In the pursuit of developing new materials for more efficient bulk-heterojunction solar cells, the blend poly (3-hexylthiophene):[ 6,6]-phenyl-C(61)-butyric acid methyl ester (P3HT:PCBM) serves as an important model system. The success of the P3HT: PCBM blend comes from efficient charge generation and transport with low recombination. There is not, however, a good microscopic picture of what causes these, hindering the development of new material systems. In this report UV photoelectron spectroscopy measurements on both regiorandom-(rra) and regioregular-(rr) P3HT are presented, and the results are interpreted using the Integer Charge Transfer model. The results suggest that spontaneous charge transfer from P3HT to PCBM occurs after heat treatment of P3HT: PCBM blends. The resulting formation of an interfacial dipole creates an extra barrier at the interface explaining the reduced (non-)geminate recombination with increased charge generation in heat treated rr-P3HT: PCBM blends. Extensive photoinduced absorption measurements using both above-and below-bandgap excitation light are presented, in good agreement with the suggested dipole formation.

Printed electronics are considered for wireless electronic tags and sensors within the future Internet-of-things (IoT) concept. As a consequence of the low charge carrier mobility of present printable organic and inorganic semiconductors, the operational frequency of printed rectifiers is not high enough to enable direct communication and powering between mobile phones and printed e-tags. Here, we report an all-printed diode operating up to 1.6 GHz. The device, based on two stacked layers of Si and NbSi2 particles, is manufactured on a flexible substrate at low temperature and in ambient atmosphere. The high charge carrier mobility of the Si microparticles allows device operation to occur in the charge injection-limited regime. The asymmetry of the oxide layers in the resulting device stack leads to rectification of tunneling current. Printed diodes were combined with antennas and electrochromic displays to form an all-printed e-tag. The harvested signal from a Global System for Mobile Communications mobile phone was used to update the display. Our findings demonstrate a new communication pathway for printed electronics within IoT applications.

High-efficiency photocatalysts are crucial for the removal of organic pollutants and environmental sustainability. In the present work, we report on a new low-temperature hydrothermal chemical method, assisted by ultrasonication, to synthesize disruptive plasmonic ZnO/graphene/Ag/AgI nanocomposites for solar-driven photocatalysis. The plasmonic nanocomposites were investigated by a wide range of characterization techniques, confirming successful formation of photocatalysts with excellent degradation efficiency. Using Congo red as a model dye molecule, our experimental results demonstrated a photocatalytic reactivity exceeding 90% efficiency after one hour simulated solar irradiation. The significantly enhanced degradation efficiency is attributed to improved electronic properties of the nanocomposites by hybridization of the graphene and to the addition of Ag/AgI which generates a strong surface plasmon resonance effect in the metallic silver further improving the photocatalytic activity and stability under solar irradiation. Scavenger experiments suggest that superoxide and hydroxyl radicals are responsible for the photodegradation of Congo red. Our findings are important for the fundamental understanding of the photocatalytic mechanism of ZnO/graphene/Ag/AgI nanocomposites and can lead to further development of novel efficient photocatalyst materials.

A ternary composite supercapacitor electrode consisting of phosphomolybdic acid (HMA), a renewable biopolymer, lignin, and polypyrrole was synthesized by a simple one-step simultaneous electrochemical deposition and characterized by electrochemical methods. It was found that the addition of HMA increased the specific capacitance of the polypyrrole-lignin composite from 477 to 682 F g(-1) ( at a discharge current of 1 A g(-1)) and also significantly improved the charge storage capacity from 6(to 128 mA h g(-1).

The electron transport in the electronic devices has significant influence on the device performance, thus current transport properties determination is highly demanded for a particular device. Herein, we report the facile hydrothermal growth method based fabrication of p-NiO/n-ZnO heterojunction. The material characterization was performed by scanning electron microscopy, X-ray diffraction, transmission electron microscopy and X-ray photo electron spectroscopy. These techniques provided the good crystal quality, pure phase of p-NiO and n-ZnO nanostructures respectively. The measured valance band offset of composite nanostructure is 2.25 eV and conduction band offset was found to be 2.58 eV. The current transport properties of the fabricated p-n junction are governed by three different I-V regions. The impedance spectroscopy was used for the determination of the role of grain boundaries at the interface.

Hexagonal c-axis oriented zinc oxide (ZnO) nanorods (NRs) with 120-300 nm diameters are synthesized via the low temperature aqueous chemical route at 80 degrees C on silver-coated glass substrates. The influence of varying the precursor solutions stirring durations on the concentration and spatial distributions of deep level defects in ZnO NRs is investigated. Room temperature micro-photoluminesnce (mu-PL) spectra were collected for all samples. Cathodoluminescence (CL) spectra of the as-synthesized NRs reveal a significant change in the intensity ratio of the near band edge emission (NBE) to the deep-level emission (DLE) peaks with increasing stirring durations. This is attributed to the variation in the concentration of the oxygen-deficiency with increasing stirring durations as suggested from the X-ray photoelectron spectroscopy analysis. Spatially resolved CL spectra taken along individual NRs revealed that stirring the precursor solutions for relatively short duration (1-3 h), which likely induced high super saturation under thermodynamic equilibrium during the synthesis process, is observed to favor the formation of point defects moving towards the tip of the NRs. In contrary, stirring for longer duration (5-15 h) will induce low super saturation favoring the formation of point defects located at the bottom of the NRs. These findings demonstrate that it is possible to control the concentration and spatial distribution of deep level defects in ZnO NRs by varying the stirring durations of the precursor solutions.

