thickness of TCP-BP-SFAC layer in two-color WOLEDs should be controlled within an appropriate range. A thick layer (a large x) of TCP-BP-SFAC adjacent to PPF layer will weaken the contribution of 4CzTPNBu, but a thin layer (a small x) may result in exciton accumulation and annihilation at 4CzTPNBu. The function of TCP-BP-SFAC layer adjacent to mCP layer in the sandwich configuration is also investigated. To verify the necessity of this TCP-BP-SFAC layer, double-layer devices WS1 with an EML of 1.5 wt% 4CzTPNBu: TCP-BP-SFAC (5 nm)/TCP-BP-SFAC (6 nm) and WS2 with an EML of 1.5 wt% 4CzTPNBu: TCP-BP-SFAC (14 nm)/TCP-BP-SFAC (6 nm) are fabricated for comparison (Supplementary Fig. and Supplementary Tables and). WS1 shows obviously decreased PE of 90.7 lm W -1 and EQE of 23.2% in comparison with W1-6. But with a thicker layer of 1.5 wt% 4CzTPNBu: TCP-BP-SFAC (14 nm), WS2 can provide comparable PE of 124.9 lm W -1 and EQE of 30.1% to those of W1-6. This result discloses this TCP-BP-SFAC layer can serve as a buffer layer to alleviate exciton annihilation, in addition to balance the carrier transport in the device.