Energy Level Lineup Research Articles

Evidence of direct Z-scheme g-C3N4/WS2 nanocomposite under interfacial coupling: First-principles study

Understanding is far from satisfactory on the high photocatalytic performances of g-C3N4/WS2 nanocomposites which are experimentally reported. Here, we investigate systematically for the first time the interfacial interactions and electronic properties of triazine-based g-C3N4/WS2 nanocomposites by first-principles calculations. The results confirm the reasonable existence of vdW interaction and typical direct Z-scheme mechanism based on the prominent interfacial binding energies and the energy level lineup at interface. Both energy band structures and work functions show that the conduction and valence band edges of the g-C3N4 layer are more negative than those of the WS2 layer after their intimate contact, respectively. Meanwhile, the interfacial interaction makes WS2 with negative charges and g-C3N4 with positive charges, forming a built-in electric field from g-C3N4 to WS2. Thus, photogenerated e––h+ pairs are separated to different places with prolonged lifetime and high redox potential. This work shines some lights on a direct Z-scheme mechanism for the g-C3N4/WS2 heterostructured photocatalyst.

Charge Transport in Pentacene-Graphene Nanojunctions.

We investigate charge transport in pentacene-graphene nanojunctions employing density functional theory (DFT) electronic structure calculations and the Landauer transport formalism. The results show that the unique electronic properties of graphene strongly influence the transport in the nanojunctions. In particular, edge states in graphene electrodes with zigzag termination result in additional transport channels close to the Fermi energy, which deeply affects the conductance at small bias voltages. Investigating different linker groups as well as chemical substitution, we demonstrate how the transport properties are furthermore influenced by the molecule-lead coupling and the energy level lineup.

Time dependence and freezing-in of the electrode oxygen plasma-induced work function enhancement in polymer semiconductor heterostructures

Indium tin oxide (ITO), Au and Pt are materials of interest as high work function contacts for organic semiconductor devices. In this paper the relative energy level line-up of these materials is investigated both as bare surfaces or part of a polymer/conductor interface. Kelvin probe (KP) measurements show that the estimated work function for Au and Pt surfaces, evaporated under normal high vacuum (HV) fabrication conditions and measured in air, can be significantly lower (by ∼0.2 to 0.9 eV) than those of clean surfaces evaporated and measured in the more demanding and clean ultra high vacuum (UHV), that are often used as reference values. The work function of all surfaces increases significantly (from ∼0.4 to >1 eV) after an oxygen plasma, but then decreases upon air exposure, with different rates for different materials. The effect of the plasma wears off most rapidly for Au whilst it is more resilient for ITO. Most interestingly, via KP and electroabsorption measurements of the built-in potential on polymer/conductor and conductor/polymer/conductor structures, we demonstrate that a plasma-induced enhancement of the work function is “frozen in” by the application of a polymer semiconductor layer over the plasma-treated surfaces and can be made to last for years by proper device encapsulation. These results have strong implications on the understanding, fabrication, design and stability of organic semiconductor devices.

Changing adsorption mode of FePc on TiO2(110) by surface modification with bipyridine

Surface modification of reactive oxide substrates to obtain a less strongly interacting template for dye adsorption may be a way to enhance performance in dye-sensitized solar cells. In this work, we have investigated the electronic and structural properties of 4,4(')-bipyridine (bipy) as modifier adsorbed on the TiO(2)(110) surface. The modified surface is then coated with iron phthalocyanine (FePc) and the properties of this heterostructure are investigated with synchrotron based photoelectron spectroscopy, x-ray absorption spectroscopy, and scanning tunneling microscopy. We find that a saturated monolayer consisting of standing bipy molecules with one nitrogen atom pointing outward is formed on the oxide surface. FePc adsorb in molecular chains along the [001] direction on top of bipy and ordered in a tilted arrangement with adjacent molecules partially overlapping. Already from the first layer, the electronic properties of FePc resemble those of multilayer films. FePc alone is oxidized on the TiO(2)(110) surface, but preadsorbed bipy prevents this reaction. The energy level lineup at the interface is clarified.

Interfacial electronic structures in an organic double-heterostructure photovoltaic cell

The electronic structure of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline/fullerene/copper-phthalocyanine double heterostructure in a typical organic photovoltaic cell was studied by photoemission spectroscopy. The results show that the traditional vacuum energy level lineup is not valid for these organic heterojunctions, which suggests formation of interface dipole and energy level bending across the interface due to charge transfer. Based on the measured energy levels using x-ray and ultraviolet photoemission spectroscopies, important parameters such as the theoretical maximum of the open circuit voltage are extracted, and their impacts on charge photogeneration process are discussed.

