Junjie Hao

Large Stokes Shift and High Efficiency Luminescent Solar Concentrator Incorporated with CuInS2/ZnS Quantum Dots.

Luminescent solar concentrator (LSC) incorporated with quantum dots (QDs) have been widely regarded as one of the most important development trends of cost-effective solar energy. In this study, for the first time we report a new QDs-LSC integrated with heavy metal free CuInS2/ZnS core/shell QDs with large Stokes shift and high optical efficiency. The as-prepared CuInS2/ZnS QDs possess advantages of high photoluminescence quantum yield of 81% and large Stocks shift more than 150u2009nm. The optical efficiency of CuInS2/ZnS QDs-LSC reaches as high as 26.5%. Moreover, the power conversion efficiency of the QDs-LSC-PV device reaches more than 3 folds to that of pure PMMA-PV device. Furthermore, the PV device is able to harvest 4.91 folds solar energy with the assistance of this new CuInS2/ZnS QDs-LSC for the same size c-Si PV cell. The results demonstrate that this new CuInS2/ZnS QDs-LSC provides a promising way for the high efficiency, nonhazardous and low cost solar energy.

Advanced principal component analysis method for phase reconstruction

Focus on the phase reconstruction from three phase-shifting interferograms with unknown phase shifts, an advanced principal component analysis method is proposed. First, use a simple subtraction operation among interferograms, two intensity difference images are obtained easily. Second, set the center region of the data of intensity difference images to zero, and then construct a covariance matrix to obtain a transformation matrix. Third, two principal components of interferograms can be determined by the Hotelling transform and then phase can be calculated from the two normalized principal components by an arctangent function. By means of the simulation calculation and the experimental research, it is proved that the phase with high precision can be obtained rapidly by the proposed algorithm.

High Efficiency and Color Rendering Quantum Dots White Light Emitting Diodes Optimized by Luminescent Microspheres Incorporating

Abstract In this research, we have developed an approach by incorporating quantum dots (QDs) with red emission into mesoporous silica microspheres through a non-chemical process and obtained luminescent microspheres (LMS). Owing to the lattice structure of LMS, QDs were effectively protected from intrinsic aggregation in matrix and surface deterioration by encapsulant, oxygen and moisture. The LMS composite has therefore maintained large extent luminescent properties of QDs, espe-cially for the high quantum efficiency. Moreover, the fabricated white light emitting diode (WLED) utilizing LMS and YAG:Ce yellow phosphor has demonstrated excellent light performance with color coordinates around (x = 0.33, y = 0.33), correlated color temperature between 5100 and 5500 K and color rendering index of Ra = 90, R9 = 95. The luminous efficiency of the WLED has reached up to a new record of 142.5 lm/W at 20 mA. LMS provide a promising way to practically apply QDs in lightings and displays with high efficiency as well as high stability.

Structural optimization for remote white light-emitting diodes with quantum dots and phosphor: packaging sequence matters

White light-emitting diodes (WLEDs) with quantum dots (QDs) and phosphor have attracted tremendous attentions due to their excellent color rendering ability. In the packaging process, QDs layer and phosphor-silicone layer tend to be separated to reduce the reabsorption losses, and to maintain the stability of QDs surface ligands. This study investigated the packaging sequence between QDs and phosphor on the optical and thermal performances of WLEDs. The output optical power and PL spectra were measured and analyzed, and the temperature fields were simulated and validated experimentally by infrared thermal imager. It was found that when driven by 60 mA, the QDs-on-phosphor type WLEDs achieved luminous efficiency (LE) of 110 lm/W, with color rendering index (CRI) of Ra = 92 and R9 = 80, while the phosphor-on-QDs type WLEDs demonstrated lower LE of 68 lm/W, with Ra = 57 and R9 = 24. Moreover, the QDs-on-phosphor type WLEDs generated less heat than that of another, consequently the highest temperature in the QDs-on-phosphor type was lower than another, and the temperature difference can reach 12.3°C. Therefore, in terms of packaging sequence, the QDs-on-phosphor type is an optimal packaging architecture for higher optical efficiency, better color rendering ability and lower device temperature.

Realization of wide circadian variability by quantum dots-luminescent mesoporous silica-based white light-emitting diodes

Human comfort has become one of the most important criteria in modern lighting architecture. Here, we proposed a tuning strategy to enhance the non-image forming photobiological effect on the human circadian rhythm based on quantum-dots-converted white light-emitting diodes (QDs-WLEDs). We introduced the limiting variability of the circadian action factor (CAF), defined as the ratio of circadian efficiency and luminous efficiency of radiation. The CAF was deeply discussed and was found to be a function of constraining the color rendering index (CRI) and correlated color temperatures. The maximum CAF variability of QDs-WLEDs was found to be dependent on the QDs peak wavelength and full width at half maximum. With the optimized parameters, the packaging materials were synthesized and WLEDs were packaged. Experimental results show that at CRIxa0>xa090, the maximum CAF variability can be tuned by 3.83 times (from 0.251 at 2700 K to 0.961 at 6500 K), which implies that our approach could reduce the number of tunable channels, and could achieve wider CAF variability.

