Work Function Of Films Research Articles

Polar Polymorphism: A New Intermediate Structure toward the Thin-Film Phase in Asymmetric Benzothieno[3,2-b][1]-benzothiophene Derivatives.

Understanding structure and polymorphism is relevant for any organic device optimization, and it is of particular relevance in 7-decyl-2-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10) since high carrier mobility in Ph-BTBT-10 thin films has been linked to the structural transformation from the metastable thin-film phase to the thermodynamically stable bilayer structure via thermal annealing. We combine here a systematic nanoscale morphological analysis with local Kelvin probe force microcopy (KPFM) that demonstrates the formation of a polar polymorph in thin films as an intermediate structure for thicknesses lower than 20 nm. The polar structure develops with thickness a variable amount of structural defects in the form of individual flipped molecules (point defects) or sizable polar domains, and evolves toward the reported nonpolar thin-film phase. The direct experimental evidence is supported by electronic structure density functional theory calculations. The structure of the film has dramatic effects on the electronic properties, leading to a decrease in the film work function (by up to 1 eV) and a considerable broadening of the occupied molecular orbitals, attributed to electrostatic disorder. From an advanced characterization point of view, KPFM stands out as a valuable tool for evaluating electrostatic disorder and the conceivable emergence of polar polymorphs in organic thin films. The emergence of polar assemblies introduces a critical consideration for other asymmetric BTBT derivatives, which may be pivotal to understanding the structure-property relationships in organic field-effect transistors (OFETs). A precise determination of any polar assemblies close to the dielectric interface is critical for the judicious design and upgrading of high-performance OFETs.

Work function and negative electron affinity of ultrathin barium fluoride films

Thin films of barium fluorides with different thicknesses were deposited on GaAs substrate by electron beam evaporation. The aim of the work was to identify the best growth conditions for the production of coatings with a low work function suitable for the anode of hybrid thermionic‐photovoltaic (TIPV) devices. The chemical composition and work function φ of the films with different thicknesses were investigated by X‐ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The lowest value of φ = 2.1 eV was obtained for the film with a thickness of ~2 nm. In the valence band spectra of the films at low kinetic energy, near the cutoff, a characteristic peak of negative electron affinity was present. This effect contributed to a further reduction of the film's work function.

Tunable optoelectronic properties of radio frequency sputter-deposited Sb2Se3 thin films: Role of growth angle and thickness

In thin film photovoltaics, antimony selenide (Sb2Se3) is making a mark as an important absorber layer for its attractive structural, optical, and electrical properties. Several deposition techniques are used to grow Sb2Se3 films, albeit growth angle-dependent physical properties of Sb2Se3 films are yet to be explored in a systematic manner. In this work, we investigate the roles of growth angle (in the range of 0°-87°) and thickness (250 and 1000 nm) in governing optoelectronic properties of Sb2Se3 films grown by radio frequency sputter deposition technique. It is interesting to note that upon increasing the growth angle, a systematic enhancement in the optical band-gap takes place. This is accompanied by a systematic reduction in the work function of these amorphous films with increasing growth angle. Subsequently, after postgrowth annealing at 573 K for 180 s (in vacuum) all Sb2Se3 films undergo a structural phase transition from amorphous to crystalline one. Following this, the band-gap and work function of these films also get modified. This study paves the way to fabricate Sb2Se3 films having tunable optoelectronic properties like absorption coefficient, band-gap, and work function. Thus, it should be extremely useful to select Sb2Se3 films having optimal thickness and optoelectronic properties to fabricate Sb2Se3 absorber layer-based multi-junction hole-blocking solar cells having improved cell efficiencies.

Work function of doped zinc oxide films deposited by ALD

Abstract

Degradation of Mg-doped zinc oxide buffer layers in thin film CdTe solar cells

Cadmium Sulphide is the conventional n-type buffer layer used in thin film Cadmium Telluride solar cells. It is well known that Cadmium Sulphide causes optical losses and sulphur diffuses into the absorber during high temperature activation. Sputter-deposited Mg-doped ZnO (MZO) has been shown to be an attractive buffer layer for Cadmium Telluride solar cells due to its transparency and tuneable band gap. It is also stable to high temperature processing and avoids diffusion of elements into the cadmium telluride absorber during the cadmium chloride activation treatment. However, degradation is observed in solar cells incorporating MZO buffer layers. Analysis of the MZO film surface potential has revealed significant fluctuations in the thin film work function once the layer is exposed to the atmosphere following deposition. These fluctuations are attributed to the high reactivity to water vapour of the MgO contained in the MZO films. This has been analysed using X-ray Photoelectron Spectroscopy to determine corresponding changes in the surface chemistry. The Zinc Oxide component is relatively stable, but the analysis shows that MgO forms a Mg(OH)2 layer on the MZO surface which forms a secondary barrier at the MZO/CdTe interface and/or at the interface between MZO and the Fluorine-doped SnO2. This affects the Fill Factor and as a consequence it degrades the conversion efficiency.

