The development of novel functional dielectric materials can open the doors to major technological innovations with societal impact. Stretchable capacitors transduce electrical into mechanical energy or vice-versa. Over the last 20 years, they have received significant interest from academia and industry. However, this technology still needs both improved dielectrics as well as conductive elastomers to achieve the desired low driving voltage and to realize devices with attractively high sensitivity. The currently most explored dielectric elastomers are polydimethylsiloxanes (PDMS). However, because of their low dielectric permittivity of only 3, the devices made of them require high voltages for operation. We synthesized polar polysiloxanes with different types and contents of polar groups, investigated their thermal and dielectric properties, and selected the most suitable groups to achieve the highest dielectric permittivity, yet sufficiently low glass transition temperature (Tg) to afford an excellent elastomer at room temperature after cross-linking. This research guided us to several promising polar polysiloxane elastomers modified with nitrile and nitroaniline groups, for which the properties were optimized. We reproducibly achieved dielectric elastomers with a dielectric permittivity of about 18. Some respond to a voltage as low as 200 V, while some give very large actuation and have a breakdown field reaching 100 V μm-1. By carefully selecting suitable synthetic chemistry, we could also achieve self-healable high permittivity elastomers. The materials can be processed into thin films by melt pressing. Stack actuators can be easily manufactured manually and give 5.4% actuation at an electric field as low as 3.2 V μm-1. Furthermore, the actuators can self-repair after a breakdown and be recycled after complete failure. A graphene nanoplatelets (GNPs) composite in PDMS as a conductive electrode was developed via in-situ polymerization. The synthesis and the processing by screen-printing were conducted solvent-free, making this composite the greenest electrode for this technology. This presentation gives an overview of recent research on improved materials for dielectric elastomer transducers (DETs) conducted at Empa. We are confident that our materials will impact fields including actuators, sensors, energy harvesting, artificial muscles, and soft robotics.
CH3NH3PbI3 perovskite solar cells are one of the most exciting technologies in the renewable energy field, resulting in over 20% power conversion efficiency. Deep understanding of the working principle is now required to turn the high efficiency solar cells into a reliable technology. In this work we have explored the role of deposition method on the crystallinity of perovskite films and its influence on the hysteresis behavior of the current-voltage characteristics. In addition Nb2O5 was used as hole blocking layer and its influence is also discussed. We have found that hysteresis is strongly dependent on both; perovskite deposition method and Nb2O5 thickness. The ideal condition where the hysteresis is suppressed or minimized was achieved by using the sequential deposition method for the perovskite semiconductor and a hole blocking layer of 50 nm.
The dielectric relaxation processes of polymethyl methacrylates that have been functionalized with Disperse Red 1 (DR1) in the side chain (DR1-co-MMA) were studied with temperature dependent impedance spectroscopy and thermally stimulated depolarization current (TSDC) techniques. Copolymers with dipole contents which varied between 10 mol% and 70 mol% were prepared. All samples showed dipole relaxations above the structural-glass transition temperature (Tg). The β-relaxation of the methyl methacrylate (MMA) repeating unit was most visible in DR1(10%)-co-MMA and rapidly vanishes with higher dipole contents. DSC data reveal an increase of the Tg by 20 °C to 125°C with the inclusion of the dipole into the polymethyl methacrylate (PMMA) as side chain. The impedance data of samples with several DR1 concentrations, taken at several temperatures above Tg, have been fitted with the Havriliak-Negami (HN) function. In all cases, the fits reveal a dielectric response that corresponds to power-law dipolar relaxations. TSDC measurements show that the copolymer can be poled, and that the induced polarization can be frozen by lowering the temperature well below the glass transition. Relaxation strengths ΔƐ estimated by integrating the depolarization current are similar to those obtained from the impedance data, confirming the efficient freezing of the dipoles in the structural glass state.
