Modification of Front Surface Antireflection of Silicon Solar Cells with Composite Metallic Nanoparticle Arrays

This work reports on a numerical simulation study of the front surface modification of silicon solar cells by using composite metallic nanoparticle arrays. On top of the front surface of the cells, composite arrays that are made of silver or gold nanoparticles are carefully designed. The radii of the nanoparticles in the composite array and the lattice constant of the array are respectively varied, and the corresponding light reflectance at the front surface of the cell is calculated in the wavelength range from 400 to 1100 nm, via the FDTD method. The results determined from the composite arrays reveal that the reflectance curve across the whole wavelength range studied presents a more uniform behavior, compared with simple arrays. It is shown that the surface plasmon excitation effect of the metallic nanoparticles plays a crucial role in the light reflectance performance at the front surface of the silicon solar cells. The dependence of the light reflectance on the direction of the incident light’s polarization is also investigated, and it is found that the polarization has little effect on the reflectance curves once the composite arrays are optimized.     

Optical Absorption of Thin Film Solar Cells with Hybrid Arranged Bottom Grating

Crystalline silicon thin film solar cells with hybrid arranged bottom grating are proposed. Optical absorption efficiency and photocurrent density are calculated to get optimized bottom grating parameters. Compared with mono arranged Ag grating or Al-doped zinc-oxide grating, hybrid arranged bottom grating could couple more near-infrared region lights into the active absorber layer. Optical absorption enhancement profiles are plotted for monolayer grating solar cells with four different bottom grating arrangements, which agree with dispersion characteristics well. The absorption enhancement profiles illustrate the mechanism of the more coupling of near-infrared lights. Electrical modeling is considered in the end and it is found that hybrid arranged bottom grating’s thin film solar cell outperforms the thin film solar cells with mono arranged bottom gratings evidently.     

Optical Waveguide Modes’ Splitting by Bi-Layer Silver Gratings in Thin Film Solar Cells

Plasmonics - A crystalline silicon thin film solar cell consisting of one-dimensional bi-layer silver back gratings was proposed in this paper. Compared with monolayer silver grating thin film...     

Polycrystalline Silicon Thin-film Solar cells with Plasmonic-enhanced Light-trapping

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To compensate for lower light absorption such physically thin devices have to incorporate light-trapping which increases their optical thickness. Light scattering by textured surfaces is a common technique but it cannot be universally applied to all solar cell technologies. Some cells, for example those made of evaporated silicon, are planar as produced and they require an alternative light-trapping means suitable for planar devices. Metal nanoparticles formed on planar silicon cell surface and capable of light scattering due to surface plasmon resonance is an effective approach.The paper presents a fabrication procedure of evaporated polycrystalline silicon solar cells with plasmonic light-trapping and demonstrates how the cell quantum efficiency improves due to presence of metal nanoparticles.To fabricate the cells a film consisting of alternative boron and phosphorous doped silicon layers is deposited on glass substrate by electron beam evaporation. An Initially amorphous film is crystallised and electronic defects are mitigated by annealing and hydrogen passivation. Metal grid contacts are applied to the layers of opposite polarity to extract electricity generated by the cell. Typically, such a ~2 μm thick cell has a short-circuit current density (Jsc) of 14-16 mA/cm², which can be increased up to 17-18 mA/cm² (~25% higher) after application of a simple diffuse back reflector made of a white paint.To implement plasmonic light-trapping a silver nanoparticle array is formed on the metallised cell silicon surface. A precursor silver film is deposited on the cell by thermal evaporation and annealed at 23°C to form silver nanoparticles. Nanoparticle size and coverage, which affect plasmonic light-scattering, can be tuned for enhanced cell performance by varying the precursor film thickness and its annealing conditions. An optimised nanoparticle array alone results in cell Jsc enhancement of about 28%, similar to the effect of the diffuse reflector. The photocurrent can be further increased by coating the nanoparticles by a low refractive index dielectric, like MgF₂, and applying the diffused reflector. The complete plasmonic cell structure comprises the polycrystalline silicon film, a silver nanoparticle array, a layer of MgF₂, and a diffuse reflector. The Jsc for such cell is 21-23 mA/cm², up to 45% higher than Jsc of the original cell without light-trapping or ~25% higher than Jsc for the cell with the diffuse reflector only.

