Elsevier

Organic Electronics

Volume 12, Issue 3, March 2011, Pages 520-528
Organic Electronics

Interfacial charge transfer and charge generation in organic electronic devices

https://doi.org/10.1016/j.orgel.2011.01.001Get rights and content

Abstract

We have recently proposed that improvement of device performance using a buffer layer of molybdenum trioxide (MoO3) originates from interfacial charge generation at an interface of MoO3 and an organic hole-transport layer [17]. However, there is no clear experimental evidence enough to support the charge generation in our recent report. In this study, from comparison of current density–voltage characteristics of organic hole-only devices and ultraviolet/visible/near-infrared absorption spectra of composite films, we can conclude that the interfacial charge generation surly occurs to realize space-charge-limited currents of a wide variety of organic hole-transport layers. Moreover, a drastic increase in current density of a bilayer device of n-type C60 and p-type N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) by using a MoO3 layer can provide the evidence of the charge generation.

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 (JV) 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 JV 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 JV 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 JV 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

JV 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 JV 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)

  • M. Kröger et al.

    Org. Electron.

    (2009)
  • K. Kanai et al.

    Org. Electron.

    (2010)
  • S. Naka et al.

    Synth. Met.

    (2000)
  • H.H. Fong et al.

    Chem. Phys.

    (2004)
  • W. Xu et al.

    Solid State Commun.

    (2008)
  • C.H. Cheung et al.

    Org. Electron.

    (2010)
  • M. Abkowitz et al.

    J. Appl. Phys.

    (1998)
  • G.G. Malliaras et al.

    J. Appl. Phys.

    (1998)
  • T.-Y. Chu et al.

    Appl. Phys. Lett.

    (2007)
  • I.D. Parker

    J. Appl. Phys.

    (1994)
  • T. Matsushima et al.

    J. Appl. Phys.

    (2008)
  • S. Tokito et al.

    J. Phys. D: Appl. Phys.

    (1996)
  • C.-W. Chen et al.

    Appl. Phys. Lett.

    (2005)
  • V. Shrotriya et al.

    Appl. Phys. Lett.

    (2006)
  • Y. Kinoshita et al.

    Appl. Phys. Lett.

    (2008)
  • G.L. Frey et al.

    Adv. Mater.

    (2002)
  • C.-W. Chu et al.

    Appl. Phys. Lett.

    (2005)
  • T. Matsushima et al.

    Appl. Phys. Lett.

    (2007)
  • H. Lee et al.

    Appl. Phys. Lett.

    (2008)
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