AVS 59th Annual International Symposium and Exhibition
    Thin Film Thursday Sessions
       Session TF-ThP

Paper TF-ThP2
Roles of MoO3 Layer for Charge Injection and Charge Generation in an Organic Light Emitting Diode

Thursday, November 1, 2012, 6:00 pm, Room Central Hall

Session: Thin Film Poster Session
Presenter: M. Kawamura, Kitami Institute of Technology, Japan
Authors: M. Kawamura, Kitami Institute of Technology, Japan
S. Yoshida, Kitami Institute of Technology, Japan
Y. Abe, Kitami Institute of Technology, Japan
Correspondent: Click to Email
It has been reported that MoO3 inserted between ITO anode and hole transport layer is useful as a hole injection layer to improve properties of organic light emitting diodes (OLEDs). However, its optimum thickness in the devices varies from sub-nanometer to 30 nm, depending on reports. In addition, as the mechanism of hole injection at the MoO3 layer and α-NPD layer, it is argued that either charge injection mechanism or charge generation mechanism is dominant. As a fundamental study to investigate the main mechanism, we investigate the influence of MoO3 thickness on properties of normal OLED device and also a device with a charge generation layer in the present work. First of all, we investigated properties of an OLED consists of ITO/MoO3/α-NPD/Alq3/LiF/Al prepared by vacuum evaporation. As a results, the best device properties were obtained when 1.0 nm thick MoO3 was inserted. Using atomic force microscopy, we found that the best device properties were obtained when the ITO surface was not completely covered with MoO3. It is considered that the incomplete coverage was preferable in the view point of energy alignment because HOMO of α-NPD (5.4 eV) is intermediate between work functions of ITO (5.0 eV) and MoO3 (5.7 eV). Then we confirmed charge generation phenomenon using device consists of ITO/Alq3/MoO3/α-NPD/Al. Current did not flow without MoO3 layer. When thickness of the MoO3 layer was above 1.0 nm, a large current flowed and the current-voltage curves were the same even the MoO3 thickness was increased to 10 nm. As the reason, it is considered that a high charge generation ratio was obtained from continuous MoO3 layer. Consequently, we propose that MoO3 work mainly for charge injection when the thickness is thin discontinuous film, and for charge generation when thick continuous film.