Introduction

Light-trapping in silicon solar cells is commonly achieved via light scattering at textured interfaces. Scattered light travels through a cell at oblique angles for a longer distance and when such angles exceed the critical angle at the cell interfaces the light is permanently trapped in the cell by total internal reflection (Animation 1: Light-trapping). Although this scheme works well for most solar cells, there are developing technologies where ultra-thin Si layers are produced planar (e.g. layer-transfer technologies and epitaxial c-Si layers) ¹ and or when such layers are not compatible with textures substrates (e.g. evaporated silicon) ². For such originally planar Si layer alternative light trapping approaches, such as diffuse white paint reflector ³, silicon plasma texturing ⁴ or high refractive index nanoparticle reflector ⁵ have been suggested.Metal nanoparticles can effectively scatter incident light into a higher refractive index material, like silicon, due to the surface plasmon resonance effect ⁶. They also can be easily formed on the planar silicon cell surface thus offering a light-trapping approach alternative to texturing. For a nanoparticle located at the air-silicon interface the scattered light fraction coupled into silicon exceeds 95% and a large faction of that light is scattered at angles above critical providing nearly ideal light-trapping condition (Animation 2: Plasmons on NP). The resonance can be tuned to the wavelength region, which is most important for a particular cell material and design, by varying the nanoparticle average size, surface coverage and local dielectric environment ^6,7. Theoretical design principles of plasmonic nanoparticle solar cells have been suggested ⁸. In practice, Ag nanoparticle array is an ideal light-trapping partner for poly-Si thin-film solar cells because most of these design principle are naturally met. The simplest way of forming nanoparticles by thermal annealing of a thin precursor Ag film results in a random array with a relatively wide size and shape distribution, which is particularly suitable for light-trapping because such an array has a wide resonance peak, covering the wavelength range of 700-900 nm, important for poly-Si solar cell performance. The nanoparticle array can only be located on the rear poly-Si cell surface thus avoiding destructive interference between incident and scattered light which occurs for front-located nanoparticles ⁹. Moreover, poly-Si thin-film cells do not requires a passivating layer and the flat base-shaped nanoparticles (that naturally result from thermal annealing of a metal film) can be directly placed on silicon further increases plasmonic scattering efficiency due to surface plasmon-polariton resonance ¹⁰.The cell with the plasmonic nanoparticle array as described above can have a photocurrent about 28% higher than the original cell. However, the array still transmits a significant amount of light which escapes through the rear of the cell and does not contribute into the current. This loss can be mitigated by adding a rear reflector to allow catching transmitted light and re-directing it back to the cell. Providing sufficient distance between the reflector and the nanoparticles (a few hundred nanometers) the reflected light will then experience one more plasmonic scattering event while passing through the nanoparticle array on re-entering the cell and the reflector itself can be made diffuse - both effects further facilitating light scattering and hence light-trapping. Importantly, the Ag nanoparticles have to be encapsulated with an inert and low refractive index dielectric, like MgF₂ or SiO₂, from the rear reflector to avoid mechanical and chemical damage ⁷. Low refractive index for this cladding layer is required to maintain a high coupling fraction into silicon and larger scattering angles, which are ensured by the high optical contrast between the media on both sides of the nanoparticle, silicon and dielectric ⁶. The photocurrent of the plasmonic cell with the diffuse rear reflector can be up to 45% higher than the current of the original cell or up to 25% higher than the current of an equivalent cell with the diffuse reflector only.     

Surface modification via wet chemical etching of single-crystalline silicon for photovoltaic application

The potential of solar cells have not been fully tapped due to the lack of energy conversion efficiency. There are three important mechanisms in producing high efficiency cells to harvest solar energy; reduction of light reflectance, enhancement of light trapping in the cell and increment of light absorption. The current work represent studies conducted in surface modification of single-crystalline silicon solar cells using wet chemical etching techniques. Two etching types are applied; alkaline etching (KOH:IPA:DI) and acidic etching (HF:HNO₃:DI). The alkaline solution resulted in anisotropic profile that leads to the formation of inverted pyramids. While acidic solution formed circular craters along the front surface of silicon wafer. This surface modification will leads to the reduction of light reflectance via texturizing the surface and thereby increases the short circuit current and conversion rate of the solar cells.     

