Photoluminescence quantum yield (PLQY) measurement: Photoluminescence quantum yields of rubrene/DBP films were measured using the de Mello method in an integrating sphere (from Labsphere). The integrating sphere was calibrated using a radiometric light source (HL-3P-INT-CAL, Ocean Insight). A 447 nm continuous wave laser was used to excite the neat rubrene/DBP (~64 nm) films. The sample was loaded into the sphere such that the 447 m laser had an approximately 8-degree incidence.