The key process of the novel UC system was triplet formation by spin inversion of the CT state during charge separation and recombination at the D/A interface (Figure ), whereas the triplet is formed by intersystem crossing in the sensitizer molecule in conventional UC (Figure ). This critical difference in the mechanism solves the problems of conventional solid-state UC. The solid-state UCs reported to date have been inefficient as a result of non-radiative triplet annihilation occurring in the sensitizer layer during inefficient triplet diffusion. To avoid these loss pathways, in most cases, the sensitizer is dispersed in matrix materials. However, the dispersion decreases the absorbance of the sensitizer layer significantly. In contrast to conventional UC, in the novel UC system, the excited state diffusing in the sensitizer layer was a singlet, and a triplet was only formed in the emitter layer near the D/A interface. Therefore, triplet loss never occurred in the sensitizer layer, which means that a pristine film with a large absorbance could be used for the sensitizer. As a result, about 90% of incident photons were absorbed in the Rub/ITIC-Cl bilayer, whereas only a small percentage can be utilized in dispersed films in conventional UC systems. This difference as well as the high IQE resulted in a 100 times increase in the EQE in the novel UC system compared to conventional systems. Furthermore, the triplet formation at the D/A interface means that intersystem crossing inside sensitizer molecules by the heavy atom effect is unnecessary for UC. The sensitizer molecules used in this study did not contain minor metals, rare-earth, or toxic elements, such as Pd, Ir, Pt, Yb, Os, Cd, or Pb. This finding is a paradigm shift for UC because a large variety of "normal organic semiconductor materials" that have already been developed for organic electronics applications can be used to realize UC provided that the energy levels of the materials meet the requirements for each energy transfer step. Moreover, triplet formation by charge separation and recombination offers the advantage of a small energy loss in UC because recent reports have proven that charge separation is possible with a minimized energy difference between S1 and the CT state of less than 50 meV, which is far smaller than the typical energy loss for intersystem crossing of approximately 0.3 eV. Our results support this point as the CT state emission in the rubrene/Y6 OLED device almost overlapped with the S1 emission of Y6, indicating that the energy difference between S1 and the CT state was negligible. This enabled a small total energy loss in the UC: the energy difference of the emission peak top between S1 of the Y6 and T1 of the rubrene was only 0.14 eV. Finally, efficient UC in the bilayer thin film enabled the demonstration of bright yellow UC emission on a flexible substrate by NIR LED excitation, as depicted in Figure . UC on flexible thin films is important for future applications in flexible solar cells, as well as bio-imaging or optogenetics applications in living organisms. Furthermore, strong laser excitation is not necessary in this system because a high EQE can be achieved under a small excitation power density. We anticipate that the significant advances of our findings will expand the applicability of UC and open a new research field based on spin inversion at the organic semiconductor interface.