Impedance spectroscopic analysis of long-lifetime blue fluorescent organic light emitting diode

Abstract

Hole and electron carrier transport dynamics of blue fluorescent organic light emitting diodes (OLEDs) were analyzed by impedance spectroscopic technique, and the relation between lifetimes and the dynamics was investigated. The OLEDs with different hole carrier transport property were fabricated by optimizing thickness of electron-blocking layer (EBL), while the electron transport level was fixed. The EBL was arranged between emission and hole transport layers (EML and HTL), and highest occupied molecular orbital level of the EBL was between those of EML and HTL. Both Cole-Cole plot and Bode-plot of imaginary part indicated that relaxation rate of hole carrier transport for the OLED with thin EBL was larger than that for the OLED with thick EBL. The relaxation rates of electron carrier transport were the same level with each other. The lifetime of the OLED with the thin EBL was longer than that of the OLED with the thick EBL. The results give us estimation that the hole carrier injection into the EML is increased for the OLED with relatively thin EBL, inducing long lifetime due to high level of hole injection into the EML even the occurrence of electron leakage from the EML to EBL.