The research efforts for silicone based elastomers with high dielectric permittivity (Ɛ’) intensified significantly in the last years since such materials would allow the construction of dielectric elastomer actuators (DEA) with low operation voltages. Polar groups can be introduced to elastomers to adjust their permittivity. The results obtained regarding the functionalization of silicones with polar nitrile (CN) and trifluoropropyl (CF3) groups are presented. Those with CN groups were synthesized via anionic polymerization of nitrile containing cyclosiloxanes or via a post-polymerization modification of functional polysiloxanes. Polysiloxanes containing CF3 groups were prepared by anionic copolymerization of 1,3,5-tris(3,3,3-trifluoropropyl)-1,3,5-trimethylcyclosiloxane with octamethylcyclotetrasiloxane. Importantly, we have found that all polysiloxanes have glass transition temperatures (Tg) well below room temperature (<-50°C). This ensures that the materials turn into true elastomers after cross-linking. In addition to this, a linear increase in Ɛ’ with increasing content of polar groups was observed with maximum values of Ɛ’ = 18 and Ɛ’ = 8.8 for polysiloxanes modified at every repeating unit with either CN or CF3 groups, respectively.
It has been the dream of many scientists to create polymeric materials with simultaneously high dielectric permittivity, low glass transition temperature, and excellent elastomeric properties. Such material would be a highly attractive dielectricum in electromechanical actuators. Within this topic we are focusing on silicones because of their excellent elastomeric properties over a wide temperature and frequency range combined with low glass transition temperatures. To increase their low permittivity, we followed different approaches which include: blending the matrix with highly polarizable conductive and polar nanofillers and chemical modification of the silicones with polar side groups. This AC340presentation will show the advantages and disadvantages of the two strategies we have been following and will provide an assessment of their future potentials.
A new type of poleable dielectric elastomer is introduced herein. The elastomer contains polymer nanoparticles with frozen molecular dipoles, which can be oriented at elevated temperatures in an electric field via poling. The aim is to provide a soft material with high, tunable optical properties suitable for actuator and flexible electronics applications. To that end poleable polymeric nanoparticles with high dipole concentrations and glass transition temperatures well above room temperature will be needed to be used as filler in an elastomer matrix. The synthesis and characterization of such particles is presented in this manuscript. Polyhydroxyethyl methacrylate (PHEMA) nanoparticles were synthesized using miniemulsion polymerization. The particles were loaded with 4-[ethyl (2-hydroxyethyl) amino]-4-nitrobenzene, usually called Disperse Red 1 (DR1), which has a large dipole moment (μ = 7.5 – 9.5 D). The maximum dipole loadings is limited by the solubility of the dipole in the monomer solutions prior to polymerization. All samples show a glass transition temperature around 95 °C. Secondary electron microscopy (SEM) revealed spherical particles, the size of which was confirmed by dynamic light scattering (DLS) measurements. A composite was prepared by dispersing the particles in polydimethyl siloxane (PDMS).
Today numerous cyanine dyes that are soluble in organic solvents are available, driven by more than a century of
research and development of the photographic industry. Several properties specific to cyanine dyes suggest that
this material class can be of interest for organic solar cell applications. The main absorption wavelength can be
tuned from the ultra-violet to the near-infrared. The unparalleled high absorption coefficients allow using very
thin films for harvesting the solar photons. Furthermore, cyanines are cationic polymethine dyes, offering the
possibility to modify the materials by defining the counteranion. We here show specifically how counterions can
be utilized to tune the bulk morphology when blended with fullerenes. We compare the performance of bilayer
heterojunction and bulk heterojunction solar cells for two different dyes absorbing in the visible and the near-infrared.
Light-induced Electron Spin Resonance (LESR) was used to study the charge transfers of light induced
excitons between cyanine dyes and the archetype fullerene C60. LESR results show good correlation with the cell
performance.
