Study of the effect of density of states distribution on carrier injection at organic/electrode interface through high-sensitivity photoemission spectroscopy and injection simulation

Shimizu, Kohei; Tokairin, Hiroshi; Nakazawa, Ryotaro; Nakamura, Ikuko; Yasuno, Shinji; Ikegami, Keitaro; Yamaguchi, Yuki; Tanaka, Yuya; Ishii, Hisao

doi:10.35848/1882-0786/ac8596

Cited by 3 publications

(4 citation statements)

References 35 publications

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“…Although many studies have shown that the use of acceptor molecules as the HIL can increase the WF of anode surfaces, it has been assumed that the EA of acceptor molecules on anodes is constant or approximately equal to E F of the anode. [ 17,32,38–40 ] The results of our experiment suggest that the acceptor molecules near the anode are anionic species and have a low EA. Moreover, an energy difference can exist between the LUMO of the acceptor molecule and E F of the anode.…”

Section: Resultsmentioning

confidence: 81%

“…When using organic acceptor molecules as the HIL, the HIB has generally been defined as the energy difference between E F of the anode and the HOMO of the organic semiconductor (HOMO energy with respect to E F , shown in Figure 2a), assuming hole injection from E F of the anode. [16][17][18][31][32][33] The WF of the ITO film with the HIL (WF ITO/HIL , shown in Figure 2a) has also been an important parameter in the analysis of the hole-injection property, since the HOMO energy with respect to E F decreases with increasing WF ITO/HIL . On the other hand, electron transfer from the HOMO of organic semiconductors to the LUMO of acceptors has also been proposed as a hole-injection mechanism.…”

Section: Resultsmentioning

confidence: 99%

“…When voltage is applied to an OLED fabricated using these stacked HILs, electrons in the HTLs move to F6TCNNQ and holes are generated efficiently. [ 32 ] Stacked HILs with high operational stability can be prepared by using the neutral F6TCNNQ as part of HILs. Since the thickness of the interlayer used in this study is only 5 nm, it is reasonable to assume that electrons are transferred from F6TCNNQ to the anode by tunnelling.…”

Section: Resultsmentioning

confidence: 99%

“…The hole-injection barrier (HIB) from the anode to an organic semiconductor has been defined as the energy difference between the Fermi level (E F ) of the anode and the highest occupied molecular orbital (HOMO) of the organic semiconductor. [16][17][18][31][32][33] This is because the hole injection from the E F of the anode into the organic semiconductor has been assumed, and the precise measurement of the EA of organic molecules has also been difficult until recently. [34,35] It is reasonable to assume that the EA of acceptor molecules, which changes in accordance with the charged states of acceptor molecules, affects hole injection as well as p-doping/charge generation.…”

Section: Doi: 101002/adma202210413mentioning

confidence: 99%

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Unlocking the Full Potential of Electron‐Acceptor Molecules for Efficient and Stable Hole Injection

et al. 2023

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Understanding the hole‐injection mechanism and improving the hole‐injection property are of pivotal importance in the future development of organic optoelectronic devices. Electron‐acceptor molecules with high electron affinity (EA) are widely used in electronic applications, such as hole injection and p‐doping. Although p‐doping has generally been studied in terms of matching the ionization energy (IE) of organic semiconductors with the EA of acceptor molecules, little is known about the effect of the EA of acceptor molecules on the hole‐injection property. In this work, the hole‐injection mechanism in devices is completely clarified, and a strategy to optimize the hole‐injection property of the acceptor molecule is developed. Efficient and stable hole injection is found to be possible even into materials with IEs as high as 5.8 eV by controlling the charged state of an acceptor molecule with an EA of about 5.0 eV. This excellent hole‐injection property enables direct hole injection into an emitting layer, realizing a pure blue organic light‐emitting diode with an extraordinarily low turn‐on voltage of 2.67 V, a power efficiency of 29 lm W−1, an external quantum efficiency of 28% and a Commission Internationale de l'Eclairage y coordinate of less than 0.10.

