Defect-driven interfacial electronic structures at an organic/metal-oxide semiconductor heterojunction

Titanium Vacancies in TiO2 Nanofibers Enable Highly Efficient Photo-Driven Seawater Splitting

Photo-driven seawater splitting is considered as one of the most promising techniques for sustainable hydrogen production. However, the high salinity of seawater would deactivate catalysts and consumes the photogenerated carriers. Metal vacancies in metal oxide semiconductors are critical to directed electron transfer and high salinity resistance, thus desirable but remains a challenge. We demonstrate a facile controllable calcination approach to synthesize TiO 2 nanofibers with rich Ti-vacancies with excellent photo/electro performances and long-time stability in photo-driven seawater splitting, including photocatalysis and photoelectrocatalysis. Experimental measurements and theoretical calculations reveal the formation of titanium vacancies, as well as its unidirectional electron trap and superior H + adsorption ability for efficient charge transfer and corrosion resistance of seawater. Therefore, the characteristics and mechanism have been proposed at an atomic-/nanoscale to clarify the generation of titanium vacancies and the corresponding interfacial electron transfer.

Interfacial co-existence of oxygen and titanium vacancies in nanostructured TiO2 for enhancement of carrier transport

The interfacial co-existence of oxygen and metal vacancies in metal oxide semiconductors and their highly efficient carrier transport have rarely been reported. This work reports on the co-existence of oxygen and titanium vacancies at the interface between TiO₂ and rGO via a simple two-step calcination treatment. Experimental measurements show that the oxygen and titanium vacancies are formed under 550 °C/Ar and 350 °C/air calcination conditions, respectively. These oxygen and titanium vacancies significantly enhance the transport of interfacial carriers, and thus greatly improve the photocurrent performances, the apparent quantum yield, and photocatalysis such as photocatalytic H₂ production from water-splitting, photocatalytic CO₂ reduction and photo-electrochemical anticorrosion of metals. A new "interfacial co-existence of oxygen and titanium vacancies" phenomenon, and its characteristics and mechanism are proposed at the atomic-/nanoscale to clarify the generation of oxygen and titanium vacancies as well as the interfacial carrier transport.

Surface Modification of Al-Doped ZnO Transparent Conducive Thin Films with Polycrystalline Zinc Molybdenum Oxide

High-work function (WF) transparent conductive thin films improve the performance of solar cells and organic light-emitting diodes by facilitating interfacial charge carrier transport. Al-doped ZnO (AZO) becomes a very promising transparent conductive material because of nontoxicity, abundant material resources, and low cost. To increase the WF of AZO without enhancing the series resistance of the device, a high-WF and low-resistance surface modifier of polycrystalline zinc molybdenum oxide (ZMO) was developed by utilizing thermal evaporation of MoO₃ on the surface of AZO and a subsequent two-step annealing treatment. The first step of air annealing causes the formation of monoclinic ZnMoO₄ nanocrystals in the ZMO modifier. This improves the WF of AZO from 3.83 to 4.86 eV by increasing the group electronegativity and cation oxidation state. Furthermore, the second step of N₂ annealing decreases the resistivity of the polycrystalline ZMO by increasing the donor states of oxygen vacancies. The surface modification effect is verified by applying the ZMO-modified AZO to the front electrode of hydrogenated amorphous silicon thin-film solar cells. The low-resistance polycrystalline ZMO modifier not only increases light harvesting in the solar cells by improving interfacial refractive index matching but also improves the open-circuit voltage by modifying the interfacial band alignment. In particular, the modifier increases the fill factor by ca. 13% by reducing the series resistance of the device. These enable a gain of ca. 23% in photoelectric conversion efficiency compared to the unmodified AZO. The results suggest the feasibility to tune the WF and conductivity of a material independently.