Low temperature aqueous chemical synthesis (LT-ACS) of zinc oxide (ZnO) nanorods (NRs) has been attracting considerable research interest due to its great potential in the development of light-emitting diodes (LEDs). The influence of the molar ratio of the zinc acetate (ZnAc): KOH as a ZnO seed layer precursor on the density of interface defects and hence the presence of non-radiative recombination centers in LT-ACS of ZnO NRs/GaN LEDs has been systematically investigated. The material quality of the as-prepared seed layer as quantitatively deduced by the X-ray photoelectron spectroscopy is found to be influenced by the molar ratio. It is revealed by spatially resolved cathodoluminescence that the seed layer molar ratio plays a significant role in the formation and the density of defects at the n-ZnO NRs/p-GaN heterostructure interface. Consequently, LED devices processed using ZnO NRs synthesized with molar ratio of 1:5M exhibit stronger yellow emission (similar to 575 nm) compared to those based on 1:1 and 1:3M ratios as measured by the electroluminescence. Furthermore, seed layer molar ratio shows a quantitative dependence of the non-radiative defect densities as deduced from light-output current characteristics analysis. These results have implications on the development of high-efficiency ZnO-based LEDs and may also be helpful in understanding the effects of the ZnO seed layer on defect-related non-radiative recombination. Published by AIP Publishing.

We demonstrate an efficient possibility to synthesize vertically aligned pure zinc oxide (ZnO) and Co-doped ZnO nanorods (NRs) using the low-temperature aqueous chemical synthesis (90 degrees C). Two different mixing methods of the synthesis solutions were investigated for the Co-doped samples. The synthesized samples were compared to pure ZnO NRs regarding the Co incorporation and crystal quality. Electron paramagnetic resonance (EPR) measurements confirmed the substitution of Co2+ inside the ZnO NRs, giving a highly anisotropic magnetic Co2+ signal. The substitution of Zn2+ by Co2+ was observed to be combined with a drastic reduction in the core-defect (CD) signal (g similar to 1.956) which is seen in pure ZnO NRs. As revealed by the cathodoluminescence (CL), the incorporation of Co causes a slight red-shift of the UV peak position combined with an enhancement in the intensity of the defect-related yellow-orange emission compared to pure ZnO NRs. Furthermore, the EPR and the CL measurements allow a possible model of the defect configuration in the samples. It is proposed that the as-synthesized pure ZnO NRs likely contain Zn interstitial (Zn-i(+)) as CDs and oxygen vacancy (V-O) or oxygen interstitial (O-i) as surface defects. As a result, Co was found to likely occupy the Zn-i(+), leading to the observed CDs reduction and hence enhancing the crystal quality. These results open the possibility of synthesis of highly crystalline quality ZnO NRs-based diluted magnetic semiconductors using the low-temperature aqueous chemical method. Published by AIP Publishing.

We propose a new facile electrochemical sensing platform for determination of urea, based on a glassy carbon electrode (GCE) modified with nickel cobalt oxide (NiCo2O4) nanoneedles. These nanoneedles are used for the first time for highly sensitive determination of urea with the lowest detection limit (1 mu M) ever reported for the non-enzymatic approach. The nanoneedles were grown through a simple and low-temperature aqueous chemical method. We characterized the structural and morphological properties of the NiCo2O4 nanoneedles by TEM, SEM, XPS and XRD. The bimetallic nickel cobalt oxide exhibits nanoneedle morphology, which results from the self-assembly of nanoparticles. The NiCo2O4 nanoneedles are exclusively composed of Ni, Co, and O and exhibit a cubic crystalline phase. Cyclic voltammetry was used to study the enhanced electrochemical properties of a NiCo2O4 nanoneedle-modified GCE by overcoming the typical poor conductivity of bare NiO and Co3O4. The GCE-modified electrode is highly sensitive towards urea, with a linear response (R-2 = 0.99) over the concentration range 0.01-5 mM and with a detection limit of 1.0 mu M. The proposed non-enzymatic urea sensor is highly selective even in the presence of common interferents such as glucose, uric acid, and ascorbic acid. This new urea sensor has good viability for urea analysis in urine samples and can represent a significant advancement in the field, owing to the simple and cost-effective fabrication of electrodes, which can be used as a promising analytical tool for urea estimation.

The temperature dependence of charge carrier mobility in conjugated polymers and their blends with fullerenes is investigated with different electrical methods, through field effect transistor (FET), space charge limited current (SCLC) and charge extraction (CELIV) measurements. Simple models, such as the Gaussian disorder model (GDM), are shown to accurately predict the temperature behavior, and a good correlation between the different measurement methods is obtained. Inconsistent charge carrier concentrations in the modeling are explained through intrinsic non-equilibrium effects, and are responsible for the limited applicability of existing numerical models. A severe extrinsic influence from water in FETs with a hydrophilic insulator interface is also demonstrated. The presence of water leads to a significant overestimate of the disorder in the materials from measurements close to room temperature and erratic behavior in the 150-350 K range. To circumvent this problem it is shown to be necessary to measure under ultra high vacuum (UHV) conditions. © 2008 Elsevier B.V. All rights reserved.

Here we report polymer diodes based on a conjugated polymer host and a dispersed molecular switch. In this case, the molecular switch is a photochromic (PC) molecule that can be reversibly switched between low and high energy gap states, triggered by exposure to ultra-violet and visible light, respectively. While dispersed inside the conjugated polymer bulk and switched to its low energy gap state, the PC molecules act as traps for holes. Solid-state blends of this PC material and conjugated polymers have been demonstrated in diodes. The state of the PC molecule controls the current density versus voltage (JV) characteristics of the resulting diode. Both poly(2-methoxy-5(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) and poly(3-hexylthiophene-2,5-diyl) (P3HT) host materials have been studied. The two conjugated polymers resulted in differing JV switching characteristics. A more pronounced JV switch is observed with MEH-PPV than with P3HT. We postulate that the PC material, while switched to its low energy gap state, act as traps in both the conjugated polymers but at different trap depth energies.

