White Stacked Electrophosphorescent Organic Light‐Emitting Devices Employing MoO₃ as a Charge‐Generation Layer

Abstract

High-efficiency white organic light-emitting devices (OLEDs) [1][2][3][4] are of interest due to their potential uses in fullcolor active-matrix displays coupled with color filters, and also as solid-state lighting sources. For full-color display backlights, high brightness is required because of loss of light in the optical films and the small aperture ratio (∼ 40 %) in the backplane of the thin-film transistors. [5] Similarly, high brightness (> 800 cd m -2 ) is required for solid-state lighting sources. To achieve both high brightness and efficiency, the stacked OLED (SOLED) [6][7][8] consisting of multiple electroluminescent elements connected in series has been introduced. More recently, high-efficiency green-light-emitting SOLEDs have been demonstrated. These devices use a transparent chargegenerating interlayer such as indium tin oxide (ITO), [9] V 2 O 5 , [9] or an organic p-n junction, [10] where the hole-transporting layer (HTL) and electron-transporting layer (ETL) are doped with FeCl 3 and Li, respectively. In the SOLED structure, the luminance at a fixed current density increases linearly with the number of stacked and independent OLED elements. This can lead to a significant improvement in lifetime, as well as external efficiency, by reducing the degradation that accompanies the high drive currents required to achieve similarly high brightness in a single-element OLED.Here we demonstrate high-efficiency white-light-emitting SOLEDs based on phosphorescent emitters with Li-doped ETLs. To the best of our knowledge, it is the first report using a vacuum-thermally deposited MoO 3 film interposed between stacked elements. Figure 1 shows the transmittance of a layer of MoO 3 . The film is more transparent in the wavelength range from 400 to 500 nm compared with previously reported V 2 O 5 interlayers, [11,12] which is essential to achieve an efficient white SOLED. Furtheremore, it is easier to handle than the corrosive and optically absorbing FeCl 3 . To our knowledge, this also is the first device that utilizes phosphor with pyrazolyl-based ligands, which provides a blue-green emission, in a white-light-emitting OLED. Figure 2 shows the proposed energy diagram of the white electrophosphorescent SOLED consisting of multiple, nearly identical white-light-emitting OLED elements, or subpixels. The highest occupied molecular orbital (HOMO) energies for each material were measured using UV photoemission spectroscopy, and the position of the lowest unoccupied molecular orbital (LUMO) energies were estimated by adding the energy corresponding to the onset of optical absorption to the HOMO energy. This procedure generally leads to a systematic underestimation of the HOMO-LUMO energy gap by 0.5 to 1.0 eV. In the white SOLED, two or three electrophosphorescent subpixels are stacked (corresponding to a 2-or 3-SOLED, respectively), with the thickness of the 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (NPD) HTL varying in each element. To obtain both high efficiency and balanced emission intensity from each phospho...

Tenfold increase in the lifetime of blue phosphorescent organic light-emitting diodes

“…Here, PQIr is co-doped at 2 vol% at different positions separated by 5 nm in the EMLs of D1-D3, with a doping layer width of 1.5 nm. The HOMO and lowest unoccupied molecular orbital energies of PQIr are at 5.0 and 2.7 eV relative to the vacuum level, respectively 24 . Owing to the low doping concentration and narrowness of the sensing layers, their presence should not significantly affect the charge transport or recombination properties in the EML.…”

Transition metal oxides on organic semiconductors

“…For example, in organic light-emitting diodes (OLEDs) [7], they are used as anode modification interlayers [3], which can substantially reduce the hole injection barrier. They are also key components of charge generation layer in tandem OLEDs [8][9][10]. In organic solar cells (OSCs) [11], they are employed as charge extraction interlayer [12][13][14] and recombination layer [15,16].…”

C60/N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine:MoO3 as the interconnection layer for high efficient tandem blue fluorescent organic light-emitting diodes

“…2,3 Tandem organic light emitting devices (OLEDs) have received considerable attention recently because of their high current efficiency and long lifetime. [4][5][6] Tandem OLED generally consists of two or more emissive units. Thus, in principle, the electroluminescence (EL) intensity of the tandem device can linearly increase with the number of emissive units.…”