The simplest way to incorporate a blue fluorophore into the device is to use a fluorescent host material that also has the desired emissive properties. This allows a simple one-EML architecture to be used; however, this one-EML design has several potential drawbacks. One such drawback relates to the difficulty in finding a set of host/dopant molecules that have the desired properties; a host must be an ambipolar charge transporter and have efficient energy transfer to dopants, and the emitter(s) must have high fluorescent quantum yield. It is also difficult to control energy transfer between the different emitters in a single-layer structure. Energy transfer from the blue to lower-energy emitters is unavoidable in a single-layer configuration, which usually leads to insufficient blue emission and downconversion losses. Energy transfer from the phosphorescent emitter back to the fluorophore is also possible if the triplet energy of the fluorophore is close to or lower than that of the phosphorescent emitters. This can be mitigated by choosing a blue emitter that has a small singlet-triplet splitting. Most molecules with this property also tend to have a reduced fluorescent quantum efficiency, although there has been progress in engineering molecules with the desired properties.31 Despite these challenges, high-efficiency, single-layer hybrid white OLEDs have been fabricated. For example, Ye et al. synthesized a sky blue fluorophore 2,8-di[4-(diphenylamino)phenyl] dibenzothiophene-S,S-dioxide and demonstrated that a single-layer hybrid white OLED using a common orange phosphorescent emitter could be fabricated with a maximum total EQE, current efficiency (CE), and power efficiency (PE) of 26.6%, , and , respectively. The spectra and EQE and power efficiency can be seen in Figs. 12(a) and 12(b), respectively.