Color-Tunable Hybrid White Organic Light-Emitting Diodes with Double Interlayers

An efficient color-tunable hybrid white organic light-emitting diode is demonstrated with double interlayers of 2,7-bis(carbazol-9-yl)-9,9-ditoylfluorene/2-(diphenylphosphoryl) spirofluorene (DMFL-CBP/SPPO1) inserted between blue fluorescent and yellow phosphorescent-emitting layers, and exhibits Commission Internationale de l’Eclairage (CIE1931) ranging from warm white (0.4368, 0.4497) to cool white (0.2781, 0.2896) with driving current density from 0.2 to 40 mA/cm. The recombination of singlet and the triplet excitons in blue fluorescent-emitting layer and yellow phosphorescent-emitting layer, respectively, can be modulated by both the thickness of these double interlayers and the applied current densities.


Introduction
Since the first practical sandwiched organic light-emitting diode (OLED) was reported by C.W. Tang in 1980s [1], thirty years have been passed with more and more attention paid in this field due to low cast, flexibility, area source, and high response [2] [3] [4].Hybrid white organic light-emitting diodes (HWO-LEDs), which posses the advantage of both the long stability of the blue fluorescent-emitting layer based OLEDs and the high electroluminescent efficiency of yellow phosphorescent-emitting layer based OLEDs, have been considered as a potential light source for lighting and display [5].

Experimental Section
After the 110 nm indium-tin-oxide (ITO) film coated glass substrate with a sheet resistance of 15 Ω/□ is treated by UV-ozone for 15 min, layers of molybdenum    When the thickness of DMFL-CBP increased, the exciton generation interface shift closer to the yellow EML, and thus leading to more exciton diffused into the yellow EML.Moreover, the efficiency of OLEDs with PO-01 as emitter is larger than that of OLEDs with BCzVBi as emitter.So, higher efficiency are achieved in the HWOLEDs with thicker DMFL-CBP.In Figure 2 To investigate the physical mechanism in these HWOLEDs, the normalized EL spectra driving at 0.2, 1, 2.5, 5, 10, 20, 40 mA/cm 2 are studied and exhibited in Figure 3 with the yellow emission peak intensity (located at 560 nm) as the reference intensity.The proportion of blue emission part in the normalized EL spectrum increased with the increasing current density in each devices.This may be caused by that more singlets are captured by the blue fluorescent emitter

Conclusion
A hybrid white light-emitting diode with widespread color CIE coordinates ranging from warm white (0.4368, 0.4497), across white (0.3289, 0.3408) to cool white (0.2781, 0.2896) is demonstrated by incorporating double interlayers of DMFL-CBP/SPPO1 between the blue fluorescent emitting layer and the yellow phosphorescent emitting layer.The color CIE coordinates can be tuned by both the thickness ratio of DMFL-CBP to SPPO1 and the driving current density, due to relative change of the exciton generation interface and the electron injecting ability into blue fluorescent emitting layer.
photometer and Keithley 2400 Source Meter were connected by software to measure the current-voltage curves, luminance, the electroluminescent (EL) spectra, CIE coordinates of the HWOLEDs.The active area of each HWOLED is 0.1 cm 2 .

Figure 2
Figure 2 shows the EL performance of the three HWOLEDs with different double interlayers of DMFL-CBP (3 nm)/SPPO1 (2 nm), DMFL-CBP (2.5 nm)/ SPPO1 (2.5 nm), DMFL-CBP (2 nm)/SPPO1 (3 nm) for Devices A, B and C, respectively.In current density-voltage-luminance curves in Figure 2(a), it can be found the thicker the DMFL-CBP, the lower the current density and the larger the luminance at each applied voltage.In Figure 2(b) and Figure 2(c), current efficiency and power efficiency increased with the increasing DMFL-CBP thickness at each applied current density, and shows the highest current efficiency and power efficiency of 36.5 cd/A and 30.1 lm/W driving at 0.2 mA/m 2 in Device A. And Devices B (C) shows maximum current efficiency and power efficiency of 27 cd/A (24.5 cd/A) and 22.7 lm/W (20.1 lm/W), respectively.These are attributed to that the exciton generation interface should be located at the interface of DMFL-CBP/SPPO1 owing to the high lowest unoccupied molecular orbital (LUMO) of DMFL-CBP (−1.29 eV) and the low highest occupied molecular orbital (HOMO) of SPPO1 (−6.5 eV) which can block electrons and holes at this interface, respectively.

Figure 1 .
Figure 1.The schematic diagram (a) and energy level graph of the HWOLEDs (Devices A, B and C).

Figure 3 .
Figure 3. (a) The normalized EL spectra of Device A; (b) The normalized EL spectra of Device B; (c) The normalized EL spectra of Device C driving at current densities of 0.2, 1, 2.5, 5, 10, 20, 40 mA/cm 2 ; (d) The EL spectra of Devices A, B, and C driving at 20 mA/cm 2 .