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Production of hydrogen through water splitting is one of the green and the most practical solutions to cope with the energy crisis and greenhouse effect. However, oxygen evolution reaction (OER) being a sluggish step, the use of precious metal-based catalysts is the main impediment toward the viability of water splitting. In this work, amorphous copper oxide and doped binary- and ternary-metal oxides (containing CoII, NiII, and CuII) have been prepared on the surface of fluorine-doped tin oxide by a facile electrodeposition route followed by thermal treatment. The fabricated electrodes have been employed as efficient binder-free OER electrocatalysts possessing a high electrochemical surface area due to their amorphous nature. The cobalt-nickel-doped copper oxide (ternary-metal oxide)-based electrode showed promising OER activity with a high current density of 100 mA cm-2 at 1.65 V versus RHE that escalates to 313 mA cm-2 at 1.76 V in alkaline media at pH 14. The high activity of the ternary-metal oxide-based electrode was further supported by a smaller semicircle in the Nyquist plot. Furthermore, all metal-oxide-based electrodes offered high stability when tested for continuous production of oxygen for 50 h. This work highlights the synthesis of efficient and cost-effective amorphous metal-based oxide catalysts to execute electrocatalytic OER employing an electrodeposition approach.

Anodes derived from SnO2 offer a greater specific capacity comparative to graphitic carbon in lithium-ion batteries (LIBs); hence, it is imperative to find a simple but effective approach for the fabrication of SnO2 . The intelligent surfacing of transition metal oxides is one of the favorite strategies to dramatically boost cycling efficiency, and currently most work is primarily aimed at coating and/or compositing with carbon-based materials. Such coating materials, however, face major challenges, including tedious processing and low capacity. This study successfully reports a new and simple WO3 coating to produce a core-shell structure on the surface of SnO2 . The empty space permitted natural expansion for the SnO2 nanostructures, retaining a higher specific capacity for over 100 cycles that did not appear in the pristine SnO2 without WO3 shell. Using WO3 -protected SnO2 nanoparticles as anode, a coin half-cell battery was designed with Li-foil as counter-electrode. Furthermore, the anode was paired with commercial LiFePO4 as cathode for a coin-type full cell and tested for lithium storage performance. The WO3 shell proved to be an effective and strong enhancer for both current rate and specific capacity of SnO2 nanoarchitectures; additionally, an enhancement of cyclic stability was achieved. The findings demonstrate that the WO3 can be used for the improvement of cyclic characteristics of other metal oxide materials as a new coating material.

The short-circuit current density and energy conversion efficiency of single-junction perovskite and perovskite/perovskite tandem solar cells can be increased by photon management. In this study, optical metasurfaces were investigated as potential light trapping structures oppose to commonly used pyramidal surface textures. Herein, metal oxide-based non-resonant metasurfaces were investigated as potential light-trapping structures in perovskite solar cells. The zinc oxide nanowire-based building blocks of the metasurface can be prepared by a templated electrodeposition through a mask of resist. The phase of the incident light can be controlled by the edge length of the subwavelength large zinc oxide nanowires. An array of zinc oxide nanowires was prepared and characterized in the current study. Three-dimensional (3D) finite-difference time-domain (FDTD) optical simulations were used to compare solar cells covered with non-resonant metasurfaces with commonly used light trapping structures. As compared to the solar cells covered with zinc oxide pyramid surface texture, solar cells with the integrated non-resonant metasurfaces exhibit almost identical quantum efficiencies and short-circuit current densities. Investigations of such metasurfaces will not only improve the photon absorption in perovskite solar cells but also reveal a pathway to make high-efficiency next-generation solar cells. Detailed guidelines for the realization of non-resonant metal oxide metasurfaces will be provided.

Polymer-fullerene blends based on poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C61-butyric-acid methyl ester (PCBM) have been extensively studied as promising bulk heterojunction materials for organic semiconductor devices with improved performance. In these donor–acceptor systems where the bulk morphology plays a crucial role, the generation and subsequent decay mechanisms of photoexcitation species are still not completely understood. In this work, we use femtosecond transient absorption spectroscopy to investigate P3HT:PCBM diodes under the influence of applied static electric fields in comparison to P3HT:PCBM thin films. At the same time, we try to present a detailed overview about work already done on these donor–acceptor systems. The excited state dynamics obtained at 638 nm from P3HT:PCBM thin films are found to be similar to those observed earlier in neat P3HT films, while those obtained in the P3HT:PCBM devices are affected by field-induced exciton dissociation, resulting not only in comparatively slower decay dynamics, but also in bimolecular deactivation processes. External electric fields are expected to enhance charge generation in the investigated P3HT:PCBM devices by dissociating excitons and loosely bound intermediate species like polaron pairs (PPs) and charge transfer (CT) excitons, which can already dissociate only due to the intrinsic fields at the donor–acceptor interfaces. Our results clearly establish the formation of PP-like transient species different from CT excitons in the P3HT:PCBM devices as a result of a field-induced diffusion-controlled exciton dissociation process. We find that the loosely bound transient species formed in this way also are reduced in part via a bimolecular annihilation process resulting in charge loss in typical donor–acceptor P3HT:PCBM bulk heterojunction semiconductor devices, which is a rather interesting finding important for a better understanding of the performance of these devices.

