Work Function Modulation Research Articles

Ultrafast exciton formation at the ZnO(1010) surface.

We study the ultrafast quasiparticle dynamics in and below the ZnO conduction band using femtosecond time-resolved two-photon photoelectron spectroscopy. Above band gap excitation causes hot electron relaxation by electron-phonon scattering down to the Fermi level E_{F} followed by ultrafast (200fs) formation of a surface exciton (SX). Transient screening of the Coulomb interaction reduces the SX formation probability at high excitation densities near the Mott limit. Located just below the surface, the SX are stable with regard to hydrogen-induced work function modifications and thus the ideal prerequisite for resonant energy transfer applications.

Poly(ethylene imine) impurities induce n-doping reaction in organic (semi)conductors.

Volatile impurities contained in polyethyleneimine (PEI), and identified as ethyleneimine dimers and trimers, are reported. These N-based molecules show a strong reducing character, as demonstrated by the change in electrical conductivity of organic (semi)conductors exposed to the PEI vapor. The results prove that electron transfer rather than a dipole effect at the electrode interface is the origin of the work-function modification by the PEI-based layers.

Investigation of phosphonic acid surface modifications on zinc oxide nanoparticles under ambient conditions

Zinc oxide (ZnO) nanoparticles have emerged as a fascinating metal oxide semiconductor nanomaterial due largely to their wide array of properties that can be altered by surface modification. For example electrical and photonic properties include a range of conductivity from metallic to insulating (n-type and p- type conductivity), wide-band gap semiconductivity, room-temperature ferromagnetism, and chemical-sensing. Recently there has been much interest in the electronic and photonic properties of ZnO nanostructures as foreseeable applications include solar cells and laser diodes. For such purposes, controlling the surface functionalization is important and can be tailored by the chemical attachment of organic acids to the surface. The oxide surface readily reacts with organics forming self-assembled alkylphosphonate films. In this study, ZnO nanoparticles were modified using self-assembly thin films with phosphonic functional head groups. The amount of organic acid used in preparation of the thin film was shown to be important to the nanoparticle surface coverage. The modified ZnO nanoparticles were then characterized using infrared spectroscopy, powder X-ray diffraction, solid-state nuclear magnetic resonance, and scanning electron microscopy-energy dispersive X-ray spectroscopy. The interfacial bonding was identified by spectroscopy analysis to be the bidentate and tridentate motifs between the phosphonic head group and the oxide surface. Work function modification was measured using Ultraviolet photoelectron spectroscopy. The influences of temperature, humidity, and solvent rinse on the stability of the surface modifications were performed.

Charge spill-out and work function of few-layer graphene on SiC(0 0 0 1)

We report on the charge spill-out and work function of epitaxial few-layer graphene on 6 H-SiC(0 0 0 1). Experiments from high-resolution, energy-filtered x-ray photoelectron emission microscopy (XPEEM) are combined with ab initio density functional theory calculations using a relaxed interface model. The work function values obtained from theory and experiments are in qualitative agreement, reproducing the previously observed trend of increasing work function with each additional graphene plane. Electron transfer at the SiC/graphene interface through a buffer layer (BL) causes an interface dipole moment which is at the origin of the graphene work function modulation. The total charge transfer is independent of the number of graphene layers, and is consistent with the constant binding energy of the SiC component of the C 1s core-level, measured by XPEEM. Charge leakage into a vacuum depends on the number of graphene layers, explaining why the experimental, layer-dependent C 1s graphene core-level binding energy shift does not rigidly follow that of the work function. Thus, a combination of charge transfer at the SiC/graphene interface and charge spill-out into the vacuum resolves the apparent discrepancy between the experimental work function and C 1s binding energy.