Plasmonic response of free charges confined in nanostructures of plasmonic materials is a powerful means for manipulating the light-material interaction at the nanoscale and hence has influence on various relevant technologies. In particular, plasmonic materials responsive in the mid-infrared range are technologically important as the mid-infrared is home to the vibrational resonance of molecules and also thermal radiation of hot objects. However, the development of the field is practically challenged with the lack of low-loss materials supporting high quality plasmons in this range of the spectrum. Here, we demonstrate that degenerately doped InN nanocrystals (NCs) support tunable and low-loss plasmon resonance spanning the entire midwave infrared range. Modulating free-carrier concentration is achieved by engineering nitrogen-vacancy defects (InN1-x, 0.017 amp;lt; x amp;lt; 0.085) in highly degenerate NCs using a nonequilibrium gas-phase growth process. Despite the significant reduction in the carrier mobility relative to intrinsic InN, the mobility in degenerate InN NCs (amp;gt;60 cm(2)/(V s)) remains considerably higher than the carrier mobility reported for other materials NCs such as doped metal oxides, chalcogenides, and noble metals. These findings demonstrate feasibility of controlled tuning of infrared plasmon resonances in a low-loss material of III-V compounds and open a gateway to further studies of these materials nanostructures for infrared plasmonic applications.

Energy barriers between the metal Fermi energy and the molecular levels of organic semiconductor devoted to charge transport play a fundamental role in the performance of organic electronic devices. Typically, techniques such as electron photoemission spectroscopy, Kelvin probe measurements, and in-device hot-electron spectroscopy have been applied to study these interfacial energy barriers. However, so far there has not been any direct method available for the determination of energy barriers at metal interfaces with n-type polymeric semiconductors. This study measures and compares metal/solution-processed electron-transporting polymer interface energy barriers by in-device hot-electron spectroscopy and ultraviolet photoemission spectroscopy. It not only demonstrates in-device hot-electron spectroscopy as a direct and reliable technique for these studies but also brings it closer to technological applications by working ex situ under ambient conditions. Moreover, this study determines that the contamination layer coming from air exposure does not play any significant role on the energy barrier alignment for charge transport. The theoretical model developed for this work confirms all the experimental observations.

Solar cells based on metal halide perovskites are one of the most promising photovoltaic technologies(1-4). Over the past few years, the long-term operational stability of such devices has been greatly improved by tuning the composition of the perovskites(5-9), optimizing the interfaces within the device structures(10-13), and using new encapsulation techniques(14,15). However, further improvements are required in order to deliver a longer-lasting technology. Ion migration in the perovskite active layer-especially under illumination and heat-is arguably the most difficult aspect to mitigate(16-18). Here we incorporate ionic liquids into the perovskite film and thence into positive-intrinsic-negative photovoltaic devices, increasing the device efficiency and markedly improving the long-term device stability. Specifically, we observe a degradation in performance of only around five per cent for the most stable encapsulated device under continuous simulated full-spectrum sunlight for more than 1,800 hours at 70 to 75 degrees Celsius, and estimate that the time required for the device to drop to eighty per cent of its peak performance is about 5,200 hours. Our demonstration of long-term operational, stable solar cells under intense conditions is a key step towards a reliable perovskite photovoltaic technology.

Thin films of conducting polypyrrole doped with large polymeric anions of polystyrene-sulphonate are electrochemically prepared to study the metal/polymer junctions. Aluminium and gold contacts are vacuum deposited to form metal/polymer/gold sandwich structures for current-voltage characterization. Photoelectron spectroscopy, using UV and X-ray photons, is carried out to investigate the possible causes of current limitation in the Al/PPy(PSS) junction.

Organic electronics based on organic semiconductors offer tremendous advantages compared to traditional inorganic counterparts such as low temperature processing, light weight, low manufacturing cost, high throughput and mechanical flexibility. Many key electronic processes in organic electronic devices, e.g. charge injection/extraction, charge recombination and exciton dissociation, occur at interfaces, significantly controlling performance and function. Understanding/modeling the interface energetics at organic-electrode/organic-organic heterojunctions is one of the crucial issues for organic electronic technologies to provide a route for improving device efficiency, which is the aim of the research presented in this thesis.

Integer charge transfer (ICT) states pre-existed in the dark and created as a consequence of Fermi level equilibrium at donor-acceptor interface have a profound effect on open circuit voltage in organic bulk heterojunction photovoltaics. ICT state formation causes vacuum level misalignment that yields a roughly constant effective donor ionization potential to acceptor electron affinity energy difference at the donor-acceptor interface, even though there is a large variation in electron affinity for the fullerene series. The large variation in open circuit voltage for the corresponding device series instead is found to be a consequence of trap-assisted recombination via integer charge transfer states. Based on the results, novel design rules for optimizing open circuit voltage and performance of organic bulk heterojunction solar cells are proposed.

Doping and insertion of interlayer are two established methods for enhancing charge injection/extraction properties at organic-electrode interface. By studying the energy level alignment behavior at low to intermediate doping levels for molecule-doped conjugated polymer/electrode interfaces, we deduce that two combined processes govern the interface energetics: (i) equilibration of the Fermi level due to oxidation (or reduction) of polymer sites at the interface as per the ICT model and (ii) a double dipole step induced by image charge from the dopant-polymer charge transfer complex that causes a shift of the work function. Such behavior is expected to hold in general for low to intermediate level doped organic semiconductor systems. The unified model is further extended to be suitable for conjugated electrolyte/electrode  interfaces, revealing the design rules for achieving the smallest charge injection/extraction barrier for both thin tunneling and thick charge transporting conjugated electrolyte interlayers.

To probe into the energy level spatial extension at interfaces, we employ the original approach of building and characterizing multilayers composed of a well-defined number of polymer monolayers with the Langmuir-Shäfer method to control polymer film uniformity and thicknesses, avoiding the problems associated with spin-coating ultrathin films. The disordered/amorphous films feature smaller, and in fact negligible, energy level bending compared to the more well-ordered films, in contradiction with existing models. It is found that that energy level bending depends on the ICT state distribution rather than the density of states of the neutral polymer chains in relation to the Fermi energy, thus taking into account the Coulomb energy associated with charging the polymer chain and transferring a charge across the interface. Based on this work, a general model for energy level bending in absence of significant doping of conjugated polymer films is proposed.

