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Plasmonic perfect meta-absobers for a-Si PV devices

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Optical and Plasmonic Perfect Meta Absorbers Literature Reviews[edit]

J. Gwamuri, D. Ö. Güney and J. M. Pearce, "Advances in Plasmonic Light Trapping in Thin-Film Solar Photovoltaic Devices", in Solar Cell Nanotechnology, Atul Tiwari (Editor), Rabah Boukherroub (Editor), Maheshwar Sharon (Editor), Wiley, ISBN: 978-1-118-68625-6 Preorder

This chapter reviews the recent promising advances in the use of plasmonic nanostructures forming metamaterials to improve absorption of light in thin-film solar photovoltaic (PV) devices. Sophisticated light management in thin-film PV has become increasingly important to ensure absorption of the entire solar spectrum while reducing semiconductor absorber layer thicknesses, which reduces deposition time, material use, embodied energy and greenhouse gas emissions, and economic costs. Metal nanostructures have a strong interaction with light, which enables unprecedented control over the propagation and the trapping of light in the absorber layer of thin-film PV. The literature is reviewed for both theoretical and experimental work on multiple nanoscale geometries of plasmonic absorbers and PV materials. Finally, the use of nanostructures to improve light trapping in PV is outlined to guide development in the future.

Theoretical/ Simulation related Articles[edit]

Abstract The study first uses numerical simulations of hexagonal triangle and sphere arrays to optimize the performance of hydrogenated amorphous silicon (a-Si:H) photovoltaic devices. The simulations indicated the potential for a sphere array to provide optical enhancement (OE) up to 7.4% compared to a standard cell using a nanosphere radius of 250 nm and silver film thickness of 50 nm. Next a detailed series of a-Si:H cells were fabricated and tested for quantum efficiency and characteristic and current–voltage (I–V) profiles using a solar simulator. Triangle and sphere array based cells, as well as the uncoated reference cells are analyzed and the results find that the simulation does not precisely predict the observed enhancement, but it forecasts a trend and can be used to guide fabrication. In general, the measured OE follows the simulated trend: (1) for triangular arrays no enhancement is observed and as the silver thickness increases the more degradation of the cell; (2) for annealed arrays both measured and simulated OE occur with the thinner silver thickness. Measured efficiency enhancement reached 20.2% and 10.9% for nanosphere diameter D = 500 nm, silver thicknesses h = 50 nm and 25 nm, respectively. These values, which surpass simulation results, indicate that this method is worth additional investigation.

Abstract Metals in the plasmonic metamaterial absorbers for photovoltaics constitute undesired resistive heating. However, tailoring the geometric skin depth of metals can minimize resistive losses while maximizing the optical absorbance in the active semiconductors of the photovoltaic device. Considering experimental permittivity data for InxGa1-xN, absorbance in the semiconductor layers of the photovoltaic device can reach above 90%. The results here also provides guidance to compare the performance of different semiconductor materials. This skin depth engineering approach can also be applied to other optoelectronic devices, where optimizing the device performance demands minimizing resistive losses and power consumption, such as photodetectors, laser diodes, and light emitting diodes.

Abstract:We review the basic physics of surface-plasmon excitations occurring at metal/dielectric interfaces with special emphasis on the possibility of using such excitations for the localization of electromagnetic energy in one, two, and three dimensions, in a context of applications in sensing and waveguiding for functional photonic devices. Localized plasmon resonances occurring in metallic nanoparticles are discussed both for single particles and particle ensembles, focusing on the generation of confined light fields enabling enhancement of Raman-scattering and nonlinear processes. We then survey the basic properties of interface plasmons propagating along flat boundaries of thin metallic films, with applications for waveguiding along patterned films, stripes, and nanowires. Interactions between plasmonic structures and optically active media are also discussed.


Abstract:We theoretically investigate the light-trapping properties of one- and two-dimensional periodic patterns etched on the front surface of c-Si and a-Si thin film solar cells with a silver back reflector and an anti-reflection coating. For each active material and configuration, absorbance A and short-circuit current density Jsc are calculated by means of rigorous coupled wave analysis (RCWA), for different active materials thicknesses in the range of interest of thin film solar cells and in a wide range of geometrical parameters. The results are then compared with Lambertian limits to light-trapping for the case of zero absorption and for the general case of finite absorption in the active material. With a proper optimization, patterns can give substantial absorption enhancement, especially for 2D patterns and for thinner cells. The effects of the photonic patterns on light harvesting are investigated from the optical spectra of the optimized configurations. We focus on the main physical effects of patterning, namely a reduction of reflection losses (better impedance matching conditions), diffraction of light in air or inside the cell, and coupling of incident radiation into quasi-guided optical modes of the structure, which is characteristic of photonic light-trapping.

