Exploring the Recombination Zone of Blue Organic Light-Emitting Diodes from Various Thickness of Emitting Layer Without Sensing Layer

Organic light emitting devices (OLEDs) with high external quantum efficiency (EQE) is indispensable for demanding solid-state lighting technologies. EQE depends on internal quantum efficiency (IQE) of the emissive layer (EML), charge balance and out coupling efficiency of the device architecture. Especially, EQE of OLEDs drops at higher luminance (called as efficiency roll-off) due to lack of exciton confinement in EML and triplet induced exciton quenching. Triplet based losses are evolved through triplet-triplet and triplet-polaron annihilation (TTA and TPA) mechanisms which cause decrement in IQE of EML. However, the inability to confine the exciton density in EML can induces a significant change in device efficiency despite having an efficient EML. Hence, it is essential to confine exciton density or recombination zone (RZ) in EML for enhanced efficiency of OLED device. In general, the change in RZ is carefully examined using a sensing layer in the EML by comparing the device electroluminescence (EL) pertaining to EML and sensing positions.<br/>Here, we presented a simple and effective way to depict the RZ movement in blue phosphorescent OLEDs (Ph-OLEDs) by varying the thickness of EML. The changes observed in EL spectra according to various EML thicknesses were investigated and its color coordinates have been compared to expose the enhanced performance of blue Ph-OLED with efficient exciton confinement in EML. RZ shifting in Blue Ph-OLEDs have been monitored by EML thickness variation and electrical analysis. From main RZ and sub RZ, we analyzed the EL intensity and concluded that the optimal point of EML thickness could be found through EL intensity. Hence, this study would be beneficial to optimize the thickness of EML to confine the RZ and appropriate selection of the EML thickness in blue OLED is essential for achieving improved performance to realize the desired efficiency.