The novel UC mechanism at the interface according to the experimental results is depicted in Figure . Firstly, the S1 excitons that were formed following the absorption of low-energy photons in the NFA sensitizer layer were separated to the CT state at the D/A interface using the HOMO energy offset between the rubrene and NFAs (Figure ). Note that the CT state had two spin states: singlet and triplet, which are denoted as CT1 and CT3, respectively. The CT state formed immediately following charge separation was CT1 owing to the spin-selection rule and the GR from CT1 did not contribute to UC emission, as discussed in the previous paragraph. Subsequently, charge pairs were separated to form free charges with a random spin state. The process thus far was exactly the same as the photoconversion mechanism of OPVs. However, in contrast to OPVs, the bilayer films did not have electrodes; thus, the free charges eventually recombined to form a CT state again. The CT1 and CT3 formation rates were 25% and 75%, respectively, owing to the spin-selection statistics of free charge recombination. The energy of CT3 could be transferred to T1 of the rubrene, following which the TTA formed S1, and finally, high-energy UC emission from the rubrene layer could be observed.