The patterning of gold and silver micro and nanostructures on rigid and flexible substrates is investigated by microcontact printed thiol based self-assembled monolayers. The aspect ratio of the noble metal micro and nanostructures is determined by interaction of the -SH head group of the CH3(CH2)19SH molecules and the surface of the noble metal. Silver micro and nanostructures with >10 times higher aspect ratios can be realized in comparison to commonly realized gold micro and nanostructures. The printing process is described, and the etching process is characterized in terms of etching window and etching selectivity. Potential electronic and photonic applications of the micro and nanostructures are discussed taking the boundary conditions of the printing process and the selected material system into consideration.

A simple wet-chemical synthesis of layered MoS2 thin films on sapphire is reported. The gap in understanding solution processed MoS2 deposition needs to be closed to exploit all its excellent properties for low-cost applications. In this work, as deposited Mo-precursor thin films were prepared based on the solubility and coating properties of Molybdenum(V) chloride in 1-Methoxy-2-propanol. Subsequent annealing of the deposited amorphous Mo-precursor films in the presence of sulfur resulted in the formation of highly crystalline layered MoS2 films on sapphire. Improved crystallinity of the deposited films was achieved by increasing the process temperature and performing the post-annealing treatment. Post-annealing at temperatures above 900 °C increased the uniformity of multilayer films, together with the increase of MoS2 grain size. For charge transport analysis, top-gate top-contact thin film transistors (TFTs) based on these solution processed MoS2 films were fabricated. Ionic liquid gating of the TFT devices exhibited n-type semiconducting behaviour with field-effect mobility as high as 12.07 cm2/Vs and Ion/off ratio ∼ 106. X-ray photoelectron spectroscopy measurements revealed that the films annealed between 900 °C and 980 °C had an average chemical composition of S/Mo ∼ 1.84. This facile liquid phase synthesis method with centimeter-scale uniformity and controllable film thickness up to 1.2 ± 0.65 nm is suitable for low-cost preparation of other transition metal dichalcogenides thin films in next-generation electronics.

The generation and decay mechanisms of polaron pairs in organic semiconductor-based optoelectronic devices under operational conditions are relevant for a better understanding of photophysical processes affecting the device performance, since the possible occurrence of a polaron pair introduces an intermediate step in exciton dissociation into fully separated charge carriers. The role played by static electric fields in polaron-pair dynamics is important but poorly understood or not investigated in detail. In this work, insights into the polaron-pair dynamics in neat poly(3-hexylthiophene-2,5-diyl) (P3HT) thin films and P3HT films sandwiched between electrical contacts with an applied external static electric field are probed using femtosecond pump-probe transient absorption spectroscopy. Asymmetric contacts result in P3HT devices with application-related diode characteristics. Consistent with the electric field-induced dissociation of oppositely charged species, we show that polaron-pair dissociation into charge carriers occurs in the P3HT device more significantly with increasing reverse bias, and that this process follows an initial instantaneous polaron-pair photoabsorption quenching due to a pronounced immediate loss of primary photoexcitation species (hot excitons). Furthermore, we show that the net-electric field present in the P3HT diode (including built-in-potential at 0 V bias) results in a more complex dynamics with new findings as compared to the neat-P3HT thin film case. Indeed, besides polaron pairs directly originating from hot excitons, we experimentally observe polaron-pair formation during exciton dissociation via a field-mediated generation process, resulting in a slower contribution to the overall decay dynamics. Moreover, unlike in the external electric field-free P3HT film, bimolecular annihilation processes clearly appear as an additional loss channel when a field is applied and hence have an impact on carrier generation performance in a working device.

Surface energies described by Hansen solubility parameters (HSP) of a synthetic organophilic fluoromica (MAE), molybdenum disulfide (MoS 2 ), and graphene are used to predict their delamination properties in a cyclic olefin copolymer (COC). This prediction suggests MAE as the most appropriate filler for COC, followed by MoS 2 and graphene. Nanocomposites are prepared via solution process and have been investigated on their delamination state using X-ray-diffraction (XRD), Raman spectroscopy , transmission electron microscopy (TEM), and angle-resolved light scattering (ARS). Nanocomposites based on MAE shown morphologies close to complete delamination. Agglomerations are detected in the case of MoS 2 and graphene. For a potential barrier application, water vapor transmission rates (WVTR) are experimentally determined. In such nanocomposites, inorganic fillers increase the diffusion length of water, known as tortuous path model, which enhances the barrier properties. Highest barrier enhancements are obtained using MAE based nanocomposites reducing the WVTR down to 40% of the initial value. The prediction of delamination based on Hansen solubility parameters is found to be a very helpful tool for selecting fillers in composites.

Two dimensional layers of dichalcogenide materials have attracted a lot of interests due to their potential applications in optoelectronics and energy storage. Hence, there is a large interest in establishing cheap, scalable processes for the production of low dimensional semiconducting dichalcogenide based films. In this work, well exfoliated MoS2 dispersions were prepared through a two-step liquid phase exfoliation process with N-methyl-pyrrolidone (NMP) and Isopropanol (IPA). The quality of the obtained MoS2 flakes was characterized by transmission electron microscopy, scanning electron microscopy, UV–Vis spectroscopy and Raman spectroscopy. For charge transport analysis, bottom-gate thin film transistors (TFTs) based on exfoliated MoS2 films were fabricated via spray coating technique. Electrical characterization of the obtained TFTs showed that adding a PMMA layer on top of the semiconductor lead to considerable improvements in the electrical performance. The analysis of the electrical characteristics suggests that the additional PMMA layer improves the charge transfer between adjacent flakes. Electrical measurements on TFTs with different channel length were used to separate the impact of the contact resistance and the channel resistance on the charge transport. The TFTs output curves showed non-linear current–voltage (I-V) characteristic. The non-linear behavior was attributed to the formation of Schottky barriers at the inter-flakes connection. In this work, we show a low-cost and scalable solution-based fabrication process that could boost the application of dichalcogenides in modern nanoelectronic devices.

