Towards Efficient and Stable Deep Blue-Light-emitting Devices by Employing HAT-CN Doped TFB-x As the Hole Injection Material

Efficient deep-blue organic light-emitting diodes have been generated by incorporating the p-type dopant hexaazatriphenylene hexacarbonitrile (HAT-CN) into poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4 '-(N-(butylphenyl)))] (TFB-x) as hole injection layer. The blend hole injection layer effectively reduced exciton quenching, markedly decreased the hole injected barrier and improved the hole injection capability. The corresponding highest occupied molecular orbital (HOMO) levels significantly reduced from -5.49 eV to -5.77 eV, and the hole mobility was dramatically increased from 1.71 x 10- 8 cm2 V- 1s- 1 to 1.92 x 10-6 cm2 V- 1s- 1, due to the increased doping concentration of HAT-CN. As expected, the OLEDs emitted deep-blue light with the CIE coordinates of (0.15, 0.14) and exhibited the apparently advanced performances, a high electroluminescence brightness of 33596 cd/m2, a luminous efficiency (LEmax) of 4.09 cd/A, and a luminance efficiency of 2.61 lm/W with a low turn-on voltage of 3.4 V based 2 % HAT-CN doped TFB-x were achieved. The device maintained a LEmax of 4.06 cd/A at a luminance of 1000 cd/m2. In addition, stable EL spectra were obtained and were nearly identical when the applied voltage was increased from 5 to 9 V. The results indicated that blending the appropriate hole-injection material can be an efficient method to improve device performance based on the large band gap of deep blue-lighting materials.