Improvement Of Photoluminescence Efficiency Research Articles

Improvement of photoluminescence efficiency and photoelectricity performances of InP quantum dots through the passivation of surface defects

At present, InP quantum dot-sensitized solar cells (QDSSCs) still show moderate power conversion efficiency (PCE), because of charge recombination on the surface defects. Herein, we report a method to reduce the surface defects of InP quantum dots by Ga-doped and ZnS shell coating, which improves the performance of InP QDSSCs. Simultaneous enhancement of photoluminescence (PL) and photo-electricity performance of Ga doped InP (Ga:InP) quantum dots (QDs), Ga:InP/ZnS core/shell QDs were achieved. The maximum PL quantum yield of Ga:InP QDs and Ga:InP/ZnS QDs reached 14.9% and 76%, respectively. The maximum PCE value of Ga:InP and Ga:InP/ZnS QDSSCs reached 3.0% and 4.5% under 1 sun illumination (AM 1.5 G, 100 mW/cm2), respectively. The J-V curve revealed that Ga doping and ZnS shell coating reduced the surface recombination and increased the cell efficiency and stability. These results provided a new approach to obtaining quantum dot materials with high luminescence properties or good solar cell performances.

Enhancing Multifunctionalities of Transition-Metal Dichalcogenide Monolayers via Cation Intercalation.

We have demonstrated that multiple functionalities of transition-metal dichalcogenide (TMDC) monolayers may be substantially improved by the intercalation of small cations (H+ or Li+) between the monolayers and underlying substrates. The functionalities include photoluminescence (PL) efficiency and catalytic activity. The improvement in PL efficiency may be up to orders of magnitude and can be mainly ascribed to two effects of the intercalated cations: p-doping to the monolayers and reducing the influence of substrates, but more studies are necessary to better understand the mechanism for the improvement in the catalytic functionality. The cation intercalation may be achieved by simply immersing substrate-supported monolayers into the solution of certain acids or salts. It is more difficult to intercalate under the monolayers interacting with substrates stronger, such as as-grown monolayers or the monolayers on 2D material substrates. This result presents a versatile strategy to simultaneously optimize multiple functionalities of TMDC monolayers.

Origin of huge photoluminescence efficiency improvement in InGaN/GaN multiple quantum wells with low-temperature GaN cap layer grown in N2/H2 mixture gas

The nominal internal quantum efficiency of InGaN/GaN multiple quantum wells significantly increases from 5.6 to 26.8%, as a low-temperature GaN cap layer is grown in N2/H2 mixture gas. Meanwhile, the room-temperature photoluminescence (PL) peak energy shows a merely 73 meV blue shift. On the basis of temperature-dependent PL characteristics analysis, the huge improvement in PL efficiency arises mainly from the “etching effect” of hydrogen, which reduces the defect density and indium segregation at the upper well/barrier interface, and consequently contributes to the decrease in the number of nonradiative recombination centers and the enhancement of carrier localization.

Improvement of photoluminescence efficiency in stacked Ge/Si/Ge quantum dots with a thin Si spacer

AbstractIn this work, two successive layers of Ge quantum dots separated by a thin Si spacer grown by ultra‐high‐vacuum chemical vapor deposition were demonstrated. With an optimal thickness of the thin Si spacer, the sandwiched Ge (13.1 ML)/Si (28 ML)/Ge (13.1 ML) quantum dots suppress the coarsening of Ge quantum dots and efficiently increase the uniformity of the quantum dots. Cross‐sectional transmission electron microscopy shows that the Si/Ge underlayers provide preferred nucleation sites for the overlayer Ge deposition. Such a modification avoids the formation of Ge superdomes and prevents the occurrence of threading dislocations even at such a thin Si spacer thickness. A stronger photoluminescence intensity of these sandwich Ge/Si/Ge quantum dots was observed compared with that of 26.2 eq‐ML Ge quantum dots without using any intermediate Si layer. Furthermore, the narrower width of the photoluminescence spectra indicates that the Ge/Si/Ge dots are more uniform compared with the 13.1 and 26.1 eq‐ML Ge quantum dots. Five‐fold bilayers of Ge/Si/Ge/Si(150 ML) have been achieved to enhance the photo‐emission efficiency. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Improvement in Photoluminescence Efficiency of GaInNAs/GaAs Quantum Wells Grown by Metalorganic Chemical Vapor Deposition for Low-Threshold 1.3 µm Range Lasers

