The effect of Ag layer thickness on the properties of WO3/Ag/MoO3 multilayer films as anode in organic light emitting diodes

Transparent conductive WO3/Ag/MoO3 (WAM) multilayer electrodes were fabricated by thermal evaporation and the effects of Ag layer thickness on the optoelectronic and structural properties of multilayer electrode as anode in organic light emitting diodes (OLEDs) were investigated using different analytical methods. For Ag layers with thickness varying between 5 and 20 nm, the best WAM performances, high optical transmittance (81.7%, at around 550 nm), and low electrical sheet resistance (9.75 Ω/cm2) were obtained for 15 nm thickness. Also, the WAM structure with 15 nm of Ag layer thickness has a very smooth surface with an RMS roughness of 0.37 nm, which is suitable for use as transparent conductive anode in OLEDs. The current density−voltage−luminance (J−V−L) characteristics measurement shows that the current density of WAM/PEDOT:PSS/TPD/Alq3/LiF/Al organic diode increases with the increase in thickness of Ag and WO3/Ag (15 nm)/MoO3 device exhibits a higher luminance intensity at lower voltage than ITO/PEDOT:PSS/TPD/Alq3/LiF/Al control device. Furthermore, this device shows the highest power efficiency (0.31 lm/W) and current efficiency (1.2 cd/A) at the current density of 20 mA/cm2, which is improved 58% and 41% compared with those of the ITO-based device, respectively. The lifetime of the WO3/Ag (15 nm)/MoO3 device was measured to be 50 h at an initial luminance of 50 cd/m2, which is five times longer than 10 h for ITO-based device.