Since TTA-UC efficiency is inherently limited by the requirement to absorb two photons to emit one photon, the maximum achievable ΦUC is 50%. ΦUC of ppy3, ppy2, and tmd are 1.4%, 2.0%, and 3.2%, respectively (Fig. ), indicating the ppy2/DBP and tmd/DBP systems had higher UC efficiencies than the ppy3/DBP system. Another important TTA-UC metric is the threshold (Ith), or the power density of incident light at which UC intensity transitions from a quadratic regime (inefficient TTA) to a linear regime (efficient TTA). Ith for the ppy3/DBP, ppy2/DBP, and tmd/DBP systems were 156, 51.5, and 115 mW/cm 2 , respectively (Fig. ), which means the ppy2/DBP and tmd/DBP systems exhibited lower thresholds to achieve efficient UC than the ppy3/DBP system. In summary, using either ppy2 or tmd as a sensitizer with DBP reduced the threshold values and increased UC quantum yields compared to using ppy3 as a sensitizer.