Molecules are known to change properties when in contact with metal surfaces. Therefore, dynamics of photoinduced molecular excitons in a semiconductor also are expected to be influenced by a metal...

A novel liquid phase synthesis of MoS 2 thin films has been achieved. Solubility of Molybdenum(V) chloride in appropriate solvent was optimized while having good coating properties essential for thin film formation. Chemical conversion of the deposited Mo-precursor films on silicon/silicon dioxide substrate to MoS 2 were obtained by annealing in presence of sulfur. This novel method allows facile upscaling process to large substrates with uniform film thickness down to 4 nm, which are of high interest for future low-cost fabrication of electronic devices. UV–Vis, Raman spectroscopy, and X-ray photoelectron spectroscopy measurements confirm the formation of MoS 2 films with a stoichiometric chemical composition of Mo/S ~ 0.47. Furthermore, X-ray diffraction and Transmission electron microscopy measurements revealed formation of polycrystalline films with random grain orientation attributed to the amorphous SiO 2 surface of the substrate. Improved crystallinity of deposited films was achieved by increasing the process temperature. Annealing at temperatures above 750 °C increased the uniformity of multilayer films, together with the increase of MoS 2 grain size to 100 nm. This simple wet-chemical synthesis approach allows upscaling, controllable film thickness and is suitable for preparation of other transition metal dichalcogenides thin films for applications in the future electronics.

A fast and compact standing wave spectrometer based on semi-transparent organic photodetectors is developed. The semi-transparent organic detectors are realized using solution-based fabrication and exhibit an overall transmittance of about 20% in the visible range of the electromagnetic spectrum. The realization of a standing wave spectrometer is achieved by combining a semi-transparent organic detector with a movable mirror controlled by a piezo micro-actuator. The operating principles of the standing wave spectrometer are demonstrated using a combined illumination from two laser sources. The advantages of standing wave sensing devices based on organic materials as well as the required material properties are presented and discussed.

A simple method to achieve tailored growth of ZnO nanorods by employing self-assembled monolayer of alkanethiol molecules on a conductive substrate is introduced. Defects or pinholes in self-assembled monolayer are used to define the nucleation sites of ZnO during electrochemical deposition. The density of ZnO nanorods is tuned by the quality of self-assembled monolayer. A corresponding growth model for the growth of ZnO on self-assembled monolayer-modified substrate is proposed. The dimensions and the nucleation density of ZnO nanorods are tailored by systematically varying the quality of self-assembled monolayer and the parameters of electrochemical deposition. Furthermore, it is shown that this method also allows for laterally patterned growth of ZnO nanorods via microcontact printing of self-assembled monolayer. Electrochemical deposition of ZnO on Au surface results in dense coverage of ZnO nanorods. Self-assembled monolayers are applied on Au to tune the density of ZnO nanorods. The pinholes in self-assembled monolayer are used to define the nucleation site of ZnO.

Metal-mesh based electrode systems are a highly conductive, versatile, and inexpensive alternative to ITO-electrodes for organic photovoltaic (OPV) cells. However, they do not offer full surface conductivity and are, therefore, usually combined with an additional conductive layer (e.g. PEDOT:PSS), which enables lateral charge-transport in the area in between the metal tracks. The sheet conductivity of this additional lateral conductive layer (LCL) needs to be carefully tuned with respect to the distance of the metal tracks to reduce short circuit current (Jsc) losses and additional series resistance due to resistive losses in the LCL material. Usually, this is done via electrical simulation or via analysis of IV-measurements of a large number of devices with different LCL sheet conductivities. Here, we present the light-biased light-beam-induced-current (LB-LBIC) measurement technique as a direct way to visualize current collection losses due to PEDOT:PSS sheet resistance in metal mesh/bus bar based OPV cells. We demonstrate that with LB-LBIC we are not only able to visualize and optimize lateral charge extraction under real-device operation conditions in OPV cells based on a high-resolution metal mesh but can measure the PEDOT:PSS sheet resistance in operating OPV devices as well.

A technique for determining the material-specific morphology of a polymer–fullerene blend is presented. This technique is applied to solution processed bulk-heterojunction organic solar cells with different weight ratios of polymer–fullerene blend using the PTB7:PCBM material system. Optical and electrical characterizations show that the light absorption increases for larger polymer (PTB7) content, while the fill factor of the fabricated solar cells is improved for larger fullerene (PCBM) content. The material-specific morphologies of polymer–fullerene bulk-heterojunctions are measured by employing AFM phase imaging. The measured AFM phase images reveal that fullerene material forms flake-like clusters which are embedded in the polymer network. The size of the flakes is increasing with larger content of fullerene material. By correlating the optical and electrical properties with the measured bulk-heterojunction morphologies, the relation between bulk-heterojunction structure and solar cell performances i...

