Lighting accounts for approximately 22 per cent of the electricity consumed in buildings in the United States, with 40 per cent of that amount consumed by inefficient (approximately 15 lm W(-1)) incandescent lamps. This has generated increased interest in the use of white electroluminescent organic light-emitting devices, owing to their potential for significantly improved efficiency over incandescent sources combined with low-cost, high-throughput manufacturability. The most impressive characteristics of such devices reported to date have been achieved in all-phosphor-doped devices, which have the potential for 100 per cent internal quantum efficiency: the phosphorescent molecules harness the triplet excitons that constitute three-quarters of the bound electron-hole pairs that form during charge injection, and which (unlike the remaining singlet excitons) would otherwise recombine non-radiatively. Here we introduce a different device concept that exploits a blue fluorescent molecule in exchange for a phosphorescent dopant, in combination with green and red phosphor dopants, to yield high power efficiency and stable colour balance, while maintaining the potential for unity internal quantum efficiency. Two distinct modes of energy transfer within this device serve to channel nearly all of the triplet energy to the phosphorescent dopants, retaining the singlet energy exclusively on the blue fluorescent dopant. Additionally, eliminating the exchange energy loss to the blue fluorophore allows for roughly 20 per cent increased power efficiency compared to a fully phosphorescent device. Our device challenges incandescent sources by exhibiting total external quantum and power efficiencies that peak at 18.7 +/- 0.5 per cent and 37.6 +/- 0.6 lm W(-1), respectively, decreasing to 18.4 +/- 0.5 per cent and 23.8 +/- 0.5 lm W(-1) at a high luminance of 500 cd m(-2).

No 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...

Organic light-emitting diodes (OLEDs) have been the subject of a significant research effort for the past two decades with a focus on devices that emit almost exclusively in the visible part of the electromagnetic spectrum.[1] Recently, there has been a growing interest in OLEDs that emit in the nearinfrared (NIR) region (700-2500 nm). [2][3][4] Applications for these NIR OLEDs are particularly interesting for nightvision-readable displays [5] and sensors.[6] The efficiency of OLEDs are markedly improved when fluorescent emissive dopants are replaced with phosphorescent heavy-metal complexes that can effectively harvest both the singlet and triplet excitons formed in electroluminescence, with wavelengths (l) ranging from the near-ultraviolet into the red (with peak emission at l max = 380-650 nm).[7-10] Herein, we report on an efficient NIR OLED that utilizes a phosphorescent Ptmetalloporphyrin dopant, with an external quantum efficiency (EQE) greater than 6 % at l max = 765 nm and a full width at half maximum of 31 nm (500 cm À1 ). Previously, two classes of phosphorescent complexes have been employed as dopants in NIR-emitting OLEDs. The first utilizes trivalent lanthanide cations (Ln 3+ ) as the emitting centers, for example, Er 3+ or Nd 3+ , chelated with chromophoric ligands to sensitize excitation-energy transfer to the lanthanide ion.[11] Schanze et al. have reported an NIR OLED utilizing Ln 3+ in conjunction with a porphyrin/polystyrene matrix, with EQE ranging from 8.0 10 À4 to 2.0 10 À4 % at approximately 1 mA cm À2 . [5] Similarly, a Nd(phenalenone) 3 -based OLED had an EQE of 0.007 % at l max = 1065 nm.[4] The second class of NIR OLEDs is transition-metal complexes, similar to those used in the visible region. A recent report of an electrophosphorescent device that used a cyclometalated [(pyrenyl-quinolyl) 2 Ir(acac)] complex as the phosphor gave l max = 720 nm and an EQE of 0.1 %. [6] A family of complexes that have shown intense absorption and emission in the red-to-NIR region of the spectrum are the metalloporphyrins. [12,13] There are a number of reports of OLEDs fabricated with [Pt(oep)], [Pt(tpp)] (oep = 2,3,7,8,12,13,17,18-ocatethylporphryin, tpp = 5,10,15,20-tetraphenylporphyrin), or analogues of these compounds as phosphorescent emitters, with emission maxima between 630 and 650 nm, [7,[14][15][16][17][18][19][20][21][22] however, there has been no apparent effort to shift the Pt-porphyrin-based OLED emission into the NIR region. Porphyrin chromophores with fused aromatic moieties at the b-pyrrole positions, for example, tetrabenzoporphyrin (bp), exhibit a bathochromic shift (relative to unsubstituted porphyrin) of the absorption and emission energy, owing to the expansion of the p-electronic system of the porphyrin core.[23] The addition of bulky groups to the meso positions of the porphyrin macrocycles with b-substituted pyrroles leads to the formation of nonplanar porphyrins, and further red-shifts the absorption spectra.[24] Coordination of a heavy-metal atom increases the rate of the int...