However, another major loss mechanism has been identified as a source of low efficiencies for solid-state UC beyond ET from the PbS QD to rubrene. This loss mechanism involves parasitic energy back transfer (BT) from the rubrene singlet to the PbS QDs, as shown in Figure . This BT process (rubrene singlet → PbS QDs) is a Förster resonance energy transfer (FRET) process which is exacerbated by high acceptor (PbS QD) concentrations leading to a severe drop in IQE for thicker PbS films deposited from solutions with concentrations beyond 5-10 mg/mL. Therefore, traditional PbS QD/rubrene devices report 1-2 monolayer thin PbS films with low absorptions on the order of 0.1-0.5% (excluding examples employing cavities). Thus far, this low absorption has presented a bottleneck to high EQEs in QD-based solidstate TTA-UC.