The influence of solvent refractive index on the photoluminescence decay of thick-shell gradient-alloyed colloidal quantum dots investigated in a wide range of delay times.

Colloidal semiconductor quantum dots have many potential optical applications, including quantum dot light-emitting diodes, single-photon sources, or biological luminescent markers. The optical properties of colloidal quantum dots can be affected by their dielectric environment. This study investigated the photoluminescence (PL) decay of thick-shell gradient-alloyed colloidal semiconductor quantum dots as a function of solvent refractive index. These measurements were conducted in a wide range of delay times to account for both the initial spontaneous decay of excitons and the delayed emission of excitons that has the form of a power law. It is shown that whereas the initial spontaneous PL decay is very sensitive to the refractive index of the solvent, the power-law delayed emission of excitons is not. Our results seem to exclude the possibility of carrier self-trapping in the considered solvents and suggest the existence of trap states inside the quantum dots. Finally, our data show that the average exciton lifetime significantly decreases as a function of the solvent refractive index. The change in exciton lifetime is qualitatively modeled and discussed.

Flexible Quantum-Dot Light-Emitting Diodes Using Embedded Silver Mesh Transparent Electrodes Manufactured by an Ultraprecise Deposition Method.

Transparent conductive electrodes (TCEs) fabricated onto flexible substrates are crucial parts of organic-light-emitting diodes (OLEDs), which are vastly utilized for display and lightning applications. Indium tin oxide (ITO), which is so far the most popular material for transparent and conductive electrodes, is found to be an unsuitable candidate for flexible devices mostly due to its brittleness. Here, we present a novel approach for the fabrication of transparent, conductive, and flexible electrodes for optoelectronic applications made of silver metal mesh by an ultraprecise deposition (UPD) method. The fabricated mesh exhibits an 80% (λ = 550 nm) optical transmittance and a sheet resistance of 11 Ω/sq. The Ag-mesh embedded into the polymer is implemented as an anode for a quantum-dot light-emitting diode (QLED) in order to assess its performance. The fabricated QLED is characterized by the maximum external quantum efficiency (EQE) of 2% and a current efficiency (CE) of 6 cd/A, reaching the maximum luminance (L) of 3200 cd/m2 at a current density of 100 mA/cm2. This method shows a fast and relatively simple approach to fabricate optoelectronic devices without the need for special treatment and sophisticated equipment.

Effect of Air Exposure of ZnMgO Nanoparticle Electron Transport Layer on Efficiency of Quantum-Dot Light-Emitting Diodes.

We demonstrate the effect of air exposure on optical and electrical properties of ZnMgO nanoparticles (NPs) typically exploited as an electron transport layer in Cd-based quantum-dot light-emitting diodes (QLEDs). We analyze the roles of air components in modifying the electrical properties of ZnMgO NPs, which reveals that H2O enables the reduction of hole leakage while O2 alters the character of charge transport due to its ability to trap electrons. As a result, the charge balance in the QDs layer is improved, which is confirmed by voltage-dependent measurements of photoluminescence quantum yield. The maximum external quantum efficiency is improved over 2-fold and reaches the value of 9.5% at a luminance of 104 cd/m2. In addition, we investigate the problem of electron leakage into the hole transport layer and show that trap-mediated electron transport in the ZnMgO layer caused by adsorbed O2 ensures a higher leakage threshold. This work also provides an insight into the possible disadvantages of device contact with air as well as problems and challenges that might occur during open-air fabrication of QLEDs.

Quantum-dot light-emitting diode with ultrathin Au electrode embedded in solution-processed phosphomolybdic acid.

We proposed to exploit phosphomolybdic acid (PMA) as a cost-efficient MoO x source for combined spin-coating/sputtering/spin-coating deposition of a MoO x /Au/MoO x (MAM) composite electrode. The bottom PMA layer provides perfect wetting conditions for ultrathin Au film sputtering and prevents the formation of gold islands on the glass surface, while the top PMA layer helps to reduce light reflection. By optimizing the thickness of ultrathin Au films and PMA layers, we achieved maximum transmittance of 79% at 550 nm and a sheet resistance of only 22 Ω sq-1 which is comparable to the resistance of ITO substrates (20 Ω sq-1). MAM multilayer was explored both as a transparent electrode and as a hole injection layer (HIL) to eliminate ITO and PEDOT:PSS from solution-processed quantum-dot light-emitting diodes (QLEDs). The fabricated MAM-based QLED shows a peak external quantum efficiency (EQE) of 2.7% and maximum brightness of 12 000 cd m-2 at 7 V. By performing bending tests of the polyethylene (PET) substrate coated with MAM electrode, we demonstrate that it is also a promising candidate for flexible transparent optoelectronics.