The development of organic electronic requires a non contact digital printing process. The European funded e-LIFT project investigated the possibility of using the Laser Induced Forward Transfer (LIFT) technique to address this field of applications. This process has been optimized for the deposition of functional organic and inorganic materials in liquid and solid phase, and a set of polymer dynamic release layer (DRL) has been developed to allow a safe transfer of a large range of thin films. Then, some specific applications related to the development of heterogeneous integration in organic electronics have been addressed. We demonstrated the ability of LIFT process to print thin film of organic semiconductor and to realize Organic Thin Film Transistors (OTFT) with mobilities as high as 4 10-2 cm2.V-1.s-1 and Ion/Ioff ratio of 2.8 105. Polymer Light Emitting Diodes (PLED) have been laser printed by transferring in a single step process a stack of thin films, leading to the fabrication of red, blue green PLEDs with luminance ranging from 145 cd.m-2 to 540 cd.m-2. Then, chemical sensors and biosensors have been fabricated by printing polymers and proteins on Surface Acoustic Wave (SAW) devices. The ability of LIFT to transfer several sensing elements on a same device with high resolution allows improving the selectivity of these sensors and biosensors. Gas sensors based on the deposition of semiconducting oxide (SnO2) and biosensors for the detection of herbicides relying on the printing of proteins have also been realized and their performances overcome those of commercial devices. At last, we successfully laser-printed thermoelectric materials and realized microgenerators for energy harvesting applications.
The success of dielectric elastomer materials in actuator technology as well as in energy harvesting is much influenced
by the material parameters, e.g. breakdown field, dielectric constant, and elastic modulus which have a direct impact on
the driving voltage. By increasing the dielectric constant of a material the activation voltage can be decreased, however
this increase is very often associated with a decrease in the breakdown field. In this proceeding, dielectric elastomer
materials based on polydimethylsiloxanes with increased strain at break and high breakdown fields are presented.
The dielectric constant (ε) of a polymer can significantly be increased by blending it with conducting fillers. Given our
interest in developing highly efficient and long-lasting actuators for muscle replacement, we set out to explore all key
issues which could help to reduce the required voltage and at the same time ensure long term stability. The presentation
describes experiments which prove that the water content in carboxylic acid-decorated phthalocyanines (Pcs), commonly
falsely referred to oligo-Pcs, is a critical factor determining the absolute value of ε. Several publications on ε values of these oligo-Pcs led to contradicting conclusions because the effect of water was not sufficiently considered. The water
content is relevant because o-Pcs are often used as fillers to increase ε of polymer matrices. This presentation also
describes an experimental evaluation on whether or not as-prepared polyaniline (PANI) and poly(divinyl benzene)-
encapsulated (PDVB) PANI can be reasonably used as high ε fillers in matrix materials. For this purpose several blends
with polystyrene-polybutadiene block copolymer gels (PS-b-PB) and polydimethyl siloxane (PDMS) were prepared and
their dielectric properties investigated. The former part of this presentation has in part already been published (D. M.
Opris et al. Chem. Mater. 20(21), 6889-6896, 2008), the latter is completely new.
In principle EAP technology could potentially replace common motion-generating mechanisms in positioning, valve
control, pump and sensor applications, where designers are seeking quieter, power efficient devices to replace
conventional electrical motors and drive trains. Their use as artificial muscles is of special interest due to their similar
properties in terms of stress and strain, energy and power densities or efficiency. A broad application of dielectric
elastomer actuators (DEA) is limited by the high voltage necessary to drive such devices.
The development of novel elastomers offering better intrinsic electromechanical properties is one way to solve the
problem. We prepared composites from cross-linked silicone elastomers or thermoplastic elastomers (TPE) by blending
them with organic fillers exhibiting a high dielectric constant. Well characterized monomeric phthalocyanines and
modified doped polyaniline (PANI) were used as filler materials. In addition, blends of TPE and an inorganic filler
material PZT were characterized as well. We studied the influence of the filler materials onto the mechanical and
electromechanical properties of the resulting mixtures. A hundredfold increase of the dielectric constant was already
observed for blends of an olefin based thermoplastic elastomer and PANI.
