Substantial efficiency enhancement in solution processed phosphorescent light emitting diode with polymer host: Efficient optimization of charge balance and processing conditions

Significant efficiency enhancement in solution processed green phosphorescent OLED (phOLED) based on Tris[2-(p-tolyl)pyridine]iridium(III) (Ir(mppy)3) has been demonstrated by carefully controlling the charge balance of the host (PVK) by doping with electron and hole transporting materials and its processing conditions. The emissive layers (EMLs) with different host combinations were annealed over a range of temperature, 80 °C–160 °C, to elucidate its effect on the morphological, luminescence properties of EML and phOLED performance. EML forms a compact structure with high RMS roughness at higher annealing temperature resulting in a strong interfacial adhesion and better charge injection. Consequently, the phOLED brightness was found to increase and a maximum value of 28,621 cd m−2 was observed at an annealing temperature of 160 °C. The compact structure of the EML, however, leads to a reduction in its photoluminescence (PL), which is significant at higher annealing temperature. Owing to this trade off, the efficiency of the phOLEDs is found to increase up to an annealing temperature of 140 °C and reduce thereafter. phOLED with PVK:PBD:TPD as host, annealed at 140 °C is found to exhibit best device property with a maximum luminous efficiency (LE) of 61.20 cd A−1 and maximum brightness of 17,274 cd m−2.