Organic semiconductors are sensitive to ambient atmosphere that can influence the energetics. The degradation effects of common PCBM film induced by oxygen and water are found to be completely different. Upon exposure to oxygen, the work function is down-shifted by ~ 0.15 eV compared to the ICT curve of the pristine PCBM film, originating from the weak interaction between the fullerene part of PCBM and oxygen, and this can be reversed by thermal treatment in vacuum. The down-shift in energetics will cause a loss in open circuit voltage at electrode interface, but aids free charge generation at donor-acceptor interface. Upon exposure to water, there is irreversible extensive broadening and bleaching of the valence electronic structure features as well as a substantial decrease of work function and ionization potential, severely degrading the transport properties.

Overall, the research results in this thesis thus give a deeper understanding of interface phenomena in organic electronics, especially regard to organic solar cells, aimed to further improve the device operation efficiency and lifetime.

In this short review the energy level alignment of interfaces involving solution-processed conjugated polymer (and soluble small molecules) films is described. Some general material properties of conjugated polymers and their solution-processed films are introduced, and the basic physics involved in energy level alignment at their interfaces is then discussed. An overview of energy level bending in (ultra)thin conjugated polymer films (often referred to as "band bending") is given and the effects of ion-containing interlayers typically used in organic electronic devices such as polymer light emitting diodes and organic bulk heterojunction solar cells are explored. The review finishes by describing a few of the available computational models useful for predicting and/or modeling energy level alignment at interfaces of solution-processed polymer films and discusses their respective strengths and weaknesses.

The semiconductor-electrode interface impacts the function and the performance of (opto) electronic devices. For printed organic electronics the electrode surface is not atomically clean leading to weakly interacting interfaces. As a result, solution-processed organic ultrathin films on electrodes typically form islands due to dewetting. It has therefore been utterly difficult to achieve homogenous ultrathin conjugated polymer films. This has made the investigation of the correct energetics of the conjugated polymer-electrode interface impossible. Also, this has hampered the development of devices including ultrathin conjugated polymer layers. Here, LangmuirShafer-manufactured homogenous mono-and multilayers of semiconducting polymers on metal electrodes are reported and the energy level bending using photoelectron spectroscopy is tracked. The amorphous films display an abrupt energy level bending that does not extend beyond the first monolayer. These findings provide new insights of the energetics of the polymer-electrode interface and opens up for new high-performing devices based on ultrathin semiconducting polymers.

The semiconductor-electrode interface impacts the function and the performance of (opto-)electronic devices. For printed organic electronics the electrode surface is not atomically clean leading to weakly interacting interfaces. As a result, solution-processed organic ultra-thin films on electrodes typically form islands due to de-wetting. It has therefore been utterly difficult to achieve homogenous ultrathin conjugated polymer films. This has made the investigation of the correct energetics of the conjugated polymer-electrode interface impossible. Also, this has hampered the development of devices including ultra-thin conjugated polymer layers. Here, we report Langmuir-Shäfer-manufactured homogenous mono- and multilayers of semiconducting polymers on metal electrodes and track the energy level bending using photoelectron spectroscopy. The amorphous films display an abrupt energy level bending that does not extend beyond the first monolayer. Our findings provide new insights of the energetics of the polymer-electrode interface and opens up for new high-performing devices based on ultra-thin semiconducting polymers.

Effects of in situ oxygen/water exposure on the energetics of [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) films are presented. For oxygen exposure, the work function is downshifted by ≈0.15 eV compared to the ideal integer charge transfer (ICT) curve for pristine PCBM, which is incompatible with significant introduction of electron trap states or p-doping. Water induces the highest occupied molecular orbital (HOMO) structure to undergo strong, irreversible modifications accompanied by a chemical interaction with PCBM.

n/a

Doping has been proved to be one of the powerful technologies to achieve significant improvement in the performance of organic electronic devices. Herein, we systematically map out the interface properties of solution-processed air-stable n-type (4(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl) doping fullerenes and fullerene derivatives and establish a universal energy level alignment scheme for this class of n-doped system. At low doping levels at which the charge-transfer doping induces mainly bound charges, the energy level alignment of the n-doping organic semiconductor can be described by combining integer charger transfer-induced shifts with a so-called double-dipole step. At high doping levels, significant densities of free charges are generated and the charge flows between the organic film and the conducting electrodes equilibrating the Fermi level in a classic "depletion layer" scheme. Moreover, we demonstrate that the model holds for both n- and p-doping of pi-backbone molecules and polymers. With the results, we provide wide guidance for identifying the application of the current organic n-type doping technology in organic electronics.

The energy offset between the electrode Fermi level and organic semiconductor transport levels is a key parameter controlling the charge injection barrier and hence efficiency of organic electronic devices. Here, we systematically explore the effect of in situ oxygen exposure on energetics in n-type conjugated polymer P(NDI2OD-T2) films. The analysis reveals that an interfacial potential step is introduced for a series of P(NDI2OD-T2) electrode contacts, causing a nearly constant downshift of the vacuum level, while the ionization energies versus vacuum level remain constant. These findings are attributed to the establishment of a so-called double-dipole step via motion of charged molecules and will modify the charge injection barriers at electrode contact. We further demonstrate that the same behavior occurs when oxygen interacts with p-type polymer TQ1 films, indicating it is possible to be a universal effect for organic semiconductOrs.

