Optimization of a Handwriting Method by an Automated Ink Pen for Cost-Effective and Sustainable Sensors

In this work, we present a do-it-yourself (DIY) approach for the environmental-friendly fabrication of printed electronic devices and sensors. The setup consists only of an automated handwriting robot and pens filled with silver conductive inks. Here, we thoroughly studied the fabrication technique and different optimized parameters. The best-achieved results were 300 mΩ/sq as sheet resistance with a printing resolution of 200 µm. The optimized parameters were used to manufacture fully functional electronics devices: a capacitive sensor and a RFID tag, essential for the remote reading of the measurements. This technique for printed electronics represents an alternative for fast-prototyping and ultra-low-cost fabrication because of both the cheap equipment required and the minimal waste of materials, which is especially interesting for the development of cost-effective sensors.TUM Graduate School and by the European Commission through the fellowship H2020-MSCA-IF-2017-794885-SELFSEN

Biocompatible Silk/Polymer Energy Harvesters Using Stretched Poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) Nanofibers

Energy harvested from human body movement can produce continuous, stable energy to portable electronics and implanted medical devices. The energy harvesters need to be light, small, inexpensive, and highly portable. Here we report a novel biocompatible device made of poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) nanofibers on flexible substrates. The nanofibers are prepared with electrospinning followed by a stretching process. This results in aligned nanofibers with diameter control. The assembled device demonstrates high mechanical-to-electrical conversion performance, with stretched PVDF-HFP nanofibers outperforming regular electrospun samples by more than 10 times. Fourier transform infrared spectroscopy (FTIR) reveals that the stretched nanofibers have a higher β phase content, which is the critical polymorph that enables piezoelectricity in polyvinylidene fluoride (PVDF). Polydimethylsiloxane (PDMS) is initially selected as the substrate material for its low cost, high flexibility, and rapid prototyping capability. Bombyx Mori silkworm silk fibroin (SF) and its composites are investigated as promising alternatives due to their high strength, toughness, and biocompatibility. A composite of silk with 20% glycerol demonstrates higher strength and larger ultimate strain than PDMS. With the integration of stretched electrospun PVDF-HFP nanofibers and flexible substrates, this pilot study shows a new pathway for the fabrication of biocompatible, skin-mountable energy devices

