Fundamental Electronic Structure of Organic Solids and Their Interfaces by Photoemission Spectroscopy and Related Methods

Ueno, Nobuo; Kera, Satoshi; Kanai, Kaname

doi:10.1002/9783527653171.ch7

Cited by 3 publications

(5 citation statements)

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“…alkali metal doping, and formation of metal-molecule and molecule-molecule interfaces with stronger chemical interactions) produces new electronic states [72][73][74][75][76][77][78][79][80][81], electronic states for superconducting phase [7,82], and in some cases enhance a wave function localization to result in a Mott-Hubbard insulator even at room temperature, giving a half-filled energy level in the gap (I, III, V) [83][84][85] (see sections 5.4.3.5). • A tiny amount of excess charges injected into the packing structure can produce larger changes in the electronic structure than supposed (formation of a very small polaron, small polaron, and large polaron, and related geometrical structure change in the molecule and molecular crystal as well) (I, III, IV) [33,43,[86][87][88] (see section 5.2.1).…”

Section: Summary Of Important Features Of Molecular Solids: the Natur...mentioning

confidence: 99%

“…• There are various phonons with largely deferent energies (in the order of 10 −1 -10 −3 eV), which originate from intramolecular vibrations and intermolecular/crystal vibrations. These phonons result in a variety of electronphonon coupling, which give various quasiparticles and hierarchical polarons with different polaron binding energies [10,11,88] that impact not only on charge transport dynamics but also produce gap states (I, III, IV) [86,87,93,94] (see section 5.2.1). • Both thin film and bulk organic semiconductor materials have highly anisotropic electronic properties, such as molecular-orientation-dependent electronic states, anisotropic charge transport, and anisotropic optical properties (I, III, IV) [54,[95][96][97][98][99] (see section 5.1.1).…”

Section: Summary Of Important Features Of Molecular Solids: the Natur...mentioning

confidence: 99%

“…high-sensitivity UPS. Here we introduce ultrahigh-sensitivity UPS results that involve large positive polarons (fully relaxed positive polaron involving all polarizations due to movements of electrons, atoms in each molecules, and molecules themselves [68,86,87]), which exist in an organic film when holes are injected into the film, as is realized by preparing the film on a high-work function substrate. Such a system was intentionally realized for the pentacene/ClAlPc/HOPG [141], as shown in figure 12 (introduced in section 5.1.2 (see figure 11)), where after deposition of the ML pentacene on the ClAlPc/HOPG substrate, electrons are transferred from the pentacene layer to the HOPG to achieve thermodynamic equilibrium of the electron system.…”

Section: The First Detection Of Invisible Band Gap States With Ultra-mentioning

confidence: 99%

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

Yang

Bussolotti

Kera

et al. 2017

J. Phys. D: Appl. Phys.

112

142

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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 bandlowest 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 wellknown 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.

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Section: Summary Of Important Features Of Molecular Solids: the Natur...mentioning

confidence: 99%

Section: Summary Of Important Features Of Molecular Solids: the Natur...mentioning

confidence: 99%

Section: The First Detection Of Invisible Band Gap States With Ultra-mentioning

confidence: 99%

See 1 more Smart Citation

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

Yang

Bussolotti

Kera

et al. 2017

J. Phys. D: Appl. Phys.

112

142

View full text Add to dashboard Cite

show abstract

“…Different values for IE and EA can be obtained from cyclic voltammetry (CV) and from direct/inverse photoelectron spectroscopy (PES/IPES). The energy gaps determined by optical absorption, photocurrent measurements, CV, scanning tunnelling spectroscopy (STS) or PES/IPES differ from each other [40][41][42][43][44][45][46]. Two main reasons for these deviations should be considered here.…”

Section: Introductionmentioning

confidence: 99%

“…First of all, the measurements are done on samples prepared by the same techniques as the films used in devices. Additionally, the creation of a hole by PES and the insertion of an electron by IPES in thin films is the closest experimental access to the transport level because the final states after PES/ IPES excitation are similar to the transport states [41,45]. These transport levels for electrons and hole are defined by the molecular radical ion, where the charge carrier is located, and the interaction with the polarisable environment [2,47].…”

Section: Introductionmentioning

confidence: 99%

Energy level alignment at planar organic heterojunctions: influence of contact doping and molecular orientation

Opitz

2017

J. Phys.: Condens. Matter

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Planar organic heterojunctions are widely used in photovoltaic cells, light-emitting diodes, and bilayer field-effect transistors. The energy level alignment in the devices plays an important role in obtaining the aspired gap arrangement. Additionally, the π-orbital overlap between the involved molecules defines e.g. the charge-separation efficiency in solar cells due to charge-transfer effects. To account for both aspects, direct/inverse photoemission spectroscopy and near edge x-ray absorption fine structure spectroscopy were used to determine the energy level landscape and the molecular orientation at prototypical planar organic heterojunctions. The combined experimental approach results in a comprehensive model for the electronic and morphological characteristics of the interface between the two investigated molecular semiconductors. Following an introduction on heterojunctions used in devices and on energy levels of organic materials, the energy level alignment of planar organic heterojunctions will be discussed. The observed energy landscape is always determined by the individual arrangement between the energy levels of the molecules and the work function of the electrode. This might result in contact doping due to Fermi level pinning at the electrode for donor/acceptor heterojunctions, which also improves the solar cell efficiency. This pinning behaviour can be observed across an unpinned interlayer and results in charge accumulation at the donor/acceptor interface, depending on the transport levels of the respective organic semiconductors. Moreover, molecular orientation will affect the energy levels because of the anisotropy in ionisation energy and electron affinity and is influenced by the structural compatibility of the involved molecules at the heterojunction. High structural compatibility leads to π-orbital stacking between different molecules at a heterojunction, which is of additional interest for photovoltaic active interfaces and for ground-state charge-transfer.

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Ultraviolet Photoelectron Spectroscopy (UPS) III: Direct Study of “Invisible” Band Gap States by Ultrahigh-Sensitivity UPS

Ueno

Sueyoshi

Bussolotti

et al. 2014

Electronic Processes in Organic Electronics

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Fundamental Electronic Structure of Organic Solids and Their Interfaces by Photoemission Spectroscopy and Related Methods

Cited by 3 publications

References 125 publications

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

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

Energy level alignment at planar organic heterojunctions: influence of contact doping and molecular orientation

Ultraviolet Photoelectron Spectroscopy (UPS) III: Direct Study of “Invisible” Band Gap States by Ultrahigh-Sensitivity UPS

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