The interface energetics and intermixing effects of the donor/acceptor bulk heterojunction (BHJ) blends of poly[2,3‐bis‐(3‐octyloxyphenyl) quinoxaline‐5, 8‐dilyl‐alt‐thiophene‐2, 5‐diyl]: [6,6]‐phenyl C71butyric acid methyl ester (TQ1:PC71BM) have been investigated using ultraviolet photoemission spectroscopy (UPS) in combination with the integer charge transfer model. The TQ1:PC71BM represents the useful model system for BHJ organic photovoltaics featuring effective charge generation and transport. It finds out that the positive integer charge state of TQ1 are equal in energy to the negative integer charge state of PC71BM, leading to a negligible potential step at TQ1:PC71BM interface and thus the vacuum level alignment. It is observed that the TQ1 accumulates on the top of TQ1:PC71BM BHJ and UPS spectra as function of various blend ratios suggest that the TQ1 mixes finely with PC71BM with the little work function modification in a wide range. In addition, no significant influence of the long‐range Coulomb interactions or the intermolecular hybridization on the occupied electronic structures is present for the well‐intermixed TQ1:PC71BM BHJs. These findings provide deep insights into the properties of BHJ blends and are beneficial for the performance optimization in organic photovoltaics.

Regular energetics at a conjugated electrolyte/electrode modifier are found and controlled by equilibration of the Fermi level and an additional interface double dipole step induced by ionic functionality. Based on the results, design rules for conjugated electrolyte/electrode modifiers to achieve the smallest charge injection/exaction barrier and break through the current thickness limitation are proposed.

We systematically show the effect of UV-light soaking on surface electronic structures and chemical states of solution processed ZnO nanoparticle (ZnONP) films in UHV, dry air and UV-ozone. UV exposure in UHV induces a slight decrease in work function and surface-desorption of chemisorbed oxygen, whereas UV exposure in the presence of oxygen causes an increase in work function due to oxygen atom vacancy filling in the ZnO matrix. We demonstrate that UV-light soaking in combination with vacuum or oxygen can tune the work function of the ZnONP films over a range exceeding 1 eV. Based on photovoltaic performance and diode measurements, we conclude that the oxygen atom vacancy filling occurs mainly at the surface of the ZnONP films and that the films consequently retain their n-type behavior despite a significant increase in the measured work function.

Organic photovoltaics are under intense development and significant focus has been placed on tuning the donor ionization potential and acceptor electron affinity to optimize open circuit voltage. Here, it is shown that for a series of regioregular-poly(3-hexylthiophene): fullerene bulk heterojunction (BHJ) organic photovoltaic devices with pinned electrodes, integer charge transfer states present in the dark and created as a consequence of Fermi level equilibrium at BHJ have a profound effect on open circuit voltage. The integer charge transfer state formation causes vacuum level misalignment that yields a roughly constant effective donor ionization potential to acceptor electron affinity energy difference at the donor-acceptor interface, even though there is a large variation in electron affinity for the fullerene series. The large variation in open circuit voltage for the corresponding device series instead is found to be a consequence of trap-assisted recombination via integer charge transfer states. Based on the results, novel design rules for optimizing open circuit voltage and performance of organic bulk heterojunction solar cells are proposed.

In this work, a new covalent grafting of chitosan on titanium and Ti6Al4V alloy surfaces is reported using chloroacetic acid as linker agent. Good results were obtained both on titanium and on Ti6Al4V alloy. The effect of the surface acid pretreatments on the subsequent functionalization with chitosan is evaluated. The morphological aspect of acid etched metal surfaces before chitosan grafting has been characterized by scanning electron microscopy (SEM). The presence of carboxylic groups on metal surfaces and then the efficiency of chitosan covalent immobilization were detected by Fourier transformed infrared-Attenuated Total Reflectance (FTIR-ATR) and X-Ray photoelectron spectroscopy (XPS). Cyclic voltammetry tests, using the functionalized titanium and Ti6Al4V samples as electrodes, were conducted in different aqueous solutions, to detect the presence of the homogeneous overlayer of chitosan on the surface, and to evaluate the importance of the carboxyl groups as linker agent.

Optical spacers based on metal oxide layers have been intensively studied in poly(3-hexylthiophene) (P3HT) based polymer solar cells for optimizing light distribution inside the device, but to date, the potential of such a metal oxide spacer to improve the electronic performance of the polymer solar cells simultaneously has not yet be investigated. Here, a detailed study of performance improvement in high efficient polymer solar cells by insertion of solution-processed ZnO optical spacer using ethanolamine surface modification is reported. Insertion of the modified ZnO optical spacer strongly improves the performance of polymer solar cells even in the absence of an increase in light absorption. The electric improvements of the device are related to improved electron extraction, reduced contact barrier, and reduced recombination at the cathode. Importantly, it is shown for the first time that the morphology of optical spacer layer is a crucial parameter to obtain highly efficient solar cells in normal device structures. By optimizing optical spacer effects, contact resistance, and morphology of ZnO optical spacers, poly[[4,8-bis[(2-ethylhexyl) oxy] benzo[1,2-b: 4,5-b] dithiophene-2,6diyl] [3-fluoro-2-[(2-ethylhexyl) carbonyl] thieno[3,4-b] thiophenediyl]] (PTB7):[6,6]-phenyl-C71-butyric acid (PC70 BM) bulk heterojunction solar cells with conversion efficiency of 7.6% are obtained in normal device structures with all-solution-processed interlayers.

Cathode interfacial layers, also called electron extraction layers (EELs), based on zinc oxide (ZnO) have been studied in polymer-blend solar cells toward optimization of the opto-electric properties. Bulk heterojunction solar cells based on poly( {4, 8-bis [(2- ethylhexyl) oxy]b enzo [1,2- b :4,5-b dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]- thieno[3,4-b]thiophenediy1}) (PTB7) and [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) were realized in regular structure with all-solution-processed interlayers. A pair of commercially available surfactants, ethanolamine (EA) and ethylene glycol (EG), were used to modify the surface of ZnO nanoparticles (NPs) in alcohol-based dispersion. The influence of ZnO particle size was also studied by preparing dispersions of two NP diameters (6 versus 11 nm). Here, we show that performance improvement can be obtained in polymer solar cells via the use of solution-processed ZnO EELs based on surface-modified nanoparticles. By the optimizing of the ZnO dispersion, surfactant ratio, and the resulting morphology of EELs, PTB7/PC70BM solar cells with a power-conversion efficiency of 8.2% could be obtained using small sized EG-modified ZnO NPs that allow the clear enhancement of the performance of solution processed photovoltaic devices compared to state-of-the-art ZnO-based cathode layers.