Uncovering the (un-)occupied electronic structure of a buried hybrid interface

The energy level alignment at organic/inorganic (o/i) semiconductor interfaces is crucial for any light-emitting or -harvesting functionality. Essential is the access to both occupied and unoccupied electronic states directly at the interface, which is often deeply buried underneath thick organic films and challenging to characterize. We use several complementary experimental techniques to determine the electronic structure of p -quinquephenyl pyridine (5P-Py) adsorbed on ZnO(1 0   -1 0). The parent anchoring group, pyridine, significantly lowers the work function by up to 2.9 eV and causes an occupied in-gap state (IGS) directly below the Fermi level E _F. Adsorption of upright-standing 5P-Py also leads to a strong work function reduction of up to 2.1 eV and to a similar IGS. The latter is then used as an initial state for the transient population of three normally unoccupied molecular levels through optical excitation and, due to its localization right at the o/i interface, provides interfacial sensitivity, even for thick 5P-Py films. We observe two final states above the vacuum level and one bound state at around 2 eV above E _F, which we attribute to the 5P-Py LUMO. By the separate study of anchoring group and organic dye combined with the exploitation of the occupied IGS for selective interfacial photoexcitation, this work provides a new pathway for characterizing the electronic structure at buried o/i interfaces.

Molecular Alignment and Electronic Structure of N,N'-Dibutyl-3,4,9,10-perylene-tetracarboxylic-diimide Molecules on MoS2 Surfaces

The molecular orientation of organic semiconductors on a solid surface could be an indispensable factor to determine the electrical performance of organic-based devices. Despite its fundamental prominence, a clear description of the emergent two-dimensional layered material-organic interface is not fully understood yet. In this study, we reveal the molecular alignment and electronic structure of thermally deposited N,N'-dibutyl-3,4,9,10-perylene-dicarboximide (PTCDI-C4) molecules on natural molybdenum disulfide (MoS₂) using near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The average tilt angle determination reveals that the anisotropy in the π* symmetry transition of the carbon K-edge (284-288 eV range) is present at the sub-monolayer regime. Supported by ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and resonant photoemission spectroscopy (RPES) measurements, we find that our spectroscopic measurements indicate a weak charge transfer established at the PTCDI-C4/MoS₂ interface. Sterical hindrance due to the C4 alkyl chain caused tilting of the molecular plane at the initial thin film deposition. Our result shows a tunable interfacial alignment of organic molecules on transition metal dichalcogenide surfaces effectively enhancing the electronic properties of hybrid organic-inorganic heterostructure devices.

Tuning the Seebeck effect in C60-based hybrid thermoelectric devices through temperature-dependent surface polarization and thermally-modulated interface dipoles

Tuning the Seebeck effect through polarization and interface dipoles.

Unveiling universal trends for the energy level alignment in organic/oxide interfaces

Organic/oxide interfaces exhibit an energy-level-alignment universal behaviour when a bias is applied. Coulomb-blockade regime is ruled by the organic electronegativity.

Interface Engineering of Metal Oxides using Ammonium Anthracene in Inverted Organic Solar Cells

In this work, by casting water-soluble ammonium anthracene on metal oxides, the organic surface modifier re-engineered the interface of the metal oxide to improve charge transport. The energy level of ammonium anthracene increased the work function of indium tin oxide (ITO), functioning as a hole-blocker (electron-transporter). Solar cells in which ITO was treated by the ammonium anthracene produced an average power conversion efficiency (PCE) of 5.8% without ZnO, the electron-transporting layer. When the ammonium anthracene was applied to ZnO, an average PCE of 8.1% was achieved, which is higher than the average PCE of 7.5% for nontreated ZnO-based devices.

