Showing papers by "Yanxia Cui published in 2017"

Tungsten based anisotropic metamaterial as an ultra-broadband absorber

Abstract: We show theoretically that an array of tungsten/germanium anisotropic nano-cones placed on top of a reflective substrate can absorb light at the wavelength range from 0.3 μm to 9 μm with an average absorption efficiency approaching 98%. It is found that the excitation of multiple orders of slow-light resonant modes is responsible for the efficient absorption at wavelengths longer than 2 μm, and the anti-reflection effect of tapered lossy material gives rise to the near perfect absorption at shorter wavelengths. The absorption spectrum suffers a small dip at around 4.2 μm where the first order and second order slow-light modes get overlapped, but we can get rid of this dip if the absorption band edge at a long wavelength range is reduced down to 5 μm. The parametrical study reflects that the absorption bandwidth is mainly determined by the filling ratio of tungsten as well as the bottom diameter of the nano-cones and the interaction between neighboring nano-cones is quite weak. Our proposal has some potential applications in the areas of solar energy harvesting and thermal emitters.

High efficiency planar Sn–Pb binary perovskite solar cells: controlled growth of large grains via a one-step solution fabrication process

Abstract: One-step solution fabrication of high-performance Sn-including perovskite solar cells (PSCs) is very challenging due to the rapid crystallization of the Sn-based perovskite layer, leading to a poor film morphology and low surface coverage. In this work, a well-controlled one-step method, assisted by a multi-step solvent treatment, is developed for the growth of a high-quality CH3NH3Pb(1−x)SnxI3 (0 ≤ x ≤ 1) perovskite film on a planar PEDOT:PSS substrate. The CH3NH3Sn0.25Pb0.75I3 perovskite films consisting of densely packed and uniformly distributed large crystal grains were obtained using sec-butyl alcohol solvent engineering and N,N-dimethylformamide solvent annealing under an N2 atmosphere. The CH3NH3Sn0.25Pb0.75I3-based PSCs with a maximum power conversion efficiency (PCE) of 12.08% and an average PCE of 11.01% were obtained. The PSCs also exhibit excellent performance reproducibility, good air stability and weak hysteresis behavior. The enhancement in the performance of the PSCs is attributed to the well-crystallized CH3NH3Sn0.25Pb0.75I3 film, resulting in simultaneous improvement in charge–carrier transport properties and reduction in charge–carrier recombination, a very promising approach to obtain high performance Sn-including perovskite solar cells.

Improved Efficiency of Organic Photovoltaic Cells by Incorporation of AuAg-Alloyed Nanoprisms

Abstract: We successfully synthesized AuAg-alloyed nanoprisms (AuAgNPrisms) through the seed mediated and galvanic replacement approaches. By introducing the synthesized AuAgNPrisms at the buffer/active interface with an optimal concentration, we increased the power conversion efficiency of PTB7:PC70BM-based organic photovoltaic cells from 6.58% to 7.11%, the current density from 13.36 to 15.01 mA/cm2, corresponding to enhancement factors of 8.1% and 12.4%, respectively. According to our systematical study, we concluded that the incorporation of AuAgNPrisms not only increases light absorption in the active layer over a broad wavelength range but improves the exciton dissociation rate at the interface between PTB7 and PC70BM. With the incorporation of AuAgNPrisms, although the increase in the leakage current is observed, the carrier mobility is also improved apparently, yielding that the reduction in the fill factor is negligible. This work could contribute to the development of high efficiency and low cost organic solar cells.

Profiling Light Absorption Enhancement in Two-Dimensional Photonic-Structured Perovskite Solar Cells

Abstract: The light absorption behavior in two-dimensional (2-D) photonic-structured active layer in lead halide perovskite solar cells (PSCs), induced by a 2-D photonic-structured TiO2 nanobowl electron extraction layer, is investigated systematically by the finite element method. The numerical results indicate that light absorption in the 2-D photonic-structured PSCs is apparently more favorable for efficient cell operation when compared to that of the planar control device having a perovskite layer thickness less than 300 nm. For optimal 2-D photonic-structured PSCs with a 100 nm thick perovskite layer, the integrated absorption efficiency reaches 65.7% over the wavelength range from 350 to 800 nm at normal incidence considering AM 1.5G solar irradiation, with enhancement of 18.4% with respect to the control cell, and being 88.3% of that of the planar PSC with 300 nm-thick perovskite layer by only 1/3 perovskite material consumption. The detailed field distribution investigations reveal that the great enhancement of light absorption in the long wavelength region is attributed to the collective impact of the different photon and plasmonic modes and their mutual coupling. This work paves the way to realize the low-cost, high-efficiency ultrathin PSCs.

Effect of spherical metallic nanoparticles in active layer on absorption enhancement in organic solar cells

Abstract: It has been demonstrated experimentally that the presence of metallic nanoparticles (MNPs) in the active layer assists in improving the power conversion efficiency of organic solar cells (OSCs), due to the combination of favorable optical as well as electrical effects. In this work, the optical effects of two different spherical MNPs (Ag and Au nanospheres) on absorption enhancement in the active layer with the optimal thickness are analyzed in detail using finite-difference time-domain simulation. The results reveal clearly that the absorption enhancement in the OSCs is dependent on both the properties of MNPs and the types of the donor/acceptor blend systems. We conclude that Au nanospheres are less effective as compared to Ag nanospheres on absorption enhancement in OSCs, and large sized MNPs are favorable for light trapping in the organic active layer due to the prominent plasmonic excitations. For a low bandgap polymer PSBTBT:PC71BM blend system incorporating Ag nanospheres, a 11.2% increase in the integrated absorption is obtained due to the excitation of magnetic and electric resonances of surface plasmons. This work could contribute to the development of high efficiency plasmonic OSCs.

