The excitation intensity dependences of the UC emission were investigated on rubrene/Y6 and rubrene/ITIC-Cl, which is another NFA. The results of the bilayer films in Figure followed the typical tendency of TTA UC. The UC emission increased quadratically at lower excitation intensities, whereas it increased linearly at higher intensities because TTA became the main decay channel under the high triplet concentration condition. The intersection of the two fitted lines represents the threshold intensity of the UC (Ith), and Ith of the rubrene/Y6 and rubrene/ITIC-Cl bilayer films were 70.6 and 111 mW/cm 2 , respectively. These values were smaller than those of conventional UC systems. The internal quantum efficiencies (IQEs) of the UC emission on the bilayer films were calculated using the rubrene PL as a reference. The IQEs of the rubrene/Y6 and rubrene/ITIC-Cl in the saturated region were 0.57% and 1.58%, respectively.