Ferroelectric Blends Research Articles

(Invited) Manipulating Photoexcitations of Flexible-Chain Polymer Semiconductors Via the Local Environment

Semiconducting:ferroelectric blends are interesting as an optoelectronic system because the strong coulombic interactions in excitons may be reduced by an enhanced polar environment provided by the ferroelectric component. Here, we demonstrate variations in the photoluminescence and charge dynamics of photo-induced absorption with model blends of the archetypal semiconducting polymer poly(3-hexylthiophene) (P3HT) and ferroelectric commodity polymer poly(vinylidene difluoride) (PVDF). This result suggests correlations between local polarity and photophysical processes of exciton dissociation and recombination, likely due to some degree of intermixing in specific blend ratios. Indeed, we see evidence of vitrification, i.e. glass formation, often leading to intermixing in these blends by varying the composition and molecular weight of the blend components. Furthermore, we will exploit the ferroelectric nano-domains, exhibited by the ter-polymer of PVDF, poly[(vinylidene fluoride‐co-trifluoro ethylene‐co-chlorotrifluoro ethylene)] [P(VDF-TrFE-CTFE)] to deliver fundamental insights into the required length scale of intermixing for photophysical processes.

Effects of configurational changes on molecular dynamics in polyvinylidene fluoride and poly(vinylidene fluoride-trifluoroethylene) ferroelectric polymers

We present a comparative study of proton dynamics in unpoled non-ferroelectric polymer polyvinylidene fluoride (PVDF) and in its trifluoroethylene containing ferroelectric copolymer (with 70/30 molar proportion), using quasi-elastic neutron scattering. The neutron data reveal the existence of two distinct types of molecular motions in the temperature range investigated. The slower motion, which is characterized in details here, is ascribed to protons jump diffusion along the polymeric carbon chains, while the faster motion could be attributed to localized rotational motion of methylene groups. At temperatures below the Curie point (Tc ∼ 385 K) of the composite polymer, the slower diffusive mode experiences longer relaxation times in the ferroelectric blend than in the bare PVDF, although the net corresponding diffusion coefficient remains comparatively the same in both polymers with characteristic activation energy of EA ≈ 27–33 kJ/mol. This arises because of a temperature dependent jump length r0, which we observe to be effectively longer in the copolymer, possibly due to the formation of ordered ferroelectric domains below Tc. Above Tc, there is no appreciable difference in r0 between the two systems. This observation directly relates the known dependence of Tc on molar ratio to changes in r0, providing fundamental insight into the ferroelectric properties of PVDF-based copolymers.

Carrier‐Selectivity‐Dependent Charge Recombination Dynamics in Organic Photovoltaic Cells with a Ferroelectric Blend Interlayer

Interfacial energetics determines the performance of organic photovoltaic (OPV) cells based on a thin film of organic semiconductor blends. Here, an approach to modulating the “carrier selectivity” at the charge collecting interfaces and the consequent variations in the nongeminate charge carrier recombination dynamics in OPV devices are demonstrated. A ferroelectric blend interfacial layer composed of a solution‐processable ferroelectric poly­mer and a wide bandgap semiconductor is introduced as a tunable electron selective layer in inverted OPV devices with non‐Ohmic contact electrodes. The direct rendering of dipole alignment within the ferroelectric blend layer is found to increase the carrier selectivity of the charge collecting interfaces up to two orders of magnitude. Transient photovoltaic analyses reveal that the increase of carrier selectivity significantly reduces the diffusion and recombination among minority carriers in the vicinity of the electrodes, giving rise to the 85% increased charge carrier lifetime. Furthermore, the carrier‐selective charge extraction leads to the constitution of the internal potential within the devices, even with energetically identical cathodes and anodes. With these carrier‐selectivity‐controlled interlayers, the devices based on various photoactive materials commonly display significant increments in the device performances, especially with the high fill factor of up to 0.76 under optimized conditions.

