Detlef Billep

Enhancement of the thermoelectric properties of PEDOT:PSS thin films by post-treatment

In this work, the thermoelectric (TE) properties of poly(3,4-ethylenedioxylthiophene):poly(styrene sulfonate) (PEDOT:PSS) thin films at room temperature are studied. Different methods have been applied for tuning the TE properties: 1st addition of polar solvent, dimethyl sulfoxide (DMSO), into the PEDOT:PSS solution; 2nd post-treatment of thin films with a mixture of DMSO and ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4). It is verified that DMSO post-treatment is more efficient than DMSO addition in improving the electrical conductivity with a trivial change in the Seebeck coefficient. The power factor is increased up to 30.1 μW mK−2 for the film with DMSO post-treatment, while the optimized power factor by DMSO addition is 18.2 μW mK−2. It is shown that both DMSO addition and post-treatment induce morphological changes: an interconnected network of elongated PEDOT grains is generated, leading to higher electrical conductivity. In contrast, for those films post-treated in the presence of EMIMBF4, an interconnected network of short and circular PEDOT grains with increased polaron density is created, resulting in the improvement in the Seebeck coefficient and a concomitant compromise in the electrical conductivity. An optimized power factor of 38.46 μW mK−2 is achieved at 50 vol% of EMIMBF4, which is the highest reported so far for PEDOT:PSS thin films to our knowledge. Assuming a thermal conductivity of 0.17 W mK−1, the corresponding ZT is 0.068 at 300 K. These results demonstrate that post-treatment is a promising approach to enhance the TE properties of PEDOT:PSS thin films. Furthermore, ionic liquid, EMIMBF4, shows the potential for tuning the TE properties of PEDOT:PSS thin films via a more environmentally benign process.

Chemical post-treatment and thermoelectric properties of poly(3,4-ethylenedioxylthiophene):poly(styrenesulfonate) thin films

We report on the modification of the thermoelectric properties of poly(3,4-ethylenedioxylthiophene):poly(styrenesulfonate) (PEDOT:PSS) thin films by means of a simple post treatment of the solid thin films realized by drop-coating. We show that the organic polar solvents, dimethyl sulfoxide and ethylene glycol as secondary dopants for PEDOT:PSS, only affect the film morphology for which a high electrical conductivity is observed. In contrast, ethanolamine (MEA) and ammonia solutions are reduction agents that improve the density of PEDOT chains in the reduced forms (polaron and neutral states), resulting in the trade-off between Seebeck coefficient and electrical conductivity. Furthermore, we show that the nature of amines determines the reduction degree: the nitrogen lone pair electrons in MEA are easier to be donated than those in ammonia solution and will therefore neutralize the PEDOT chains.

Silicon mirror arrays fabricated by using bulk and surface micromachining

Recently scanning actuator arrays have developed using metal, e.g. aluminium, or polysilicon as mirror material. Design and technology of micro mirror arrays mad of monocrystalline silicon are discussed in this paper as well as experimental results characterizing the arrays. Micro mirror arrays with up to 1000 simultaneously movable electrostatically operated cells convenient for continuous scanning with frequencies of several hundred Hz up to some kHz will be presented. The technological approaches consist of the use of silicon wet- and dry-etching, wafer bonding and metallization. A novel modified BESOI technology with CMP, wafer bonding with buried refractory metal electrodes and sacrificial layer etching has ben developed and will be discussed. The design process is based on simple analytical calculations of the mechanical behavior, the fluid flow surrounding the movable mirror and the electrostatic field as well as numerical simulations by means of the finite element method and network analysis. Furthermore, some experimental methods to characterize the electro-mechanical behavior of micro mirror arrays are discussed. In order to evaluate theoretical models describing the behavior, the natural frequencies, the damping coefficients and the frequency transfer function are measured. The adaptation of the model parameters leads to more accurate values simulating the behavior.

