Reduction Of Work Function Research Articles

Interface engineering for efficient fullerene-free organic solar cells

We demonstrate the role of zinc oxide (ZnO) morphology and addition of an acceptor interlayer to achieve high efficiency fullerene-free bulk heterojunction inverted organic solar cells. Nanopatterning of the ZnO buffer layer enhances the effective light absorption in the active layer, and the insertion of a twisted perylene acceptor layer planarizes and decreases the electron extraction barrier. Along with an increase in current homogeneity, the reduced work function difference and selective transport of electrons prevent the accumulation of charges and decrease the electron-hole recombination at the interface. These factors enable an overall increase of efficiency to 4.6%, which is significant for a fullerene-free solution-processed organic solar cell.

Structural and electrical characterization of CoTiN metal gates

As the gate size continues to decrease in nanoscale transistors, having metal gates with amorphous or near amorphous structures can potentially reduce grain-induced work function variation. Furthermore, amorphous materials are known to have superior diffusion barrier properties, which can help prevent work function change due to the diffusion of metals in contact with the gate. In this work we show that with the addition of cobalt, thin films of polycrystalline TiN become more amorphous with a smaller grain size. Cox(TiN)1-x films, where x = 60–80%, appear to consist of nanocrystals embedded in an amorphous matrix, and are thermally stable with no significant crystallization up to an annealing temperature of at least 600 °C. Reducing the nitrogen gas flow ratio during sputter deposition from 9% to 2.5% further decreases the films' crystallinity, which is apparent by more sparse and even smaller nanocrystals. In addition to being partially amorphous, these CoTiN films also exhibit good thermal stability, low resistivity, low roughness, and have the potential for atomic layer deposition compatibility. Even though these materials are not completely amorphous, their small crystal size and amorphous matrix can potentially reduce work function variation and improve their diffusion barrier property. These properties make CoTiN a good candidate as a gate material for future nanoelectronic devices and technology.

Organometallic dimers: application to work-function reduction of conducting oxides.

The dimers of pentamethyliridocene and ruthenium pentamethylcyclopentadienyl mesitylene, (IrCp*Cp)2 and (RuCp*mes)2, respectively, are shown here to be effective solution-processable reagents for lowering the work functions of electrode materials; this approach is compared to the use of solution-deposited films of ethoxylated poly(ethylenimine) (PEIE). The work functions of indium tin oxide (ITO), zinc oxide, and gold electrodes can be reduced to 3.3-3.4 eV by immersion in a toluene solution of (IrCp*Cp)2; these values are similar to those that can be obtained by spin-coating a thin layer of PEIE onto the electrodes. The work-function reductions achieved using (IrCp*Cp)2 are primarily attributable to the interface dipoles associated with the formation of submonolayers of IrCp*Cp(+) cations on negatively charged substrates, which in turn result from redox reactions between the dimer and the electrode. The electrical properties of C60 diodes with dimer-modified ITO cathodes are similar to those of analogous devices with PEIE-modified ITO cathodes.

Armchair Boron Nitride nanotubes—heterocyclic molecules interactions: A computational description

Abstract Ab-initio calculations using density functional theory (DFT) are used to investigate the non-covalent interactions between single wall armchair boron nitride nanotubes (BNNTs) with open ends and several heterocyclic molecules: thiophene (T; C4H4S), benzothiophene (BT; C8H6S) and dibenzothiophene (DBT; C12H8S). In the armchair model the nanotubes exhibit (n, n) chirality; here we consider n = 5. The exchange-correlation energies are treated according to the Hamprecht-Cohen-Tozer-Handy functional in the generalized gradient approximation (HCTH-GGA). A base function with double polarization is used. The geometry optimization of (5,5) BNNT-X; X = T, BT and DBT has been carried out using the minimum energy criterion in 5 different configurations of the molecules adsorbed on the nanotube. Our computer simulations have found that the preferential adsorption site of the molecule on the nanotube surface is the parallel configuration for BT and DBT, and at one nanotube end for the T fragment, with all cases having physical interactions. The polarity exhibits an increase which favors the possible dispersion, provided the electrons are polarized. The nanostructure functionalization increases the chemical reactivity which in turn enhances interactions between the molecule and the nanotube. The BNNT-dibenzothiophene work function reduction as compared with the pristine case yields the improvement of the field emission properties.

