By inserting lithium fluoride (LiF) between solution-processed MoO3 with optimal thickness on top of super yellow poly-(p-phenylenevinylene) (SY-PPV), the efficiency of the SY-PPV fluorescent-based devices can be significantly improved by more than two-fold. Despite the increased driving voltage, the device showed a current and a luminance efficiency up to 22.8 cd A-1 and 14.3 lm W-1 respectively, which is a more than a two-fold increase in efficiency compared to the control device using LiF/Al at a brightness of 1000 cdm-2. Ultraviolet photoelectron spectroscopy (UPS) is used to analyze the energy alignment between SY-PPV and the solution processed MoO3 and MoO3/LiF/Al interfaces. We found that the solution processed MoO3 using diluted sodium hydroxide has relatively low ionization energy (IA), electron affinity (EA) and work function decreasing with increasing thickness of MoO3. However, the optical bandgap increases with increasing spin-speed. A large energetic barrier is always present between the SY-PPY and deep lying valence band of MoO3. This is supported by suppression of hole current in hole dominating devices. The ability of thin MoO3 (∼2 nm) acting as a hole blocking layer while allowing electrons to be transported across the layer and a large upward vacuum shift appeared to be the origin of efficiency enhancement of SY-PPV light-emitting diode when MoO3/LiF/Al is used.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics
- Acoustics and Ultrasonics
- Surfaces, Coatings and Films