This article reviews experimental studies on ‘bridging electronic structure and charge transport property of organic semiconductors’ performed using ultraviolet photoelectron spectroscopy (UPS) and related methods mainly in Chiba University, Japan, in particular on the investigation of the origin and the role of electronic states existing in the highest occupied molecular orbital band–lowest unoccupied molecular orbital band (HOMO–LUMO) gap. We summarize experimental observations including direct measurements of ‘invisible’ gap states with ultrahigh sensitivity UPS, which demonstrate that there exist intrinsic gap states in organic semiconductors. We firstly describe the nature of organic molecular solids to understand features of organic semiconductors because such intrinsic gap states are a result of the interplay of these features, which give the principal difference between the organic semiconductor and inorganic counterpart. We then discuss (i) the origin and role of the band gap states in relation to intermolecular interaction/band dispersion and electron–phonon coupling, (ii) the Fermi level pinning issue in organic semiconductors, and (iii) the method of computing the Fermi level position within the HOMO–LUMO gap for experimental groups. The gap states of organic semiconductors appear easily when a weak perturbation is applied to the organic system, namely by contact with other material, by injecting a charge, by elevating temperature, and by exposure to 1 atm gas. What we finally found is that tailing states of HOMO and LUMO always exist, and their energy distributions must not be symmetric; they thus produce a larger Fermi level shift from the mid gap position than previously thought. Furthermore, as shown by computational work, Fermi level pinning, which is a well-known phenomena in semiconductor devices field, occurs in weakly interacting organic/conductor systems without any gap states if the system temperature is not zero (T  >  0). We also describe the experimental knowhow of the ultrahigh-sensitivity UPS of organic systems, which are very fragile upon ultraviolet-light irradiation, and some updates on our previously published results.

Origin and role of gap states in organic semiconductor studied by UPS: as the nature of organic molecular crystals

Substantial variations in the electronic structure and thus possibly conflicting energetics at interfaces between hybrid perovskites and charge transport layers in solar cells have been reported by the research community. In an attempt to unravel the origin of these variations and enable reliable device design, we demonstrate that donor-like surface states stemming from reduced lead (Pb0) directly impact the energy level alignment at perovskite (CH3NH3PbI3–xClx) and molecular electron acceptor layer interfaces using photoelectron spectroscopy. When forming the interfaces, it is found that electron transfer from surface states to acceptor molecules occurs, leading to a strong decrease in the density of ionized surface states. As a consequence, for perovskite samples with low surface state density, the initial band bending at the pristine perovskite surface can be flattened upon interface formation. In contrast, for perovskites with a high surface state density, the Fermi level is strongly pinned at the con...

Surface State Density Determines the Energy Level Alignment at Hybrid Perovskite/Electron Acceptors Interfaces.

A simplistic design of a self-powered UV-photodetector device based on hybrid reduced-graphene-oxide (r-GO)/gallium nitride (GaN) is demonstrated. Under zero bias, the fabricated hybrid photodetector shows a photosensivity of ∼85% while the ohmic contact GaN photodetector with an identical device structure exhibits only ∼5.3% photosensivity at 350 nm illumination (18 μW/cm2). The responsivity and detectivity of the hybrid device were found to be 1.54 mA/W and 1.45 × 1010 Jones (cm Hz½ W−1), respectively, at zero bias with fast response (60 ms), recovery time (267 ms), and excellent repeatability. Power density-dependent responsivity and detectivity revealed ultrasensitive behaviour under low light conditions. The source of the observed self-powered effect in the hybrid photodetector is attributed to the depletion region formed at the r-GO and GaN quasi-ohmic interface.

Ultrasensitive self-powered large area planar GaN UV-photodetector using reduced graphene oxide electrodes

Intermolecular single-electron transfer on electrically insulating films is a key process in molecular electronics1–4 and an important example of a redox reaction5,6. Electron-transfer rates in molecular systems depend on a few fundamental parameters, such as interadsorbate distance, temperature and, in particular, the Marcus reorganization energy
 7
 . This crucial parameter is the energy gain that results from the distortion of the equilibrium nuclear geometry in the molecule and its environment on charging8,9. The substrate, especially ionic films
 10
 , can have an important influence on the reorganization energy11,12. Reorganization energies are measured in electrochemistry
 13
 as well as with optical14,15 and photoemission spectroscopies16,17, but not at the single-molecule limit and nor on insulating surfaces. Atomic force microscopy (AFM), with single-charge sensitivity18–22, atomic-scale spatial resolution
 20
 and operable on insulating films, overcomes these challenges. Here, we investigate redox reactions of single naphthalocyanine (NPc) molecules on multilayered NaCl films. Employing the atomic force microscope as an ultralow current meter allows us to measure the differential conductance related to transitions between two charge states in both directions. Thereby, the reorganization energy of NPc on NaCl is determined as (0.8 ± 0.2) eV, and density functional theory (DFT) calculations provide the atomistic picture of the nuclear relaxations on charging. Our approach presents a route to perform tunnelling spectroscopy of single adsorbates on insulating substrates and provides insight into single-electron intermolecular transport. An atomic force microscope is used as an ultralow current sensor to measure the reorganization energy of a single naphthalocyanine molecule during a redox reaction on a NaCl film.