Взаємодія кисню та гадолінію з Si(100)-2×1. Утворення системи з роботою виходу 1 еВ

Changes in the electronic properties of the Si(100) surface, when a multilayer structure of oxidized Gd atoms is created on it, have been studied, by using the electron spectroscopy methods. It is shown that, after a number of adsorption cycles of Gd and oxygen atoms on the Si(100)-2×1 surface at room temperature and the annealing of the obtained structure at 600 ∘C, the work function decreases from 4.8 to less than 1 eV. The work function reduction at larger numbers of processing cycles is shown to be accompanied by the oxidation of Gd and Si atoms and a gradual decrease of the Si concentration in the near-surface region. The obtained results are explained by the formation of an O–Gd dipole layer on the surface.

Ab initiostudies of Cs on GaAs (100) and (110) surfaces

GaAs with an atomic monolayer of Cs is one of the best known photoemissive materials. The results of density functional theory calculations of Cs adsorption on the GaAs(100)-$(4\ifmmode\times\else\texttimes\fi{}2)$ gallium-terminated reconstructed surface and the GaAs(110) surface are presented in this work. Coverage of up to 4 Cs ${\mathrm{atoms}/\mathrm{nm}}^{2}$ on GaAs surfaces has been studied to predict the work-function reduction and adsorption energies accurately. The high mobility of Cs atoms on the (110) surface allows formation of ordered structures, whereas the low mobility of Cs of the (100) surface causes amorphous growth.

Spontaneous doping of two-dimensional NaCl films with Cr atoms: aggregation and electronic structure.

Scanning tunneling microscopy (STM) experiments combined with density functional theory (DFT) calculations reveal that deposited Cr atoms replace either Na or Cl ions, forming substituting dopants in ultrathin NaCl/Au(111) films. The Cr dopants exchange electrons with the support thus changing the electronic properties of the film and in particular the work function. The Cr atoms spontaneously aggregate near the edges of the bilayer (2L) NaCl islands, forming a new phase in the insulator with a remarkably dense population of Cr dopants. The spectra of differential conductance yield evidence that, compared to the undoped or Cr-poor 2L NaCl films on Au(111), the Cr-rich region shows different interface states, shifted image-potential states, and a reduced work function. This demonstrates the potential of doping ultrathin films to modify their adsorption properties in a desired manner.

Properties of Extremely Narrow Gaps Between Electrodes of a Molecular Transistor

An electron transport through the extremely small gaps (1–5 nm wide) formed in narrow and thin gold nanowires by the electromigration method is studied in this work at various temperatures. A careful investigation of the final stage of a gap formation has shown a quantum character of a nanowire conductivity on this stage. Analysis of the electron transport characteristics through the resulting gaps was carried out. It shows that regimes of both a direct tunneling between electrodes and a “cold” emission into a barrier region were realized. Significant reduction of electron work function for gold electrodes of prepared gaps is revealed.

Investigation of the effect of conjugated oligoelectrolyte as a cathode buffer layer on the photovoltaic properties

A series of conjugated oligoelectrolytes consisting of fluorene and phenothiazine, 6,6',6″,6‴,6′′′′,6′′′′′-(9H,9′H,9′′H-[2,3′:6′,2′′-terfluorene]-9,9,9′,9′,9′′,9′′-hexayl)hexakis(N,N,N-trimethylhexan-1-aminium)bromide (FFiF),6,6′,6′′,6′′′-((10-(4-(trimethylammonio)butyl)-10H-phenothiazine-3,7-diyl)bis(9H-fluorene-9,9,2-triyl))tetrakis(N,N,N-trimethylhexan-1-aminium)bromide(FPiF) and 6,6',6”,6”'-((10-hexyl-10H-phenothiazine-3,7-diyl)bis(9H-fluorene-9,9,2-triyl))tetrakis(N,N,N-trimethylhexan-1-aminium) bromide (FPF), are utilized as the cathode interfacial layer in conventional-type polymer solar cells based on P3HT and PCBM. Insertion of these interfacial layers between P3HT/PCBM active layer and the metal cathode increases overall power conversion efficiency from 2.76% (reference device) to 2.81%, 3.04%, and 3.37% of devices with FPiF, FFiF, or FPF-coated Al, respectively, primarily due to the reduction of the metal work function through the introduction of conjugated oligoelectrolytes (COEs). This result comes from the difference moiety (i.e., phenothiazine or fluorene) and arrangement of sides chains with quaternary ammonium bromide (QAB) on trimer backbones. The conformation of FFiF is expected to be not exactly in a zigzag arrangement, while the zigzag conformation is expected for the alternated oligomer based on phenothiazine and fluorene (FPiF and FPF). Therefore, the number of accumulated QAB in the side chains of FPiF at the surface of cathode will be smaller than that of FFiF or FPF. The larger amount QAB groups gathering near the cathode layer leads to the bigger in reduction of metal work function. Additionally, FPF coated Al shows the lower work function than that of FFiF coated Al, even though FFiF contains more QAB groups accruing at cathode layer. Thus the device with FPF modified-Al displays the best performances among the devices.