고성능 유기 다이오드를 위한 유기물/금속 계면 연구

학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2014. 2. 이창희.Organic electronics have received a great attention for next generation electronics due to lots of advantages such as easy patterning, flexibility, light-weight, and potential of large-area application. In spite of such a lot of advantages, however, there are still many issues that need to be improved such as charge injection, mobility, lifetime, operational stability, reliability, and uniformity. One of the important issues to make high performance diode is to improve charge injection efficiency. In this thesis, we investigate organic/metal interface of the diode to enhance charge injection efficiency and demonstrate high performance organic diodes. Two major methods are used to improve device performance: improved charge injection by electrical annealing and reduced hole injection barrier by using permanent dipole moment of self-assembled monolayer (SAM). First, we investigate the effect of electrical annealing on pentacene diode to which electrical annealing has not been applied because it cannot have ionic species. By using molybdenum trioxide (MoO3) instead of ionic species, electrical annealing can be applied to thermally deposited device which is advantageous for fabricating high performance devices. After electrical annealing, The turn-on voltage is reduced from approximately 1.3 V to 0.2 V and current at 3 V is increased from approximately 0.2 mA to 1 mA without increase of the reverse-bias current. In addition to MoO3 as a hole injection layer, 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) and copper hexadecafluorophthalocyanine (F16CuPc), which have deep highest occupied molecular orbital (HOMO) levels, show electrical annealing effect but poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), 4,4,4-tris(N-3-methylphenylamino)triphenylamin (m-MTDATA), and copper phthalocyanine (CuPc) do not affect electrical annealing. the cutoff frequency was increased from 10.5 MHz to 85.7 MHz. There is no improvement of current or reduction of turn-on voltage by using thermal annealing only, indicating that electric field plays an important role for electrical annealing. From the time of flight secondary ion mass spectrometry (ToF-SIMS) and impedance spectra, we conclude that the device performance of the pentacene diode is improved by electrical annealing due to the creation of the pentacene:MoO3 mixed layer. The mixed layer effectively increases charge injection by reducing small potential barrier which causes the turn-on voltage of current–voltage (I–V) characteristics and the RC-component at Au/MoO3/pentacene interface of impedance spectra. Note that because this uniform and thin pentacene:MoO3 mixed layer cannot be formed by thermal evaporation, electrical annealing is the best technique to form the uniform, thin, and gradual pentacene:MoO3 mixed layer for improving device performance. After electrical annealing, Al penetration into the pentacene layer was also observed. Because Al was deposited on the polycrystalline pentacene, Al spikes are formed at the pentacene grain boundary. These Al spikes can induce a higher electric field, facilitating the penetration of Al. Therefore, the penetrated Al may create rod-like structures that can be modeled as constant phase element (CPE). Second, we investigated the structure–property relationship of pentacene on gold and SAM-treated gold along the vertical direction. From the photoelectron spectrometer, the work function of gold, thiophenol (TP)-modified gold and pentafluorobenzenethiol (PFBT)-modified gold is measured to be 4.78, 4.67, and 5.02 eV, respectively. PFBT-treated gold effectively lower the injection barrier between the anode and the active layer, the forward-bias current density of the diode with PFBT-treated gold is much higher than that with pristine gold and finally current density of 100 A/cm2 is obtained at 3 V. In addition, the rectification ratio of the diode is founded to be 7.47 × 105 at 1 V, and 1.05 × 107 at 2.8 V. The 3-dB frequency, in terms of voltage, of the rectifier which is composed of the diode and a capacitor is obtained to be 1.24 GHz. Finally, Vout of 3.8 V at 1 GHz is obtained when input voltage of 10 V is applied. From the X-ray diffraction (XRD), atomic force microscope (AFM), and Raman analysis, pentacene molecules on gold exhibit lying-down orientation and those on PFBT-treated gold exhibit standing-up orientation. These structure differences change the electrical property. The mobility, calculated by space charge limited current (SCLC), of the pentacene film on gold and PFBT-treated gold is measured to be 6.82 × 10-4 and 0.114 cm2V-1s-1, respectively. The XRD patterns and vertical scanning electron microscope (SEM) images show that pentacene on gold exhibits the entangled and disordered structure whereas pentacene on PFBT-treated gold exhibits dense and ordered structure. This poor molecular ordering for pentacene on gold can limit charge transport property, resulting that the mobility of the pentacene film on gold is smaller than that of on PFBT-treated gold.Abstract i Contents v List of Figures xi List of Tables xvii Chapter 1 Introduction 1 1.1 Improved Charge Injection by Electrical Annealing 6 1.2 Improved Charge Injection by Self-Assembled Monolayer 9 1.3 Outline of Thesis 11 Chapter 2 Experimental Methods 13 2.1 Materials 13 2.1.1 Chemical Structures of Organic Materials 13 2.1.2 Preparation of SAMs Solutions 15 2.2 Device Fabrication Methods 16 2.2.1 Preparation of Pentacene Diodes for Electrical Annealing 16 2.2.2 Preparation of Pentacene Diodes with SAM-treated Gold 18 2.2.3 Measurement Setup of Pentacene Rectifiers 20 2.3 Device Characterization Methods 21 2.3.1 Carrier Transport in Organic Semiconductors 21 2.3.2 I–V characteristics measurement 23 2.3.3 Frequency Response of Pentacene Rectifiers 24 2.3.4 Mobility Measurements 26 2.3.5 Impedance Spectroscopy 28 2.3.6 ToF-SIMS Measurement 29 2.3.7 Raman Spectroscopy 29 2.3.8 Other Characterization Methods 30 Chapter 3 Improved Injection Efficiency of Organic Diodes by Electrical Annealing 31 3.1 Systematic Investigation into Improved Device Performance of Pentacene Diodes after Electrical Annealing 33 3.1.1 I–V Characteristics of Pentacene Diodes Applying Constant Voltage or Constant Current 33 3.1.2 Current Characteristics of Pentacene Diodes with Electrical Annealing 35 3.1.3 I–V Characteristics of Pentacene Diodes with Various HILs 39 3.1.4 Frequency Characteristics of Pentacene Rectifiers 42 3.1.5 Thermal Annealing Effect on Pentacene Diodes 44 3.1.6 Hysteresis of the Pentacene Diodes before and after Electrical Annealing 45 3.2 Investigation into Proper Mechanism of Electrical Annealing 46 3.2.1 ToF-SIMS Measurements 46 3.2.2 Pentacene Diodes with MoO3 Doped Pentacene as a HIL 48 3.2.3 Impedance Spectroscopy 53 3.3 Summary 61 Chapter 4 High Performance Pentacene Diodes based on SAM-treated Gold 63 4.1 Effective Work Function of SAM-treated Gold 65 4.1.1 Photoelectron Spectrometer Measurement 65 4.2 Structural study of pentacene on SAM-treated gold 67 4.2.1 Morphological Study Using AFM 67 4.2.2 XRD Analysis 70 4.2.3 DFT Simulations of Single Pentacene Molecule for Raman Spectroscopy 71 4.2.4 Raman Spectra of Pentacene on SAM-treated Gold 74 4.2.5 Mobility of Pentacene on SAM-treated Gold Extracted by SCLC 80 4.2.6 Mobility of Pentacene on SAM-treated Gold Extracted by Photo-CELIV 83 4.2.7 SEM Images of Pentacene on SAM-treated Gold 85 4.3 Electrical Performance of Pentacene Diodes with SAM-treated Gold 89 4.3.1 J–V Characteristics of Pentacene Diodes with SAM-treated Gold 89 4.3.2 Frequency Performances of Pentacene Rectifiers with SAM-treated Gold 92 4.4 Demonstration of UHF Operating Pentacene Rectifiers 95 4.4.1 Design of Antennas for UHF RFID tags 95 4.4.2 Loop Antenna Fabrication Using Screen Printing 97 4.4.3 Demonstration of Pentacene Rectifiers Operating at 500 MHz 99 4.5 Summary 101 Chapter 5 Conclusion 103 Bibilography 107 Publication 117 Abstract in Korean 123Docto