Efficient and stable inverted polymer solar cells prepared via air exposure

The performance of the inverted polymer solar cells (PSCs) were examined with solution-processed TiO2 nanoparticles (TiO2 NPs) as electron transport layer (ETL) and vapor deposited MoO3 as hole transport layer (HTL). The performance of the inverted PSCs based on P3HT:PCBM heterojunction was closely depend on the thickness of interface layer. The high power conversion efficiency (PCE) of 3.90% was achieved when the thickness of MoO3 is 5 nm. The interesting thing is that the PCE of these kinds of inverted PSCs were dramatically enhance after exposing them in air. The increase of the Ag work function, the reduction of MoO3 film work function, and the enlarged P3HT:PCBM grain size are deduced to the main reasons for the improvement of the devices’ performance. The very high PCE of 4.51% was achieved after the device was exposed in air for 144 h. In addition, we also measured the stability of this kind of devices in air for 1008 h. The devices show excellent stability and the PCE of the devices could maintain about 4.0% after 1008 h exposure. The results provide a new facile method for fabrication of highly efficient and stable inverted PSCs.

Investigations on RF-magnetron sputtered Co3O4 thin films regarding the solar energy conversion properties

Co3O4 samples have been deposited using RF-magnetron sputtering from an oxide target. In situ x-ray and ultraviolet photoelectron spectroscopy and x-ray diffraction have been carried out to identify the phase and composition of the films. The work function of films showed variation of about 1 eV depending on the oxygen content of the sputter gas while the Fermi level position in the band gap was invariant towards different deposition parameters. Optical transmission spectroscopy in the UV/VIS/NIR regime showed two optical transitions at 0.8 eV and 1.7 eV. However, the optical absorption from these transition does not seem to aid in the conduction of charge carriers as has been revealed by conductivity measurements in a linear 4-point-setup. Diodes were prepared in a glass FTO NiO Au-stacking geometry. They show poor photovoltaic behaviour with a short-circuit current of 0.33 mA cm−1 and an open-circuit voltage of 0.15 V, resulting in an overall efficiency of . The limitation of as an absorber in an All-Oxide solar cell can be related to poor transport properties combined with defect states in the band gap and Fermi level pinning at interfaces.

Stoichiometry engineering of ternary oxide ultrathin films:BaxTi2O3on Au(111)

Ternary oxide ${\mathrm{Ba}}_{x}{\mathrm{Ti}}_{2}{\mathrm{O}}_{3}$ ultrathin films on Au(111) substrates $(x\ensuremath{\le}2/3)$ have been studied using a joint experimental and theoretical approach, including the use of scanning tunneling microscopy (STM), first principles calculations, and Monte Carlo simulations. The films are created by first covering the Au(111) substrate with a ${\mathrm{Ti}}_{2}{\mathrm{O}}_{3}\phantom{\rule{0.28em}{0ex}}(2\ifmmode\times\else\texttimes\fi{}2)$ honeycomb (HC) network and then evaporating low concentrations of Ba atoms onto this film. STM imaging shows that the Ba atoms adsorb individually in the hollow sites of the HC network. Depending on the Ba coverage $x$, which ranges from 0 to 2/3, two ordered phases can be identified at $x=1/3$ and $x=2/3$. A disordered labyrinthlike phase is observed for values of $x$ between 1/3 and 2/3. Theoretical modeling shows that the structural character of these films is driven by the charge transfer that occurs between the electropositive Ba atoms and the electronegative ${\mathrm{Ti}}_{2}{\mathrm{O}}_{3}$/Au substrate. This results in a number of calculated effects including an increase in the film rumpling and a reduction of the film work function with increasing $x$. The evolution of the structure of the thin films as a function of Ba coverage can be described by a lattice gas model with first-, second-, and third-neighbor Ba-Ba repulsive interactions. The range of the dipolar interactions is a key factor in understanding the behavior of Ba ordering. The structural and electronic flexibility, which can be engineered through stoichiometry, temperature, or support control, makes these ultrathin films promising materials for applications related to adsorption or reactivity, or as template supports for the growth of size-selected clusters.

Electro-Optic Modulation Induced Enhancement in Photocatalytic Activity of N-Doped TiO2 Thin Films

In the presence of ultraviolet light, TiO2 photocatalyst finds a lot of applications in degradation of organic and inorganic particles present in environment as well as in water. TiO2 nanoparticles can not be utilized effectively in presence of solar light since solar radiation has very little contribution in UV region. The photoresponse of TiO2 can be shifted to the visible region by nitrogen doping. In the nitrogen-doped TiO2 lattice, the oxygen vacancies are formed below the conduction band in the form of donor energy level. These vacancies increase with increasing nitrogen concentration in TiO2, which results in an increase in the trapping rate. This leads to enhancement of photocatalytic activity of nitrogen-doped TiO2 film. Nitrogen doping results in the creation of defects in nitrogen-doped TiO2, which in other way leads to a modification of the work function of thin films. The observed changes in work function of these films, measured using scanning Kelvin probe microscopy, indicates the shifting of Fermi level with the introduction of defects. The study of work function as well as surface properties of thin films helps in better understanding of the photo activity of nitrogen-doped TiO2 film.