In situ reactivation of low-temperature thermionic electron emission from nitrogen doped diamond films by hydrogen exposure

Nitrogen doped, hydrogen terminated diamond films have shown a work function of less than 1.5eV and thermionic electron emission (TE) has been detected at temperatures less than 500°C. However, ambient exposure or extended operation leads to a deterioration of the emission properties. In this study thermionic electron emission has been evaluated for as-received surfaces and for surfaces after 18months of ambient exposure. The initial TE current density of the freshly deposited diamond film was ~5×10−5A/cm2 at 500°C. In contrast, the initial TE current density of a film aged for 18months was ~1.8×10−9A/cm2 at 500°C. The decreased emission current density is presumed to be a consequence of oxidation, surface adsorption of contaminants and hydrogen depletion from the surface layer. In situ reactivation of the aged film surface was achieved by introducing hydrogen at a pressure of 1.3×10−4mbar and using a hot filament of a nearby ionization gauge to generate atomic and/or excited molecular hydrogen. After 2h of exposure with the sample at 500°C, the surface exhibited a stable emission current density of ~2.3×10−6A/cm2 (an increase by a factor of ~1300). To elucidate the reactivation process thermionic electron energy distribution (TEED) and XPS core level spectra were measured during in situ hydrogen exposure at 5×10−8mbar. During the isothermal exposure it was determined that atomic or excited hydrogen resulted in a much greater increase of the TE in comparison to exposure to molecular hydrogen. During exposure at 400°C the surface oxygen was substantially reduced, the TEED cut-off energy, which indicates the effective work function, decreased by ~200meV, and the TE intensity increased by a factor of ~100. The increase in thermionic emission with hydrogen was ascribed to the reactivation of the surface through the formation of a uniform surface dipole layer and a reduction of the surface work function.

Enhancement of the Efficiency and Stability of Organic Photovoltaic Devices via the Addition of a Lithium-Neutralized Graphene Oxide Electron-Transporting Layer

Lithium-neutralized graphene oxide (GO-Li) was spin coated between the photoactive layer and the metal oxide electron-transporting layer (ETL) as an additional interlayer in organic photovoltaic devices. The introduction of GO-Li leads to a superior interface between the ETL and the photoactive layer. Combined with the reduced work function (WF) of GO-Li (4.3 eV), which is a perfect match with the fullerene acceptor material LUMO level, PCDTBT:PC71BM-based air-processed devices with a GO-Li layer exhibited a significant enhancement in their power conversion efficiency (PCE) from 5.51 to 6.29% (14.2% increase over that of comparable devices without the graphene-based interfacial layer). Furthermore, the GO-Li device exhibited stability higher than that of the device without the interlayer due to the fact that the GO-Li acts as an internal shield against humidity, protecting the air sensitive polymers and improving the lifetime of the devices.

Induced work function changes at Mg-doped MgO/Ag(001) interfaces: Combined Auger electron diffraction and density functional study

The properties of MgO/Ag(001) ultrathin films with substitutional Mg atoms in the interface metal layer have been investigated by means of Auger electron diffraction experiments, ultraviolet photoemission spectroscopy, and density functional theory (DFT) calculations. Exploiting the layer-by-layer resolution of the Mg KL_23 L_23 Auger spectra and using multiple scattering calculations, we first determine the interlayer distances as well as the morphological parameters of the MgO/Ag(001) system with and without Mg atoms incorporated at the interface. We find that the Mg atoms incorporation drives a strong distortion of the interface layers and that its impact on the metal/oxide electronic structure is an important reduction of the work function (0.5 eV) related to band-offset variations at the interface. These experimental observations are in very good agreement with our DFT calculations which reproduce the induced lattice distortion and which reveal (through a Bader analysis) that the increase of the interface Mg concentration results in an electron transfer from Mg to Ag atoms of the metallic interface layer. Although the local lattice distortion appears as a consequence of the attractive (repulsive) Coulomb interaction between O2- ions of the MgO interface layer and the nearest positively (negatively) charged Mg (Ag) neighbors of the metallic interface layer, its effect on the work function reduction is only limited. Finally, an analysis of the induced work function changes in terms of charge transfer, rumpling, and electrostatic compression contributions is attempted and reveals that the metal/oxide work function changes induced by interface Mg atoms incorporation are essentially driven by the increase of the electrostatic compression effect.

