Dependence of the Color Tunability on the H2Pc Thickness in DC-Voltage-Driven Organic Light-Emitting Diodes

Dependence of the color tunability on the metal free Phthalocyanine (H2Pc) layer thickness in DC-voltage-driven organic light-emitting diodes (OLEDs) was investigated. A H2Pc layer was employed as a blue/red emission layer, which was prepared on an Alq3 green emission layer. The thickness of the H2Pc 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 H2Pc and Alq3. 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 H2Pc thin film thicknesses. In addition, the recombination rates at the interfaces between the hole transport layer and Alq3 and the H2Pc/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 H2Pc layer in the CTOLED. It was verified that the thermally treated H2Pc thin film layer acted as a barrier that prevented electrons from being transferred to the Alq3 at low applied voltages, resulting in white color emission with temperature tunability. The CTOLED with a 20 nm of H2Pc layer demonstrated the best stable interface state and stability, resulting in the lowest driving voltage, relatively high luminance, and optimal light emission uniformity, respectively.