Epitaxial hexagonal boron nitride on Ir(111): A work function template

Hexagonal boron nitride (h-BN) is a prominent member in the growing family of two-dimensional materials with potential applications ranging from being an atomically smooth support for other 2D materials to templating growth of molecular layers. We have studied the structure of monolayer h-BN grown by chemical vapour deposition on Ir(111) by low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) experiments and state-of-the-art density functional theory (DFT) calculations. The lattice-mismatch between the h-BN and Ir(111) surface results in the formation of a moir\'e superstructure with a periodicity of $\sim$29 \AA\ and a corrugation of $\sim$0.4 \AA. By measuring the field emission resonances above the h-BN layer, we find a modulation of the work function within the moir\'e unit cell of $\sim$0.5 eV. DFT simulations for a 13-on-12 h-BN/Ir(111) unit cell confirm our experimental findings and allow us to relate the change in the work function to the subtle changes in the interaction between boron and nitrogen atoms and the underlying substrate atoms within the moir\'e unit cell. Hexagonal boron nitride on Ir(111) combines weak topographic corrugation with a strong work function modulation over the moir\'e unit cell. This makes h-BN/Ir(111) a potential substrate for electronically modulated thin film and hetero-sandwich structures.

Improved performance of P3HT:PCBM photovoltaic devices with ITO treated by hydrogen peroxide.

The properties of poly(3-hexylthiophene):(6,6)-phenyl C61 butyric acid methyl ester (P3HT:PCBM) organic photovoltaic devices (OPVs) with indium tin oxide (ITO) anode treated by hydrogen peroxide are investigated. The optimized hydrogen peroxide solution has a concentration of 10% and ITO is treated for 15 min. The modification of ITO anode results in an enhancement of the power conversion efficiency (PCE) of the device, which is attributed to the increase of the photocurrent. The performance enhancement is attributed to the work function modification of the ITO substrate through the strong oxygenation of hydrogen peroxide, and then the charge collection efficiency is improved.

Influence of the Ionic Functionalities of Polyfluorene Derivatives as a Cathode Interfacial Layer on Inverted Polymer Solar Cells

In this work, we synthesized water-soluble polyfluorene derivatives (WPFs) with anionic and/or cationic side chains, which were used as an indium tin oxide (ITO) cathode interfacial layer in inverted polymer solar cells. Three WPFs (WPFN+, WPFZW, and WPFS-) were obtained via Suzuki coupling reactions. Their solubility in polar solvents allowed the WPFs to be used as interfacial layers in inverted polymer solar cells (I-PSCs). Among the WPF-modified ITO electrodes, WPFN+ (with ammonium side chains)-modified ITO can be used as a cathode for electron extraction, while WPFS- (with sulfonate side chains)-modified ITO cannot extract electrons in I-PSCs based on poly(3-hexylthiophene): [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PC61BM). The electron extraction of WPF-modified ITO can mainly be attributed to the different dipole formations at the WPF/ITO interfaces, based on the types of ionic groups on the side chains of the polyfluorene. In addition, we observed that the extent of ITO work-function modification was not always exactly correlated with the device performance based on the results obtained using a WPFZW (with ammonium and sulfonate side chains)-modified ITO electrode.

Interface electronic structures of reversible double-docking self-assembled monolayers on an Au(111) surface

Double-docking self-assembled monolayers (DDSAMs), namely self-assembled monolayers (SAMs) formed by molecules possessing two docking groups, provide great flexibility to tune the work function of metal electrodes and the tunnelling barrier between metal electrodes and the SAMs, and thus offer promising applications in both organic and molecular electronics. Based on the dispersion-corrected density functional theory (DFT) in comparison with conventional DFT, we carry out a systematic investigation on the dual configurations of a series of DDSAMs on an Au(111) surface. Through analysing the interface electronic structures, we obtain the relationship between single molecular properties and the SAM-induced work-function modification as well as the level alignment between the metal Fermi level and molecular frontier states. The two possible conformations of one type of DDSAM on a metal surface reveal a strong difference in the work-function modification and the electron/hole tunnelling barriers. Fermi-level pinning is found to be a key factor to understand the interface electronic properties.

