White-light-emitting organic electroluminescent devices based on interlayer sequential energy transfer

Abstract

We demonstrate efficient, molecular organic white-light-emitting devices using vacuum-deposited thin films of red luminescent [2-methyl-6-[2-(2,3,6,7-tetrahydro-1H, 5H-benzo [ij] quinolizin-9-yl) ethenyl]-4H-pyran-4-ylidene] propane-dinitrile (DCM2), doped into blue-emitting 4, 4′ bis [N-1-napthyl-N-phenyl-amino]biphenyl (α-NPD), and green-emitting tris-(8-hydroxyquinolinato) aluminum(III) (AlQ3). The luminescent layers are separated by a hole-blocking layer of 2,9-dimethyl, 4,7-diphenyl, 1,10-phenanthroline (BCP), whose thickness is on the order of a typical Förster transfer radius of 30–40 Å. Excitons formed on α-NPD sequentially transfer their energy via a Förster mechanism to AlQ3 across the BCP layer, and from AlQ3 to DCM2. This interlayer sequential energy transfer results in partial excitation of all three molecular species, thereby producing white light emission. The thickness of the blocking layer and the concentration of DCM2 in α-NPD permit the tuning of the device spectrum to achieve a balanced white emission with Commission Internationale d’Eclairage chromaticity coordinates of (0.33, 0.33). The spectrum is largely insensitive to the drive current, and the devices have a maximum luminance of 13 500 cd/m2. At a luminance of 100 cd/m2, the quantum and power efficiencies are 0.5% and 0.35 lm/W, respectively.

Engineering fused coumarin dyes: a molecular level understanding of aggregation quenching and tuning electroluminescence via alkyl chain substitution

“…Organic compounds having emission covering the whole visible region (400 nm to 700 nm) are lacking in the literature and only a few reports are available. [3][4][5][6] The most commonly exploited approaches for white light emissions 7 include multilayered device fabrication by consecutive evaporation of different emitting compounds, [7][8][9][10][11] spin coating of a blend of different soluble emitters, 12 utilizing excimer or exciplex emission, 13 use of organic-metal complexes in a hybrid device structure 5,14 and phosphorescent emitters with a suitable host. 15 The device structure also plays an important role for white emission.…”

Hole transport in the organic small molecule material α-NPD: evidence for the presence of correlated disorder

“…1͑a͔͒ is widely used in OLEDs as a hole-injection layer, 28 hole-transport layer, [29][30][31][32][33][34] electron-blocking layer, 35 blueemitting layer, 36,37 and as a host material in mixed emitting layers. 31,[38][39][40] The relatively high glass transition temperature, 95°C, is viewed as beneficial to the OLED stability. In none of the earlier studies of the hole-mobility of ␣-NPD, based on the steady-state current density versus voltage ͓J͑V͔͒ curves, 16,24,41,42 time-of-flight measurements [43][44][45][46][47][48][49] or impedance measurements, 50 the carrier-density dependence of the mobility was taken into account.…”

“…This material ͓also called NPB, see Fig. 1͑a͔͒ is widely used in OLEDs as a hole-injection layer, 28 hole-transport layer, [29][30][31][32][33][34] electron-blocking layer, 35 blueemitting layer, 36,37 and as a host material in mixed emitting layers. 31,[38][39][40] The relatively high glass transition temperature, 95°C, is viewed as beneficial to the OLED stability.…”

Characterization of white electroluminescent devices fabricated using conjugated polymer blends

“…In small molecule devices, a red light emitting material is co-deposited with blue and/or green light emitting materials. [21][22][23] Kido et al have reported that solution-processed polymer devices using composites of blue (B), green (G), and red (R) emitting dyes and poly(vinylcarbazole) emit white light. 24 In both vacuum-deposited small molecule devices and solution-processed polymer devices, control of the energy transfer between the red, green, and blue dyes is essential and usually requires the introduction of very low levels of doping controls.…”