Substrate effects on the electronic properties of an organic/organic heterojunction

The electronic structures of copper-phthalocyanine/tris(8-hydroxyquinoline) aluminum (CuPc∕Alq3) heterojunctions on Mg and indium tin oxide (ITO) substrates have been studied by photoemission spectroscopy. While the typical vacuum energy level lineup occurred at the CuPc∕Alq3 junction on ITO, the same junction formed on Mg displayed vastly different electronic structures, showing a 0.5eV band bending associated with the formation of a space charge layer. The substrate effects were explained by the proximity of the Mg’s Fermi level to the lowest unoccupied molecular orbital of CuPc, resulting in spontaneous charge transfer. The results show the feasibility of tuning the electronic properties of an organic heterojunction via the Fermi level of the substrate.

Microscopic study of electrical transport through individual molecules with metallic contacts. II. Effect of the interface structure

We investigate the effect on molecular transport due to the different structural aspects of metal-molecule interfaces. The example system chosen is the prototypical molecular device formed by sandwiching the phenyl dithiolate molecule (PDT) between two gold electrodes with different metal-molecule distance, atomic structure at the metallic surface, molecular adsorption geometry and with an additional hydrogen end atom. We find the dependence of the conductance on the metal-molecule interface structure is determined by the competition between the modified metal-molecule coupling and the corresponding modified energy level lineup at the molecular junction. The results of the detailed microscopic calculation can all be understood qualitatively from the equilibrium energy level lineup and the knowledge of the voltage drop across the molecular junction at finite bias voltages.

Electronic line-up in light-emitting diodes with alkali-halide/metal cathodes

The electronic nature of metal-semiconductor contacts is a fundamental issue in the understanding of semiconductor device physics, because such contacts control charge injection, and therefore play a major role in determining the electron/hole population in the semiconductor itself. This role is particularly important for organic semiconductors as they are generally used in their pristine, undoped form. Here, we review our progress in the understanding of the energy level line-up in finished, blue-emitting, polyfluorene-based light-emitting diodes, which exploit LiF and CsF thin films in combination with Ca and Al to obtain cathodes with low injection barriers. We have used electroabsorption measurements, as they allow the noninvasive determination of the built-in potential when changing the cathode. This provides precious experimental information on the alteration of the polymer/cathode interfacial energy level line-up. The latter is found to depend strongly on the electrode work function. Thus, the Schottky–Mott model for the energy level alignment is found to be a better first-order approximation than those models where strong pinning or large interface dipoles determine the alignment (e.g., Bardeen model), except for electrodes that extensively react with the polymer, and introduce deep gap states. In addition, we show results that validate the approximation of rigid tilting of polymer energy levels with bias (for biases for which no significant injection of carriers occurs). To investigate further the consequences of the electronic line-up on device operation, we complemented the electroabsorption measurements with characterization of the emissive and transport properties of the light-emitting diodes, and confirmed that the cathodic barrier lowering in CsF/Ca/Al and LiF/Ca/Al electrodes leads to the best improvements in electron injection. We found that luminance and overall current are greatly affected by the barrier-reducing cathodes, indicating a truly bipolar transport, with comparable electron and hole currents. We also found significant indications of CsF/Ca/Al cathodes strongly reacting with the polymer, which is suggestive of CsF dissociation and diffusion in the bulk of the polymer.

Energy level line-up in polymer light-emitting diodes via electroabsorption spectroscopy

The influence of carrier-injecting layers on the built-in potential of polymer light-emitting diodes (PLEDs) is analysed by means of electroabsorption spectroscopy. Two promising materials on which research has recently focused are: first, poly(ethylene-dioxythiophene)/poly(styrene sulphonic acid) (PEDOT:PSS), a doped conducting polymer employed as hole injector; and secondly, lithium fluoride (LiF), which has been shown to enhance electron injection of Al-based cathodes. In the first samples LEDs with an indium tin oxide (ITO) anode were compared to those incorporating a PEDOT:PSS layer (in an ITO/PEDOT:PSS bilayer anode). In the second set of samples the comparison was between LEDs with an aluminium cathode and those incorporating a LiF layer (in an Al/LiF bilayer cathode) as a function of LiF thickness. Electroabsorption spectroscopy results supported by work function measurements clearly prove that both PEDOT:PSS and LiF introduce a large reduction in the barrier height at the anode and cathode interfaces, respectively. The ensuing enhancement in carrier injection is considered to be responsible for the marked improvement in device performance (lower operating voltages, higher luminous efficiencies and longer lifetimes).