BaTiO₃ 강유전체 나노입자가 첨가된 OLED 특성 연구.

Abstract

Basically, Electroluminescence phenomenon used in OLED device was strongly depends on its charge injection characteristics. For this reason, the various solutions have been proposed to improve the carrier injection characteristics. In this study, we doped nano-size ferroelectric material in organic light-emitting layer, in order to form a local field at the interface, and investigated how it affects on the charge injection characteristics. We designed devices based on the simple structure of OLED ( ITO / PFO / Al ). Ferroelectric nano-size ceramic powder; BaTiO₃ was dispersed with different weight ratio in PFO emission-layer. Four types of specimens were designed: 0wt%. 10wt%. 20wt% and 30wt% respectively. Then their I-V-L characteristics were measured using Keithley 2400 Sourcemeter and Minolta CS-2000 Spectroradiometer. Doped nano-size ferroelectric ceramic powder had its own surface charge. These charge could induced local field. Therefore potential barrier at the interface of PFO and Al could be influenced. For 10wt% doped specimen, current density was rising 2.8 times higher at 5V applying voltage and 1.8 times higher at 10V applying voltage. It means that with very low driving voltage like 5V, carrier injection into PFO layer was easily achieved than non-doped device. Over-doping as 20wt% and 30wt%, device current density characteristics were rather reduced than that of non-doped device. It might be explained that the closer distance of more-doped particles, form relative strong local fields due to the superposition of these local fields between particles. Therefore over doped PFO layer act as semi-insulation layer. So consequently, current density might be lowered. With same 21V driving voltage, luminance of 10wt% doped OLED was 2 times higher than that of non-doped device. Well controlled doping ratio and dispersion of particles in OLED could improve the current density characteristics of device. Moreover, this particles act as traps in organic layer. So I-V characteristics of our devices were well matched to Trap charge limited current (TCLC) model.