Charge transport, carrier balance, and blue electrophosphorescence in diphenyl[4-(triphenylsilyl)phenyl]phosphine oxide devices

Diphenyl[4-(triphenylsilyl)phenyl]phosphine oxide (EMPA1) displays a wide highest occupied molecular orbital-lowest unoccupied molecular orbital gap (4.1 eV), singlet (4.3 eV) and triplet (3.4 eV), and an electron-dominated charge transport that follows a trap-free space charge limited model with an average electron mobility of 5.7× 10^-6 cm² V^-1 s^-1 and a hole mobility of 1.1× 10^-6 cm² V^-1 s^-1. At high driving voltages (>6 V), ambipolar charge transport is observed, resulting in a balanced charge density in the active layer. Highly efficient blue phosphorescent organic light-emitting diodes were realized, showing a high external quantum efficiency (21%) and a luminance efficiency of 45 cd/A using a bis[2-(4′, 6′ -difluorophenyl)- pyridinato- N, C 2′] iridium(III) picolinate dopant.