DEVELOPMENT OF PIEZOELECTRIC ENERGY HARVESTING SYSTEM FOR LOW-FREQUENCY VIBRATIONS

Harvesting energy from vibration sources has attracted the interest of researchers for the past three decades. Researchers have been working on the potential of achieving self-powered MEMS scale devices. Piezoelectric cantilever harvesters have caught the attention in this field because of the excellent combination of high-power density and compact structure. The main objective of this thesis is to develop a novel and optimum piezoelectric harvester system using lumped parameter model (LPM) for given vibration sources. The finite element model (FEM) is used in this work as an original approach to be utilized for optimal design optimization. Three types of validations are accomplished to solidify the use of FEM in mimicking the distributed parameter model (DPM) for linearly tapered piezoelectric cantilevers. The first two validations are accomplished using beam deflection and relative transmissibility functions. Comparisons between the FEM and the DPM developed by the literature are performed. The third validation is carried for an electromechanical piezoelectric cantilever in FEM. Results confirmed the effectiveness of the developed FEM. A number of significant contributions are achieved while fulfilling the aim of this work. First, a dimensionless parameter, Power Factor (PF), is derived and used to understand the impact of the geometry on the piezoelectric harvester performance. The PF showed an optimum performance at a taper ratio of 0, taking the full length of the cantilever and thickness ratio of 0.7. Second, the accuracy of the LPM for linearly tapered piezoelectric harvesters and optimal design are investigated. Results indicated that the percentage of the deflection error between the LPM and the FEM reaches 9% when the taper ratio is zero. However, when tip-mass to cantilever ratios are larger than 2, the error decreases to less than 0.5% leading to more accurate results in the vibrational response of the beam. Further studies on the accuracy are accomplished using the relative transmissibility function. Results showed that as the taper ratio decreases towards zero, the percentage error of using the LPM to predict the vibration response increases significantly to 55%. These results lay the foundation for the third contribution of developing correction factors for tapered and optimal piezoelectric cantilever harvesters using FEM. Comparisons of the corrected LPM and FEM for different configurations are examined. Results indicated that as the taper ratio decreases, the surface power density increases. However, the developed optimal design exhibits the highest surface power density of 1.40×104 [(mW/g2)/ m2] which is 16.4% more than the best following shape of a taper ratio 0.2 and 58% more than the taper ratio 1. Furthermore, a parametric study of the optimal design is performed to scrutinize the effect of various parameters on the harvester performance. Finally, detailed criteria for designing the optimal piezoelectric harvester for different conditions are structured