Analysis of the electrical properties of an electron injection layer in Alq₃-based organic light emitting diodes

We investigated the carrier transfer and luminescence characteristics of organic light emitting diodes (OLEDs) with structure ITO/HAT-CN/NPB/Alq3/Al, ITO/HAT-CN/NPB/Alq₃/Liq/Al, and ITO/HAT-CN/NPB/Alq₃/LiF/Al. The performance of the OLED device is improved by inserting an electron injection layer (EIL), which induces lowring of the electron injection barrier. We also investigated the electrical transport behaviors of p-Si/Alq₃/Al, p-Si/Alq₃/Liq/Al, and p-Si/Alq₃/LiF/Al Schottky diodes, by using current-voltage (I-V ) and capacitance-voltage (C-V ) characterization methods. The parameters of diode quality factor n and barrier height φ_b were dependent on the interlayer materials between Alq₃ and Al. The barrier heights φ_b were 0.59, 0.49, and 0.45 eV, respectively, and the diode quality factors n were 1.34, 1.31, and 1.30, respectively, obtained from the I-V characteristics. The built in potentials V_bi were 0.41, 0.42, and 0.42 eV, respectively, obtained from the C-V characteristics. In this experiment, Liq and LiF thin film layers improved the carrier transport behaviors by increasing electron injection from Al to Alq₃, and the LiF schottky diode showed better I-V performance than the Liq schottky diode. We confirmed that a Liq or LiF thinfilm inter-layer governs electron and hole transport at the Al/Alq₃ interface, and has an important role in determining the electrical properties of OLED devices.