Wenzhu Gao

Down-conversion monochromatic light-emitting diodes with the color determined by the active layer thickness and concentration of carbon dots

We report a series of monochromatic light-emitting diodes based on monodisperse carbon dots with an emission color ranging from blue to red, which is determined by the thickness of the down-conversion layers and the carbon dot doping concentration in the polymer matrix. We further demonstrate the potential of CDs for fabrication of relief graphical patterns with anti-counterfeiting security.

Single layer graphene electrodes for quantum dot-light emitting diodes

Single layer graphene was employed as the electrode in quantum dot-light emitting diodes (QD-LEDs) to replace indium tin oxide (ITO). The graphene layer demonstrated low surface roughness, good hole injection ability, and proper work function matching with the poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) layer. Together with the hole transport layer and electron transport layer, the fabricated QD-LED showed good current efficiency and power efficiency, which were even higher than an ITO-based similar device under low current density. The result indicates that graphene can be used as anodes to replace ITO in QD-LEDs.

PbSe nanocrystal solar cells using bandgap engineering

Benefiting from the strong quantum confinement, PbSe nanocrystals allow their bandgap and absorption edge to be tuned to optimize the absorption of the solar radiation. Here, bandgap engineering-based photovoltaic devices were designed, fabricated, and characterized using two-size PbSe nanocrystals. The fabricated two size particle photovoltaic devices showed 12.8% higher power conversion efficiency compared to that of the single-particle devices, as a result of the enhanced photon absorption and the improved charge transfer.

Colloidal PbSe Solar Cells with Molybdenum Oxide Modified Graphene Anodes.

With good electrical conductivity, optical transparency, and mechanical compliance, graphene films have shown great potential in application for photovoltaic devices as electrodes. However, photovoltaic devices employing graphene anodes usually suffer from poor hole collection efficiency because of the mismatch of energy levels between the anode and light-harvesting layers. Here, a simple solution treatment and a low-cost solution-processed molybdenum oxide (MoOx) film were used to modify the work function of graphene and the interfacial morphology, respectively, yielding highly efficient hole transfer. As a result, the graphene/MoOx anodes demonstrated low surface roughness and high electrical conductivity. Using the graphene/MoOx anodes in PbSe nanocrystal solar cells, we achieved 1 sun power conversion efficiency of 3.56%. Compared to the control devices with indium tin oxide anodes, the graphene/MoOx-based devices show excellent performance, demonstrating the great potential of the graphene/MoOx anodes for use in optoelectronics.

Planar temperature sensing using heavy-metal-free quantum dots with micrometer resolution

This work describes a micrometer resolution and plane-array temperature-sensing method using the photoluminescence (PL) of ZnCuInS/ZnSe/ZnS quantum dots (QDs). Heavy-metal-free QDs were directly deposited on a printed circuit board to analyze the surface temperature of the devices on the board. An optical fiber monochromator and a high-powered microscope were employed to fabricate a system which could collect temperature-dependent QD emissions from the micrometer area for the temperature measurements. This system realizes the imaging of the surface temperature distribution in the planar micrometer area. Temperature sensitivity of the PL intensity reached 0.66% °C(-1), and the relative error was less than 2%.

ZnCuInS/ZnSe/ZnS quantum dot-based downconversion light-emitting diodes and their thermal effect

The quantum dot-based light-emitting diodes (QD-LEDs) were fabricated using blue GaN chips and red-, yellow-, and green-emitting ZnCuInS/ZnSe/ZnS QDs. The power efficiencies were measured as 14.0 lm/W for red, 47.1 lm/Wf or yellow, and 62.4 lm/W for green LEDs at 2.6 V. The temperature effect of ZnCuInS/ZnSe/ZnS QDs on these LEDs was investigated using CIE chromaticity coordinates, spectral wavelength, full width at half maximum (FWHM), and power efficiency (PE). The thermal quenching induced by the increased surface temperature of the device was confirmed to be one of the important factors to decrease power efficiencies while the CIE chromaticity coordinates changed little due to the low emission temperature coefficients of 0.022, 0.050, and 0.068 nm/°C for red-, yellow-, and green-emitting ZnCuInS/ZnSe/ZnS QDs. These indicate that ZnCuInS/ZnSe/ZnS QDs are more suitable for downconversion LEDs compared to CdSe QDs.