Fabrication of miniaturized devices is usually time-consuming, costly, and the materials commonly used limit the structures that are possible to create. The techniques most often used to make microsystems involve multiple steps, where each step takes considerable time, and if only a few systems are to be made, the price per device becomes excessive. This thesis describes how a simple syringebased 3D-printer, in combination with an appropriate choice of materials, can reduce the delay between design and prototype and simplify fabrication of microsystems. This thesis suggest two types of materials that we propose be used in combination with 3D-printing to further develop microsystems for biology and biochemistry.

Analytical applications in biology and biochemistry often contain electrodes, such as in gel electrophoresis. Faradaic (electrochemical) reactions have to occur at the metal electrodes to allow electron-to-ion transduction through an electrolyte-based system to drive a current when a potential is applied to the electrodes in an electrolyte-based system. These electrochemical reactions at the electrodes, such as water electrolysis, are usually problematic when miniaturizing devices and analytical systems. An alternative to metal electrodes can be electrochemicallyactive conducting polymers, e.g. poly(3,4-ethylenedioxythiophene) (PEDOT), which can be used to reduce electrolysis when driving a current through water-based systems. Paper 1 describes gel electrophoresis where the platinum electrodes were replaced with the conductive polymer PEDOT, without affecting the separation.

Manufacturing and prototyping of microsystems can be simplified by using 3Dprinting in combination with a sacrificial material. A sacrificial template material can further simplify bottom-up manufacturing of more complicated forms such as protruding and overhanging structures. We showed in paper 2 that polyethylene glycol (PEG), in combination with a carbonate-based plasticizer, functions well as a 3D-printable sacrificial template material. PEG2000 with between 20 wt% and 30 wt% ethylene carbonate or propylene carbonate has properties advantageous for 3D-printing, such as shear-thinning rheology, mechanical and chemical stability, and easy dissolution in water.

There are multiple applications for polymers: our bodies are built of them, plastic bags and boxes used for storage are composed of them, as are the shells for electronics, TVs, computers, clothes etc. Many polymers are cheap, and easy to manufacture and process which make them suitable for disposable systems. The choice of polymer to construct an object will therefore highly influence the properties of the object itself. The focus of this thesis is the application of commonly used polymers to solve some challenges regarding integration of electrodes in electrokinetic devices and 3D printing.

The first part of this thesis regards electrokinetic systems and the electrodes’ impact on the system. Electrokinetic systems require Faradaic (electrochemical) reactions at the electrodes to maintain an electric field in an electrolyte. The electrochemical reactions at the electrodes allow electron-to-ion transduction at the electrode-electrolyte interface, necessary to drive a current at the applied potential through the system, which thereby either cause flow (electroosmosis) or separation (electrophoresis). These electrochemical reactions at the electrodes, such as water electrolysis, are usually problematic in analytical systems and systems applied in biology. One solution to reduce the impact of water electrolysis is by replacing metal electrodes with electrochemically active polymers, e.g. poly(3,4-ethylenedioxythiophene) (PEDOT). Paper 1 demonstrates that PEDOT electrodes can replace platinum electrodes in a gel electrophoretic setup. Paper 2 reports an all-plastic, planar, flexible electroosmotic pump which continuously transports water from one side to the other using potentials as low as 0.3 V. This electroosmotic pump was further developed in paper 3, where it was made into a compact and modular setup, compatible with commercial microfluidic devices. We demonstrated that the pump could maintain an alternating flow for at least 96 h, with a sufficient flow of cell medium to keep cells alive for the same period of time.

The second part of the thesis describes the use of 3D printers for manufacturing prototypes and the material requirements for 3D printing. Protruding and over-hanging structures are more challenging to print using a 3D printer and usually require supporting material during the printing process. In paper 4, we showed that polyethylene glycol (PEG), in combination with a carbonate-based plasticizer, functions well as a 3D printable sacrificial template material. PEG2000 with between 20 and 30 wt% dimethyl carbonate or propylene carbonate have good shear-thinning rheology, mechanical and chemical stability, and water solubility, which are advantageous for a supporting material used in 3D printing.

The advances presented in this thesis have solved some of the challenges regarding electrokinetic systems and prototype manufacturing. Hopefully this will contribute to the development of robust, disposable, low-cost, and autonomous electrokinetic devices.

Recent advances in microfluidic devices put a high demand on small, robust and reliable pumps suitable for high-throughput applications. Here we demonstrate a compact, low-cost, directly attachable (clip-on) electroosmotic pump that couples with standard Luer connectors on a microfluidic device. The pump is easy to make and consists of a porous polycarbonate membrane and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) electrodes. The soft electrode and membrane materials make it possible to incorporate the pump into a standard syringe filter holder, which in turn can be attached to commercial chips. The pump is less than half the size of the microscope slide used for many commercial lab-on-a-chip devices, meaning that these pumps can be used to control fluid flow in individual reactors in highly parallelized chemistry and biology experiments. Flow rates at various electric current and device dimensions are reported. We demonstrate the feasibility and safety of the pump for biological experiments by exposing endothelial cells to oscillating shear stress (up to 5 dyn/cm2) and by controlling the movement of both micro- and macroparticles, generating steady or oscillatory flow rates up to ± 400 μL/min.

