We next investigated the photophysical behavior of both emitters in the OLED relevant host, mCP, which has a suitably high triplet energy of 2.91 eV. First, we identified 2 wt% as the optimal concentration by measuring the FPL of spin-coated films of varying concentration from 1 wt% to 10 wt% in mCP. We then compared the FPL of 2 wt% films in a range of hosts such as bis[2-(diphenylphosphino)phenyl]ether oxide (DPEPO) and 4,4'-bis(9-carbazolyl)-1,1'biphenyl (CBP), and the FPL in mCP was found to be the highest at 86%. (Figure ). The u.