TADF Sensitizers: Drawbacks and Opportunities. As is evident from this study, using TADF compounds as the sensitizers in TTA-UC holds great promise. The most obvious advantage compared to other sensitizers yielding decent FUC in vis-to-UV TTA-UC (i.e., Ir complexes and quantum dots containing heavy metals such as Cd and Pb) is that TADF compounds are purely organic, consisting only of earth-abundant, non-toxic elements. They are, thus, well-suited for future large-scale operation, which is not the case for Ir complexes, despite possessing promising photophysical properties otherwise. Additionally, due to the OLED community's increasing interest in TADF compounds during the last decade there is a huge variety of available molecules with different energy levels and triplet excited state lifetimes, of which the latter in many cases are orders of magnitude longer than those found in e.g., Ir complexes. Making the best use of existing TADF compounds in TTA-UC schemes is, however, not straight-forward. The sought-after qualities for use in OLEDs differ significantly from what is needed in a typical sensitizer, meaning that current TADF design in many cases has gravitated towards compounds not suitable for TTA-UC. One crucial benefit in both contexts is the access to small singlet-triplet energy splittings (DES-T). In OLEDs the excited states are created by means of electricity, and the resulting distribution is dictated by spin-statistics, leading to 75% triplets and 25% singlets (Figure ). Highly efficient rISC to generate a higher fraction of emissive singlets is, thus, one of the most important properties of TADF compounds in the context of OLEDs, and is a process that is sped up in molecules with small DES-T (generally, krISC µ exp[-DES-T/kBT]). In TTA-UC a small DES-T enables larger apparent anti-Stokes shifts since the initial energy loss during the ISC event is smaller than in typical sensitizers containing heavy metals. Once the triplet has been populated it is instead beneficial if rISC is inefficient since the generated exciton should be transferred to the annihilator instead of returning to the singlet manifold. A too small DES-T might therefore inhibit efficient TET even if the annihilator concentration is kept high. An intermediate DES-T (0.1 eV < DES-T < 0.2 eV), enabling relatively large apparent anti-Stokes shifts and slow rISC simultaneously, should be favored.