Efficient solution-processed small-molecule single emitting layer electrophosphorescent white light-emitting diodes

Efficient single emitting layer electrophosphorescent white organic light-emitting diodes (WOLEDs) based on small molecules were fabricated by solution process. A solution processible wide-gap compound 9,9-bis[4-(3,6-di-tert-butylcarbazol-9-yl)phenyl] fluorene (TBCPF) was used as the host material and 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxidiazolyl]phenylene (OXD-7) was doped to enhance the electron transport in the emission layer. The optimized single active layer WOLED with three phosphorescent dopants showed a maximum luminance efficiency of 20.8 cd A−1 and a maximum luminance of 26,600 cd m−2. And the maximum efficiency of double-layer WOLEDs with an additional hole-blocking/electron-transporting layer was further improved to 29.6 cd A−1. The color of the devices shifted slightly with the bias voltage. The energy transfer processes and the origin of the white emission in the single emitting layer were also discussed based on the measurement of the transient photoluminescence.