Free falling picoliter and sub-picoliter droplets generated by an ultrasonic atomizer are studied from a top-view perspective with a high-speed camera while falling on a heated surface. Due to the small droplet size processes are dominated by surface tension (Re = 0.5, We = 10−4, Oh = 2 × 10−2) resulting in almost perfect spherical shapes. Our presented model analyzes the droplets top-view appearance and allows the extraction of static and dynamic advancing and receding contact angles. In addition, the extracted evaporation rate allows to determine the critical heat flux (CHF) and Leidenfrost point (LFP). The evaporation regimes: nucleate boiling, transition boiling and film-boiling can be clearly defined by CHF and LFP. The presented model assumes the droplet evaporation starts with a shape corresponding to the static advancing contact angle Θa and continues to the static receding contact angle Θr with pinned contact line, i.e. with a constant visible droplet area. Afterward, it evaporates with a dynamic receding contact angle Θdr until the end of its lifetime. The initial advancing contact angle is calculated from the volume of the arriving droplet in air and the measured contact area directly after landing on the surface. The analysis yields Θa = 37.2 ± 3.3° and Θdr = 9.1 ± 1.6° for DI-water on silicon with natural oxide layer at T = 110°C. Furthermore, different complex scenarios of interactions between droplets are observed for sessile droplets formation and during the evaporation process.

A large lateral size and low dimensions are prerequisites for next generation electronics. Since the first single layer MoS2 transistor reported by Kis's group in 2011, layered transition metal dichalcogenides (TMDs) have been demonstrated to be the ideal candidate for next generation electronics. However, the development of large scale and low cost growth techniques is a crucial step towards TMDs’ inclusion in modern electronics and photoelectronics. In this work we develop a cheap, wet chemical, and environment friendly deposition process for two dimensional MoS2 flakes with extended size. For our deposition process, ammonium tetrathiomolybdate (ATTM) dissolved in deionized water was used as precursor solution and was deposited on a SiO2/Si substrate through a Langmuir–Blodgett like deposition process. To our knowledge, this is the first time MoS2 flakes have been grown in an aqueous solution. Large-sized MoS2 flakes exceeding 150 μm in lateral size were obtained after thermal decomposition. Thicknesses ranging from a monolayer to 5 monolayers were confirmed by AFM and Raman spectroscopy. Further investigations revealed that the quality of the produced flakes strongly depends on the post growth thermal treatment and its atmosphere. This simple and nontoxic deposition method is suitable for the preparation of large (hybrid) transition metal dichalcogenide nanostructures for applications in next generation electronics.

Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) are promising candidates for future electronics. Currently, the growth of TMD large area thin films/flakes is one of the biggest challenges. A novel method for the growth of ultra-thin and large area WS2 monolayer flakes has been developed by introducing a solution-based temperature-dependent process. This two-dimensional WS2 growth process is low cost and environmentally friendly. WO3 flakes are grown at the air–liquid interface using ammonium tetrathiotungstate ((NH4)2WS4, ATTW) as WS2 precursor. The process requires a moderate activation temperature as no flakes are formed at room temperature. Successful growth of flakes was observed in an aqueous solution of the precursor at a temperature between 70 °C and 90 °C. These flakes could be transferred to any substrate by a controlled dip-coating process. Large 2D WS2 flakes with a lateral size of up to 100 μm were obtained after sulfurization. The thickness ranged from a WS2 monolayer to five monolayers, as verified by atomic force microscope. The chemical reaction mechanism behind the formation of the flakes was investigated by FTIR, Raman, UV–Vis and x-ray photoelectron spectroscopy. The initial flakes were found to be made of WO3, which were successfully converted to WS2 by a post annealing step at 500 °C–900 °C. This simple and environmentally friendly growth technique can be used to produce large WS2 flakes for next generation electronics.

Diketone passivation of defect states at the surface of solution-processed zinc oxide thin film transistors is investigated. Trifluoroacetylacetophenone is used as the basis for passivating molecules due to its easy chemical variability on the phenyl ring. Electron withdrawing or donating substituents can be introduced at the phenyl ring to vary the electron density at the ketone anchoring positions of the passivating molecule. The relative change of electron density is quantified by DFT calculations for different substituents. Experimentally thin zinc oxide films are prepared by aqueous spray pyrolysis, while passivating diketones are deposited in vacuum on the ZnO surface. XPS and AFM measurements confirm the successful deposition of passivating molecules at the zinc oxide surface. Electrical characterization of passivated zinc oxide transistors is carried out for different exposure times. All diketones improve transistor stability for positive and negative bias stress. The passivated transistors show improved electrical properties with increased electron mobility and decreased disorder parameter. Finally, threshold voltages shift systematically to negative values and hysteresis clearly reduces. The experimentally determined electrical properties of passivated zinc oxide transistors show a systematic correlation to the effective electron density at the oxygen anchor atoms of diketones and higher charge results in a better performance.

Impedance and transient current measurements on metal–insulator–semiconductor (MIS) capacitors are used as tools to thoroughly investigate the bulk and interface electronic transport properties of semiconducting polymers, i.e. poly(3-hexylthiophene) (P3HT). Distinct features were observed at both interfaces, i.e. metal–semiconductor and semiconductor–insulator. The results revealed a dispersive transport in the bulk due to the band tail of the localized states, presence of interface states at the interface between the insulator and the semiconductor and formation of a less conductive small layer at the interface semiconductor–metal contact due to intrusions of sputtered Au particles. Effects of self-assembled monolayers (SAMs) treatments of the gate insulating dielectric were investigated showing that treating the gate dielectric with either ozone or hexamethyldisilazane (HMDS) or octyltrichlorosilane (OTS) alter not only the interface semiconductor–insulator but the bulk properties as well. An exponential density of states with a width parameter of 38–58 meV depending on the surface treatment was found to be representative of the band tail of P3HT. Though both OTS and HMDS treatments slightly increase the density of interface states, only OTS treated samples showed a decrease in disorder parameter of the bulk. The latter fact can be attributed to an increase of the grain size due to a favored π-π stacking film growth. An outcome explaining the already reported increase of the lateral mobility and decrease of the vertical mobility observed upon OTS treatment of the gate insulating dielectric in poly(3-hexylthiophene) based devices.

