A blue organic light emitting diodes with graded junction

Compared to the conventional hetero-junction OLED (Cell-HJ), a blue OLED with a graded junction in the interface between hole transporting layer (HTL) and emitted layer (EML) exhibited six times higher stability. The study of hole-only devices with graded junction and hetero-junction indicated that graded junction benefit to the hole injection, implying the actual hole barrier decrease. Improvement of lifetime is attributed to the decrease of actual hole barrier, which decrease the produced joule heat in the interface, thus decrease the local aggregation of molecular. However, excess of holes in the device with graded junction caused the decrease of efficiency. In order to increase the electron injection, a step barrier for electron injection was made and the balance of hole and electron was improved, resulting in the increase of efficiency.