A Comparative Investigation of a Pentacene Layer on Gold and PMMA in Bottom-Contact Pentacene Thin Film Transistors

This study demonstrates an attempt to fabricate the 25 µm channel-length bottom-contact pentacene TFTs using thermal evaporation through shadow mask technique and to investigate crystal structure, morphology and electronic structure of the pentacene layer deposited at the same time on gold (Au) source-drain electrodes and Poly(methyl methacrylate) (PMMA) gate-dielectric of the TFTs. The pentacene layers with thicknesses of 50 nm were deposited at the evaporation rates of 0.1, 0.5, 1.0 and 1.5 nm/min at substrate temperatures of 60 °C. These conditions were employed with and without preheating at 80 °C before deposition. Preheating at 80 °C was found to improve quality of the pentacene film on the PMMA gate-dielectric. Using the deposition rate of 1 nm/min and the substrate temperature of 60 °C, best performance of TFTs were obtained. At this deposition condition, pentacene film work function was found to be higher on the Au source-drain electrodes than on the PMMA gate-dielectric.

Effect of deposition condition and UV-ozone post-treatment on work function of DC magnetron sputtered AZO thin films

Highly transparent and conductive aluminum-doped zinc oxide (AZO) thin films were successfully deposited on glass by DC magnetron sputtering. The effects of Al-doping content (1–4 at.%), substrate temperature (250–350 °C) and ultraviolet (UV)-ozone treatment time (0–20 min) on AZO film work function were systematically investigated. It was revealed that the AZO work function increased with the decrease of Al content, while it was almost insensitive to the substrate temperature. Moreover, the UV-ozone treatment significantly improved AZO work function, while the post-treatment time had no effect on AZO work function. Two factors, [OZnAl]/([Zn] + 1.5[Al]) ratio and carbon contamination on film surface, affected the AZO work function. It was suggested that AZO films with high [OZnAl]/([Zn] + 1.5[Al]) ratio and reduced carbon concentration, possessed high work function.

Polyaniline‐Based Conducting Polymer Compositions with a High Work Function for Hole‐Injection Layers in Organic Light‐Emitting Diodes: Formation of Ohmic Contacts

It is a great challenge to develop solution-processed, polymeric hole-injection layers (HILs) that perform better than small molecular layers for realizing high-performance small-molecule organic light-emitting diodes (SM-OLEDs). We have greatly improved the injection efficiency and the current efficiency of SM-OLEDs by introducing conducting polymer compositions composed of polyaniline doped with polystyrene sulfonate and perfluorinated ionomer (PFI) as the HIL. During single spin-coating of conducting polymer compositions, the PFI layer was self-organized at the surface and greatly increased the film work function. It enhanced hole-injection efficiency and current efficiency by introducing a nearly ohmic contact and improving electron blocking. Our results demonstrate that solution-processed polyaniline HILs with tunable work functions are good candidates for reducing process costs and improving OLED performance.

Low content indium-doped zinc oxide films with tunable work function fabricated through magnetron sputtering

Low content indium-doped zinc oxide (LC-IZO) films have been prepared through radio-frequency (RF) magnetron sputtering. The work functions of the LC-IZO films were successfully tuned up by modulating the deposited conditions, such as indium content, substrate temperature, bias voltage and film thickness. The work functions of the films may be modulated in a large range between 4.59 eV and 5.56 eV. The change of the work function may be the results induced by the change in the surface state. The results may be helpful for widening the application of LC-IZO films.

Work-function engineering of carbon nanotube transparent conductive films

We developed a method that allows control of the work function of carbon nanotube (CNT) transparent conductive films on a flexible sheet of plastic. The approach involves the deposition of small amount of metals, such as aluminum, on CNT transparent conductive films followed by a measure of the work function of the films using in situ ultraviolet photoelectron spectroscopy. Core-level spectra of the films were collected in order to investigate the chemical reaction when a small amount of aluminum was deposited on their surface in a stepwise manner. The measurements revealed that deposition of less than 0.5 nm of aluminum was enough to control the work function of CNT transparent conductive films.