Syringe-based 3D-printing is a powerful additive manufacturing method for fabricating short runs (small volumes) of components from multiple materials with a wide range of viscosities. However, objects that are hollow or not in complete contact with the printer’s stage are difficult to fabricate. Using a sacrificial template as a supporting layer enables bottom-up construction of complex structures. Template materials based on polyethylene glycol (PEG) plasticized with organic carbonates to tune their rheological (shear-thinning) and thermal (crystallization) properties have been evaluated, including results from rheometry, differential scanning calorimetry, dissolution rate, chemical compatibility with  polydimethylsiloxane (PDMS), and general functionality in a syringe-based 3D-printer. A family of such blends yields material that is easily printed, is stable over time, is soluble in water, and can support other materials and larger structures without collapsing. These mixtures are proposed for use with other extrudable or mouldable materials to enable 3D-printed devices with complex unsupported geometries.

In nearly all cases, electrophoresis in gels is driven via the electrolysis of water at the electrodes, where the process consumes water and produces electrochemical by-products. We have previously demonstrated that p-conjugated polymers such as poly(3,4-ethylenedioxythiophene) (PEDOT) can be placed between traditional metal electrodes and an electrolyte to mitigate electrolysis in liquid (capillary electroosmosis/electrophoresis) systems. In this report, we extend our previous result to gel electrophoresis, and show that electrodes containing PEDOT can be used with a commercial polyacrylamide gel electrophoresis system with minimal impact to the resulting gel image or the ionic transport measured during a separation.

A large-area, fabric-like pump would potentially have applications, for example, in controlling water transport through a garment, such as a rain jacket, regardless of the external temperature and humidity. This paper presents an all-plastic, flexible electroosmotic pump, constructed from commercially available materials: A polycarbonate membrane combined with the electrochemically active polymer poly(3,4-ethylenedioxythiophene) polystyrene sulfonate that actively transports water using an electric potential that can be supplied by a small battery. By using electrochemically active polymer electrodes instead of metal electrodes, the electrochemical reaction that drives flow avoids the oxygen and hydrogen gas production or pH changes associated with water electrolysis. We observe a water mass flux up to 23 mg min(-1) per cm(2) polycarbonate membrane (porosity 10-15%), at an applied potential of 5 V, and a limiting operating pressure of 0.3 kPa V-1, similar to previously reported membrane-based electroosmotic pumps.

La0.7Sr0.3MnO3 polycrystalline manganite thin films were grown on silicon (Si) substrates covered by SiOx amorphous native oxide. Curie temperatures of about 325 K were achieved for 70-nm-thick films. Strong room temperature XMCD signal was detected indicating high spin polarization at the surface. Cross-sectional TEM images show sharp interface between SiOx and manganite without signature of chemical reaction at the interface. Unusual sharp splitting of the manganite film was observed: on the top of a transition layer characterized by low crystalline order, a magnetically robust layer is formed. © 2007 Elsevier B.V. All rights reserved.

Polycrystalline La0.7Sr0.3MnO3 manganite thin films were grown on silicon substrates covered by SiOx amorphous native oxide. Unusual splitting of the manganite layer was found: on the top of an intermediate layer characterised by lower crystalline order, a magnetic robust layer is formed. Curie temperatures of about 325 K were achieved for 70 nm thick films. A strong room temperature XMCD signal was detected indicating high spin polarisation near the surface. © 2005 Elsevier Ltd. All rights reserved.

Here, we report on devices based on patterned thin films of the conducting polymer system poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulphonic acid) (PEDOT:PSS) combined with patterns of solid electrolyte. The key device functionalities base on the updating of the RedOx state of PEDOT. This results in control of the electronic properties of this conjugated polymer, i.e. the conductivity and optical properties are updated. Based on this we have achieved electric current rectifiers, transistors and display cells. Also, matrix addressed displays will be presented. Electrochemical switching is taking place when the oxidation and reduction potentials are overcome respectively. Therefore, these devices operate at voltage levels less then 2 Volts. Low voltage operation is achieved in devices not requiring any extremely narrow dimensions, as is the case for field effect driven devices. All devices reported can or has been made using standard printing techniques on flexible carriers.

Macroporous gelatin beads (CultiSpher™ microcarriers) provide a convenient method for rapidly and reliably introducing cells cultured ex situ into a microfluidic device, where the spheres create a 3D environment for continued cell proliferation. We demonstrate the usefulness of this technique with a proof-of-concept viability analysis of cardiac cells after treatment with doxorubicin. © 2014 the Partner Organisations.

Thin film iron-tetracyanoethylene Fe(TCNE) _x , x∼2, as determined by photoelectron spectroscopy, was grown in situ under ultra-high vacuum conditions using a recently developed physical vapor deposition-based technique for fabrication of oxygen- and precursor-free organic-based molecular magnets. Photoelectron spectroscopy results show no spurious trace elements in the films, and the iron is of Fe²⁺ valency. The highest occupied molecular orbital of Fe(TCNE) _x is located at ∼1.7 eV vs. Fermi level and is derived mainly from the TCNE⁻ singly occupied molecular orbital according to photoelectron spectroscopy and resonant photoelectron spectroscopy results. The Fe(3d)-derived states appear at higher binding energy, ∼4.5 eV, which is in contrast to V(TCNE)₂ where the highest occupied molecular orbital is mainly derived from V(3d) states. Fitting ligand field multiplet and charge transfer multiplet calculations to the Fe L-edge near edge X-ray absorption fine structure spectrum yields a high-spin Fe²⁺ (3d⁶) configuration with a crystal field parameter 10Dq∼0.6 eV for the Fe(TCNE) _x system. We propose that the significantly weaker Fe-TCNE ligand interaction as compared to the room temperature magnet V(TCNE)₂ (10Dq∼2.3 eV) is a strongly contributing factor to the substantially lower magnetic ordering temperature (T _C ) seen for Fe(TCNE) _x -type magnets.

