High efficiency and color quality undoped phosphorescent white organic light-emitting diodes based on simple ultrathin structure in exciplex

White organic light-emitting diodes (WOLEDs) with ultra-thin emitting layers have shown considerable potential in solid-state lighting fields due to the simplified structures. However, such WOLEDs with high efficiency, high color rendering index (CRI) and stable spectral emission are still facing challenges. In this paper, we demonstrated a high efficiency and high CRI phosphorescent WOLED by strategically adjusting the positions of red, green and blue phosphorescent ultra-thin layers in exciplex emitter. By investigating the processes of energy transfer and utilization of triplet excitons, a high efficiency, high CRI and stable spectrum emission warm WOLED was successfully fabricated. The maximum external quantum efficiency (EQE), power efficiency (PE) and current efficiency (CE) reached 26.1%, 50 lm W−1,and 40 cd A−1, respectively, and the CRI was as high as 80 in the luminance range from 1000 cd m−2 to 30000 cd m−2 with stable emission spectra, exhibiting superior comprehensive EL performances. This work also indicates the potential of undoped ultra-thin layer structures to fabricate high performance WOLEDs for lighting applications.