We report a full series of blue, green and red quantum-dot-based light-emitting devices (QD-LEDs), all with high external quantum efficiencies over 10%. We show that the fine nanostructure of quantum dots-especially the composition of the graded intermediate shell and the thickness of the outer shell-plays a very important role in determining QD-LED device performance due to its effects on charge injection, transport and recombination. These simple devices have maximum current and external quantum efficiencies of 63 cd A −1 and 14.5% for green QD-LEDs, 15 cd A −1 and 12.0% for red devices, and 4.4 cd A −1 and 10.7% for blue devices, all of which are well maintained over a wide range of luminances from 10 2 to 10 4 cd m −2 . All the QD-LEDs are solution-processed for ease of mass production, and have low turn-on voltages and saturated pure colours. The green and red devices exhibit lifetimes of more than 90,000 and 300,000 h, respectively. Since their inception about three decades ago 1-3 , semiconductor quantum dots have been intensively investigated because of their unique optical properties, including size-controlled tunable emission wavelength (known as the 'quantum confinement effect'), narrow emission spectra, high luminescent efficiency and colloidal-based synthesis process 4-7 . All these attractive characteristics make quantum dots excellent candidates for the development of next-generation display technologies. Quantum dot-based lightemitting diodes (QD-LEDs) have been demonstrated recently, and may offer many advantages over conventional LED and organic LED (OLEDs) technologies in terms of colour purity, stability and production cost, while still achieving similar levels of efficiency. To date, however, the electroluminescence efficiencies of QD-LEDs have remained significantly below those of OLEDs, despite steady progress in recent years [8][9][10][11][12][13][14][15][16][17] . Recently, an efficient deep-blue QD-LED has been reported that makes use of solutionprocessed poly(3,4-ethylenedioxythiophene):polystyrene sulphonate (PEDOT:PSS) and poly(N-vinyl carbazole) (PVK) as its hole injection and transport layers (HIL and HTL), respectively, and ZnO nanoparticles as its electron transport layer (ETL), and achieves a maximum external quantum efficiency (η EQE ) of 7.1% (ref. 15). The same device structure was also used to achieve a green QD-LED with an η EQE of 12.6% (ref. 17). Highly efficient red QD-LEDs with η EQE = 18-20% have been realized using an inverted device structure containing a vacuum-deposited HIL and HTL 16 , and also in another arrangement using a thin insulating layer to obtain an enhanced charge balance 18 . These are the first times that the performances of QD-LEDs have been comparable to those of state-of-the-art phosphorescent OLEDs 19-21 .It is noted that although high efficiencies have been achieved with blue (B), green (G) and red (R) QD-LEDs, these singlecolour QD-LEDs, developed by different research groups, commonly involve very different quantum dot preparation procedures (fo...
We demonstrate double-heterostructure copper phthalocyanine/C60 organic photovoltaic cells with series resistances as low as 0.1 Ω cm2. A high fill factor of ∼0.6 is achieved, which is only slightly reduced at very intense illumination. As a result, the power conversion efficiency increases with the incident optical power density, reaching a maximum of (4.2±0.2)% under 4–12 suns simulated AM1.5G illumination. The cell performance is accurately described employing an analysis based on conventional semiconductor p–n junction diodes. The dependence of the series resistance on the device area suggests the dominance of the bulk resistance of the indium-tin-oxide anode as a limiting factor in practical cell efficiencies.
The foaming solutions were prepared by mixing a cationic surfactant (tetradecyltrimethylamonium bromide, TTAB) or an anionic surfactant (sodium dodecylsulfate, SDS), with water, titanium ethoxide, and HCl. Typically, titanium ethoxide was added to an aqueous solution of TTAB (35 wt.-%) or SDS (15 wt.-%) in order to reach a proportion of 10 wt.-%. Then, the pH of the solution was adjusted to pH = 1 by adding HCl (37 %). The mixture was subjected to strong stirring for 30 min to homogenize the solution and to evaporate ethanol produced by the hydrolysis of titanium alkoxide. A particulate sol could be obtained by aging for 20 h. Foam was obtained by bubbling nitrogen through a porous glass disk into perfluorohexane in a 2.5 cm-diameter, 60 cm-high Plexiglas column. Different porosity glass disks (100±160 lm, 40±100 lm, 16±40 lm, or 10±16 lm) could be used to introduce nitrogen into the foaming solution. The reaction took place inside the Plexiglas column. During the reaction, the foam was wetted from above with the foaming solution. Imposing a sol flux Q at the top of the foam allowed the imposition of a constant and homogeneous liquid fraction to the entire sample. Varying the sol flux Q at the top of the foam varied the liquid fraction, and thus tuned the morphology of the foam. Metastable foams were recovered at the top of the column with a spatula and stored in a beaker. Then, the foam was immediately treated with an aqueous ammonia solution (20 wt.-%) with a pipette in order to promote titanium dioxide condensation. The quantity of ammonia used during the process depended upon the foam-liquid fraction. Typically, we used 0.5 mL of ammonia solution for 100 mL of foam and a sol flux of 0.024 g s ±1 , so the ratio was 2 mL/ 100 mL for a sol flux of 0.160 g s ±1 . The final foams were then frozen overnight and lyophilized for 5 h. The resulting hybrid organic±inor-ganic monolith-type materials were then thermally treated at 500 C in order to obtain the anatase structure of TiO 2 , or at 900 C to obtain the rutile structure. The heating rate was 2 C min ±1 , with a first Plateau at 200 C for 2 h. The cooling process was uncontrolled and depended upon oven cooling. The final inorganic scaffolds were then analyzed.Transmission electron microscopy (TEM) experiments were performed with a Jeol 2000 FX microscope (acceleration voltage of 200 kV). The samples were prepared as follows: TiO 2 scaffolds in a powder state were deposited on a copper grid coated with a Formvar/ carbon membrane. Scanning electron microscopy (SEM) observations were performed with a Jeol JSM-840A SEM operating at 10 kV. The specimens were gold-coated or carbon-coated prior to examination. Mesoscale surface areas and pore characteristics were obtained with a Micromeritics ASAP 2010 instrument, employing the Brunauer±Em-mett±Teller (BET) method. Prior to performing the nitrogen adsorption±desorption measurements, the macrocellular-foam monoliths were reduced to a powder state. Small-angle X-ray experiments were carried on with an 18 kW rotating-anod...
We demonstrate high-efficiency organic photovoltaic cells by stacking two hybrid planar-mixed molecular heterojunction cells in series. Absorption of incident light is maximized by locating the subcell tuned to absorb long-wavelength light nearest to the transparent anode, and tuning the second subcell closest to the reflecting metal cathode to preferentially absorb short-wavelength solar energy. Using the donor, copper phthalocyanine, and the acceptor, C60, we achieve a maximum power conversion efficiency of ηP=(5.7±0.3)% under 1 sun simulated AM1.5G solar illumination. An open-circuit voltage of VOC⩽1.2V is obtained, doubling that of a single cell. Analytical models suggest that power conversion efficiencies exceeding 6.5% can be obtained by this architecture.
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