Thermionic emission from phosphorus (P) doped diamond nanocrystals supported by conical carbon nanotubes and ultraviolet photoelectron spectroscopy study of P-doped diamond films

Materials with low work function values (<2eV) are highly in demand for low temperature thermionic electron emission, which is a key phenomenon for waste heat recovery applications. Here we present the work function reduction of phosphorus (P) doped (i) diamond nanocrystals grown on conical carbon nanotubes (CCNTs) and (ii) diamond films grown on silicon substrates. Thermionic emission measurements from phosphorus doped diamond crystals on CCNTs resulted in a work function value of 2.23eV. The CCNTs provide the conducting backbone for the P-doped diamond nanocrystals and the reduced work-function is interpreted as due to the presence of midband-gap state and no evidence for negative electron affinity was seen. However, ultraviolet photoelectron spectroscopy studies on phosphorus doped diamond films yielded a work function value of ~1.8eV with a negative electron affinity (NEA) value of 1.2eV. Detailed band diagrams are presented to support the observed values for both cases.

Understanding the formation and evolution of interdiffusion grown organolead halide perovskite thin films by thermal annealing

Organolead trihalide perovskites are emerging excellent active materials for thin film solar cells. Here, the formation and evolution of methylammonium lead iodide perovskite thin films grown by the low-temperature thermal annealing induced interdiffusion process are investigated. It is found that thermal annealing not only drives the formation of perovskite but also affects the morphology, optoelectronic properties and correlated device performance. Thermal annealing at 105 °C quickly drives the formation of phase-pure perovskite in a short time of 15 min, and continued thermal annealing up to two hours continuously increases the perovskite crystallinity and grain size without losing film continuity or coverage. The measured Hall mobility increases monotonically to 36.0 cm2 V−1 s−1 upon annealing, which is correlated with the increased crystallinity and grain size. Device efficiencies increase with an increased short circuit current density and fill factor with a longer annealing time up to two hours, and the highest device efficiency of 13.4% is achieved. It is found that the reduction of work function in the perovskite films, caused by the increased annealing duration, is linearly correlated with the open circuit voltage loss, which points out a path for the further increase of the device efficiency.

Spin-polarization properties and electronic structure of the ordered c(2 × 2) MnCu/Cu(1 1 0) surface alloy

The spin-ordering and electronic properties of the c(2 × 2) MnCu/Cu(1 1 0) surface alloy are investigated by means of ab initio density-functional calculations. We first address the magnetic ground state and the robustness of the spin-polarization properties. The lowest-energy state is found to be ferromagnetic with a very low Curie temperature, showing that the paramagnetic phase should be established in this system at room temperature. The calculated trends obtained for the various spin structures considered indicate that the local Mn-spin moment and resulting reduced work function as well as Mn-outward buckling should persist in the paramagnetic phase. We then address the electronic surface-band structure of the paramagnetic phase close to the Fermi energy, in connection with the interpretation of recent angle-resolved-photoemission-spectroscopy experiments at room temperature. Our calculations account for an intriguing new surface-band feature observed experimentally near the point upon alloy formation, and provide a microscopic interpretation for this feature and for the alloy-induced changes in the Cu(1 1 0) Shockley surface state.

Tuning the energy gap of conjugated polymer zwitterions for efficient interlayers and solar cells

ABSTRACTNarrow band gap conjugated polymer zwitterions (CPZs) were synthesized by Suzuki polymerization and characterized to understand their electronic properties and utility as cathode modification layers in solar cells. The polymers were prepared from diketopyrrolopyrrole (DPP) and iso‐indigo monomers containing sulfobetaine (SB) pendant groups, benefiting from an ion‐rich aqueous phase in the polymerizations. UV–vis absorption spectroscopy revealed the optical energy gap value for the CPZs, ranging from 1.7 to 1.2 eV. Ultraviolet photoelectron spectroscopy of the CPZs as thin layers on Ag metal showed that the pendent zwitterions impart an interfacial dipole (Δ) to the metal and a work function reduction of ∼0.9 eV. OPVs fabricated using a conventional bulk heterojunction (BHJ) device architecture of ITO/PEDOT:PSS/(PTB7:PC71BM)/CPZ/Ag led to dramatic improvements in power conversion efficiency (PCE) values relative to devices having bare Ag cathodes (PCE &lt; 2% for bare Ag vs. 6.7–7.7% for CPZ/Ag). The benzothiadiazole (BT)/DPP polymer denoted as PT2BTDPPSB gave an optimal PCE of 7.7% in a conventional BHJ OPV device architecture fabricated on a Ag cathode. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 327–336