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Section: Resultsmentioning

confidence: 81%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Doi: 101002/adma202210413mentioning

confidence: 99%

See 2 more Smart Citations

Unlocking the Full Potential of Electron‐Acceptor Molecules for Efficient and Stable Hole Injection

et al. 2023

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Protein Electronic Energy Transport Levels Derived from High‐Sensitivity Near‐UV and Constant Final State Yield Photoemission Spectroscopy

Fereiro,

Tomita,

Bendikov

et al. 2024

Small Methods

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Proteins are attractive as functional components in molecular junctions. However, controlling the electronic charge transport via proteins, held between two electrodes, requires information on their frontier orbital energy level alignment relative to the electrodes’ Fermi level (EF), which normally requires studies of UV Photoemission Spectroscopy (UPS) with HeI excitation. Such excitation is problematic for proteins, which can denature under standard measuring conditions. Here high‐sensitivity soft UV photoemission spectroscopy (HS‐UPS) combined with Constant Final State Yield Spectroscopy (CFS‐YS) is used to get this information for electrode/protein contacts. Monolayers of the redox protein Azurin, (Az) and its Apo‐form on Au substrates, have HOMO onset energies, obtained from CFS‐YS, differ by ≈0.2 eV, showing the crucial role of the Cu redox centre in the electron transport process. It is found that combined HS‐UPS/CFS‐YS measurements agree with the Photoelectron Yield Spectroscopy (PYS), showing potential of the HS‐UPS + CFS‐YS as a powerful tool to characterize and map the energetics of a protein‐electrode interfaces, which will aid optimizing design of devices with targeted electronic properties, as well as for novel applications.

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Reliable measurement of the density of states including occupied in-gap states of an amorphous In–Ga–Zn–O thin film via photoemission spectroscopies: Direct observation of light-induced in-gap states

Nakazawa,

Matsuzaki,

Shimizu

et al. 2024

Journal of Applied Physics

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Illumination stress (IS) and negative bias under illumination stress (NBIS) cause considerable device instability in thin-film transistors based on amorphous In–Ga–Zn–O (a-IGZO). Models using in-gap states are suggested to explain device instability. Therefore, to provide reliably their density of states (DOS), this study investigated the valence band, conduction band, and in-gap states of an a-IGZO thin film. The DOS of in-gap states was directly determined in a dynamic range of six orders of magnitude through constant final state yield spectroscopy (CFS-YS) using low-energy and low-flux photons. Furthermore, light irradiation irreversibly induced extra in-gap states near the Fermi level and shifted the Fermi level to the vacuum level side, which should be related to the device instability due to IS and NBIS. Hard x-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy using synchrotron radiation observed the large DOS of in-gap states near the Fermi level as in previous works. Here, we reveal that they are not intrinsic electronic states of undamaged a-IGZO, but induced by the intense measurement light of synchrotron radiation. This study demonstrates that CFS-YS is useful for determining the reliable DOS of the in-gap states for samples that are sensitive to light irradiation. The absorption spectrum measured through photothermal deflection spectroscopy is interpreted based on DOS directly determined via photoemission spectroscopies. This indicates that the line shape in the energy region below the region assigned to the Urbach tail in previous works actually roughly reflects the DOS of occupied in-gap states.

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Study of the effect of density of states distribution on carrier injection at organic/electrode interface through high-sensitivity photoemission spectroscopy and injection simulation

Cited by 3 publications

References 35 publications

Unlocking the Full Potential of Electron‐Acceptor Molecules for Efficient and Stable Hole Injection

Unlocking the Full Potential of Electron‐Acceptor Molecules for Efficient and Stable Hole Injection

Protein Electronic Energy Transport Levels Derived from High‐Sensitivity Near‐UV and Constant Final State Yield Photoemission Spectroscopy

Reliable measurement of the density of states including occupied in-gap states of an amorphous In–Ga–Zn–O thin film via photoemission spectroscopies: Direct observation of light-induced in-gap states

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