Comparison of Nanohole-Type and Nanopillar-Type Patterned Metallic Electrodes Incorporated in Organic Solar Cells.

Abstract: Both the nanohole- and nanopillar-type patterned metallic electrodes (PMEs) have been introduced in organic solar cells (OSCs) for improving device performances experimentally, but there is few work addressing the similarities and differences between them. In this theoretical work, we systematically compare the impact of the nanohole- and nanopillar-type PMEs on the performance of an OSC based on hybridized cavity resonances. By optimizing the geometrical parameters of each PME, we obtained an interesting result that the integrated absorption efficiencies in the active layer with different optimized PMEs are almost the same (both are equal to 82.4%), outperforming that of the planar control by 9.9%. Though the absorption enhancement spectra of the two different optimal devices are similar as well, the mechanisms of light trapping at the corresponding enhancement peaks are distinct from each other. In a comprehensive view, the nanopillar-type PME is suggested to be applied in the present system, since its optimal design has a moderate filling ratio, which is much easier to fabricate than its counterpart. This work could contribute to the development of high-efficiency OSCs.

Absorption Enhancement in Thin Organic Solar Cells With MoO3/Ag/MoO3 Transparent Anode Based on Short-Pitched Metallic Grating

Abstract: The multilayer film consisting of MoO3/Ag/MoO3 (shortened as MAM) is very promising as an alternative of indium-tin oxide (ITO) to work as the transparent anode in organic solar cells (OSCs). In MAM-based thin OSCs, the absorption of light is quite poor. Here, we propose to apply a short-pitched metallic grating into the MAM-based OSC for improving the absorption in its thin active layer. Numerical calculations reveal that the obtained enhancement of integrated absorption in the active layer is more than 500% with respect to the equivalent planar control. The field distributions at the two peaks of the normalized absorption spectrum indicate that propagating surface plasmon polaritons (SPPs), as well as the horizontal metal-insulator-metal (MIM) waveguide mode, play the main role of trapping light into the active material. It is expected that the proposal could contribute to the development of the efficient ITO free thin OSCs.

Flexible wide-angle wide spectrum efficient absorber and manufacturing method thereof

Abstract: The present invention belongs to the efficient electromagnetic wave absorber field, and concretely provides a flexible wide-angle wide spectrum efficient absorber and a manufacturing method thereof. About the flexible wide-angle wide spectrum efficient absorber, structures, namely of moth eye structures, being similar to micro-nano bulges on moth 's eyes in periodic arrangement mode are uniformly distributed at the upper surface of a substrate made by a flexible polymer, the height of each moth eye structure is 210-230 nm from the top to the bottom, the distance between adjacent moth eye structures is 390-410 nm, a metal reflecting layer, a dielectric layer and a metal nanoparticle layer are deposited on the moth eye structures in order, the metal of the metal reflecting layer is metal with high reflectivity, and the metal of the metal nanoparticle layer is precious metal. The present invention provides a simple and economical method for preparation of the flexible wide-angle wide spectrum efficient absorber on the patterning substrate.

High power continuous wave second harmonic generation in a phase matching insensitive MgO doped LiNbO_3 waveguide

Abstract: This paper reports an efficient method of using a chirped grating to reduce the phase-matching sensitivity and increase the output power of continuous wave second harmonic generation in a MgO doped LiNbO3 waveguide. It was found that using a chirped grating with decreasing periods from 6.96 μm to 6.95 μm significantly improved the phase mismatch tolerance and increased the second harmonic generation output power from 0.64 W (using uniform grating) to 1.13 W. This design was shown to lead to a phase matching insensitive waveguide. This indicates that there is a possibility of obtaining higher output power with the MgO doped LiNbO3 waveguide.

Enhanced light out-coupling efficiency and reduced efficiency roll-off in phosphorescent OLEDs with a spontaneously distributed embossed structure formed by a spin-coating method

Abstract: A simple and low-cost spin-coating method was proposed to form a spontaneously distributed embossed structure. With such an embossed structure produced by Al2O3 nanospheres (NPs) with different diameters, the out-coupling efficiency is increased in all viewing directions without introducing significant spectral changes and the efficiency roll-off is suppressed at the same time. When the concentration of Al2O3 ethanol solution is chosen as 0.08 mg ml−1, a peak current efficiency (CE) of 52.1 cd A−1, peak power efficiency (PE) of 32.7 l m W−1, and peak external quantum efficiency (EQE) of 20.9% are obtained. From the maximum efficiency value to the case where the brightness is 3000 cd m−2, the roll-off of CE, PE and EQE for device B are only 6.53%, 26.30% and 6.22%, respectively. The improved properties are attributed to greatly enhanced light extraction and an enlarged charge carrier recombination region.