Narrowband emission and enhanced stability in top-emitting OLEDs with dual resonant cavities

Capping layers (CPLs) are commonly employed in top-emitting organic light-emitting diodes (TEOLEDs) due to their ability to optimize color purity, enhance external light out-coupling efficiency, and improve device stability. However, the mismatch in refractive index between CPLs and thin film encapsulation (TFE) often induces light trapping. This study introduces a novel approach by combining a low refractive index material, lithium fluoride (LiF), with the traditional TFE material, silicon nitride (SiN), to form a combined CPL (LiF/SiN), resulting in improved light outcoupling and light reflection properties. The significant refractive index contrast between LiF and SiN can facilitate enhanced light extraction by redirecting internally reflected light through evanescent waves. Moreover, the LiF/SiN CPLs function as a secondary resonant cavity, leading to reduced emission spectral bandwidth and enhanced light extraction compared to the control TEOLEDs that only incorporate the primary cavity of organic active layers. As a result, incorporating the LiF/SiN CPLs significantly increases current efficiency from 125.0 cd A^-1 to 163.6 cd A^-1 for green devices, from 71.2 cd A^-1 to 110.1 cd A^-1 for red devices, and from 43.1 cd A^-1 to 53.1 cd A^-1 for blue devices, with the corresponding full width at half maximum decreased from 20 nm to 10 nm, 26 nm to 14 nm, and 21 nm to 12 nm, respectively, demonstrating the compatibility of the CPLs with different color devices. Notably, an LT₉₅ lifetime of 51 300 hours for green devices was achieved when tested at 1000 cd m^-2. Utilizing narrow-band light emission without spectral overlap of each color enables the generation of purer and more vivid colors for display.