Light Conversion Efficiency Enhancement of Modified Quantum Dot Films Integrated With Micro SiO 2 Particles

Photoluminescence quantum dots (QDs) have been considered as a kind of promising light converting materials with high luminous efficiency, tunable spectrum, and narrow emission. However, the current light conversion efficiency (LCE) of QD films is at a relative low level, which will result in many problems, such as inferior luminous performance, severe self-heating, etc. To enhance the LCE of QD films, SiO2 particles were doped into QD films by physical blending, for their remarkable light scattering effect. The LCE enhancement after adding SiO2 particles were studied by experiments. Experimental results showed that the SiO2 modified QD films improved the LCE up to 63.45%, which was 103.88% higher than conventional QD films. Besides, as the SiO2 particles diameters increased, the optimal mass fractions of SiO2 particles for top LCE would decrease.

Precise optical modeling of quantum dots for white light-emitting diodes

Quantum dots (QDs)-based white light-emitting diodes (QDs-WLEDs) have been attracting numerous attentions in lighting and flat panel display applications, by virtue of their high luminous efficacy and excellent color rendering ability. However, QDs’ key optical parameters including scattering, absorption and anisotropy coefficients for optical modeling are still unclear, which are severely against the design and optimization of QDs-WLEDs. In this work, we proposed a new precise optical modeling approach towards QDs. Optical properties of QDs-polymer film were obtained for the first time, by combining double integrating sphere (DIS) system measurement with inverse adding doubling (IAD) algorithm calculation. The measured results show that the typical scattering, absorption and anisotropy coefficients of red emissive QDs are 2.9382u2009mm−1, 3.7000u2009mm−1 and 0.4918 for blue light, respectively, and 1.2490u2009mm−1, 0.6062u2009mm−1 and 0.5038 for red light, respectively. A Monte-Carlo ray-tracing model was set-up for validation. With a maximum deviation of 1.16%, the simulated values quantitatively agree with the experimental results. Therefore, our approach provides an effective way for optical properties measurement and precise optical modeling of QDs for QDs-WLEDs.

Optically Active CdSe-Dot/CdS-Rod Nanocrystals with Induced Chirality and Circularly Polarized Luminescence

Ligand-induced chirality in semiconductor nanocrystals (NCs) has attracted attention because of the tunable optical properties of the NCs. Induced circular dichroism (CD) has been observed in CdX (X = S, Se, Te) NCs and their hybrids, but circularly polarized luminescence (CPL) in these fluorescent nanomaterials has been seldom reported. Herein, we describe the successful preparation of l- and d-cysteine-capped CdSe-dot/CdS-rods (DRs) with tunable CD and CPL behaviors and a maximum anisotropic factor ( glum) of 4.66 × 10-4. The observed CD and CPL activities are sensitive to the relative absorption ratio of the CdS shell to the CdSe core, suggesting that the anisotropic g-factors in both CD and CPL increase to some extent for a smaller shell-to-core absorption ratio. In addition, the molar ratio of chiral cysteine to the DRs is investigated. Instead of enhancing the chiral interactions between the chiral molecules and DRs, an excess of cysteine molecules in aqueous solution inhibits both the CD and CPL activities. Such chiral and emissive NCs provide an ideal platform for the rational design of semiconductor nanomaterials with chiroptical properties.

Thermal analysis of white light-emitting diodes structures with hybrid quantum dots/phosphor layer

This study quantitatively analyzed the optical and thermal performances of remote quantum dots (QDs)-based white light-emitting diodes (QDs-WLEDs) with the same spectra power distribution (SPD) but reverse packaging structures. The output optical power and PL spectra were measured and analyzed by an integrating sphere system, and the temperature fields were simulated by combing optical measurement with thermal simulation, finally the temperature fields were validated by infrared thermal imager. It was found that when achieved identical SPD, the QDs-on-phosphor type achieved LE of 112.2 lm/W, while the phosphor-on-QDs type demonstrated lower LE of 101.4 lm/W. Moreover, the QDs-on-phosphor type generated less heat than that of another, consequently the highest temperature in the QDs-on-phosphor type was lower than another, and the temperature difference can reach 11.2°C at driving current of 160 mA. Therefore, the QDs-on-phosphor type is an optimal packaging architecture for higher optical efficiency and lower device temperature.

White light-emitting diodes with enhanced luminous efficiency and high color rendering using separated quantum dots@silica/phosphor structure

White light-emitting diodes (WLEDs) composed of blue LED chip, yellow phosphor and red quantum dots (QDs) are considered as potential alternative for next generation artificial light source, with their high luminous efficiency (LE) and color rendering index (CRI). However, the poor compatibility of QDs/silicone and the optical reabsorption effect between the mixed QDs/phosphor particles severely hinder the wide utilization of QDs-WLEDs. Therefore, in this letter, a separated QDs/phosphor structure was proposed to eliminate the reabsorption energy losses, and a silica shell was coated onto the QDs surface to solve the compatibility problem between QDs and silicone. With CRI > 90 and R9 > 90, the newly proposed QDs@silica nanoparticles based WLEDs (QSNs-WLEDs) demonstrated a LE enhancement of 14.6 % over conventional mixed type WLEDs. Benefit from the silica shell coating, the QSNs-WLEDs also show high stability under different driving current.