Interfacial charge transfer and charge generation in organic electronic devices
Graphical abstract
Research highlights
► Space-charge-limited currents of various organic layers are observed using thin MoO3. ► Electrical analysis enables estimation of hole mobilities of organic layers. ► Absorption spectroscopy reveals charge transfer occurring between organic and MoO3. ► Charge generation mechanism is proposed for enhanced device performance.
Introduction
Large charge injection barriers are frequently present at electrode/organic heterojunction interfaces in organic electronic devices, making precisely clarifying charge transport mechanisms of organic thin films difficult because observed current density–voltage (J–V) characteristics are controlled by both charge injection and transport processes [1], [2], [3]. Also, the charge injection barriers are problematic for overall performance of organic electronic devices [4], [5]. To solve the injection barrier problem, transition metal oxide, molybdenum trioxide (MoO3) has been used as a buffer layer between a metal electrode and an organic layer in organic electronic devices, such as organic light-emitting diodes (OLEDs) [6], [7], organic solar cells [8], [9], and organic thin-film transistors [10], [11]. Previously, we have demonstrated that use of a MoO3 buffer layer with a specific thickness of 0.75 nm between an anode layer of indium tin oxide (ITO) and a hole-transport layer (HTL) of N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) leads to an increase in current density by about four orders of magnitude and appearance of a space-charge-limited current (SCLC) of α-NPD [12], indicating that the injection barrier at the ITO/α-NPD interface is completely negligible. This specific MoO3 thickness of 0.75 nm is much smaller than those previously used for OLEDs. Power consumption and operational lifetimes of OLEDs are markedly improved by using the 0.75 nm MoO3 buffer layer as well [5]. Thus, the very thin MoO3 buffer layer is useful for establishing fundamental physics of charge transport in organic films and for developing the organic electronic devices.
Electronic states of multilayer films and composite films of organic and MoO3 have been intensively investigated using ultraviolet photoelectron spectroscopy, inverse photoelectron spectroscopy, and X-ray photoelectron spectroscopy [13], [14], [15], [16]. These leading papers have shown that: (1) charge transfer between electrode/MoO3/organic interfaces induces large vacuum level shifts, minimizing a hole injection barrier height, (2) gap states are generated inside band gaps of α-NPD and MoO3, and (3) a vacuum-deposited MoO3 layer works as an electron conductor due to donating electrons from gap states to a conduction band. We have recently proposed that the improved device characteristics originate from interfacial charge generation at a MoO3/HTL interface [17]. However, there is no clear experimental evidence enough to support the charge generation in our recent report. In this study, from comparison of J–V characteristics of organic hole-only devices and ultraviolet/visible/near-infrared (UV–vis–NIR) absorption spectra of composite films, we can conclude that the interfacial charge generation surly occurs to realize SCLCs of a wide variety of organic HTLs. Moreover, a drastic increase in current density of a bilayer device of n-type C60 and p-type α-NPD by using a MoO3 layer can provide the evidence of the charge generation.
The interfacial charge generation mechanism we have recently proposed [17] is composed of (i) electron transfer from a lower-ionization-energy HTL to a higher-work-function MoO3 layer to form electron–hole pairs at the MoO3/HTL interface, (ii) separation of the electron–hole pairs under an external electric field, (iii) transit of resultant electrons and holes to corresponding electrodes through a conduction band (or gap states) of MoO3 and a hole-transport level of α-NPD, and repetition of the charge transfer (i), the charge separation (ii), and the charge transit (iii) for steady-state current flow (Fig. 1). For a well-known standard hole injection mechanism without a MoO3 buffer layer, HTL-to-ITO electron extraction (transfer) under a local high electric field and separation of electron–hole pairs are repeated in a manner almost similar to the charge generation mechanism mentioned above [15]. The difference between the charge generation and the charge injection is whether the charge transfer occurs or not under no electric field condition. Using the charge generation mechanism, a well-known charge injection barrier is no longer considered important and J–V characteristics are controlled by a SCLC. Since the HTL-to-MoO3 electron transfer characteristics are expected to be related to a difference between an ionization energy of an HTL and a work function of a MoO3 layer, a relationship between J–V characteristics and electron transfer characteristics is investigated in this study when ionization energies of HTL materials are systematically changed (see Fig. 2).