Size dependences in the adsorptive properties of the surface of ytterbium nanofilms deposited on silicon: The CO-Yb-Si(111)7 × 7 system

This paper reports on a study of the adsorption of CO molecules on the surface of ytterbium nanofilms of different thicknesses, which were sublimed on Si(111)7 × 7 at room temperature. Dependences of two types were investigated: the surface concentration of adsorbed molecules vs. CO dose expressed in langmuirs and the work function of films vs. CO dose. It was shown that the behavior of these dependences is mediated by size effects and Friedel oscillations generated by the ytterbium-silicon interface. Both effects exert an influence on the binding of CO molecules to the surface. At low molecule concentrations, this binding is effected through lone electron pairs localized at the carbon ends of the molecules. These electrons form a donor-acceptor bond to the vacant 5d level of the metal, with the level dropping below the Fermi level. At high CO molecule concentrations, the pattern becomes more complex; indeed, the enhanced Coulomb interaction gives rise to a partial transfer of electrons from the 5d level to the vacant 2π* orbital of CO molecules.

MoO x modified ZnGaO based transparent conducting oxides

We report here the growth of high work function bilayered structures of thin MoOx (2.0<x<2.75) layer (few nanometers) on Zn0.95Ga0.05O films by pulsed laser deposition (PLD) on glass and sapphire substrates for transparent electrode applications. It was found that the films were highly textured along [0001] direction. The crystalline quality of the films deposited at different substrate temperatures was investigated by x-ray diffraction, transmission electron microscopy (TEM) imaging, and selected area diffraction pattern (SAED). In the MoOx layer, molybdenum exists in Mo4+, Mo5+, and Mo6+ oxidation states, and the ratio of (Mo4++Mo5+) to Mo6+ was determined to be ∼2:1. The bilayer films showed good optical transparency (≥80%) and low resistivity of ∼10−4 Ω cm. Different transport behavior of the MoOx/ZnGa0.05O films grown at different Ts (substrate temperature) was observed in temperature-dependent resistivity measurements. The bilayer film at higher Ts showed metallic conductivity behavior down to 113 K. Moreover, a blueshift of the absorption edge in the transmission spectrum was observed with the increase in Ts, indicating an increase in the carrier concentration. It was observed that the ZnGa0.05O films with ultrathin MoOx (∼1–2 nanometers) overlayer showed a higher work function (varying from 4.7 to 5.1 eV) as compared to the single layer ZnGa0.05O film work function (∼4.4 eV). A correlation between the surface work function and MoOx layer thickness is observed. The higher work function of the MoOx overlayer is envisaged to improve the transport of the carriers across the heterojunction in a solid state device, thus resulting an increase in device efficiency.

Characterization of resistivity and work function of sputtered-TaN film for gate electrode applications

Tantalum nitride (TaN) films were prepared by reactive sputtering in a gas Ar and N2 for gate electrode applications. Resistivity, crystallinity, and work function of the films were investigated as a function of nitrogen flow rate. As the nitrogen flow rate increased from 0 to 20 sccm, the resistivity of as-deposited TaN films increased from 132 to 1.4×105 μΩ cm. With a nitrogen flow rate of 8 and 10 sccm, the fcc TaN phase was obtained. The work function of the TaN films was investigated using TaN-gated nmetal–oxide–semiconductor capacitors with SiO2 gate dielectrics of various thicknesses. As the nitrogen flow rate increased from 4 to 12 sccm, the work function decreased from 4.1 to 3.4 eV for as-deposited films. After annealing at 950 °C for 1 min, the work function increased to 4.5–4.7 eV, with less dependency on the nitrogen flow rate.

On the origin of metal film work function changes under electrochemical modification

The catalytic activity and selectivity of metals can be reversibly altered by supplying promoters electrochemically. This has been achieved by using a catalyst as the working electrode in a solid electrolyte electrochemical cell. In this letter we review the accepted explanation for the relationship between the catalyst work function ( eφ W) and the potential difference between the working and reference electrodes of the electrochemical cell ( V WR). We show that this explanation is not correct. A theoretical derivation for the relationship found between eφ W and V WR in some experiments is not presently available. For the moment, it must be considered only an experimental finding.

Dynamic work function shift in cold cathode emitters using current carrying thin films

We analyze a new cold cathode emitter which consists of a thin wide band gap semiconductor material sandwiched between a metallic material, and a low work function semimetallic thin film. We show that under forward bias operation the electrons captured in the low work function material are responsible for an effective reduction of the semimetallic film work function, together with a substantial increase of the cathode emitted current. The dynamic work function shift is shown to increase with the amount of injected current. Potential material candidates are suggested to achieve low-voltage (<20 V), room-temperature cold cathode operation with emission currents approaching several hundred A/cm2 and large efficiencies.

Fermi levels of FCC and FCT hydrides of Ti and Zr

Measurements of the work functions of films of TiH x and ZrH x during the FCC/FCT transition have given the change of Fermi level between these two phases of the hydrides. The results agree with modern calculations of the band structures of these hydrides.