KEYWORDS: Interfaces, Dewetting, Coating, Solar cells, Thin films, Ions, Atomic force microscopy, Photovoltaics, Heterojunctions, Thin film solar cells
The details of the arrangement of mixtures of semiconducting materials in thin-films have a major influence on the
performance of organic heterojunction solar cells. Here, we exploit the phenomenon of spinodal dewetting during spin
coating of blends of PCBM and a cyanine dye for the design of phase separated morphologies with increased interfacial
area. AFM snapshots of as-prepared films and after selective dissolution suggest that the solution separates into transient
bilayers, which destabilize due to long-range intermolecular interactions. We propose that film destabilization is
effectively driven by electrostatic forces that build up due to mobile ions that cross the junction and dissolve partially in
PCBM. The resulting morphology type is mainly dependent on the ratio between the layer thicknesses, whereas the
dominant wavelengths formed are determined by the absolute film thickness. Solar cells were fabricated from films with
known structure and a power conversion efficiency of η = 0.29 % was measured for a vertically segregated film
consisting of a cyanine layer covering the anode and an upper phase composed of dewetted PCBM domains. We explain
the merits of this structure in contrast to a lateral separated blend morphology where the efficiency was 3 times smaller.
Significant progress is being made in the photovoltaic energy conversion using organic semiconducting materials. One
of the focuses of attention is the nanoscale morphology of the donor-acceptor mixture, to ensure efficient charge
generation and loss-free charge transport at the same time. Using small molecule and polymer blend systems, recent
efforts highlight the problems to ensure an optimized relationship between molecular structure, morphology and device
properties. Here, we present two examples using a host/guest mixture approach for the controlled, sequential design of
bilayer organic solar cell architectures that consist of a large interface area with connecting paths to the respective
electrodes at the same time. In the first example, we employed polymer demixing during spin coating to produce a rough
interface: surface directed spinodal decomposition leads to a 2-dimensional spinodal pattern with submicrometer features
at the polymer-polymer interface. The second system consists of a solution of a blend of small molecules, where phase
separation into a bilayer during spin coating is followed by dewetting. For both cases, the guest can be removed using a
selective solvent after the phase separation process, and the rough host surface can be covered with a second active,
semiconducting component. We explain the potential merits of the resulting interdigitated bilayer films, and explore to
which extent polymer-polymer and surface interactions can be employed to create surface features in the nanometer
range.
A novel cyanine-fullerene dyad is used as active layer in a heterostructure photovoltaic cell based on PEDOT and buckminsterfullerene C60. The photocurrent spectrum of the device matches the absorption spectrum of the film and shows monochromatic quantum efficiencies of 5.5% and fill factors above 0.35 at white light irradiation intensities up to 310 mW/cm2. By blending a long wavelength absorbing cyanine with the dyad, the photocurrent spectrum extends to 800 nm with appreciable monochromatic quantum efficiency. Sensitization by cyanine dyes demonstrates the possibility to achieve photovoltaic response in the near infrared.
We present a theoretical and experimental study of a multilayer organic light emitting device (OLED) with a partially doped emission layer. An extended version of our established "MOLED" device model is used to understand the effects of the partially doped layer on the transport behavior and on the radiative charge recombination distribution as a function of applied bias. A step by step discussion of the possible mechanisms that can be introduced by doping and the resulting changes on the device properties is presented. We have found that under certain conditions the recombination zone is split into two zones leading to an emission color change with increasing voltage.
By using the yellow emitting laser dye derivative 4-(dicyanomethylene)-2-methyl-6-{2-[(4-diphenylaminophenyl]ethyl}-4H-pyran (DCM-TPA) with electron trapping capabilities as a dopant in a standard organic light emitting device, we have achieved high quantum efficiency with excellent color saturation. Furthermore, for this special case a blue-shift of the emission color is observed for increasing bias due to the appearance of a double peak structure of the recombination zone.
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