Tuning electrical parameters of graphene/p-type polycrystalline silicon Schottky diodes by ultraviolet irradiation

The present work reports the fabrication and detailed electrical properties of graphene/p-type polycrystalline silicon (poly-Si) Schottky diodes with and without ultraviolet irradiation. Ultraviolet treatment may lead to the reduced work function of graphene, thus increasing the Schottky barrier height at the graphene/poly-Si interface. Compared to the graphene/poly-Si Schottky diodes without ultraviolet treatment, the ultraviolet-treated graphene/poly-Si Schottky diodes exhibit lower ideality factor and higher responsivity. The sensitivity of the work function of graphene to the ultraviolet irradiation time provides an opportunity to tune the electrical parameters of graphene/poly-Si Schottky diodes. The ability to controllably tune the work function of graphene is essential for optimizing the efficiency of optoelectronic and electronic devices.

Facile modification of Cu source–drain (S/D) electrodes for high-performance, low-voltage n-channel organic thin film transistors (OTFTs) based on C60

Exploring suitable electrode materials with sufficiently low work function, ambient stability and low-cost is of great technological importance to the development of n-channel OTFTs. Here, we show that the work function of Cu can be effectively reduced from 4.65eV to 4.28eV through surface modification via simply spin-coating a thin layer of branched polyethylenimine (PEI). By exploiting a high-capacitance density gate dielectric (200nF/cm2), low-voltage (3V) C60 TFTs with electron mobility (μe) of 3.2cm2/Vs are demonstrated with PEI modified Cu as source–drain (S/D) electrodes. In contrast, the device with Cu S/D electrodes possesses μe of only 1.0cm2/Vs. The improvement in electrical performance of the PEI modified device is attributed to the efficient electron injection at the Cu/C60 interface which resulted from the reduction in work function of Cu. Moreover, upon PEI modification, the bias stability of the device can be obviously enhanced as compared to the unmodified one, and the resultant device exhibits an excellent thermal stability up to 200°C without appreciable degradation in mobility. The facile modification of low-cost Cu as S/D electrodes for high-performance n-channel OTFTs as well as the low-voltage operation will pave the way for large scale manufacturing of organic electronics.

Work function reduction by BaO: Growth of crystalline barium oxide on Ag(001) and Ag(111) surfaces

Ultrathin films of barium oxide were grown on Ag(001) and Ag(111) using the evaporation of Ba metal in an O2 atmosphere by molecular beam epitaxy. Ultraviolet photoemission spectroscopy reveals that films consisting of predominantly BaO or BaO2 result in Ag(001) work function reductions of 1.74eV and 0.64eV, respectively. On the Ag(001) surface, Ba oxide growth is initiated by two-dimensional nucleation of epitaxial BaO, followed by a transition to three-dimensional dual-phase nucleation of epitaxial BaO and BaO2. Three-dimensional islands of primarily BaO2(111) nucleate epitaxially on the Ag(111) substrate leaving large patches of Ag uncovered. We find no indication of chemical reaction or charge transfer between the films and the Ag substrates. These data suggest that the origin of the observed work function reduction is largely due to a combination of BaO surface relaxation and an electrostatic compressive effect.

Ethoxylated polyethylenimine as an efficient electron injection layer for conventional and inverted polymer light emitting diodes

We report inverted light emitting devices using ethoxylated polyethylenimine (PEIE) as a single electron injection layer for indium tin oxide cathode, which possess comparable efficiency to those using ZnO/PEIE double electron injection layers. Implementation of a PEIE layer between light emitting polymer layer and aluminum has been shown to significantly enhance device efficiency as well. Improvement of device efficiency can be attributed to increased electron injection due to the reduced work function of PEIE modified cathode as well as the hole blocking effect of PEIE layer. Furthermore, PEIE serves as an efficient electron injector for a range of light emitting polymers with wide distribution of energy levels.