Formation of Ohmic Carrier Injection at Anode/Organic Interfaces and Carrier Transport Mechanisms of Organic Thin Films
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1154-B10-92
Formation of Ohmic Carrier Injection at Anode/Organic Interfaces and Carrier Transport Mechanisms of Organic Thin Films
Toshinori Matsushima, Guang-He Jin, Yoshihiro Kanai, Tomoyuki Yokota, Seiki Kitada, Toshiyuki Kishi, and Hideyuki Murata * School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan *Corresponding author. Tel.: +81 761 51 1531; fax: +81 761 51 1149; E-mail address: [email protected]
ABSTRACT We have shown that hole mobilities of a wide variety of organic thin films can be estimated using a steady-state space-charge-limited current (SCLC) technique due to formation of Ohmic hole injection by introducing a very thin hole-injection layer of molybdenum oxide (MoO3) between an indium tin oxide anode layer and an organic hole-transport layer. Organic hole-transport materials used to estimate hole mobilities are 4,4',4"-tris(N-3-methylphenyl-Nphenyl-amino)triphenylamine (m-MTDATA), 4,4',4"-tris(N-2-naphthyl-N-phenylamino)triphenylamine (2-TNATA), rubrene, N,N´-di(m-tolyl)-N,N´-diphenylbenzidine (TPD), and N,N´-diphenyl-N,N´-bis(1-naphthyl)-1,1´-biphenyl-4,4´-diamine (α-NPD). These materials are found to have electric-field-dependent hole mobilities. While field dependence parameters (β) estimated from SCLCs are almost similar to those estimated using a widely used time-offlight (TOF) technique, zero field SCLC mobilities (µ0) are about one order of magnitude lower than zero field TOF mobilities.
INTRODUCTION Organic light-emitting diodes (OLEDs) have been developed due to their high potentials for use in low-cost, mechanically flexible, light-weight display and lighting applications. Multilayer OLEDs are typically composed of an indium tin oxide (ITO) anode, an organic holetransport layer (HTL), an emitting layer, an electron-transport layer, and a metal cathode. In general, a large hole injection barrier height of several hundred meV is present between an ITO layer and an HTL, which causes an increase in driving voltage of OLEDs. Various organic and inorganic hole-injection layers (HILs) have been inserted between an ITO and an HTL to reduce the driving voltages [1-3]. Recently, we have demonstrated that the use of a 0.75 nm HIL of molybdenum oxide (MoO3) inserted between an ITO and an HTL of N,N´-diphenyl-N,N´-bis(1naphthyl)-1,1´-biphenyl-4,4´-diamine (α-NPD) leads to the formation of an Ohmic contact at the ITO/MoO3/α-NPD interfaces and the observation of a space-charge-limited current (SCLC) of α-NPD [4]. This MoO3 thickness of 0.75 nm is much thinner than the previously reported values. Moreover, marked improvements of driving voltages, power conversion efficiencies, and operational stability of OLEDs have been realized using the very thin MoO3 HIL [5,6].
Presence of charge-carrier injection barriers at electrode/organic interfaces generally makes it difficult to investigate carrier transport mechanisms in organic films because observed currents are governed by both carrier injection and transport [7]. In this stu
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