a the FPL was measured using an integrating sphere under nitrogen (lexc =340 nm). b Obtained at 298 K, lexc = 340 nm. c Full-width at half-maximum; d Obtained from the onset of the steady-state PL spectrum at 77 K. e Obtained from the onset of the delayed emission spectrum (1-10 ms) at 77 K (lexc = 340 nm). f DEST = E(S1) -E(T1). g Measured at 300 K under vacuum, lexc = 379 nm. To explore the potential of DDiKTa-A as an emitter in OLEDs, we turned our attention to assessing the photophysical properties in mCP thin films, as this host has a suitably high triplet energy (ET = 3 eV) to confine the excitons onto the emitter. We first explored the dependence of both the SS PL and FPL of DDiKTa-A on the doping concentration in mCP (Figure ). Under N2, the FPL decreased from 92 to 74% upon increasing the doping concentration from 2 to 10 wt% in mCP, while the emission red-shifted from 562 to 588 nm, indicating that the emitter tends to aggregate and that these aggregates quench the emission. Therefore, using only the 2 wt% films, we next screened the properties in two other high triplet energy hosts, PPT and DPEPO (Figure ). In both phosphine oxide-based hosts, the 2 wt% films emit at around 600 nm while the FPL are 69 and 50%, respectively. The observed red-shifting of the emission from orange in mCP (lPL = 562 nm, FWHM = 75 nm) to red (lPL = 600 nm, FWHM = 102 nm) in these two hosts can be attributed to the increased stabilization of the LRCT state with increasing host polarity. Due to the high FPL and narrower emission in the 2 wt% doped films in mCP, we carried out the subsequent photophysical studies using this film. The ΔEST of DDiKTa-A in this film is 0.16 eV (Figure ), which is of similar magnitude to that measured in 2-MeTHF glass. The emission is quenched in the presence of air, dropping from a FPL of 92 to 80%. There is the characteristic temperature dependence of the delayed emission that is indicative of TADF (Figure ). The TR PL decay at RT shows multiexponential kinetics under vacuum, the average τP and τd are 15.1 ns and 279 μs, The kinetics rate constants for the 2 wt% doped films in mCP are collected in Table and the kRISC is 1.9 × 10 4 s -1 for DDiKTa-A, which is similar to that determined in 2wt% doped film in mCP for DiKTa (kRISC = 2.5 × 10 4 s -1 ). Considering the small decrease in FPL (from 92 to 73%) with an increase in doping concentration from 2 to 10 wt% in spin-coated mCP film, the emissive layers containing different doping concentrations of 2, 5, and 10 wt% in OLEDs were fabricated to assess how device performance is affected by doping concentration. The device with 5 wt% showed the best performance (Figure ). The device emitted at an electroluminescence peak maximum, lEL, of 572 nm and was comparably broader to that observed in the PL spectrum (FWHM of 69 nm), Figure To address its rather inefficient exciton harvesting, we proceeded to fabricate a hyperfluorescence device (HF device), identifying 4CzIPN as a suitable assistant dopant. The emissive layer of the optimized HF device consisted of a 5:10:85 ratio for DDiKTa-A/4CzIPN/mCP. The photophysical properties are shown in Figure -S19, and the spin-coated film emits at λPL at 577 nm, has a τp of 32 ns and τd of 149 μs and, importantly a ΦPL of 98%. The corresponding kRISC of this film is 5.47×10 4 s -1 (Table ). The HF device emitted at lEL of 574 nm (FWHM of 75 nm), which is a little broader than the device with only DDiKTa-A. The efficiency of the HF device was noticeably improved, with an EQEmax of 24.3% and the device showed a significantly reduced efficiency rolloff of 7.4% at 100 cd m -2 (EQE100 of 22.5%). This device represents one of only a handful of reported carbonyl-based orange/red MR-TADF OLEDs (Table and Figure ). ).