Thin film transistors (TFT) with an indium based mixed oxide semiconductor are investigated for titanium–gold top-contacts. It is noticed that upon post annealing, in order to remove chemical residuals from top-contact lift-off steps, oxidation of titanium occurs depending on the annealing conditions. Mobility of the TFT is strongly affected by contact oxidation arising from this post lift-off annealing process. Oxidation of the top-contact is facilitated by adsorbed surface oxygen or out-diffusing oxygen from the semiconductor depending on the post lift-off annealing conditions. A passivation layer that binds effectively to surface vacancies and removes adsorbed oxygen species from the semiconductor surface is demonstrated. The combinations of this passivation layer with relatively low temperature and short post lift-off annealing in an oxygen deficient environment result in significantly reduced contact oxidation and subsequently better transistor performance. Contact resistance as low as 90 Ω cm and mobility as high as 5.3 cm 2 /V s are obtained for solution processed mixed metal oxide semiconductor in top-contact geometry.

The optics of axial silicon nanowire solar cells is investigated and compared to silicon thin-film solar cells with textured contact layers. The quantum efficiency and short circuit current density are calculated taking a device geometry into account, which can be fabricated by using standard semiconductor processing. The solar cells with textured absorber and textured contact layers provide a gain of short circuit current density of 4.4 mA/cm2 and 6.1 mA/cm2 compared to a solar cell on a flat substrate, respectively. The influence of the device dimensions on the quantum efficiency and short circuit current density will be discussed.

A new method for electropolymerization of polythiophenes as anode buffer layer in combination with a PEDOT:PSS buffer in P3HT–PCBM solar cells is introduced. This approach uses a stack of PEDOT:PSS and electropolymerized polythiophene (ePT) as an anode buffer layer. A significant improvement in the short circuit current is seen in solar cells with additional thin buffer layers of ePT grown on top of PEDOT:PSS. The cells with ePT buffer layer grown with constant potential mode compared to solar cells without additional ePT buffer layer have an 18.7% increase in power conversion efficiency. However, the cells with ePT grown with cyclic potential mode result in an additional enhancement in short circuit current with a further increase of 11.7% in power conversion efficiency. This improvement is attributed to better quality of the buffer layer obtained by cyclic potential electropolymerization technique with respect to film quality and area coverage.

In this work the influence of unstructured charge-extraction/transport layers on solar cell photocurrent measurements is investigated. Photovoltaic cells incorporating unstructured conductive layers exhibit a significant amount of photoactive area spreading. This can lead to a large error in determining solar cell short circuit current density and fill factor from I - V measurements. To counteract this effect an analytical model is developed to describe the photoactive area spreading. The developed model requires only solar cell contact geometry and transport layer sheet resistance in order to calculate the spreading current from measured I - V data. Thus total removal of unwanted spreading current from experimental data and calculation of realistic solar cell efficiency is achieved. As an example this model is applied to unstructured metal mesh based organic solar cells, and presented data show that it yields more accurate results compared to the conventional shadow masking approach.

This paper outlined how to control density and shape of electrodeposited ZnO nanorods to achieve high scattering properties. Light scattering at nanostructured metal–semiconductor interfaces is a proven method to improve absorption in photovoltaic devices. Adjustment of nanostructure shape and mean distance is critical to achieve efficient light scattering. A simple model is introduced that predicts maximal suppression of the specular transmitted light, resulting in maximal light scattering. This model predicts in an ideal case, 50 % nanostructure coverage of the electrode. Furthermore, an optimal nanostructure height has been determined depending on the incident wavelength and the refractive indices. Experimentally, the crystal density on ITO substrates was adjusted by pulsing the deposition potential, thus, removing the requirement for an additional seeding layer. The solution of the diffusion equation indicated a break-to-pulse ratio of at least 2.4 for an efficient control of the crystal density during pulsed electrodeposition. In addition, the structure height was set by varying the number of pulse cycles. Such tailored ZnO nanostructures showed a suppression of the specular transmitted light beam of 83.5 % and a diffusive forward scattering efficiency of 39 % at a wavelength of 406 nm. Thus, the optical absorption of e.g. an 80-nm thick polymeric active layer of P3HT could be increased by 47 % by applying such tailored ZnO nanostructures.

Successful deposition of ZnO layers from non-toxic solvent by utilizing a perfume atomizer is demonstrated. The adsorption edge of the zinc oxide films was found to be 3.22 eV which is in good agreement with literature. In addition it is found that the homogeneity of the films increases in side geometry with increasing distance to the perfume atomizer due to the droplet size distribution along the x-axis of the aerosol. The films were used to fabricate ZnO–TFTs. A dominating influence of the grain sizes can be excluded by correlating atomic force microscopy (AFM) images to the electrical properties of the transistors deposited in different geometries but a strong influence of the transistor performance on the growth rate was found. The increase in performance with decreasing growth rate was attributed to a longer reaction time decreasing the impurity level in the films. The linear mobility, the on-set voltage and the on–off current ratio are found to be 5 cm2 V−1 s−1, 0 V and 106 for small growth rates, respectively. Hence the transistors show high mobility and an excellent switching behavior.