Abstract:We study numerically the photon input efficiency of silicon solar cells due to gold plasmonic nanoparticles deposited on the cells. At low densities, when collective effects in light scattering by the nanoparticle ensemble are negligible, the absorption dependence increases linearly for a significant range of the solar spectrum. Collective effects lead to the input efficiency saturates, reaches its maximum and then decreases with nanoparticle density. The maximal input efficiency depends on the photon wavelength, nanoparticle shape and size, their distance to the cell, and the cell thickness, and can reach ~95% in thick solar cells. Finally, we show that aluminum nanoparticles improve the input efficiency in comparison with gold nanoparticles.

Abstract: Establishing the fundamental limit of nanophotonic light-trapping schemes is of paramount importance and is becoming increasingly urgent for current solar cell research. The standard theory of light trapping demonstrated that absorption enhancement in a medium cannot exceed a factor of 4n2 ∕ sin2 θ, where n is the refractive index of the active layer, and θ is the angle of the emission cone in the medium surrounding the cell. This theory, however, is not applicable in the nanophotonic regime. Here we develop a statistical temporal coupled-mode theory of light trapping based on a rigorous electromagnetic approach. Our theory reveals that the conventional limit can be substantially surpassed when optical modes exhibit deep-subwavelength-scale field confinement, opening new avenues for highly efficient next-generation solar cells.

Abstract: We theoretically and numerically study the absorption effect and the heat generation in plasmonic metamaterials under light radiation at their plasmonic resonance. Three different types of structures, all possessing high-performance absorption for visible lights, are investigated. The main aim of this work is to present an intuitive and original understanding of the high-performance absorption effects. From the macroscopic electromagnetic point of view, the effective-medium approach is used to describe the absorption effects of the plasmonic metamaterials. On the other hand, the field distributions and heat generation effects in such plasmonic nanostructures are investigated, which also provides a satisfactory qualitative description of such absorption behavior based upon the microscopic perspective.

 -Theoretically and numerically study of absorption effect & heat generation metamaterials under light radiation at their plasmonic resonance.
 -Three different types of structures all possessing high-performance absorption for visible lights investigated.
 -Perfect absorber features; - broad spectrum , blackbody like behaviour from IR - Visible (400-700nm)
                                - Narrow band response - selective emitter.
 -Ag nano particles (very small spheres) embedded in a dielectric matrix (plasmonic blackbody) with A > 90%.
 -Wide range of incident were reported achieved frome 240 - 850 nm.
 - A two dimensional design of an ultra-thin, wide-angle perfect absorber for infrared light is proposed.
 - Design based on plasmonic meta-material absorbing IR @ absorption rate > 99.9 % .
 - Proposed fixtures include; Wide range of incident angles and tunable resonant wavelength @ sub-wavelenght size.
 - Angular dependence of the absorption for different polarizations of the incident radiation remained above 90% (P & S-polarization)for incidence angles up to 45◦.
 - large field enhancement inside the meta-material, COMSOL was used to simulate optical properties.
 - A theoretical paper.

Abstract:Recent research in the rapidly emerging field of plasmonics has shown the potential to significantly enhance light trapping inside thin-film solar cells by using metallic nanoparticles. In this article it is demonstrated the plasmon enhancement of optical absorption in amorphous silicon solar cells by using silver nanoparticles. Based on the analysis of the higher-order surface plasmon modes, it is shown how spectral positions of the surface plasmons affect the plasmonic enhancement of thin-film solar cells. By using the predictive 3D modeling, we investigate the effect of the higher-order modes on that enhancement. Finally, we suggest how to maximize the light trapping and optical absorption in the thin-film cell by optimizing the nanoparticle array parameters, which in turn can be used to fine tune the corresponding surface plasmon modes.

