All-exciplex-based white organic light-emitting diodes by employing an interface-free sandwich light-emitting unit achieving high electroluminescence performance

In this work, we proposed a novel and simple all-exciplex-based white organic light-emitting diodes (WOLEDs) with a sandwich light-emitting unit, where the light-emitting unit is composed of a yellow exciplex layer sandwiched between two blue exciplex layers, and no interfaces are formed in different exciplex layers due to the design of shared donor or acceptor materials. Based on this structure, a series of all-exciplex-based WOLEDs were developed, and the emission spectra and device performance could be easily adjusted and improved by simply changing the thickness of the blue exciplex layers on both sides or the ratio of donor to acceptor of the sandwiched yellow exciplex layer. As a consequence, the optimized two-color WOLEDs achieve an ultra-high color rendering index (CRI) of 88, to the best of our knowledge and is among the highest values for a two-color white device reported in the literature. The device efficiency was also improved by the use of a thermally activated delayed fluorescent exciplex. For example, the maximum external quantum efficiency for the resulting WOLEDs reached 5.21%, exceeding the theoretical limit (5%) for traditional fluorescent OLEDs. Moreover, the device structure can be further simplified by using an interfacial exciplex to replace the bilateral co-doped blue exciplex layers, and the corresponding white devices also obtained nearly the same EL performance. We believe this work paves way for the development of simplified yet high-performance all-exciplex-based WOLEDs.