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...

The detailed characterization of solution‐derived nickel (II) oxide (NiO) hole‐transporting layer (HTL) films and their application in high efficiency organic photovoltaic (OPV) cells is reported. The NiO precursor solution is examined in situ to determine the chemical species present. Coordination complexes of monoethanolamine (MEA) with Ni in ethanol thermally decompose to form non‐stoichiometric NiO. Specifically, the [Ni(MEA)2(OAc)]+ ion is found to be the most prevalent species in the precursor solution. The defect‐induced Ni3+ ion, which is present in non‐stoichiometric NiO and signifies the p‐type conduction of NiO, as well as the dipolar nickel oxyhydroxide (NiOOH) species are confirmed using X‐ray photoelectron spectroscopy. Bulk heterojunction (BHJ) solar cells with a polymer/fullerene photoactive layer blend composed of poly‐dithienogermole‐thienopyrrolodione (pDTG‐TPD) and [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM) are fabricated using these solution‐processed NiO films. The resulting devices show an average power conversion efficiency (PCE) of 7.8%, which is a 15% improvement over devices utilizing a poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL. The enhancement is due to the optical resonance in the solar cell and the hydrophobicity of NiO, which promotes a more homogeneous donor/acceptor morphology in the active layer at the NiO/BHJ interface. Finally, devices incorporating NiO as a HTL are more stable in air than devices using PEDOT:PSS.

Significant progress in power conversion efficiencies and stabilities of polymer solar cells has been achieved using semiconducting metal oxides as charge extraction interlayers. Both n-and p-type transition metal oxides with good transparency in the visible as well as infrared region make good Ohmic contacts to both donors and acceptors in polymer bulk heterojunction solar cells. Their compatibility with roll-to-roll processing makes them very attractive for low cost manufacturing of polymer solar cells. In this review, we will present the recent results on synthesis and characterization of these metal oxides along with the device performance of the solar cells using these metal oxides as interlayers.

Interface recombination induced by the defect states in zinc‐oxide‐nanoparticle‐based electron extraction layer is reported as a significant loss‐mechanism of photocurrent collection. By choosing appropriate UV–ozone treatment conditions on the zinc oxide layer, inverted polymer solar cells show reduced interface recombination and thus improved power conversion efficiencies of up to 8.1%.

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