaller bulk resistance compared to that of undoped polymer electrolyte. A small amount of ZnO-NPs was found to enhance the proton-conduction significantly; the highest obtainable room-temperature ionic conductivity was 4.71 × 10−4 S/cm. The effect of ZnO-NP content on the transport parameters of the prepared proton-conducting NSPEs was investigated using the Rice–Roth model; the results reveal that the increase in ionic conductivity is due to an increment in the number of proton ions and their mobility.

Thin coatings of hafnon abate oxidative recession of SiC fibers

AbstractThe oxidation behavior of SiC fibers coated with (a) undoped polysilazane and (b) precursors containing a mixture of polysilazane and hafnium butoxide in equal weight fractions, is reported. The coatings were prepared by repetitive cycles of nanolayer depositions, as reported in recent publications. The oxidation experiments were carried out at 1400°C in ambient air (Boulder, CO) for up to 100 hours. The extent of degradation of SiC was measured by the recession in the diameter of the fibers as a function of time. The fibers with undoped polymer precursor recessed significantly, whereas the fibers coated with HfSiCNO remained essentially unchanged. These results are in agreement with earlier work from our laboratory where the resilience of hafnon and zircon, as well as hafnia and zirconia, against high‐temperature corrosion in streaming humid environments had been highlighted.

Quinoid Conductive Polymer for High-Performance Lithium- and Sodium-Ion Batteries

Most conductive polymers only transmit electricity with dopants. Herein, we reported an undoped polymer PDAPI which was synthesized via a single-step reaction. Without any dopants, PDAPI powder rev...