Flexible white top-emitting organic light-emitting diode with a MoO _x roughness improvement layer

In this paper, an MoO_x film is deposited on a polyethylene terephthalate (PET) substrate as a buffer layer to improve the surface roughness of the flexible PET substrate. With an optimized MoO_x thickness of 100 nm, the surface roughness of the PET substrate can be reduced to a very small value of 0.273 nm (much less than 0.585 nm of the pure PET). Flexible white top-emitting organic light-emitting diodes (TEOLEDs) with red and blue dual phosphorescent emitting layers are constructed based on a low-reflectivity Sm/Ag semi-transparent cathode. The flexible white emission exhibits the best luminance and current injection characteristics with the 100-nm-thick MoO _x buffer layer and this result indicates that a smooth substrate is beneficial to the enhancement of device electrical and electroluminescence performances. However, the white TEOLED with a 50-nm-thick MoO_x buffer layer exhibits a maximum current efficiency of 4.64 cd/A and a power efficiency of 1.9 lm/W, slightly higher than those with a 100-nm MoO_x buffer layer, which is mainly due to an obvious intensity enhancement but limited current increases in 50-nm MoO_x-based white TEOLED. The change amplitudes of the Commission International de l'Eclairage (CIE) chromaticity coordinates are less than (0.016, 0.005) for all devices in a wide luminance range over 100 cd/m², indicating an excellent color stability in our white flexible TEOLEDs. Additionally, the flexible white TEOLED with an MoO_x buffer layer shows excellent flexibility to withstand more than 500 bending times under a curvature radius of approximately 9 mm. Research demonstrates that it is mainly attributed to the high surface energy of the MoO_x buffer layer, which is conducible to the improvement of the surface adhesion to the PET substrate and the Ag anode.