Potentiometry in Gas Phase

Properties of organic semiconductors (OS) are controlled by the primary and secondary doping1,2. These highly versatile materials have found sensing application in modern potentiometric sensors for ions as well as for potentiometric sensing of gases. Operation of the equilibrium ion sensors is governed by the modulation of the potential at the interface between the solution and the OS, according to the Nikolskij-Eisenman equation. Exchange of ions with the OS can be seen as a form of primary doping of the latter. For the operation of such potentiometric sensors a stable and well-defined external reference electrode is needed2.When a dry, solid state OS is exposed to a gas or a vapor that can donate or accept electrons, a charge transfer complex is formed between the OS and the gas. Such interaction is called secondary doping, which results in modulation of work function (WF) of the OS. Such interaction can be measured with a Kelvin Probe or with a field-effect transistor. From the sensing point of view the biggest difference and advantage of WF-FET vis-à-vis ion sensors is the fact that no external reference electrode is needed because silicon is used as reference and its WF is stable under normal operating conditions. That fact alone makes the WF-FETs ideally suited for miniaturization, leading to gas-sensing arrays3.Jiri Janata and Mira Josowicz, J.Solid State Electrochem. 13 (2009) 41-49, “Organic Semiconductors in Potentiometric Gas Sensors”Jiri Janata, Principles of Chemical Sensors, 2nd Edition, Springer, 2009, New YorkJiri Janata, Coll.Czechoslov..Chem.Commun. 2009, 74, 1623–1634, Potentiometry in gas phase

The Unusual Mechanism of Partial Fermi Level Pinning at Metal–MoS2 Interfaces

Density functional theory calculations are performed to unravel the nature of the contact between metal electrodes and monolayer MoS2. Schottky barriers are shown to be present for a variety of metals with the work functions spanning over 4.2-6.1 eV. Except for the p-type Schottky contact with platinum, the Fermi levels in all of the studied metal-MoS2 complexes are situated above the midgap of MoS2. The mechanism of the Fermi level pinning at metal-MoS2 contact is shown to be unique for metal-2D-semiconductor interfaces, remarkably different from the well-known Bardeen pinning effect, metal-induced gap states, and defect/disorder induced gap states, which are applicable to traditional metal-semiconductor junctions. At metal-MoS2 interfaces, the Fermi level is partially pinned as a result of two interface behaviors: first by a metal work function modification by interface dipole formation due to the charge redistribution, and second by the production of gap states mainly of Mo d-orbitals character by the weakened intralayer S-Mo bonding due to the interface metal-S interaction. This finding would provide guidance to develop approaches to form Ohmic contact to MoS2.

A futuristic approach towards interface layer modifications for improved efficiency in inverted organic solar cells

Inverted polymer Solar Cells of the classical poly (3-hexylthiophene) (P3HT):(6,6)-phenyl-C61butyric acid methyl ester (PC61BM) blend on indium tin oxide substrates were fabricated, which shows improved device performance, by using a facile solution–processed ZnO-polyelectrolytes [poly (diallyldimethylammonium chloride) (PDADMAC), Poly (acrylic acid sodium salt) (PAS), poly (4-styrenesulfonic acid) (PSS), and Polyvinylpyrrolidone (PVP)] nanocomposite as a cathode interface layer compared to devices using pristine ZnO as cathode buffer layer in ambient conditions. The devices with different combinations of polyelectrolyte with ZnO show different improvements in the device efficiency. The combinations of ZnO with PVP and PDADMAC show highest amount of improvements in the efficiency by a factor of ∼17–19. The improvement of the efficiency may be due to various phenomena, such as the passivation of ZnO surface as well as bulk traps, work function modification, improved energy level alignment, improved electronic coupling of the inorganic/organic interface, improved light harvesting, and decrease of surface as well as bulk charge recombination in the device. The introduction of polyelectrolyte into ZnO inhibits the aggregation of ZnO nanoparticles yielding the large area ZnO nanoclusters; and hence, forming the uniform film of ZnO resulting in the modifications of morphology as well as electronic structure of ZnO-polyelectrolyte nano-composite favouring better electronic coupling between cathode and active layer and hence enhancing the current and, consequently, the efficiency. This simple low temperature ZnO-polyelectrolyte nanocomposite based protocol proposed for cathode interface layer modification may be very much useful for roll to roll industrial manufacturing of organic solar cells.