Surface Structure Modification of ZnO and the Impact on Electronic Properties

Zinc oxide (ZnO) is a widely utilized, versatile material implemented in a diverse range of technological applications, particularly in optoelectronic devices, where its inherent transparency, tunable electronic properties, and accessible nanostructures can be combined to confer superior device properties. ZnO is a complex material with a rich and intricate defect chemistry, and its properties can be extremely sensitive to processing methods and conditions; consequently, surface modification of ZnO using both inorganic and organic species has been explored to control and regulate its surface properties, particularly at heterointerfaces in electronic devices. Here, the properties of ZnO are described in detail, particularly its surface chemistry, along with the role of defects in governing its electronic properties, and methods employed to modulate the behavior of as-grown ZnO. An outline is also given on how the native and modified oxide interact with molecular materials. To illustrate the diverse range of surface modification methods and their subsequent influence on electronic properties, a comprehensive review of the modification of ZnO surfaces at molecular interfaces in hybrid photovoltaic (hPV) and organic photovoltaic (OPV) devices is presented. This is a case study rather than a progress report, aiming to highlight the progress made toward controlling and altering the surface properties of ZnO, and to bring attention to the ways in which this may be achieved by using various interfacial modifiers (IMs).

Hybridization-Induced Carrier Localization at the C60 /ZnO Interface

Electronic coupling and ground-state charge transfer at the C60 /ZnO hybrid interface is shown to localize carriers in the C60 phase. This effect, revealed by resonant X-ray photoemission, arises from interfacial hybridization between C60 and ZnO. Such localization at carrier-selective electrodes and interlayers may lead to severely reduced carrier harvesting efficiencies and increased recombination rates in organic electronic devices.

Comparison of the Impact of Zinc Vacancies on Charge Separation and Charge Transfer at ZnO/Sexithienyl and ZnO/Fullerene Interfaces

The impact of surface zinc vacancies on charge transfer and charge separation at donor/ZnO and acceptor/ZnO interfaces is identified via density functional theory calculations. The results show their effect to be related to the stronger internal electric field present near these vacancies. Thus, such surface defects can have a significant negative impact on the performance of hybrid solar cells using ZnO as electron acceptors.

Spectroscopy and control of near-surface defects in conductive thin film ZnO

The electronic structure of inorganic semiconductor interfaces functionalized with extended π-conjugated organic molecules can be strongly influenced by localized gap states or point defects, often present at low concentrations and hard to identify spectroscopically. At the same time, in transparent conductive oxides such as ZnO, the presence of these gap states conveys the desirable high conductivity necessary for function as electron-selective interlayer or electron collection electrode in organic optoelectronic devices. Here, we report on the direct spectroscopic detection of a donor state within the band gap of highly conductive zinc oxide by two-photon photoemission spectroscopy. We show that adsorption of the prototypical organic acceptor C60 quenches this state by ground-state charge transfer, with immediate consequences on the interfacial energy level alignment. Comparison with computational results suggests the identity of the gap state as a near-surface-confined oxygen vacancy.

Densely-packed ZnTPPs Monolayer on the Rutile TiO2(110)-(1×1) Surface: Adsorption Behavior and Energy Level Alignment

The adsorption of a densely packed Zinc(II) tetraphenylporphyrin monolayer on a rutile TiO₂(110)-(1×1) surface has been studied using a combination of experimental and theoretical methods, aimed at analyzing the relation between adsorption behavior and barrier height formation. The adsorption configuration of ZnTPP was determined from scanning tunnel microscopy (STM) imaging, density functional theory (DFT) calculations and STM image simulation. The corresponding energy alignment was experimentally determined from X-ray and UV-photoemission spectroscopies and inverse photoemission spectroscopy. These results were found in good agreement with an appropriately corrected DFT model, pointing to the importance of local bonding and intermolecular interactions in the establishment of barrier heights.

