The spin coated substrates were then loaded into an Angstrom Engineering thermal evaporation chamber. All evaporations were performed under a pressure of less than 1E-05 mbar. DBP-doped (1% v/v) rubrene evaporations were performed by co-evaporating the chemicals from two different sources. For a 1% doping, the DBP was evaporated at a rate of ~0.02-0.03 Å/s while the rubrene was evaporated at ~2-3 Å/s. Both DBP and rubrene were evaporated from resistively heated alumina crucibles. The rubrene/DBP films had a final thickness of ~64 nm based on profilometry. This rubrene thickness showed the best performance for our UC devices.