Reorganization energy upon charging a single molecule on an insulator measured by atomic force microscopy.

We studied the influence of oxygen on the electronic trap states in a pentacene thin film. This was done by carrying out gated four-terminal measurements on thin-film transistors as a function of temperature and without ever exposing the samples to ambient air. Photo-oxidation of pentacene is shown to lead to a peak of trap states centered at 0.28 eV from the mobility edge, with trap densities of the order of ${10}^{18}\text{ }{\text{cm}}^{\ensuremath{-}3}$. As the gate voltage is ramped up, these trap states are occupied at first and cause a reduction in the number of free carriers at a given gate voltage. Moreover, the exposure to oxygen reduces the mobility of the charge carriers above the mobility edge. We correlate the change of these transport parameters with the change of the essential device parameters, i.e., subthreshold performance and effective field-effect mobility. This study supports the assumption of a mobility edge for charge transport and contributes to a detailed understanding of an important degradation mechanism of organic field-effect transistors. Deep traps in an organic field-effect transistor reduce the effective field-effect mobility by reducing the number of free carriers and their mobility above the mobility edge.

/pdf/oxygen-related-traps-in-pentacene-thin-films-energetic-2ttzt99s74.pdf

Oxygen-related traps in pentacene thin films: Energetic position and implications for transistor performance

Photoelectron spectroscopy on the charge reorganization energy and small polaron binding energy of molecular film

Tetraphenyldibenzoperiflanthene (DBP) is a promising candidate as a component of highly efficient organic photovoltaic cells and organic light-emitting diodes. The structural properties of thin films of this particular lander-type molecule on Ag(111) were investigated by complementary techniques. Highly ordered structures were obtained, and their mutual alignment was characterized by means of low-energy electron diffraction (LEED). Scanning tunneling microscopy (STM) images reveal two slightly different arrangements within the first monolayer (ML), both describable as specific herringbone patterns with two molecules per unit cell whose dibenzoperiflanthene framework is parallel to the surface. In contrast, single DBP molecules in the second ML were imaged with much higher intramolecular resolution, resembling the shape of the frontier orbitals in the gas phase as calculated by means of density functional theory (DFT). Further deposition leads to the growth of highly ordered bilayer islands on top of the f...

Self-Assembly of Tetraphenyldibenzoperiflanthene (DBP) Films on Ag(111) in the Monolayer Regime

We report on the results of experimental and theoretical studies on the electronic structure of gas-phase diindenoperylene (DIP) and DIP-monolayer (ML) on Cu(111). Vapor-phase ultraviolet photoelectron spectroscopy (UPS) was realized for 11.3 mg of DIP, giving reference orbital energies of isolated DIP, and UPS and inverse photoemission spectroscopy of DIP-ML/graphite were performed to obtain DIP-ML electronic states at a weak interfacial interaction. Furthermore, first-principles calculation clearly demonstrates the interfacial rearrangement. These results provide evidence that the rearrangement of orbital energies, which is realized in HOMO–LUMO and HOMO–HOMO−1 gaps, brings partially occupied LUMO through the surface-induced aromatic stabilization of DIP, a pure hydrocarbon molecule, on Cu(111).

https://iopscience.iop.org/article/10.7567/APEX.9.045201/pdf

Charge transfer states appear in the π-conjugated pure hydrocarbon molecule on Cu(111)

Erratum to ‘‘First-principles measurements of charge mobility in organic semiconductors: Valence hole–vibration coupling in organic ultrathin films” [Prog. Surf. Sci. 84 (2009) 135–154]

Erratum to ‘‘First-principles measurements of charge mobility in organic semiconductors: Valence hole-vibration coupling in organic ultrathin films” [Prog. Surf. Sci. 84 (2009) 135-154]

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