Broadband Absorption Enhancement Research Articles

Omnidirectional and broadband absorption enhancement from trapezoidal Mie resonators in semiconductor metasurfaces.

Light trapping in planar ultrathin-film solar cells is limited due to a small number of optical modes available in the thin-film slab. A nanostructured thin-film design could surpass this limit by providing broadband increase in the local density of states in a subwavelength volume and maintaining efficient coupling of light. Here we report a broadband metasurface design, enabling efficient and broadband absorption enhancement by direct coupling of incoming light to resonant modes of subwavelengthscale Mie nanoresonators defined in the thin-film active layer. Absorption was investigated both theoretically and experimentally in prototypes consisting of lithographically patterned, two-dimensional periodic arrays of silicon nanoresonators on silica substrates. A crossed trapezoid resonator shape of rectangular cross section is used to excite broadband Mie resonances across visible and near-IR spectra. Our numerical simulations, optical absorption measurements and photocurrent spectral response measurements demonstrate that crossed trapezoidal Mie resonant structures enable angle-insensitive, broadband absorption. A short circuit current density of 12.0 mA/cm2 is achieved in 210 nm thick patterned Si films, yielding a 4-fold increase compared to planar films of the same thickness. It is suggested that silicon metasurfaces with Mie resonator arrays can provide useful insights to guide future ultrathin-film solar cell designs incorporating nanostructured thin active layers.

Incorporating silver-SiO2 core-shell nanocubes for simultaneous broadband absorption and charge collection enhancements in organic solar cells

A new type of silver nanoparticles, silver-SiO2 core-shell nanocubes (Ag NCs@SiO2), is developed for simultaneous broadband absorption and efficient charge collection enhancements in organic solar cells (OSCs), thereby improving short-circuit current density and power conversion efficiency (PCE). The effect of Ag NCs@SiO2, present at the bottom of active layer (PTB7:PC70BM), on the performance of OSCs were analyzed systematically. The results indicate that the broadband absorption enhancement in the OSCs is realized mainly due to the excitation of the strong localized surface plasmon resonance surrounding the Ag NCs@SiO2, supported by the theoretical simulation. While the efficient charge collection efficiency is attributed primarily to the improvement in vertical phase separation of donor/acceptor in the active layer, associated with the process integration of Ag NCs@SiO2 in the cell fabrication. The use of Ag NCs@SiO2 benefits the absorption enhancement and also prevents the exciton quenching and carrier charge recombination loss that would occur in cells with pure Ag NCs. The combination of these advantages results in an average PCE of 7.84% for cells with Ag NCs@SiO2, corresponding to a 13.6% increase in the efficiency as compared to that of the structurally identical control cell without Ag NCs@SiO2.

Engineering non-radiative anapole modes for broadband absorption enhancement of light.

In this paper, we propose a novel, frequency- and angularly- broadband approach to achieve absorption rate enhancement in high-index dielectric nanostructures through the engineering of non-radiative anapole modes. We employ multipolar decomposition of numerically computed current distributions and analyze the far-field scattering power of multipole moments. By leveraging the destructive interference of electric dipole and toroidal dipole moments, we design non-radiating anapole modes and demonstrate significantly enhanced absorbed power in silicon and germanium nanostructures. We demonstrate wide wavelength tunability of the anapole-driven peak absorption enhancement for nano-disks and square nano-pixel geometries, which can be conveniently fabricated with current lithography. Finally, by combining nano-disks and nano-pixels of different sizes into functional surface units, we design nanostructured arrays with enhanced bandwidth and absorption rates that can be useful for the engineering of broadband semiconductor photodetectors driven by controllable anapole responses.

High Efficiency Organic Solar Cells Achieved by the Simultaneous Plasmon‐Optical and Plasmon‐Electrical Effects from Plasmonic Asymmetric Modes of Gold Nanostars

The plasmon-optical effects have been utilized to optically enhance active layer absorption in organic solar cells (OSCs). The exploited plasmonic resonances of metal nanomaterials are typically from the fundamental dipole/high-order modes with narrow spectral widths for regional OSC absorption improvement. The conventional broadband absorption enhancement (using plasmonic effects) needs linear-superposition of plasmonic resonances. In this work, through strategic incorporation of gold nanostars (Au NSs) in between hole transport layer (HTL) and active layer, the excited plasmonic asymmetric modes offer a new approach toward broadband enhancement. Remarkably, the improvement is explained by energy transfer of plasmonic asymmetric modes of Au NS. In more detail, after incorporation of Au NSs, the optical power in electron transport layer transfers to active layer for improving OSC absorption, which otherwise will become dissipation or leakage as the role of carrier transport layer is not for photon-absorption induced carrier generation. Moreover, Au NSs simultaneously deliver plasmon-electrical effects which shorten transport path length of the typically low-mobility holes and lengthen that of high-mobility electrons for better balanced carrier collection. Meanwhile, the resistance of HTL is reduced by Au NSs. Consequently, power conversion efficiency of 10.5% has been achieved through cooperatively plasmon-optical and plasmon-electrical effects of Au NSs.

