To improve the EQEs of UC devices towards realizing their potential, we pursued improvements to the various components contributing to the EQE. The EQE of the TTA-UC process can be broken down as the product of absorption and internal quantum efficiency (IQE). The IQE can be further broken down as the product of the efficiencies of energy transfer (φET), TTA (φTTA), and photoluminescence (φPL) (see Equation ). As shown in Figure , efficient energy transfer (ET) from PbS QDs to rubrene is essential to maximize the triplet density in rubrene. This energy transfer in the forward direction (PbS QDs → rubrene) is of the Dexter type, i.e., the energy transfer rate is exponentially dependent on the distance between the donor (PbS QDs) and acceptor (rubrene). The insulating organic ligand shell (typically, oleic acid) adhered to the PbS QDs is known to hinder the ET efficiency due to increased spatial distance between the donor and acceptor. This issue has been commonly overcome through the use of shorter ligands or extractor/mediator ligands on the QDs.