A Facile Self-assembly Synthesis of Hexagonal ZnO Nanosheet Films and Their Photoelectrochemical Properties

Here, large-scale and uniform hexagonal zinc oxide (ZnO) nanosheet films were deposited onto indium tin oxide (ITO)-coated transparent conducting glass substrates via a facile galvanic displacement deposition process. Compared with other commonly used solution methods, this process avoids high temperature and electric power as well as supporting agents to make it simple and cost-effective. The as-fabricated ZnO nanosheet films have uniform hexagonal wurtzite structure. The photoelectrochemical (PEC) cell based on ZnO nanosheet film/ITO photoelectrode was also fabricated and its performance was improved by optimizing the solution concentration. A higher photocurrent density of ~500 μA cm^-2 under AM 1.5 G simulated illumination of 100 mW cm^-2 with zero bias potential (vs. Ag/AgCl electrode) was obtained, which may ascribe to the increased surface-to-volume ratio of disordered ZnO nanosheet arrays. Our developed method may be used to deposit other oxide semiconductors, and the ZnO nanosheet film/ITO PEC cell can be used to design low-cost optoelectronic and photoelectrochemical devices.

Chemical Interaction, Space-charge Layer and Molecule Charging Energy for a TiO2/TCNQ Interface

Three driving forces control the energy level alignment between transition-metal oxides and organic materials: the chemical interaction between the two materials, the organic electronegativity and the possible space charge layer formed in the oxide. This is illustrated in this study by analyzing experimentally and theoretically a paradigmatic case, the TiO₂(110) / TCNQ interface: due to the chemical interaction between the two materials, the organic electron affinity level is located below the Fermi energy of the n-doped TiO₂. Then, one electron is transferred from the oxide to this level and a space charge layer is developed in the oxide inducing an important increase in the interface dipole and in the oxide work-function.

Disrupted Attosecond Charge Carrier Delocalization at a Hybrid Organic/Inorganic Semiconductor Interface

Despite significant interest in hybrid organic/inorganic semiconductor interfaces, little is known regarding the fate of charge carriers at metal oxide interfaces, particularly on ultrafast time scales. Using core-hole clock spectroscopy, we investigate the ultrafast charge carrier dynamics of conductive ZnO films at a hybrid interface with an organic semiconductor. The adsorption of C60 on the ZnO surface strongly suppresses the ultrafast carrier delocalization and increases the charge carrier residence time from 400 attoseconds to nearly 30 fs. Here, we show that a new hybridized interfacial density of states with substantial molecular character is formed, fundamentally altering the observed carrier dynamics. The remarkable change in the dynamics sheds light on the fate of carriers at hybrid organic/inorganic semiconductor interfaces relevant to organic optoelectronics and provides for the first time an atomistic picture of the electronically perturbed near-interface region of a metal oxide.

Electronic properties and Schottky barriers at ZnO-metal interfaces from first principles

First principles calculations were performed to study the interface electronic structure and the Schottky barrier heights (SBHs) of ZnO-metal interfaces. Different kinds of metals were considered with different chemistries on the polar (0 0 0 1) and (0 0 0 1¯) ZnO surfaces. The projection of the density of states on the atomic orbitals of the interface atoms reveals that two kinds of interface electronic states appear: states due to the chemical bonding which appear at well defined energies and conventional metal-induced gap states associated with a smooth density of states in the bulk ZnO band gap region. The relative weight and distribution of the two classes of states depend on both the ZnO substrate termination and on the metal species. SBHs are found to be very sensitive to the specific interface chemical bonding. In particular, it is possible to note the occurrence of either Schottky barriers or Ohmic contacts. Our results have been compared with experiments and with available phenomenological theories, which estimate the SBH from few characteristic material parameters. Finally, the electronic and structural contributions to the SBH have been singled out and related to the different charge transfers occurring at the different interfaces.

Band offset at TiO2/MDMO PPV and TiO2/PEDOT PSS interfaces studied using photoelectron spectroscopy

We report band alignment and band offset studies across the interfaces of hetero-structures of TiO₂ with MDMO PPV and PEDOT PSS using photoelectron spectroscopy.

Work function shifts of a zinc oxide surface upon deposition of self-assembled monolayers: a theoretical insight

We have investigated at the DFT level the way the work function of ZnO is affected upon deposition of self-assembled monolayers made of 4-tert-butylpyridine and various benzoic acids.