Zinc oxide thin films were prepared by pulsed spray pyrolysis (SP) from aqueous zinc acetate precursor solution. The deposition behavior of the solution with regard to the pool boiling curve of the solvent was investigated to gain insights of the deposition mechanism which leads to homogeneous and reproducible ZnO layers. The Leidenfrost effect was found to play an important role since the Leidenfrost temperature has to be exceeded to form high quality layers. It is concluded that 3D nucleation of ZnO nano-crystals takes place out of the gaseous phase by a CVD-like process on ITO as well as on SiO2 substrates. Crystal orientation and surface roughness of the zinc oxide layer are found to depend strongly on the substrate. An increasing grain size with film thickness is observed. These findings were utilized to fabricate and investigate the semiconducting properties of the films in ZnO thin film transistors (TFTs). The mobility exceeded 12 cm2 V−1 s−1, the on-set was at 1 V and the on–off current ratio was found to be higher than 108. Hence the morphology and the electrical parameters of the ZnO films deposited by pulsed spray pyrolysis from non-toxic aqueous zinc acetate solution above the Leidenfrost point show excellent properties for electronic applications.

A model is presented that describes the gate-voltage-dependent contact resistance and channel-length-dependent charge carrier mobility of small-molecule-based organic thin-film transistors in top and bottom drain/source contact configuration.

An atomistic model which provides a consistent picture of the formation of gap states in the prototype material pentacene is presented and the influence of these states on the operation of thin-film transistors. Gap states are formed by an electrochemical process. The required formation energy depends on the electrochemical potential which can be controlled by the applied bias voltage.

In this study, we report on the dependence of the charge carrier mobility μ on the organic semiconductor thickness in organic field-effect transistors (OFETs) fabricated in top-gate architecture on flexible and transparent plastic foils as substrates with regio-regular poly-(3-hexylthiophene) (rr-P3HT) as organic semiconductor and poly(methyl methacrylate) (PMMA) as gate insulator. The gate-voltage dependence of the charge carrier mobility is taken into account by analyzing the mobility according to the Vissenberg and Matters model for charge transport in disordered systems. The transistors exhibit high mobility of almost 0.01 cm 2 /(Vs) and long-term stability. After a strong mobility increase by two orders of magnitude with P3HT thickness for very thin films of several nanometer, saturation was observed around 50 nm thickness. It was observed that the exponent of the mobility’s gate-voltage dependence increases from 1.2 to 3.2 when decreasing the semiconductor thickness. A larger exponent is related to a wider distribution of localized states in the tail of an exponential density of states and thus to higher disorder of the system. This finding explains the low mobility in these very thin films and is confirmed by phase contrast AFM measurements, which demonstrate the low degree of order in thin P3HT films.

A patterning method was developed that allows for combining the advantages of a low cost microcontact printing process on rigid and flexible substrates with the advantages of conventional semiconductor processing. The patterning approach combines printing of self-assembled monolayers with selective dewetting while being compatible with conventional semiconductor processes. Alkyl thiol self-assembled monolayers were printed on gold and silver films on rigid and flexible substrates. The printed regions turn hydrophobic while the bare regions remain hydrophilic. The hydrophilic regions of the surface were selectively wetted by a polymer like poly-methyl methacrylate. The selectively patterned poly-methyl methacrylate is used as etch resist while the bare regions of the gold film were patterned by a potassium iodide/iodine solution. The potassium iodide/iodine etchant is compatible with conventional semiconductor processing in contrast to the commonly used ferri/ferrocyanide etching solutions. The method allows for patterning of gold and silver films with submicron dimension on flexible and rigid substrates. Organic thin-film transistors were realized with drain and source fabricated by the proposed approach. The performance of the organic transistor is comparable to devices fabricated by photolithography.

The scaling behavior and the influence of the device geometry on the switching speed of organic thin film transistors were investigated. Thiophene transistors were characterized and the influence of contact effects and overlap capacitance was studied. Fundamental scaling limits of the switching frequencies were derived. The maximum transfer frequency is limited by the device design and the specific contact resistance between the drain/source electrode and the organic semiconductor. Finally, strategies will be presented how to realize organic transistors operating at frequencies of up to 100 MHz.

A universal microstructuring approach was developed, which facilitates the patterning of surfaces by a combination of microcontact printing (mCP) and selective surface dewetting/wetting. Self-assembled monolayers (SAMs) were patterned on glass or silicon substrates by μCP. The regions coated by the SAMs turn hydrophobic, whereas the uncoated regions stay hydrophilic. Such functionalized surfaces facilitate selective deposition of polymers or resists. Polymethyl methacrylate and prepolymer polyurethane were selectively deposited on the hydrophilic regions of the substrate. The hydrophobic regions of the substrate stay uncoated. Subsequently, the resist was used to lift-off metallic microstructures in order to realize micro coils and electrodes for radio frequency information tags. The printed electrodes were used to define drain and source contacts of organic thin film transistors. The device characteristic of the organic transistors will be presented.

The influence of environmental conditions on the electronic structure and the device stability of polycrystalline pentacene transistors were investigated. Electrical in situ measurements of pentacene TFTs were carried out to study the influence of dry oxygen on the device operation. The polycrystalline pentacene thin film transistors were fabricated by organic molecular beam deposition (OMBD) on thermal oxide dielectrics. Exposing the pentacene film to oxygen leads to the creation of acceptor-like states deep in the bandgap. The acceptor-like states cause a reduction of the subthreshold slope and a shift of the onset of the drain current to positive gate voltages. The threshold voltage and the mobility of the transistors are not affected by the dry oxygen. A simple model will be described which directly correlates the onset voltage of the transistors with the acceptor concentration in the pentacene film.