Room temperature ferromagnetic ordering is reported in Ni–tetracyanoethylene (TCNE) thin films fabricated on Au substrates using physical vapor deposition (PVD) under ultra high vacuum conditions. This technique enables the preparation of very clean films without having any kind of contamination from oxygen-containing species, solvents or precursor molecules. Film stoichiometry was obtained from X-ray photoelectron spectroscopy (XPS) measurements. XPS derived stoichiometry points to a 1 : 2 ratio between Ni and TCNE resulting in Ni(TCNE)_x, x ≈ 2. No evidence of pure Ni metal in the in situ grown films was present in the XPS or the ultraviolet photoelectron spectroscopy (UPS) measurements within the detection limits of the techniques.

The effect of Li-doping in poly(para-phenylenevinylene) (PPV) based light emitting devices has been studied. In a standard structure with an indium tin oxide (ITO) anode, poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT-PSS)-layer and an active PPV-layer, the effects of a thin (around 1 Å) Li-layer and a thin layer, (50 Å), of a large bandgap polymer, poly(2,5-diheptyl-1,4-phenylene-alt-1,4-naphthylene) (P14NHP) between the PPV and the aluminum cathode have been studied in terms of IV-characteristics and efficiency. The Li-atoms dope the interfacial layer of the PPV as seen by photoelectron spectroscopy. A thin layer of Li improves the charge balance by decreasing the energy barrier for injection of electrons for the Al/Li/PPV/PEDOT-PSS/ITO device. The efficient electron injection originates from a Fermi level alignment between the doped polymer and the aluminum cathode, which reduces the energy barrier. A thin layer of the large bandgap polymer P14NHP, between the PPV and Al contact, increases the light output and efficiency by blocking the holes. In addition, it may also reduce the light quenching by moving the region of recombination away from the Al-contact. The addition of a Li-layer on top of P14NHP leads to an increase of the quantum efficiency, because of better electron injection. © 2002 Elsevier Science B.V. All rights reserved.

The chemical and electronic structure of three different, strictly alternating copolymers, poly(2,5-diheptyl-1,4-phenylene-alt-1,4-naphthylene) (P14NHP), poly(2,5-diheptyl-1,4-phenylene-alt-2,6-naphthylene) (P26NHP) and poly(2,5-diheptyl-1,4-phenylene-alt-9,10-anthrylene) (P910AHP), have been studied by photoelectron spectroscopy and optical absorption spectroscopy. The experimental results have been analyzed using the results of quantum chemical calculations. In the geometrical structure of all three of the polymers there are large torsion angles between the phenylene unit and the naphthylene or anthrylene units. These large torsion angles lead to localization of the p-electron wave functions, and minimal conjugation along the polymer backbone. For all three polymers, the highest occupied molecular orbital is completely localized to the naphthylene or anthrylene unit. The frontier molecular orbital wave functions are very reminiscent of the highest occupied orbitals of the isolated naphthalene or anthracene molecules. The optical absorption spectra of all three polymers verify the existence of large optical band gaps, consistent with the large torsion angels. The first several optical transitions in the polymers are also very reminiscent of the transitions in single naphthalene and anthracene molecules.

Near-edge X-ray absorption fine structure spectroscopy spectra of ethylenedioxythiophene has been recorded in gas phase at the carbon K-edge, sulphur L-edge and oxygen, K-edge. The experimental data has been interpreted with the help of a modified density functional code deMon. The good agreement between the calculated spectra and the measured one allows us to assign all observed resonances. The existence of p* resonances in both the measured and the calculated OK-edge adsorption spectrum demonstrate that the delocalized p-system of the thiophene part of the molecule is extended up to the oxygen atoms of the molecule. © 2004 Elsevier B.V. All rights reserved.

Stable junctions between a single carbon chain and two single-wall carbon nanotubes were produced via coalescence of functionalized fullerenes filled into a single-wall carbon nanotube and directly imaged by in situ transmission electron microscopy. First principles quantum chemical calculations support the observed stability of such molecular junctions. They also show that short carbon chains bound to other carbon structures are cumulenes and stable semiconductors due to Peierls-like distortion. Junctions like this can be regarded as archetypical building blocks for all-carbon molecular electronics.

Organic electronics is a rapidly evolving field with vast number of applications having high potential for commercial success. Although a great progress has been made, many organic electronic applications: organic light-emitting diodes (OLEDs), organic fieldeffect transistors (OFETs), organic solar cells, etc; still require further optimization to fulfill the requirements for successful commercialization. For many applications, available at this time organic materials do not provide satisfactory performance and stability, which hinders the possibility of a large-scale production. Therefore, the key ingredient needed for a successful improvement in performance and stability of organic electronic devices is in-depth knowledge of physical and chemical properties of molecular and polymeric materials. Since many applications encompass several thin film layers made of organics, and often also inorganic materials, the understanding of both organic-organic and hybrid interfaces is yet another important issue necessary for the successful development of organic electronics.

The research presented in this thesis is based mainly on photoelectron spectroscopy, which is an experimental technique especially suited to study both surfaces and interfaces of materials. In the thesis, the properties of one of the most successful polymeric materials, poly(3,4-ethylenedioxythiophene), often abbreviated as PEDOT, have been extensively studied. The research was done in close cooperation with an industrial partner – AGFA Gevaert, Belgium. The study was focused on the exploration of the intrinsic properties of the material, such as stability, morphology and conductivity. In addition, however, a possibility of alternation of these properties was also explored. This thesis reports also about investigations of the properties of various organic-organic and hybrid interfaces. The energy level alignment at such interfaces plays important role in charge injection and performance of the thin film organic-based devices. The conditions for different energy level alignment regimes at the various interfaces have been studied. The studies on interfaces were performed in close collaboration with the R&D division of DuPont Corporation, USA. This work led to the significant advances in understanding of the interface energetics and properties of industryrelevant organic materials, as represented not only by published scientific papers, but also patent applications.