A quality improvement of the III–V dilute nitride semiconductor alloy, GaInNAs, grown by metalorganic chemical vapor deposition (MOCVD) on a GaAs substrate is reported for 1.3 µm-wavelength lasers. GaInNAs wafers were grown at various growth temperatures, V/III ratios, and growth rates. The photoluminescence (PL) efficiency of GaInNAs/GaAs quantum wells (QWs) was increased by lowering the growth temperature and increasing the V/III ratio in the growth conditions conventionally used for nitrogen (N)-free GaInAs/GaAs QW growth. These conditions are important for realizing high PL efficiency because they prevent the inhomogeneity of the immiscible alloy of GaInNAs. It was also observed that the optimal window for the growth temperature, V/III ratio, and growth rate for the GaInNAs is narrower than that of N-free GaInAs QWs. After careful optimization of the growth conditions, GaInNAs/GaAs QW lasers with various emission wavelengths were fabricated. Low-threshold current densities of 0.17 kA/cm2/well, 0.18 kA/cm2/well, and 0.44 kA/cm2/well are obtained for emission wavelengths of 1.25 µm, 1.30 µm, and 1.34 µm, respectively. The results obtained for growth conditions and lasing characteristics are useful in further improving 1.3 µm or longer wavelength GaInNAs lasers grown by MOCVD.

Improvement in photoluminescence efficiency of Si 1− xGe x alloy nanocrystals embedded in SiO 2 matrices by P doping

The effects of P doping on photoluminescence (PL) properties of Si 1− x Ge x alloy nanocrystals (nc-Si 1− x Ge x ) embedded in SiO 2 thin films were studied. P doping resulted in a drastic decrease in the electron spin resonance (ESR) signals, which are assigned to the Si and Ge dangling bonds at interfaces between nc-Si 1− x Ge x and matrices (Si and Ge P b centers). It was found that, with increasing P concentration, the signal from Ge P b centers is first quenched, and then the quenching of the signal from Si P b centers starts. The quenching of the ESR signals was accompanied by a drastic enhancement of PL intensity. By further increasing P concentration, PL intensity became weaker. In this P concentration range, optical absorption due to the intravalley transition of free electrons generated by P doping appears.

Improvement of Photoluminescence Efficiency in Poly( p -Phenylene Vinylene) Induced by Laser Irradiation

In this work, we report a strong enhancement of photoluminescence (PL) intensity in PPV cast films upon laser irradiation in air, which competes with other quenching process related with formation of carbonyl groups. This increase of PL is dependent on laser intensity and film thickness. We observed that PL intensity increases up to 300% without significant changes in peak positions. This effect was accompanied by a blue shift of the absorption spectra and by the incorporation of carbonyl groups detected by infrared measurements.

Improvement in photoluminescence efficiency of SiO2 films containing Si nanocrystals by P doping: An electron spin resonance study

SiO 2 and phosphosilicate glass (PSG) films containing Si nanocrystals (nc-Si) as small as a few nanometers were studied by electron spin resonance (ESR) and photoluminescence (PL), and the correlation between the two measurements was examined. It is shown that the incorporation of nc-Si in SiO2 results in the drastic increase in the ESR signal; the signal is assigned to the Si dangling bonds at the interfaces between nc-Si and matrices (Pb centers). The ESR signal becomes weaker by doping P into SiO2 matrices, i.e., by using PSG as matrices. By increasing the P concentration, the ESR signal decreases further. By decreasing the ESR signal, the low-energy PL peak at 0.9 eV decreases, while the band-edge PL at 1.4 eV increases. These results suggest that the 0.9 eV peak is related to Pb centers, and that the decrease in the density of the Pb centers by P doping brings about an improvement in the band-edge PL efficiency of nc-Si.

Photoluminescence from Si nanocrystals dispersed in phosphosilicate glass thin films: Improvement of photoluminescence efficiency

Photoluminescence (PL) from Si nanocrystals (nc-Si) dispersed in phosphosilicate glass thin films was studied. It was found that, at room temperature, the 1.4 eV PL due to the recombination of electron-hole pairs in nc-Si becomes intense as the P concentration increases. At low temperatures, an additional peak related to defects at interfaces between nc-Si and the matrix was observed at about 0.9 eV. In contrast to the 1.4 eV peak, the 0.9 eV peak became weaker with increasing P concentration and almost disappeared at a P concentration of 1.5 mol %. These results suggest that the number of interface defects decreases with increasing P concentration and that this decrease leads to an improvement of the band-edge PL of nc-Si.