Effect of Light Incident Angle on Fano Resonance Loss Mediation in Silver Nanoparticles Integrated Thin Silicon Wafers

In this work, we have studied the role of light incidence angle in the suppression of Fano resonance loss for mediating broadband light incoupling. At light normal incidence angle, the transmission loss of silver nanoparticles (Ag NPs) integrated 100 μm silicon (Si) wafer is reduced to ~?4 from ~?11%, and total reflectance is reduced to 32.7 from 38.7%. The reduction in reflectance is observed only in surface plasmon off-resonance region of the NPs, but the reflectance is enhanced slightly in surface plasmon resonance (SPR) region. With the change in light incident angles, we have observed a reduction in reflectance from NPs integrated silicon wafer at SPR region. At 40° light incident angle, the reflectance is reduced to 21 from 38.7% due to minimization of the Fano resonance in SPR region of the NPs. The Fano resonance loss reduction is explained based on modification in NPs’ dipole and quadrupole modes hybridization at the silicon interface with different light incident angles instead of normal incidence. Experimental observations are validated by simulating Ag NPs’ near-fields and angular distribution of far-fields at the silicon interface, scattering efficiency spectra at different light incident angles by finite difference time domain calculations.     

Tuning Plasmon Resonances for Light Coupling into Silicon: a “Rule of Thumb” for Experimental Design

For Si thin-film solar cells to become efficient, schemes to increase the optical absorption in the films are necessary. Scattering of light using plasmonic resonances in metal nanoparticles has been suggested as a feasible route. When placed on a dielectric layer on the front of a solar cell, such metal nanoparticles can scatter a large fraction of the incident light into the solar cell at the resonance wavelength, and hence increase the light collection. However, many related effects may lead to a reduction in photocurrent. Thus, nanoparticle plasmon resonances must be optimized in order to improve the overall light collection. From an experimentalist’s point of view, simple and fast experimental design tools should be explored. In this work, we investigate the plasmon-related photocurrent enhancements for Si test-solar cells with a number of different metal nanoparticle shapes and materials placed on top of a dielectric layer. The spectral position of the photocurrent-enhancement onset is compared to plasmon resonance calculations based on a fairly simple model. Despite the fact that the optical interactions in nanoparticle solar cell configurations can be quite complex, the photocurrent enhancement in the investigated test-solar cells can be predicted qualitatively well for particles with a plasmon resonance in the visible spectrum. This simple and fast model can be used as a rule of thumb in designing nanoparticle arrays for a specific photocurrent enhancement profile.     

Improving Solar Cells’ Light Trapping by the Low Loss Interface Photonic Crystals

Improving the silicon layer’s optical absorption is a key research point for crystalline silicon based thin film solar cells. Light trapping is a method widely adopted to achieve this research purpose. In this paper, we propose low loss interface photonic crystals layer (IPC), which is sandwiched between the crystalline silicon layer and the cover layer. The low loss interface photonic crystals layer could boost the light trapping efficiency significantly. The mechanism is that the smaller refraction index difference between silicon layer and the low loss interface photonic crystals layer could reduce the light’s interface reflection. Taking advantage of the coupling calculation by optical and electrical simulations, solar cell’s absorption efficiency and electrical performance parameters are obtained. Compared with optimized reference group, the maximum output power of the proposed solar cell could be improved by 6.44%. The result indicates that the proposed low loss interface photonic crystals layer is applicable for light’s trapping in crystalline silicon thin film solar cells.

     

Surface Plasmon Enhancement of Optical Absorption in Thin-Film Silicon Solar Cells

Strong electromagnetic field enhancement that occurs under conditions of the surface plasmon excitation in metallic nanoparticles deposited on a semiconductor surface is a very efficient and promising tool for increasing the optical absorption within semiconductor solar cells and, hence, their photocurrent response. The enhancement of the optical absorption in thin-film silicon solar cells via the excitation of localized surface plasmons in spherical silver nanoparticles is investigated. Using the effective medium model, the effect of the nanoparticle size and the surface coverage on that enhancement is analyzed. The optimum configuration and the nanoparticle parameters leading to the maximum enhancement in the optical absorption and the photocurrent response in a single p-n junction silicon cell are obtained. The effect of coupling between the silicon layer and the surface plasmon fields on the efficiency of the above enhancement is quantified as well.     

Plasmon Hybridization in Silver Nanoislands as Semishells Arrays Coupled to a Thin Metallic Film

We obtained experimentally strong plasmon interactions between localized surface plasmon with delocalized surface plasmon polaritons in a new nanosystem of silver semishells island film arrays arranged as a closed-packing structure coupled to an adjacent thin silver film. We show that plasmon interactions for such a nanosystem exhibits two pronounced resonances and interpret the coupling in terms of Fano resonances. The higher energy resonance is identified as a symmetric hybridization mode between localized plasmon resonances in the island semishell array and surface plasmon polaritons in the metal film and while the lower energy resonance is identified as a corresponding anti-symmetric hybridization mode. Increasing the size of the particle arrays enhances and red shifts the resonances. We show that adding a dielectric spacer between the semishell island array and the metal film results in a red shifting of the resonances and introduce an additional high energy spectral peak. The effect of the spacer layer is interpreted as a reduced hybridization and the generation of additional localized surface plasmon resonances.     

