We report on an all-solution-processed fabrication of highly efficient green quantum dot-light-emitting diodes (QLEDs) with an inverted architecture, where an interfacial polymeric surface modifier of polyethylenimine ethoxylated (PEIE) is inserted between a quantum dot (QD) emitting layer (EML) and a hole transport layer (HTL), and a MoO hole injection layer is solution deposited on top of the HTL. Among the inverted QLEDs with varied PEIE thicknesses, the device with an optimal PEIE thickness of 15.5 nm shows record maximum efficiency values of 65.3 cd/A in current efficiency and 15.6% in external quantum efficiency (EQE). All-solution-processed fabrication of inverted QLED is further implemented on a flexible platform by developing a high-performing transparent conducting composite film of ZnO nanoparticles-overcoated on Ag nanowires. The resulting flexible inverted device possesses 35.1 cd/A in current efficiency and 8.4% in EQE, which are also the highest efficiency values ever reported in flexible QLEDs.

The performance of quantum dot light-emitting diodes (QD-LEDs) was investigated for different hole transport layers with small molecules and polymers: poly(4-butyl-phenyl-diphenyl-amine), poly-N-vinylcarbazole (PVK), N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-diphenyl-4,4'-diamine, 4,4',4″-tris(N-carbazolyl)-triphenyl-amine (TCTA), and 4,4'-bis(carbazole-9-yl)biphenyl (CBP). The electroluminescence performance of QD-LEDs was considerably improved by adding small molecules (TCTA or CBP) having high hole mobilily to the polymer hole transport material (PVK). The maximal current efficiency of QD-LED-based PVK was improved by 27% upon addition of 20 wt % TCTA due to the hole injection improvement. The lower turn-on voltage, the higher current density, and the higher luminance were achieved by addition of TCTA. The maximal luminance of 40900 cd/m(2) and the highest current efficiency of 14.0 cd/A with the narrow full width at half-maximum (<35 nm) were achieved by the best hole transport layer.

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In this work, we developed a charge control sandwich structure around QD layers for the inverted QLEDs, the performance of which is shown to exceed that of the conventional QLEDs in terms of the external quantum efficiency (EQE) and the current efficiency (CE). The QD light-emitting layer (EML) is sandwiched with two ultrathin interfacial layers: one is a poly(9-vinlycarbazole) (PVK) layer to prevent excess electrons, and the other is a polyethylenimine ethoxylated (PEIE) layer to reduce the hole injection barrier. The sandwich structure resolves the imbalance between injected holes and electrons and brings the level of balanced charge carriers to a maximum. We demonstrated the highly improved performance of 89.8 cd/A of current efficiency, 22.4% of external quantum efficiency, and 72 814 cd m of maximum brightness with the solution-processed inverted QLED. This sandwich structure (PVK/QD/PEIE), as a framework, can be applied to various QLED devices for enhancing performance.

Fluorescence of semiconductor quantum dots (QDs) can be tuned by engineering the band gap via size and composition control and further doping them with impurity ions. Targeting on highly bright white-emissive I-III-VI -type copper gallium sulfide (Cu-Ga-S, CGS) host QDs with the entire visible spectral coverage of blue to red, herein, Mn(2+) ion doping, through surface adsorption and lattice diffusion is fulfilled. Upon doping a distinct Mn emission from (4)T1-(6)A1 transition successfully appears in white photoluminescence (PL) of undoped CGS/ZnS core/shell QDs and with varying Mn concentration a systematic white spectral evolution of CGS:Mn/ZnS QDs is achievable with high PL quantum yield retained. The origins of white PL of CGS:Mn/ZnS QDs that is well decomposed into three emission bands are appropriately assigned. The resulting single-phased, doped QDs are then employed as near-UV-to-white down converters for the fabrication of white light-emitting diodes (LEDs). Electroluminescent properties of white QD-LEDs depending on Mn concentration of CGS:Mn/ZnS QDs and forward current are also discussed in detail.