For future low cost electronics, high-performance organic thin-film transistors (OTFT) are highly desirable. One possible route towards device optimization is the downscaling of the channel length L into the sub-micrometer regime which allows higher operation frequency, better integration and enhanced currents. Reducing the channel length decreases the effect of grain boundaries on transport in polycrystalline organic films, but in return the metal–organic contact resistance becomes more important. Furthermore, sub-micrometer devices require a thinner insulator layer to maintain a proper scaling of the ratio between the longitudinal and transversal electrical field in the channel. We present a systematic study of transistors based on vacuum-deposited dihexylquaterthiophene (DH4T), an n-type Si gate, a SiO 2 insulator layer and metal drain/source contacts with channel lengths L from 50 μm down to 50 nm. When varying the contact metal (Ti/Au, Ti/Pt and Pd), we observe a disadvantageous influence of the Ti adhesion layer, which can be partially eliminated by reducing the Ti layer thickness. For channel lengths in the sub-micrometer range a strong beneficial effect occurs when reducing the SiO 2 thickness from 100 to 30 nm. Our devices with 30 nm oxide layers show standard FET characteristics (i.e., no short-channel effects), a high carrier mobility of about 4 × 10 −2 cm 2 /V s and an on/off current ratio of 10 5 and beyond for channel lengths down to 180 nm. The field-effect is still well observable even for channel lengths down to 50 nm.

We use the interference of deformation-potential- and Frohlich-induced Raman scattering from LO phonons in zincblende materials to identify the in-plane axes of zincblende (100) surfaces and interfaces. This procedure, which allows e.g. to distinguish between the [011] and the [0–11] direction, is demonstrated experimentally for n- and p-type GaAs(100). The main potential of the method is its application to heterostructures. Here it allows the unambiguous assignment of polarized Raman scattering features from surfaces and interfaces to their characteristic symmetry directions. Two examples are presented: (i) localized vibrations of CdSe monolayers, embedded in BeTe stacks, and (ii) vibration modes of Te dimers on clean (2×1) reconstructed BeTe(100) surfaces.

We report on the application of protective capping layers for nonultrahigh vacuum (UHV) transport of BeTe layers. It allows the subsequent recovery of the clean BeTe(100) surface by thermal decapping in UHV. For capping tellurium/selenium double layers with a thickness of about 0.2 μm are employed. After decapping by annealing which results in the desorption of Se at 150 °C and of Te at 260 °C, the low-energy electron diffraction pattern confirms the reestablishment of (2×1) surface reconstruction of BeTe(100). Subsequent annealing to 520 °C results in (3×1) reconstruction. In situ monitoring by Raman spectroscopy and reflection anisotropy spectroscopy is used to control the decapping process and the subsequent annealing-induced change in surface reconstruction.

The optics of core / shell nanowire solar cells was investigated. The optical wave propagation was studied by finite difference time domain simulations using realistic interface morphologies. The interface morphologies were determined by a 3D surface coverage algorithm, which provides a realistic film formation of amorphous silicon films on zinc oxide nanowire arrays. The influence of the nanowire dimensions on the interface morphology and light trapping was investigated and optimal dimensions of the zinc oxide nanowire were derived.

Submonolayer cyclic epitaxy was used for the first time to obtain a bulk layer of BeCdSe solid solution 100 nm thick with a Be content close to 46%, which corresponded to the composition lattice-matched to the GaAs substrate. In addition, low-temperature lasing at a wavelength of 460 nm with a threshold power density of about 40 kW/cm2 in a structure with multiple ZnSe/BeCdSe quantum wells was also attained for the first time. The band-bending parameter in the BeCdSe solid solutions was estimated at 4.5 eV.

The structural and electronic properties of semiconductor interfaces crucially influence the behaviour of modern nanoscale heterostructures. We apply optical spectroscopy (Raman spectroscopy and electroreflectance (ER)) with special reference to the interface-induced vibronic signature and electronic interband transitions in wide-band gap II–VI materials. This material system allows the combination of materials with extremely different vibronic and electronic properties, which have to be matched at the interfaces. As examples we present the analysis of ZnSe/BeTe superlattices (SL) and CdSe monolayers, embedded between BeTe barriers, grown on GaAs(100) substrates. Both systems exhibit type II electronic band alignment with a rather high valence band offset of about 1 eV and an electron confinement potential in the range of 2 eV. Since these II–VI compounds contain no common elements, two types of interface bond configurations are possible: either Be–Se or Zn–Te (Cd–Te). We show that the properties of the type-II spatially indirect transition are completely governed by this interface layer. The normal and inverted electronic interface signature can be addressed individually in ER by applying an external dc electric field or by using polarised light with polarisation either along the [110] or [10] direction. In addition the chemical nature of the interface is analysed, using its phonon properties in Raman spectroscopy: They are completely altered for different interface bonding conditions in the case of embedded monolayers. Moreover, the ER results from the epilayers are also used for the deduction of the band offset at the interface substrate–epilayer and Schottky barrier of the outer interfaces (epilayer–metal).

The lattice dynamical properties of cubic CdS/ZnS(e) and hexagonal CdSe/CdS superlattices are investigated by means of Raman and FTIR spectroscopy. Due to the good lateral and in-depth homogeneity of the layer thicknesses folded longitudinal acoustic phonons are observed for all three superlattice systems. The determined frequencies agree quite well with calculations based on the Rytov model. For the optical phonons strain-induced energy shifts become important. Besides, we observe higher order confined longitudinal optical phonons in CdS/ZnSe superlattices. Our experimental results are compared with calculations based on a linear-chain model showing that in this AB/CD type superlattice real interface modes can be observed.