Integration of piezoelectric polymer transducers into microsystems for sensing applications

A polymer-based sensor for low frequency acceleration detection is fabricated by using microinjection molding technologies. Finite Element simulations and characterization of the sensing functionality are done. Due to an out-of-plane acceleration a force is applied to a seismic mass (length and width each 3.2 mm, thickness 1 mm), which leads to a deformation of a connected plate with dimensions of 1 mm × 1 mm × 50 μm. Thus, charge separation at the electrodes of integrated piezoelectric polyvinylidene fluoride (PVDF) copolymer sheets occur and can be measured as sensor signal. A charge sensitivity of 0.57 pC/g is determined which is in good agreement with the simulation results. A resonance frequency of 660 Hz was measured. Furthermore, the sensor concept as well as preparation technologies to assemble a compound structure containing piezoelectric layers and the system integration by micro injection molding are discussed. In addition, different bonding techniques for the assembly of the functional components are investigated and described.

Pulsed DC magnetron sputtered piezoelectric thin film aluminum nitride – Technology and piezoelectric properties

Pulsed DC magnetron sputtered aluminum nitride (AlN) thin films are prepared on several seed layers and at different sputtering conditions. The piezoelectric c-axis (002) orientation of the AlN is analyzed with X-ray diffraction method. The transverse piezoelectric coefficient d 31 is determined with a Laser-Doppler-Vibrometer at cantilevers and membranes by analytical calculations and finite element method. Additionally, thin film AlN on bulk silicon is used to characterize the longitudinal piezoelectric charge coefficient d 33.

Application of micromirror arrays for Hadamard transform optics

The paper presents a novel kind of Hadamard transform optic. First investigations are made with a micro mirror array in a Hadamard transform spectrometer (HTS) whereby the usually used detector array is replaced by the micro mirror array. All the mirrors are imaged onto a single detector. The measurement is performed using a Hadamard matrix, i.e. while each detector reading a certain combination of mirrors given by the matrix is reflecting the light towards the detector. All the rest of them are reflecting the light beside it. The consequence is an improvement of the signal to noise ratio (SNR). The novelty of the realized spectrometer is that in contrast to other applications the mirrors are not statically switched but they are forced to oscillate at their resonant frequency. By this way a special Hadamard matrix can be used that improves the SNR best.

Precise determination of piezoelectric longitudinal charge coefficients for piezoelectric thin films assisted by finite element modeling

A determination method to identify the piezoelectric longitudinal charge coefficient d₃₃ for thin films on substrate is presented. Finite Element (FE) simulations and measurements at PZT on bulk silicon show correlations of the substrate characteristics and electrode size subject to the displacement of an electric actuated piezoelectric material. FE models are used to identify the d₃₃ independent of the substrate and electrode characteristic. Thereby the piezoelectric longitudinal charge coefficient of 1.1 μm PLD deposited Pb(Zr_0.52Ti_0.48)O₃ (PZT) is calculated d₃₃ = 272.5 pm/V and a small standard deviation of σ = 4 pm/V for different sample geometries is analyzed.

Yet another tuning fork gyroscope

This paper reports on the development of a micromechanical vibrating mass tuning fork gyroscope with coupled drive and sense modes. The mechanical structure has been designed to have a first anti-phase drive mode and a second anti-phase sense mode. The frequencies of all other modal modes are higher. The micromechanical structure is fabricated with a cavity SOI technology called BDRIE [1]. The analog ASIC for very low noise readout (<; 50 nV/rtHz) is placed on the MEMS and is connected symmetrically with wire bonds. MEMS and ASIC have been co-designed to best fit physically and electrically to each other.

Wake up MEMS for inertial sensors

A piezoelectric MEMS with wake up function, Power down interrupt generator (PDIG), for inertial sensor systems is reported. The aluminum nitride based MEMS generates electric signals intrinsically in result of an inertial accelerations. The PDIG is optimized for maximum charge and voltage sensitivity. The charge sensitivity for a single electrode designed PDIG is measured with 40.1 pC/g and the maximum voltage sensitivity with 1.273 V/g. The device shows a high linearity and reproducibility. The PDIG is merged with a CMOS electronic, designed for low current consumption in sleeping mode. The minimum acceleration to wake up the system is adjustable and measured with 0.08 g.