Dependence of the Color Tunability on the H[sub.2]Pc Thickness in DC-Voltage-Driven Organic Light-Emitting Diodes

Dependence of the color tunability on the metal free Phthalocyanine (H[sub.2]Pc) layer thickness in DC-voltage-driven organic light-emitting diodes (OLEDs) was investigated. A H[sub.2]Pc layer was employed as a blue/red emission layer, which was prepared on an Alq[sub.3] green emission layer. The thickness of the H[sub.2]Pc layer varied from 5 to 30 nm, with a step of 5 nm. The fabricated color-tunable OLEDs (CTOLEDs) were subjected to a thermal treatment layer for 2 min at a temperature of 120 °C to improve the interface properties, especially between H[sub.2]Pc and Alq[sub.3]. The current density-voltage-luminance characteristics and Commission Internationale de L'Eclairage (CIE) coordinates of the CTOLEDs with and without thermal treatment were measured, and their energy band diagrams were analyzed with respect to the H[sub.2]Pc thin film thicknesses. In addition, the recombination rates at the interfaces between the hole transport layer and Alq[sub.3] and the H[sub.2]Pc/electron transport layer of the CTOLEDs with and without thermal treatment were theoretically investigated using a technology-computer-aided design (TCAD) program. The experimental and theoretical results showed that the emission color temperature from cool white to warm white at a low voltage can be controlled by adjusting the thickness of the H[sub.2]Pc layer in the CTOLED. It was verified that the thermally treated H[sub.2]Pc thin film layer acted as a barrier that prevented electrons from being transferred to the Alq[sub.3] at low applied voltages, resulting in white color emission with temperature tunability. The CTOLED with a 20 nm of H[sub.2]Pc layer demonstrated the best stable interface state and stability, resulting in the lowest driving voltage, relatively high luminance, and optimal light emission uniformity, respectively.