All-plastic solar cells with a high photovoltaic dynamic range

We report on semitransparent air-processed all-plastic solar cells, fabricated from vacuum-free processes, comprising two polymer electrodes, a polymeric work-function modification layer and a polymer:fullerene photoactive layer. The active layer and the top PEDOT:PSS electrode were prepared by sequential film-transfer lamination on polyethylenimine-modified PEDOT:PSS bottom electrodes. The transferring of films offers ease of layer patterning and the misalignment of defects in the different layers resulting from the additive film transfer lamination process yields high shunt resistance values of 108 ohm cm2. Consequently, all-plastic solar cells fabricated with this process exhibit very low reverse bias dark current and can operate in the photovoltaic quadrant with light irradiance varying over five orders of magnitude. The analysis of the values of the open-circuit voltage as a function of light irradiance over that wide dynamic range points toward an ideality factor of n = 1.82 and a reverse saturation current density of 6.2 × 10−11 A cm−2 for solar cells with an active layer comprised of a blend of poly(3-hexylthiophene) and an indene fullerene bis-adduct.

Work Function Modulation and Thermal Stability of Reduced Graphene Oxide Gate Electrodes in MOS Devices

Work function (WF) tuning of the contact electrodes is a key requirement in several device technologies, including organic photovoltaics (OPVs), organic light-emitting diodes (OLEDs), and complementary metal oxide semiconductor (CMOS) transistors. Here, we demonstrate that the WF of the gate electrode in an MOS structure can be modulated from 4.35 eV (n-type metal) to 5.28 eV (p-type metal) by sandwiching different thicknesses of reduced graphene oxide (rGO) layers between top contact metals and gate dielectric SiO2. The WF of the gate electrode shows strong dependence on the rGO thickness and is seen to be nearly independent of the contact metals used. The observed WF modulation is attributed to the different amounts of oxygen concentrations in different thicknesses of rGO layers. Importantly, this oxygen concentration can also be varied by the reduction extent of the graphene oxide as experimentally demonstrated. The results are verified by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy analyses. The obtained WF values are thermally stable up to 800 °C. At further high temperatures, diffusion of metal through the rGO sheets is the main cause for WF instability, as confirmed by cross-sectional high-resolution transmission electron microscopy analysis. These findings are not limited to MOS devices, and the WF modulation technique has the potential for applications in other technologies such as OLEDs and OPVs involving graphene as conducting electrodes.

Work function modification of the (111) gold surface covered by long alkanethiol-based self-assembled monolayers

Many recent experimental studies have demonstrated that the deposition of self-assembled monolayers on a metal electrode can significantly modulate its work function and hence charge injection properties in organic-based devices. In this context, we model here by means of Density Functional Theory (DFT) calculations the deposition of long saturated and partially fluorinated alkanethiol chains on the gold (111) surface and explore the shift in the work function while changing the size and degree of fluorination of the molecular backbone in order to contribute to the definition of design rules. Among all the derivatization schemes, only the introduction of a terminal fluoromethyl unit leads to an appreciable change in the work function shift, further accompanied by a reversal of its sign. The results show pronounced odd-even effects with different origins.

Work function tuning of tin-doped indium oxide electrodes with solution-processed lithium fluoride

Solution-processed lithium fluoride (sol-LiF) nanoparticles synthesized in polymeric micelle nanoreactors enabled tuning of the surface work function of tin-doped indium oxide (ITO) films. The micelle reactors provided the means for controlling surface coverage by progressively building up the interlayer through alternating deposition and plasma etch removal of the polymer. In order to determine the surface coverage and average interparticle distance, spatial point pattern analysis was applied to scanning electron microscope images of the nanoparticle dispersions. The work function of the sol-LiF modified ITO, obtained from photoelectron emission yield spectroscopy analysis, was shown to increase with surface coverage of the sol-LiF particles, suggesting a lateral depolarization effect. Analysis of the photoelectron emission energy distribution in the near threshold region revealed the contribution of surface states for surface coverage in excess of 14.1%. Optimization of the interfacial barrier was achieved through contributions from both work function modification and surface states.

