Tieqiang Zhang

Color-Switchable Electroluminescence of Carbon Dot Light-Emitting Diodes

Carbon-dot based light-emitting diodes (LEDs) with driving current controlled color change are reported. These devices consist of a carbon-dot emissive layer sandwiched between an organic hole transport layer and an organic or inorganic electron transport layer fabricated by a solution-based process. By tuning the device structure and the injecting current density (by changing the applied voltage), we can obtain multicolor emission of blue, cyan, magenta, and white from the same carbon dots. Such a switchable EL behavior with white emission has not been observed thus far in single emitting layer structured nanomaterial LEDs. This interesting current density-dependent emission is useful for the development of colorful LEDs. The pure blue and white emissions are obtained by tuning the electron transport layer materials and the thickness of electrode.

High photocurrent PbSe solar cells with thin active layers

Thin active layers in solar cells normally have lower defect density with easier procedure and lower cost than thick ones. PbSe nanocrystal (NC)-based solar cells with thin layers were fabricated using a ligand exchange procedure. An optical spacer was confirmed to be effective in improving the distribution of light field in the devices, which could enhance the photon absorption of the thin active layers. Meanwhile, the transportation of electrons and holes was balanced and optimized by tuning the particle size and the layer thickness, which demonstrated a high short-circuit current of 32.2 mA cm−2 and a 1 sun power conversion efficiency of 4.12%. Besides, the devices with smaller PbSe NCs preserved the high efficiency for tens of hours, which is different from previous studies using large NCs.

Real-time and on-chip surface temperature sensing of GaN LED chips using PbSe quantum dots

PbSe quantum dots (QDs) were employed as real-time and on-chip temperature sensors to monitor the surface temperature of GaN LED chips. The temperature-dependent photoluminescence spectra were achieved and confirmed to be a good method for surface temperature sensing in a micro- to nano-region. The nanosized QD sensors did not influence the LED emission spectrum due to their infrared emission and little absorption. The surface temperature of GaN LED chips was analyzed at different working times and voltages. The temperature sensitivity characterized by the photoluminescence peak position of PbSe QDs was found to be 0.15 nm °C(-1) in a range of 30-120 °C and the precision was determined to be ± 3 °C. The QD surface temperature sensors were confirmed to have good reversibility and repeatability.

Colloidal PbSe quantum dot-solution-filled liquid-core optical fiber for 1.55 μm telecommunication wavelengths.

We have studied the optical properties of PbSe colloidal quantum dot-solution filled hollow core multimode silica waveguides as a function of quantum dot-solution concentration, waveguide length, optical pump power and choice of organic solvent in order to establish the conditions to maximize near infrared spontaneous emission intensities. The optical performance was compared and showed good agreement with a simple three level system model for the quantum dots confined in an optical waveguide. Near infrared absorption-free solvent of tetrachlorethylene was confirmed to be a good candidate for the waveguide medium due to the enhancement of output intensity from the liquid-core fiber compared to the performance in toluene-based fiber. This approach demonstrates a useful method for early characterization of quantum dot materials in a waveguide test-bed with minimal material processing on the colloidal nanoparticles.

Multicolor fluorescent light-emitting diodes based on cesium lead halide perovskite quantum dots

High quantum yield, narrow full width at half-maximum and tunable emission color of perovskite quantum dots (QDs) make this kind of material good prospects for light-emitting diodes (LEDs). However, the relatively poor stability under high temperature and air condition limits the device performance. To overcome this issue, the liquid-type packaging structure in combination with blue LED chip was employed to fabricate the fluorescent perovskite quantum dot-based LEDs. A variety of monochromatic LEDs with green, yellow, reddish-orange, and red emission were fabricated by utilizing the inorganic cesium lead halide perovskite quantum dots as the color-conversion layer, which exhibited the narrow full width at half-maximum (<35 nm), the relatively high luminous efficiency (reaching 75.5 lm/W), and the relatively high external quantum efficiency (14.6%), making it the best-performing perovskite LEDs so far. Compared to the solid state LED device, the liquid-type LED devices exhibited excellent color stability a...

Near-infrared light emitting diodes using PbSe quantum dots

Near infrared light emitting diodes (NIR LEDs) were fabricated employing blue GaN chips as the excitation source and PbSe quantum dots as the NIR emitting materials. Quantum dots with different emitting wavelengths were selected to fabricate three NIR LEDs corresponding to two typical applications of illumination and optical communication. The variation of emission peak and full width at half-maximum of the devices were investigated under different voltage bias, and the highest external quantum efficiency of 2.52% was achieved, which was comparable to those commercial InGaAsP LEDs and visible quantum dot electroluminescence LEDs.

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.

Temperature effect on colloidal PbSe quantum dot-filled liquid-core optical fiber

The optical property variation induced by temperature change for PbSe quantum dot (QD)-filled hollow core fiber was investigated as a function of particle size, waveguide length, and doping concentration. The temperature coefficients of emission peak and intensity in QD-filled fiber were obviously size-dependent. The fiber filled with 4.5 nm PbSe QDs had better temperature stability in emission wavelength. The mechanism of the output loss in fiber was established, based on the thermal quenching of QD luminescence and the guided mode leakage arising from the temperature dependent refractive indexes of the fiber core and the cladding. (C)2014 Optical Society of America

Tunable near-Infrared Luminescence of PbSe Quantum Dots for Multigas Analysis

Multigas sensing is highly demanded in the fields of environmental monitoring, industrial production, and coal mine security. Three near-infrared emission wavelengths from PbSe quantum dots (QDs) were used to analyze the concentration of three gases simultaneously through direct absorption spectroscopy, including acetylene (C2H2), methane (CH4), and ammonia (NH3). The corresponding lower detection limits for the three gases were 20, 100, and 20 ppm, respectively, with an accuracy of 2%. This study demonstrates that QDs with tunable emissions have great potential for simultaneous and uninterfered multiplex gas analysis and detection due to the advantages of the easy tunability of multiplex emitting wavelengths from QDs.

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.