Optical study of thin-film photovoltaic cells with apparent optical path length

Extending the insufficient optical path length (OPL) in thin-film photovoltaic cells (PVs) is the key to achieving a high power conversion efficiency (PCE) in devices. Here, we introduce the apparent OPL (AOPL) as a figure of merit for light absorbing capability in thin-film PVs. The optical characteristics such as the structural effects and angular responses in thin-film PVs were analyzed in terms of the AOPL. Although the Lambertian scattering surface yields a broadband absorption enhancement in thin-film PVs, the enhancement is not as effective as in thick-film PVs. On the other hand, nanophotonic schemes are introduced as an approach to increasing the single-pass AOPL by inducing surface plasmon resonance. The scheme using periodic metal gratings is proved to increase the AOPL in a narrow wavelength range and specific polarization, overcoming the Yablonovitch limit. The AOPL calculation can be also adopted in the experimental analysis and a maximum AOPL of 4.15d (where d is the active layer thickness) is exhibited in the absorption band edge region of PTB7:PC70BM-based polymer PVs.

Omnidirectional broadband absorption enhancement in laterally assembled quadrangle silicon nanowire solar cells

In this paper, effects of geometry and spatial placement of quadrangle silicon nanowires (NWs) on the performance of laterally assembled NW solar cells are investigated. Two different approaches are proposed to reach the broadband absorption enhancement in these types of solar cells. In the first method, a semi-periodic array is used to have a better utilization of the optical antenna effect. The current density can be enhanced as a result of a compromise between the diffraction and optical antenna effects. In the second approach, multiple NWs with different geometries are employed to use the cavity modes of the various NWs. The best current density is achieved for the combination of both of these methods. The different parameters in each structure are selected using the optimization algorithm. Finally, a multilayer structure with the optimized dimensions is proposed to obtain the maximum achievable current density in ultrathin solar cells. The broadband absorption enhancement in the proposed structure is preserved for a wide range of incident angles. Using the multilayer NW arrays, it is possible to improve the absorption enhancement of solar cells without introducing more absorbing material.

Enhanced Absorption in Laser-Crystallized Silicon Thin Films on Textured Glass

This paper reports enhanced absorption in laser-crystallized Si thin films on abrade-textured glass. The Si film on textured glass has large high-quality grains of several hundred micrometers in width and up to several centimeters in length. A combination of glass texturing, rear Si texturing, white paint rear reflector, and front moth-eye antireflection foil on a 10-μm-thick Si film (on $\text{SiO}_{x}/\text{SiN}_{x}/\text{SiO}_{x}$ intermediate layer) shows a large broadband absorption enhancement with potential short-circuit current densities up to 29.5 mA/cm2 (26.1% enhancement as compared with a planar reference).

Broadband near-infrared absorption enhancement in Si substrate via random distributed Ag nanoparticles

A broadband near-infrared (NIR) absorption from 1200 to 2500 nm in Si substrate is investigated by Ag film deposition and subsequent thermal annealing. Under thermal annealing, Ag films can crack and shrink into Ag nanoparticles (NPs). Annealing temperature greatly affects the size, distribution status and morphology of Ag NPs, resulting in different NIR absorption. The influence of the size and distribution of Ag NPs on NIR absorption originated from localized surface plasmon resonance (LSPR) is theoretically simulated by a finite difference time domain method. The band width of LSPR belongs to different type Ag NPs are discussed.

Design of optimized wave-optical spheroidal nanostructures for photonic-enhanced solar cells

The interaction of light with wavelength-sized photonic nanostructures is highly promising for light management applied to thin-film photovoltaics. Several light trapping effects come into play in the wave optics regime of such structures that crucially depend on the parameters of the photonic and absorbing elements. Thus, multi-parameter optimizations employing exact numerical models, as performed in this work, are essential to determine the maximum photocurrent enhancement that can be produced in solar cells.Generalized spheroidal geometries and high-index dielectric materials are considered here to model the design of the optical elements providing broadband absorption enhancement in planar silicon solar cells. The physical mechanisms responsible for such enhancement are schematized in a spectral diagram, providing a deeper understanding of the advantageous characteristics of the optimized geometries. The best structures, composed of TiO2 half-spheroids patterned on the cells' top surface, yield two times higher photocurrent (up to 32.5mA/cm2 in 1.5µm thick silicon layer) than the same devices without photonic schemes.These results set the state-of-the-art closer to the theoretical Lambertian limit. In addition, the considered light trapping designs are not affected by the traditional compromise between absorption enhancement versus current degradation by recombination, which is a key technological advantage.