Large work function reduction by adsorption of a molecule with a negative electron affinity: Pyridine on ZnO$(10\bar{1}0)$(101¯)

Using thermal desorption and photoelectron spectroscopy to study the adsorption of pyridine on ZnO(1010), we find that the work function is significantly reduced from 4.5 eV for the bare ZnO surface to 1.6 eV for one monolayer of adsorbed pyridine. Further insight into the interface morphology and binding mechanism is obtained using density functional theory. Although semilocal density functional theory provides unsatisfactory total work functions, excellent agreement of the work function changes is achieved for all coverages. In a closed monolayer, pyridine is found to bind to every second surface Zn atom. The strong polarity of the Zn-pyridine bond and the molecular dipole moment act cooperatively, leading to the observed strong work function reduction. Based on simple alignment considerations, we illustrate that even larger work function modifications should be achievable using molecules with negative electron affinity. We expect the application of such molecules to significantly reduce the electron injection barriers at ZnO/organic heterostructures.

Control of Surface Work Function by Depositing Insulating Oxide Capping Layers

We have studied the work function modulation in pulsed laser deposited nonpolar MgO (100) and polar MgO (100) and LaAlO3 (100) insulating oxide films on Nb:SrTiO3 (100) and (111) substrates using Kelvin probe measurements. The work function modulation in MgO/Nb:SrTiO3 is ∼1 eV, which may be understood by the “electron compression model”, where the leaked-out electrons from Nb:SrTiO3 surface are pushed back by MgO layer, reducing the surface dipole moment. By forming double insulating top layer in LaAlO3/nondoped SrTiO3/Nb:SrTiO3 (100), we have realized the work function as low as ∼2.2 eV, which would be originated in the formation of high-density electrons at the LaAlO3/nondoped SrTiO3 interface and can be very close to the top surface. The present results demonstrate that chemically stable oxides can be used as low work function devices employing heterostructure form.

Photoinduced work function modifications and their effect on photoelectron spectroscopy

We investigate the effect of a spatially varying work function on photoemission experiments. It is demonstrated that a photoinduced work function change when probed by ultraviolet and two-photon photoemission spectroscopy can have pronounced effects on photoemission spectra. These effects are simulated by a simple model that reproduces the data remarkably well and allows for quantitative interpretation of the modified low energy region of the photoemission spectra. These findings are highly relevant when discussing work function determinations by photoemission spectroscopy and moreover may have substantial impact on the energy level alignment of molecule-metal or -semiconductor interfaces.

Anticorrelation between the Evolution of Molecular Dipole Moments and Induced Work Function Modifications.

We explore the limits of modifying metal work functions with large molecular dipoles by systematically increasing the dipole moment of archetype donor–acceptor molecules in self-assembled monolayers on gold. Contrary to intuition, we find that enhancing the dipoles leads to a reduction of the adsorption-induced change of the work function. Using atomistic simulations, we show that large dipoles imply electronic localization and level shifts that drive the interface into a thermodynamically unstable situation and trigger compensating charge reorganizations working against the molecular dipoles. Under certain circumstances, these are even found to overcompensate the effect that increasing the dipoles has for the work function.

Role of Oxygen in the Work Function Modification at Various Stages of Chemically Synthesized Graphene

Correlation between the work function and oxygen content at various stages during synthesis of graphene by chemical route is presented. The values of work function in terms of contact surface potential difference (CPD) were estimated using a scanning Kelvin probe method. Oxygen content was determined using energy dispersive X-ray absorption (EDAX), and the data was further supported using the Rutherford back scattering method in oxygen resonance mode. Results indicated a linear correlation between the oxygen content and measured values of CPD at various stages during the chemical synthesis of graphene.