We have determined the dielectric function (DF) of beryllium chalcogenides between 2.5 and 25 eV using an ellipsometer operating with synchrotron radiation in the VUV range. The VUV ellipsometer is, in particular, useful for the characterization of wide band gap semiconductors since the spectral range of spectrometers equipped with conventional light sources is limited to below 6 eV. Beryllium chalcogenides are a relatively new class of material with still unknown optical and electronic properties. Here we determine the dielectric function of BeTe and BexZn1—xSe for the full composition range (0 ≤ x ≤ 1). We verify that they are wide band gap semiconductors with their direct band gaps at 4.2 eV (BeTe) and 5.55 eV (BeSe).

We investigated the growth of CdS/ZnSe type II quantum-well structures, produced by MBE for the first time. Epitaxy was carried out with compound sources on GaAs(0 0 1) substrates. The best results were achieved on ZnSe buffer layers and at rather low substrate temperatures ( T =170°C). Photoluminescence measurements showed intense luminescence bands in the energy range from 2.0 to 2.65 eV depending on the quantum-well thickness. The PL bands are quite narrow (16–43 meV). X-ray diffraction profiles of superlattices revealed satellite peaks up to the fourth order. The critical thickness for the coherent growth of CdS on ZnSe was determined by detecting the quenching of the PL intensity as a function of the quantum-well thickness; it is about 4.5 nm.

In this paper, the fabrication and modeling of thin-film transistors (TFTs) based on random network of single-walled carbon nanotubes (SWCNTs) are presented. The thin film is obtained by dispersing 99% semiconductive SWCNTs with an effective deposition technique at room temperature that combines vacuum filtration and silanization of substrate. The case of TFT structures with channel length varying from 2 to 50 μm is experimentally studied. In the device with channel length equal to 8 μm, an apparent mobility of 40.75 cm2 $/$V·s, a current density of 0.06 μm, and $I_{{\rm ON}}/$$I_{{\rm OFF}}$ ratio of $1.8 \times 10^4 $ have been measured. In order to obtain a numerical model as close as possible to the real structure, a 3-D model for the thin-film layer is developed, rather than the 2-D type commonly used in the literature, to reproduce the electric transport properties of the CNTs network in the channel of TFTs devices. The spatial arrangement of the random network CNTs in the channel of the thin-film structure is also analyzed by considering the percolation theory. According to this theory, an exponent of the power law equal to $\alpha = 1.7$ is experimentally detected, indicating that the devices operate close to the percolation region. A good agreement is found between the transport characteristics of the simulated and fabricated devices. The adopted model opens new routes to understand the transport properties of the film. The proposed fabrication approach can be easily transformed to large areas leading to a suitable use in industrial application.

Room-temperature (RT) operation of optically pumped GaN-GaInN distributed-feedback (DFB) lasers is demonstrated in the spectral range from 400 to 415 nm. The ridge-waveguide distributed feedback lasers were realized by dry etching on a GaN-GaInN double heterostructure. Laser emission was observed in the entire spectral range of optical gain, which was determined by amplified spontaneous emission (ASE) measurements. The wavelength shift of the distributed feedback laser emission with temperature was investigated. An array of DFB lasers with different grating periods allowed to obtain the spectral dependence of the laser threshold and the effective refractive index of the GaN-GaInN lasers. The lowest laser threshold we achieved was 0.95 MW/cm/sup 2/.

In this paper, the fabrication of thin film transistor based on randomized network of single walled carbon nano tubes (SWCNTs-TFT) is presented. The randomized network is obtained by deposition of dispersed SWCNTs on the substrate with a novel technique combining vacuum filtration and silanization of substrate. This approach, which is compatible with all kind of substrates, allows a fabrication process at room temperature that is capable to overcome the high temperature procedure for CNTs deposition. The drain and source electrodes of the TFT are based on an interdigitated electrode (IDE) with 8 μm channel length and 3mm channel width. The obtained device shows output performance with an apparent mobility of 40.7 cm2/Vs, current density 0.05 μA/μm and ION /IOFF ratio 2×103. A comparison of the model describing the SWCNTs-TFT with that of (metal-oxide-semiconductor) MOS-like device confirms a p-type behavior. The proposed approach can be easily transformed to large areas leading to a suitable use in low cost industrial application.

The charge transport in organic semiconductors is still not completely understood. We use electroreflectance spectroscopy to investigate charge transport in organic field effect devices in order to obtain optical information on the charges within the charge accumulation layer at the organic semiconductor-insulator interface. Here, the reflectance geometry allows an analysis of devices prepared on opaque substrates. The recorded spectra were analyzed based on optical layer stack simulations to extract quantitatively the change of the dielectric function due to the charge injection of the accumulation layer in poly(3-hexylthiophene) (P3HT). For the simulation the anisotropic optical response of spin-coated P3HT layers is experimentally determined by spectroscopic ellipsometry. Using the developed theoretical approach the characteristic change of three different preparations of the organic semiconductor-insulator interface is analyzed. Laterally resolved measurements close to an injecting contact have revealed characteristic spectral changes in dependence of the preparation conditions, and significant spectral changes within the first $90\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$ from the contact were found, which were attributed to an energetic relaxation of charges during the charge transport process.