Solution processed single-emissive-layer white organic light-emitting diodes based on fluorene host: Balanced consideration for color quality and electroluminescent efficiency

Cost-effective fabrication of white organic light-emitting diodes (WOLED) is meaningful toward commercial application of environment-friendly solid-state lighting sources. Electroluminescent efficiency and color quality are two opposite performance characteristics facing solution processed WOLEDs requiring balanced consideration. Herein, a recently synthesized molecule of 4,4'-(9,9'-(1,3-phenylene)bis(9H-fluorene-9,9-diyl))bis(N,N-diphenylaniline) (DTPAFB) is introduced as a host material for solution processed all-phosphor WOLEDs, embracing four well-known molecules which are blue iridium (III) bis(2-(4,6-difluorophenyl)pyridinato-N,C²)(picolinate) (FIrpic), green iridium (III) bis[2-(2-pyridinyl-N)phenyl-C](2,4-pentanedionato-O²,O⁴) [Ir(ppy)₂(acac)], and orange iridium (III) bis(2-phenyl-benzothiazole-C²,N)(acetylacetonate) [Ir(bt)₂(acac)] plus a home-made red phosphor of iridium (III) tris(1-(2,6-dimethylphenoxy)-4-(4-chlorophenyl)phthalazine) [Ir(MPCPPZ)₃]. Illumination quality white light with high brightness, high efficiency, suitable correlated color temperature (CCT), high color-rendering index (CRI), and stable electroluminescent (EL) emission is obtained. A stable white emission with a CRI over 70, Commission Internationale de L'Eclairage (CIE) of (0.37, 0.42), and high EL efficiency of 19.6 lm W^-1 at high luminance of 2000 cd m^-2 for blue/orange complementary color WOLEDs is demonstrated. The optimized red/green/blue three primary color WOLEDs show improved CRI up to 81, moderate high efficiency of 25.8 cd A^-1, 14.4 lm W^-1, and EQE of 13.9%. Furthermore, the red/green/blue/orange four primary color WOLEDs show the optional balance between color quality and EL efficiency with high CRI of around 81-83 and medium CCT of 3755-3929 K which is warm and soft to human eyes. At an illumination relevant luminance of 1000 cd m^-2, the total power efficiency reaches 33.6 lm W^-1, and still remains 30.2 lm W^-1 at 3000 cd m^-2, approaching the efficiency of state-of-the-art fluorescent-tube (40-70 lm W^-1), potentially suitable as an environment-friendly solid-state lighting source. This work indicates that developing high performance host materials and highly efficient phosphors and carefully combining them with common phosphors is an effective way toward high performance WOLEDs.