Broadband optical absorption enhancement of N719 dye in ethanol by gold–silver alloy nanoparticles fabricated under laser ablation technique

The formation of gold–silver alloy nanoparticles (Au–Ag alloy NPs) by a two-step process with a pulsed Nd:YAG laser without any additives is presented. Mixtures of Au and Ag colloidal suspensions were separately obtained by 1064-nm laser ablation of metallic targets immersed in ethanol. Subsequently, the as-mixed colloidal suspensions were reirradiated by laser-induced heating at the second-harmonic generation (532 nm) for different irradiation periods of time. The absorption spectra and morphology of the colloidal alloys were studied as a function of exposure time to laser irradiation. Transmission electron microscopy revealed the formation of monodispersed spherical nanoparticles with a homogeneous size distribution in all the synthesized samples. UV–vis and photoluminescence spectroscopy measurements were also employed to characterize the changes in the light absorption and emission of N719 dye solution with different concentrations of Au–Ag colloidal alloys, respectively. The localized surface plasmon resonance (LSPR) of Au–Ag alloy NPs enhanced the absorption and fluorescence peak of the dye solution. The mixture of dye molecules with a higher concentration of alloy NPs exhibited an additional coupling of dipole moments with the LSPR, thereby contributing to the improvement of the optical properties of the mixture.

Broadband Absorption Enhancement in Solar Cells with an Atomically Thin Active Layer

We numerically demonstrate broadband absorption enhancement in the visible in monolayer MoS2 placed on a photonic crystal slab, backed by a perfect electric conductor mirror. The absorption enhancement arises from interaction between the MoS2 and the guided resonances of the photonic crystal slab. Through consideration of light trapping theory and parametric studies of the design parameters, it is shown that the optimal photonic crystal slab has a grating period that is subwavelength to most of the frequency region of interest, has large air holes, and is thin. The optimal design features an average absorption of 51% at normal incidence in the visible above the band gap of MoS2 (400–690 nm) and is less than 100 nm thick. We also comment on the angular response of the optimal device.

Adjustable optical response of amorphous silicon nanowires integrated with thin films

We experimentally demonstrate a new optical platform by integrating hydrogenated amorphous silicon nanowire arrays with thin films deposited on transparent substrates like glass. A 535 nm thick thin film is anisotropically etched to fabricate vertical nanowire arrays of 100 nm diameter arranged in a square lattice. Adjusting the nanowire length, and consequently the thin film thickness permits the optical properties of this configuration to be tuned for either transmission filter response or enhanced broadband absorption. Vivid structural colors are also achieved in reflection and transmission. The optical properties of the platform are investigated for three different etch depths. Transmission filter response is achieved for a configuration with nanowires on glass without any thin film. Alternatively, integrating thin film with nanowires increases the absorption efficiency by ∼97% compared to the thin film starting layer and by ∼78% over nanowires on glass. The ability to tune the optical response of this material in this fashion makes it a promising platform for high performance photovoltaics, photodetectors and sensors.

Nanoparticle loading effects on the broadband absorption for plasmonic-metal@semiconductor-microsphere photocatalyst

Encapsulating a single plasmonic-nanoparticle inside a large semiconductor-microsphere could substantially enhance its broadband absorption [Sun et al., ACS Catal. 4 (2014) 4269–4276]. However, the multi-nanoparticle loading effect is unclear and hinders the practical catalyst design. Herein, we study the nanoparticle loading effect of plasmonic-metal@semiconductor-microsphere photocatalyst on its broadband absorption in the visible light spectrum. Finite-element-study suggests that the broadband absorption holds a universal logarithmic relation with the number of nanoparticles at moderate loading range. Higher loading brings the nanoparticles in proximity, inducing plasmonic-coupling effect which causes red-shift on the absorption spectrum and rendering the logarithmic increment no longer valid. Based on the universal scaling of plasmonic-coupling, an analytical expression to estimate the loading rate at coupling-onset (below which no plasmonic-coupling presents) is derived for arbitrary plasmonic-metal@semiconductor-microsphere (for instance, the loading rate is 10.92wt% for 30nm-Au@0.6μm-TiO2 microsphere with 399 broadband absorption enhancement at coupling-onset). Further considering the diminishing increment nature of the logarithmic relation, the actual loading is recommended to be lower than that at coupling-onset to achieve better cost-efficiency (with loading 30% lower than that at coupling-onset, the broadband absorption reduces only 10%). These findings could provide the experimentalists useful guidelines in the development of the plasmonic-metal@semiconductor-microsphere photocatalyst.