Morphology Optimization of Silver Nanoparticles Used to Improve the Light Absorption in Thin-Film Silicon Solar Cells

Plasmonics - Silver nanoparticle (NP) arrays are used as antireflection coating to enhance light trapping capability of thin-film silicon solar cells. In this paper, we theoretically investigate...     

Metallic Grating-Assisted Fiber Optic SPR Sensor with Extreme Sensitivity in IR Region

We theoretically propose a surface plasmon resonance (SPR)-based fiber optic refractive index (RI) sensor. A surface plasmon exciting metallic grating formed with the alternation of indium tin oxide (ITO) and silver (Ag) stripes is considered on the core of the fiber. A thin film of silicon is used as an overlay. Silicon film not only protects the metallic grating from oxidation but also enhances the field to improve the device sensitivity. The sensor is characterized in terms of sensitivity, detection accuracy (DA), figure of merit (FoM), and quality factor (QF). The maximum sensitivity in the RI range 1.33 to 1.38 refractive index unit (RIU) is reported to be?~25 µm/RIU in infra-red region of investigation.

     

Optical Absorption Enhancement in Freestanding GaAs Thin Film Nanopyramid Arrays

Although III–V compound semiconductor multi‐junction cells show the highest efficiency among all types of solar cells, their cost is quite high due to expensive substrates, long epitaxial growth and complex balance of system components. To reduce the cost, ultra‐thin films with advanced light management are desired. Here effective light trapping in freestanding thin film nanopyramid arrays is demonstrated and multiple‐times light path enhancement is realized, where only 160 nm thick GaAs with nanopyramid structures is equivalent to a 1 μm thick planar film. The GaAs nanopyramids are fabricated using a combination of nanosphere lithography, nanopyramid metal organic chemical vapor deposition (MOCVD) growth, and gas‐phase substrate removal processes. Excellent optical absorption is demonstrated over a broad range of wavelengths, at various incident angles and at large‐curvature bending. Compared to an equally thick planar control film, the overall number of photons absorbed is increased by about 100% at various incident angles due to significant antireflection and light trapping effects. By implementing these nanopyramid structures, III–V material usage and deposition time can be significantly reduced to produce high‐efficiency, low‐cost thin film III–V solar cells.     

Discrete Optical Field Manipulation by Ag-Al Bilayer Gratings for Broadband Absorption Enhancement in Thin-Film Solar Cells

Plasmonic gratings have been widely used for light harvesting in thin-film solar cells (TFSCs). However, the detrimental parasitic metal absorption loss limits the actual light absorption in the active layer and reduces the power conversion efficiency. In this paper, it is found that the localized surface plasmon resonance (LSPR) used to increase long-wavelength light absorption has significant field concentration around the bottom corners of metal gratings, but the field distribution for the short-wavelength absorption band localizes around the top corners of gratings. Due to the differences between the spatial field distributions and the related mechanisms of metal loss, discrete optical field manipulation is proposed to suppress the ohmic loss mainly associated with LSPR and the interband transition loss associated with metal materials by using Ag-Al bilayer gratings, where Ag has a small absorption coefficient and Al has a high plasmon frequency. Fifteen to forty percent improvements of photocurrents in TFSCs with Ag-Al bilayer gratings are observed in simulation compared to the ones with single-layer metal gratings. This combined metal nanostructure scheme suppresses the loss issue of metal and extends the application potential of plasmonic light-harvesting techniques.     

Influences of Nanostructure Arrays on Light Absorption in Amorphous Silicon Thin-Film Solar Cells

By means of finite-difference time-domain (FDTD) numerical method, we investigate the possibility to enhance the light absorption in solar cells by employing different nanostructures. The solar cells are made of 100-nm-thick amorphous silicon (α-Si). The impacts of gold nanohole arrays, dielectric nanosphere arrays, and gold nanoparticle arrays on the light absorption are simulated, compared, and analyzed. The results show that gold nanohole arrays functioning as the back reflective layer, dielectric nanosphere arrays, and gold nanoparticle arrays can significantly enhance the light absorption for the solar cells, and the former two can increase the short-circuit current by more than 40 %, showing a great potential to improve the utilization efficiency of solar energy.     