Section snippets
Experimental
HTL materials used in this study are 4,4′,4′′-tris(N-3-methylphenyl-N-phenyl-amino)triphenylamine (m-MTDATA), fac-tris(2-phenylpyridinato)iridium(III) [Ir(ppy)3], 4,4′,4′′-tris(N-2-naphthyl-N-phenyl-amino)triphenylamine (2-TNATA), N,N′-di(m-tolyl)-N,N′-diphenyl benzidine (TPD), rubrene, α-NPD, 2,4,6-tricarbazolo-1,3,5-triazine (TRZ-2), and 4,4′-bis(carbazol-9-yl)-2,2′-biphenyl (CBP) (the chemical structures of the HTL molecules are shown in Fig. 2). Hole-only device structures and bilayer
Current density–voltage characteristics
The ionization energies of the HTL materials are measured as summarized in Table 1. The work functions of ITO, MoO3, and Al are also measured respectively to be 5.02 ± 0.02, 5.68 ± 0.03, and 3.58 ± 0.02 eV. The work function of MoO3 corresponds to a difference between a vacuum level and a Fermi level right below a conduction band edge [16]. The ionization energies and the work functions we measured here may be slightly different from actual values because they are measured in air [18]. In fact, the
Conclusions
J–V characteristics of organic hole-only devices having a MoO3 buffer layer and UV–vis–NIR absorption spectra of composite films of organic and MoO3 are investigated when HTL materials with different ionization energies are systematically changed. We find that SCLCs of a wide variety of HTLs are observed and their hole mobilities can be estimated using a SCLC analysis when ionization energies of HTLs are lower than a work function of MoO3. From comparison of the J–V characteristics and the
Acknowledgements
This work is supported by Grants-in-Aid for Scientific Research (Nos. 21760005, 20241034, and 20108012). Part of this work is based on “Development of the Next Generation Large-scale Organic Electroluminescence Display Basic Technology (Green IT Project)” with New Energy and Industrial Technology Development Organization (NEDO).
References (39)
- et al.
Org. Electron.
(2009) - et al.
Org. Electron.
(2010) - et al.
Synth. Met.
(2000) - et al.
Chem. Phys.
(2004) - et al.
Solid State Commun.
(2008) - et al.
Org. Electron.
(2010) - et al.
J. Appl. Phys.
(1998) - et al.
J. Appl. Phys.
(1998) - et al.
Appl. Phys. Lett.
(2007) J. Appl. Phys.
(1994)
J. Appl. Phys.
J. Phys. D: Appl. Phys.
Appl. Phys. Lett.
Appl. Phys. Lett.
Appl. Phys. Lett.
Adv. Mater.
Appl. Phys. Lett.
Appl. Phys. Lett.
Appl. Phys. Lett.
Cited by (76)
Structural dynamics upon photoinduced charge transfer in N,N,N′,N′-tetramethylmethylenediamine
2023, Spectrochimica Acta - Part A: Molecular and Biomolecular SpectroscopyCharge-transfer complexes and their applications in optoelectronic devices
2021, Materials Today EnergyPhotocatalytic properties of SnO<inf>2</inf>/MoO<inf>3</inf> mixed oxides and their relation to the electronic properties and surface acidity
2021, Journal of Photochemistry and Photobiology A: ChemistryInverted organic light-emitting devices using a charge-generation unit as an electron injector
2020, Organic ElectronicsCitation Excerpt :Compared to the absorption spectrum of NPB, the additional absorption bands centered at around 485 nm and 1320 nm can be clearly observed from the NPB/MoO3 sample, indicating that the charge-transfer complex has been formed at the NPB/MoO3 interface. According to the early report [49–51], the additional bands can be attributed to the absorption of NPB cations caused by the ground state charge transfer from NPB to MoO3. The results indicate that the charge carriers generate indeed at the NPB/MoO3 interface through the electron transfer from the HOMO level of NPB to the CB level of MoO3.