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Ferroelectrets, with their internal bipolar charges on the surfaces of the voids can be viewed as a collection of a large number of individual tiny electret ionization chambers. Electrets in ionization chambers are discharged upon irradiation with ionizing radiation. The discharge of cellular propylene is studied upon irradiation with alpha-radiation from a 226Ra source, while in-situ measuring the piezoelectric effect. Piezoelectric coefficients are derived from piezoelectric thickness extension resonances and from the second harmonic current upon application of a sinusoidal voltage signal. The piezoelectric coefficient gives a direct measure of the effective internal surface charge density within the voids of the cellular material. Compared to electret ionizing chambers, a much better temporal stability of charges in ferroelectrets is observed. The small air volume in the voids of cellular polypropylene is believed to account for the remarkable charge stability.

Organic field-effect transistors (OFETs) consist of a gate dielectric and an organic semiconductor film. The performance of organic electronic devices substantially depends on the dielectric properties of the insulating gate layer. Only a few key parameters, primarily the dielectric constant and the resulting device capacitance, have been regarded to be of central importance. Many insulating layers are however not simple dielectrics, an example are space charge gate electrets with internally trapped charges. Space charge gate electrets affect the electrical characteristics of diodes and transistors in a much more sophisticated manner, they are for example the key element in flash memories. We present impedance measurements of an organic metal-insulator-semiconductor (MIS) diode and corresponding measurements of a related organic field effect transistor. Both devices have a comparable design, with polyvinylalcohol as gate electret and the methanofullerene PCBM as organic semiconductor. Pronounced electret effects of charge injection and trapping are observed by impedance measurements of the MIS structure and these effects are found equally expressed in the electrical characteristics of the OFET configuration, reflecting first steps towards organic flash memories

Cellular space charge polymer electrets significantly enlarge the basis of pyro- and piezoelectric as well as ferroic materials. Since the first review of the field has been published in this journal (R. Gerhard-Multhaupt, Less can be more: Holes in polymers lead to a new paradigm of piezoelectric materials for electret transducers, IEEE Trans. Dielectr. Electr. Insul. Vol.9, pp.850-859, 2002), progress has been achieved in the understanding, preparation and characterization of cellular space charge electrets. Advanced applications arise in stacked microphones, artificial sonar systems, and in the combination of piezo- and ferroelectrets with flexible electronics. In the present survey an update of the developments since 2002 is provided

Polymer electrets from the polyvinylidene family are highly suitable materials for plastic electronics. The high dielectric constant of such polymers in both the ferro- and non-ferroelectric phase is most suitably employed in gate dielectrics in organic field-effect transistors. The pyro- and piezoelectric response makes ferroelectric polymer electrets also an ideal choice for organic temperature, infrared radiation and pressure sensors. Here we report steps towards transparent organic pyroelectric infrared sensors: The infrared sensor element is based on poly (vinylidene fluoride), the top contact pentacene transistor on poly(vinylidene-fluoride tetrafluoroethylene hexafluoropropylene) and poly(vinyl cinnamate) as gate dielectric. Steps for combining the ferroelectric polymer sensors and transistors to a fully organic temperature and infrared sensing element are outlined. The sensor concept may be easily scaled up to the production of sensor arrays for thermal imaging

Cellular space-charge electrets have recently emerged as a new class of materials for electromechanical devices, offering chances for a wide range of applications and challenges for materials optimization. However, many fundamental and applied aspects of the physics of these novel materials are not yet explored. Here we summarize our present understanding of the (quasi)-piezo- and -pyroelectricity in such materials. In contrast to the dipole-density piezoelectricity in ferroelectric polymers, the piezoelectric-like response of cellular polymers is intrinsic, with positive d/sub 33/ and negative d/sub 31/ and d/sub 32/ piezoelectric-like coefficients. Similarities with ferroelectric materials are outlined, especially switching of "polarization" and (quasi)-piezoelectricity. First steps towards patterned charging of cellular polymers are reported, an immediate consequence of the possibility for "polarization"-switchingin cellular materials. The results on cellular space-charge electrets suggest that well-known electret devices like microphones may be seen in a new light. Examples include dielectric and electromechanical hysteresis loops obtained with a commercially available electret microphone. In view of the results, cellular polymers may henceforth be called "ferroelectrets" and their material behavior "ferroelectretic". From an applied point of view, the performance of a Fresnel zone plate for focussing ultrasound is shown. Such a device may pave the way for a simple tool in nondestructive materials inspection, and demonstrates the large potential of cellular polymers for applications.

Organic materials with non-centrosymmetric chromophores are known to be susceptible to a number of photochemical processes, including reversible isomerization reactions as well as irreversible photo-oxidation or photo- reduction reactions. Reversible isomerization is the basis for a variety of applications, such as photo-induced poling, optical data storage and optical grating formation. The irreversible processes that involve the destruction of the chromophores have been found useful for the fabrication of optical waveguides, but they also limit the life times of polymeric photonic devices. In this paper, it is demonstrated that dielectric measurements allow for an in-depth investigation of non-reversible chromophore degradation processes in a typical side-chain polymer. The time- and temperature-dependent dielectric function of the polymer at 1 kHz enables us to follow the chromophore-degradation kinetics and to monitor the bleaching depth as a function of time at room and elevated temperatures

The preparation and characterization of novel piezoelectric and pyroelectric electrets is discussed with respect to their electret properties. Charged heterogeneous electrets, like closed-cell microporous polypropylene (pp) foams and 'soft-hard' fluoropolymer hybrids are shown to yield large quasi-static and dynamic piezoelectric responses, indistinguishable from true piezoelectricity. Piezoelectric coefficients comparable to that of ceramic counterparts have been achieved. For the investigation of the piezoelectric and pyroelectric properties of the electret materials, the thermal-pulse technique has been extended towards temperature-dependent investigations and measurements on planar, cylindrical or spherical samples. The potential of the thermal-pulse technique not only for the measurement of electric-field and polarization distributions, but also for acoustical, electromechanical and pyroelectric properties is outlined.

One of the critical issues with poled photonic polymers is the thermal stability of the nonlinear optical effect in these dipolar electrets. The temporal decrease of the nonlinear optical coefficients is caused by dipolar-relaxation and molecular-degradation processes. In this paper, it is demonstrated that dielectric measurements allow for an in-depth investigation of dipole-degradation processes in a typical side-chain photonic polymer. The results yield an estimate of effective device lifetimes, which are in some cases dominated by irreversible chemical degradation rather than by reversible dipolar relaxation.