Design of Plasmonic Nanoparticles for Efficient Subwavelength Light Trapping in Thin-Film Solar Cells

This paper explores geometry-sensitive scattering from plasmonic nanoparticles deposited on top of a thin-film amorphous silicon solar cell to enhance light trapping in the photo-active layer. Considering the nanoparticles as ideal spheroids, the broadband optical absorption by the silicon layer is analyzed and optimized with respect to the nanoparticle aspect ratio in both the cases of resonant (silver) and nonresonant (aluminum) plasmonic nanostructures. It is demonstrated how the coupling of sunlight with the semiconductor can be improved through tuning the nanoparticle shape in both the dipolar and multi-polar scattering regimes, as well as discussed how the native oxide shell formed on the nanospheroid surface after the prolonged action of air and moisture affects the light trapping in the active layer and changes the photocurrent generation by the solar cell.     

Fabrication of Ag Nanoparticles Embedded in Al:ZnO as Potential Light-Trapping Plasmonic Interface for Thin Film Solar Cells

Incident photon conversion efficiency of the absorbing materials at either side of a thin film solar module can be enhanced by integrating a plasmonic interface. Silver nanoparticles represent a good candidate that can be integrated to a thin film solar cell for efficient light-trapping. The aim of this work is to fabricate plasmonically active interface consisting of Ag nanoparticles embedded in Al:ZnO that has the potential to be used at the front surface and at the back reflector of a thin film solar cell to enhance light-trapping and increase the photoconversion efficiency. We show that Ag can readily dewet the Al:ZnO surface when annealed at temperatures significantly lower than the melting temperature of Ag, which is beneficial for lowering the thermal budget and cost in solar cell fabrication. We find that such an interface fabricated by a simple dewetting technique leads to plasmonic resonance in the visible and near infrared regions of the solar spectrum, which is important in enhancing the conversion efficiency of thin film solar cells.     

Plasmon-Enhanced Light Trapping to Improve Efficiency of TiO2 Nanorod-Based Dye-Sensitized Solar Cell

Localized surface plasmon resonance incurred by silver nanoparticles is used to enhance the photoelectric conversion efficiency of a TiO₂ nanorod-based dye-sensitized solar cell (DSSC). Improved light transmission is observed experimentally in silver nanoparticle-coated FTO glass. The transmission data are used to explore the effect on electrical parameters of DSSC using theoretical model. Current density increased from 11.7 to 12.34 mA/cm² and open-circuit voltage increased from 704 to 709.5 mV. Overall efficiency enhancement of 6.67 % is observed in TiO₂ nanorod-based DSSC due to plasmon-induced light trapping.     

Tailoring Absorption in Metal Gratings with Resonant Ultrathin Bridges

We present a theoretical analysis of the effects of short range surface plasmon polariton excitation on subwavelength bridges in metal gratings. We show that localized resonances in thin metal bridges placed within the slit of a free-standing silver grating dramatically modify transmission spectra and boost absorption regardless of the periodicity of the grating. Additionally, the interference of multiple localized resonances makes it possible to tailor the absorption properties of ultrathin gratings, regardless of the apertures’ geometrical size. This tunable, narrow band, enhanced–absorption mechanism triggered by resonant, short-range surface plasmon polaritons may also enhance nonlinear optical processes like harmonic generation, in view of the large third-order susceptibility of metals.     

Characterization of Grating Coupled Surface Plasmon Polaritons Using Diffracted Rays Transmittance

A method to sense the excitation of surface plasmon polariton (SPP) on metallic grating device using the transmitted signal will be presented. The grating transmittance signal will be fully characterized varying the light incident angle and azimuthal grating orientation by means of the SPP vector model and rigorous coupled-wave analysis simulation. Simulation results will be compared with experimental measurements obtained with a 635 nm wavelength laser in the transverse magnetic polarization mode. The laser will light grating devices in contact with either air or water through a customized microfluidic chamber. A characterization of the diffracted rays will show the relationship between the grating coupling configuration and the Kretschmann one. In fact, the diffracted ray affected by SPP resonance is transmitted with an output angle which is the same incident angle that should be used to excite SPP in Kretschmann configuration. Lastly, the grating parameters (amplitude and metal thickness) impact on transmittance signal will be analyzed with respect to the order zero reflectance signal.