Periodically Diameter-Modulated Semiconductor Nanowires for Enhanced Optical Absorption.

A diameter-modulated silicon nanowire array to enhance the optical absorption across broad spectral range is presented. Periodic shape engineering is achieved using conventional semiconductor processes and the unique optical properties are analyzed. The periodicity in the diameter of the silicon nanowires enables stronger and more closely spaced optical resonances, leading to broadband absorption enhancement.

Enhanced Broadband Light Trapping in c-Si Solar Cells Using Nanosphere-Embedded Metallic Grating Structure

A hexagonal nanosphere (NS)-embedded back plasmonic grating structure is proposed to improve the light absorption of crystalline silicon (c-Si) solar cells. These structures are simple, can be deposited toward the final stages of device processing, and involve no increase in the surface area of the semiconductor layer. Experimental fabrication of this structure on a 200-μm c-Si wafer has been realized using silver, resulting in broadband absorption enhancement in the near-infrared region. In corresponding to the optical measurement, a maximum potential photocurrent density enhancement of around 2.23 mA/cm2 has been predicted, compared with the reference with a planar metal reflector. Three-dimensional finite-element method numerical simulations were also performed on smooth and irregular surface geometries of the NSs. Our results demonstrate that while both configurations perform better than a planar reflector, the irregular nanofeatures on the gratings can adjust the optical resonance to ensure that the light is more efficiently scattered into the Si, which would significantly improve its optical absorption. These structures have the potential to be used as rear metal contacts, in addition to performing the function of a light-trapping layer without increasing the fraction of the metal component.

Nano-bio hybrids of plasmonic metals/photosynthetic proteins for broad-band light absorption enhancement in organic solar cells

We prepare novel nano-bio hybrids by employing Ag nanoprisms as surface plasmons to couple with light-harvesting complex II (LHCII) for broad-band optical enhancement in organic photovoltaics. The results open the possibility to use metal nanoparticles for tuning the photophysical properties of integral biomolecules in a well-defined way.

Light Absorption Enhancement in Laser-Crystallized Silicon Thin Films on Textured Glass

Liquid-phase crystallization of Si films on textured glass by line-focus diode laser produces a material with large high-quality grains of several hundred micrometers in width and up to centimeters in length, similar to the films crystallized on planar glass. A combination of glass texturing, rear Si texturing, white paint rear reflector, and a front moth-eye antireflection foil on a 7-μm absorber results in significant broadband absorption enhancement in the Si film with potential short-circuit current densities in solar cells of $\text{27.8} \;\text{mA/cm}^{\text{2}}$ or 36.3% enhancement compared with a planar reference film without light-trapping features.

Optimal design of laterally assembled hexagonal silicon nanowires for broadband absorption enhancement in ultrathin solar cells

Design approaches to carry out broadband absorption in laterally assembled hexagonal silicon nanowire (NW) solar cells are investigated. Two different methods are proposed to improve the current density of silicon NW solar cells. It is observed that the key to the broadband absorption is disorder and irregularity. The first approach to reach the broadband absorption is using multiple NWs with different geometries. Nevertheless, the maximum enhancement is obtained by introducing irregular NWs. They can support more cavity modes, while scattering by NWs leads to broadening of the absorption spectra. An array of optimized irregular NWs also has preferable features compared to other broadband structures. Using irregular NW arrays, it is possible to improve the absorption enhancement of solar cells without introducing more absorbing material.

Effect of symmetry in periodic nanostructures on light trapping in thin film solar cells

We perform detailed theoretical analysis on the relation between the symmetry in periodic nanostructures and its light-trapping performance in thin film photovoltaics. The symmetry-breaking approach for light trapping implicitly assumes that the increase in the number of resonances would enhance broadband absorption. However, our calculations show that this assumption is not entirely true. We find that the correlation between the number of resonances and the spectral average absorption is only rough. By breaking the symmetry systematically, we identify the symmetry-breaking steps that significantly enhance broadband absorption. Our results show that these symmetry-breaking steps do not always agree with the steps that increase the number of resonances. However, both the level of absorptance and the number of resonances in general increase by symmetry breaking, which is in agreement with previously reported results. Our results suggest that the development of experimental symmetry-breaking techniques in light-trapping nanostructures should not be guided solely based on simple group theory analysis.

Organic Solar Cells: High‐Performance Organic Solar Cells with Broadband Absorption Enhancement and Reliable Reproducibility Enabled by Collective Plasmonic Effects (Advanced Optical Materials 9/2015)

Organic Solar Cells: High‐Performance Organic Solar Cells with Broadband Absorption Enhancement and Reliable Reproducibility Enabled by Collective Plasmonic Effects (Advanced Optical Materials 9/2015)