singlet (S 1 ) and triplet (T 1 ) states. Due to the competing radiative and nonradiative modes of decay of the S 1 excitons (resulting from the energy gap law), the development of orange-red TADF materials is significantly more difficult than that of blue and green TADF materials. [12] Orange-red TADF emitters require strong donors and acceptors with high rigidity and large, conjugated planar structures to suppress nonradiative decay, which is necessary to obtain orange-red emissions while maintaining a high PLQY. However, such rigidity imparts the structure with poor solubility in ordinary organic solvents, hindering their use as solutionprocessed OLEDs. In addition, the large conjugate-planar structure also leads to severe concentration quenching of excitons, which adversely affects device performance. Furthermore, when compared to vacuum evaporation devices, solution-processed devices are generally more difficult to inject holes into the emission layer (EML), resulting in poor carrier balance factor and inferior performance. This is chiefly due to the lack of a hole transport layer (HTL) to effectively suppress triplet exciton quenching at the hole-injection layer (HIL)/EML interface. [13] The aforementioned aspects make the development of solution-processed, orange-red, TADF OLEDs with an electroluminescence (EL) peak longer than 580 nm a decidedly challenging prospect. [14][15][16][17][18] So far, solution-processed, orange-red, TADF OLEDs with high external quantum efficiency (EQE) are uncommon and always accompanied by a high driving voltage, which leads to low power efficiency (PE). For example, Zeng et al. reported a solution-processable, orange-red, TADF emitter designated as NAI_R3, with the corresponding device achieving a maximum external quantum efficiency (EQE max ) of 22.5% with a turn-on voltage (V on ) of 7 V at 1 cd m −2 , which was groundbreaking at the time. [15] However, this ultrahigh V on results in a low maximum power efficiencies (PE max ) of only 9.4 lm W −1 . On the other hand, state-of-the-art values for corresponding vacuumdeposited, orange-red, TADF OLEDs include an EQE max of over 30%, with a much lower V on of 2.6 V. [16] Chen and co-workers developed tDBBPZ-DPXZ, an orange-red TADF emitter that can be used for both vacuum evaporation and solution processes. [17] The corresponding vacuum-deposited device exhibited an EQE max of 17.0% and a relatively low V on of 3.5 V, while the solution-processed device with the same EML compositionThe development of high-performance, solution-processed, orange-red organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) emitters is a challenging endeavor. In this study, two orange-red TADF emitters, namely 2DMAC-DBP-2tBuCz and 2SPAC-DBP-2tBuCz, are developed by a novel donor-acceptor-functional-group (D-A-R) molecular design strategy. This design makes the molecules highly soluble and inhibits concentration quenching of excitons, rendering the emitter suitable for use in devices with high concentration to b...

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