Organic ferroelectric field-effect transistor with P(VDF-TrFE)/PMMA blend thin films for non-volatile memory applications

Among the many ferroelectrics polymers, poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TrFE)] has drawn a great attention with its promising memory properties such as large remanent polarization, good fatigue and retention properties. Since the ferroelectric layer plays a critical role in the operation of memory device, it is important to control the structure of ferroelectric thin film. In order to improve the performance of a ferroelectric device, in this contribution we employed P(VDF-TrFE) layers blended with various compositions of an amorphous poly(methyl methacrylate) (PMMA) from 0 wt.% to 20 wt.%. In metal/ferroelectric/metal capacitor structure, we observed the decrease of remanent polarization from 9.13 μC/cm 2 to 4.7 μC/cm 2 and increase of coercive voltage from 9.5 V to 15.2 V with PMMA. Polarization switching time of the ferroelectric blend films estimated from the switched polarization vs. time curves increases with the amount of PMMA. Furthermore, ferroelectric field-effect transistors (FeFETs) based on ferroelectric P(VDF-TrFE) and PMMA blend films with single-crystalline tri-isopropylsilylethynyl pentacene (TIPS-PEN) channels show that both ON and OFF currents of the transistors are maintained with the PMMA contents despite the reduction of remanent polarization of P(VDF-TrFE)/PMMA films.

Relaxor ferroelectric behavior and structural evaluation in electron-irradiated P (vinylidene fluoride-trifluoroethylene) copolymer blends

The effect of high-energy electron-irradiation on binary ferroelectric blends of poly(vinylidene fluoride-trifluoroethylene) 56/44 and 80/20 mol% has been studied in a broad dose ranging from 0 to 120 Mrad. It was found that the blends transformed from a normal ferroelectric to relaxor ferroelectric, obeying the Vogel-Fulcher law, after a proper electron dose by dielectric constants and loss measurements. X-ray diffraction shows significant changes in the ferroelectric-to-paraelectric phase transition behavior from all-trans to trans-gauche conformation after irradiation. In differential scanning calorimetry (DSC) thermograms, the Curie temperature and melting temperature decrease with the dose, indicating that there exists a strong lattice coupling between the two copolymers.

Thermal and structural properties of high-energy electron irradiated poly(vinylidene fluoride-trifluoroethylene) copolymer blends

Thermal and structural properties of high-energy electron irradiated ferroelectric blends by poly(vinylidene fluoride-trifluoroethylene) 56/44 and 80/20 mol% copolymers have been investigated by thermally stimulated depolarization current (TSDC), differential scanning calorimetry (DSC) and X-ray diffraction measurements. The temperature peaks observed in TSDC spectra of the blends decrease with the irradiation dose, associating phase transitional characters of the parent copolymers. And their distribution shows the existence of two types of crystallite, which become more clear after irradiation, demonstrating that the miscibility in the crystalline region for their large compositional discrepancy (24 mol% in their VF 2 contents) in the blend. In DSC thermograms, it is found that the Curie temperature and melting temperature decrease with the dose. The ferroelectric-to-paraelectric (F–P) transition temperatures and enthalpies in the blends merge into one with the increasing of dose, which indicates a strong lattice coupling. X-ray diffraction shows some degree of mixing inside a crystal lattice due to significant changes in the F–P phase transition behavior from all-trans to trans-gauche conformation in nanometer range after irradiation.

Thermally Stimulated Depolarization Current in Ferroelectric Blends of Copolymers of Vinylidene Fluoride with Trifluoroethylene

Ferroelectric blends of vinylidene fluoride (VDF)/trifluoroethylene (TrFE) copolymer with different molar ratio of 56/44, 60/40, 63/37, 67/33, 70/30, 73/27 and 77/23 were fabricated by casting from methyl ethyl ketone solution. Thermally stimulated depolarization current (TSDC) of the samples has been measured on the modified apparatus of thermal electric analyzer. Wide angle X-ray diffraction shows two peaks in the region of 2θ = 18–20° for all the blends, which indicates that the blends have the same crystal lattice structure as the copolymers. DSC results present that the blends display the phase transition characters of both copolymers of 77/23 and 56/44. TSDC spectra of the blends exhibit the characters of the both components at different polarization temperatures.

Thermally Stimulated Depolarization Current in Ferroelectric Blends of Copolymers of Vinylidene Fluoride with Trifluoroethylene

Ferroelectric blends of vinylidene fluoride (VDF)/trifluoroethylene (TrFE) copolymer with different molar ratio of 56/44, 60/40, 63/37, 67/33, 70/30, 73/27 and 77/23 were fabricated by casting from methyl ethyl ketone solution. Thermally stimulated depolarization current (TSDC) of the samples has been measured on the modified apparatus of thermal electric analyzer. Wide angle X-ray diffraction shows two peaks in the region of 2θ = 18–20° for all the blends, which indicates that the blends have the same crystal lattice structure as the copolymers. DSC results present that the blends display the phase transition characters of both copolymers of 77/23 and 56/44. TSDC spectra of the blends exhibit the characters of the both components at different polarization temperatures.