Here, f is the spin-statistical factor, FTET is the TET efficiency (ISC included), FTTA is the TTA quantum yield, and FF the annihilator fluorescence quantum yield. Since two low-energy photons are needed to afford one highly energetic singlet, FTTA (and subsequentially FUC,g) has a theoretical maximum of 50%. The internal, or generated, UC quantum yield (referred to as FUC,g) was determined alongside that of the external quantum yield (FUC). The difference between these mainly lie in that secondary inner-filter effects are accounted for when calculating FUC,g, which affects both the spectral shape and the peak intensities (Figure ). Reabsorption is accounted for by using the output coupling yield, Fout, with FUC = FUC,g ´ Fout. A lower value for Fout indicates stronger reabsorption of UC emission by the sample. Another factor that had to be dealt with was that of sensitizer degradation. This is a common issue in vis-to-UV UC systems, and a challenge also faced by the organic light-emitting diode (OLED) community when working with TADF materials in general. Upon 405 nm cw excitation 4CzBN suffered from degradation, which manifested itself both in changes of the absorption spectrum and in loss of fluorescence over time (Figure ). When paired with an annihilator species the UC emission intensity typically went down over time, even though efficient TET attenuated the sensitizer degradation (Figure ).