On the other hand, TADF materials are generally designed to have highly twisted electron donor-acceptor (D-A) structures to acquire small ∆ESTs. Such kind of twisted D-A structures can also endow the molecules with ambipolar carrier transport ability, which is favorable for achieving carrier balance and thus high exciton recombination efficiencies in devices. Therefore, TADF materials can behavior as hosts as well as sensitizers for various kinds of luminescent materials to greatly improve device performances because of enhanced efficiencies for exciton production and utilization. The conventional fluorescence emitters, who can only harness singlet excitons, particularly benefit from doping in TADF materials, rendering much higher EL efficiencies than those in common host materials. Combining the high color purity and stability of fluorescence emitters, all-fluorescence WOLEDs using TADF materials as both hosts and sensitizers possess superior competitiveness and merits to conventional all-fluorescence WOLEDs based on common fluorescence emitters, bringing about new opportunities for the advancement of WOLEDs. For white light illumination, power conservation is a vital index for WOLEDs, which can be reflected by the power efficiency (PE). And the PE at operating luminance is a more accurate parameter for the evaluation of WOLED performances. However, most of the efficient WOLEDs encounter sharp declines in PEs at high luminance, and effective approaches to improve PEs and reduce PE roll-offs are received relatively less attention. The exciton utilization of the emitters and the carrier injection and transport of the devices play critical roles in determining PEs, which should be carefully addressed in the design of energy-conserving WOLEDs by properly selecting not only luminescent materials but also host materials. Currently, several new hosts are demonstrated to be promising candidates for WOLEDs. Organic semiconductors generally exhibit asymmetric carrier transport property, in which the mobility of holes is generally much faster than electrons. The n-type hosts with high T 1 energy levels can erase the barrier of electron injection and balance the transports of electrons and holes, which is conducive to lowering driving voltages of the devices (Fig. ). However, such kind of n-type hosts are difficult to design, and there are only very limited cases of n-type hosts successfully applied in WOLEDs to boost PEs. Besides, intermolecular charge transfer host, namely exciplex host, is reported as an effective alternative for solving the power consumption problem, because their ambipolar carrier transport and easy carrier injection can ensure smooth recombination of excitons in emitting layer (EML), rendering low turn-on voltages for WOLEDs. Nevertheless, the fabrication complexity involving multielement doping technique often results in failure to acquire desired device performances based on exciplex host. The application of TADF materials as hosts is another promising avenue to enhance PEs owing to their narrower energy gaps compared with conventional hosts as well as much better exciton harvesting ability. The ideal multifunctional TADF materials for both hosts and emitters require not only balanced ambipolar carrier transport but also suppressed exciton annihilation and emission quenching in neat films, which is challenging because most reported TADF materials suffer from serious losses of emission and exciton in neat films, and thus have to be doped in other conventional hosts when applied in OLEDs. In consequence, up to now, the successful cases of highperformances WOLEDs based on TADF materials with comparable or superior PEs to those of the best phosphorescence materials engaged WOLEDs are extremely rare. In our previous work, a robust sky-blue delayed fluorescence molecule, (4-(spiro[acridine-9,9'-fluoren]-10yl)phenyl)(9-(3,5-di(carbazol-9-yl)phenyl)carbazol-3-yl)methanone (TCP-BP-SFAC), with balanced ambipolar carrier transport and excellent PL efficiency in neat film was created, which provided record-high EQEs of 26.1% and 38.6% in nondoped and doped OLEDs, respectively. These intriguing properties enable TCP-BP-SFAC to function as host and emitter simultaneously in WOLEDs. In this contribution, a simple sandwich configuration consisting of an orange EML located between two sky-blue EMLs is proposed to boost the EL efficiencies of allfluorescence two-color WOLEDs (Fig. ), in which the neat film of TCP-BP-SFAC (Fig. ) is adopted as sky-blue EML, and the doped film of an efficient orange TADF emitter 2,3,5,6-tetrakis(3,6-di-(tert-butyl)carbazol-9-yl)-1,4dicyanobenzene (4CzTPNBu) blended in TCP-BP-SFAC host functions as orange EML. The sandwich configuration effectively alleviates exciton quenching caused by the electron-trapping effect of 4CzTPNBu. Thanks to the balanced ambipolar transportation and high solid-state efficiency of TCP-BP-SFAC, the record-beating PE of 130.7 lm W -1 is successfully achieved in all-fluorescence two-color warm-white devices. For further improving the color quality, a red EML comprised of a red fluorescence dopant 5,10,15,20-tetraphenylbisbenz indeno[1,2,3-cd:1′,2′,3′lm]perylene (DBP) doped in TCP-BP-SFAC host is introduced, which is sensitized by an efficient out-of-phase configuration via Förster energy transfer (FET) as shown in Fig. . The all-fluorescence three-color WOLEDs achieve nearly the same EQEs of 30.8% as two-color devices, as well as high PEs over 106 lm W -1 , which are also pioneering among all-fluorescence three-color WOLEDs reported so far. Importantly, the PEs at operating luminance of 100 and 1000 cd m -2 are kept at satisfactory levels for both two-color and three-color devices, indicating they are practicable devices for illumination. The in-depth working mechanisms behind these devices are thoroughly studied and the effective design strategy for high-performance all-fluorescence WOLEDs is elucidated in detail, in consideration of the electron-trapping effect and host-tuning effect. Moreover, the operational lifetimes to 50% (LT50) at the initial luminance of 100 cd m -2 reach over 50000 h, validating the presented design strategies for allfluorescence WOLEDs are of high potential for practial application.