Abstract:A metamaterials-based approach to making a wide-angle absorber of infrared radiation is described. The technique is based on an anisotropic Perfectly Impedance Matched Negative Index Material (PIMNIM). It is shown analytically that a sub-wavelength in all three dimensions PIMNIM enables absorption of close to 100% for incidence angles up to 45 deg to the normal. A specific implementation of such frequency-tunable PIMNIM based on plasmonic metamaterials is presented. Applications to infrared imaging and coherent thermal sources are described.

Design of plasmonic back structures for efficiency enhancement of thin-film amorphous Si solar cells [9]

Abstract: Metallic back structures with one-dimensional periodic nanoridges attached to a thin-film amorphous Si (a-Si) solar cell are numerically studied. At the interfaces between a-Si and metal materials, the excitation of surface-plasmon polaritons leads to obvious absorption enhancements in a wide near-IR range for different ridge shapes and periods. The highest enhancement factor of the cell external quantum efficiency is estimated to be 3.32. The optimized structure can achieve an increase of 17.12% in the cell efficiency.

Abstract: We theoretically demonstrate a polarization-independent nanopatterned ultra-thin metallic structure supporting short-range surface plasmon polariton (SRSPP) modes to improve the performance of organic solar cells. The physical mechanism and the mode distribution of the SRSPP excited in the cell device were analyzed, and reveal that the SRSPP-assisted broadband absorption enhancement peak could be tuned by tailoring the parameters of the nanopatterned metallic structure. Three-dimensional finite-difference time domain calculations show that this plasmonic structure can enhance the optical absorption of polymer-based photovoltaics by 39% to 112%, depending on the nature of the active layer (corresponding to an enhancement in short-circuit current density by 47% to 130%). These results are promising for the design of organic photovoltaics with enhanced performance.

Abstract:We describe an ultrathin solar cell architecture that combines the benefits of both plasmonic photovoltaics and traditional antireflection coatings. Spatially resolved electron generation rates are used to determine the total integrated current improvement under AM1.5G solar illumination, which can reach a factor of 1.8. The frequency-dependent absorption is found to strongly correlate with the occupation of optical modes within the structure, and the improved absorption is mainly attributed to improved coupling to guided modes rather than localized resonant modes.

Abstract:We propose an approach for enhancing the absorption of thin-film amorphous silicon solar cells using periodic arrangements of resonant dielectric nanospheres deposited as a continuous film on top of a thin planar cell. We numerically demonstrate this enhancement using 3D full field finite difference time domain simulations and 3D finite element device physics simulations of a nanosphere array above a thin-film amorphous silicon solar cell structure featuring back reflector and anti-reflection coating. In addition, we use the full field finite difference time domain results as input to finite element device physics simulations to demonstrate that the enhanced absorption contributes to the current extracted from the device. We study the influence of a multi-sized array of spheres, compare spheres and domes and propose an analytical model based on the temporal coupled mode theory.

 -polarized broad band plane wave source used and absorption calculated in both transverse magnetic and electric polarizations      
 - wavelengths range from 300 - 840 nanometers used.
 - absorption generates photocurrent in the blue part of the spectrum for the a-Si layer.
 - Results from 3D FDTD modeling as input to the 3D finite-element device physics simulations to simulate the optical- electrical performances.   
 - Flat cell results; Jph=12.47 mA/cm2, overall efficiency of 7.61%, Jsc of 9.27 mA/cm2, Voc of 0.990 V, and FF of 0.830..
 - Resonant SiO2 spheres cell results; Jph, is 14.05 mA/cm2,  Jsc of 10.37 mA/cm2,Voc equals 0.993 V. FF is 0.831 
 - carriers generated at the resonant wavelengths contribute to the overall Jsc.
 - external quantum efficiency improved at resonant wavelength of 665 nm from (the flat case) 0.32 to 0.66. 
 - total optical generation rate same for TE and TM polarizations.
 - Multi-sized sphere structure simulation; 16% Jsc enhancement compared to no sphere case, but similar enhancement as mono-sized spheres .
 - Conclusions: Enhanced absorption results in charge carriers that contribute to the electrical current generated by the cell.
 - Dielectric resonant spheres offer better enhancement than domes.'

   

Abstract: This paper explores geometry-sensitive scattering from plasmonic nanoparticles deposited on top of a thin-film amorphous silicon solar cell to enhancelight 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.

Abstarct:Recently, thin film solar cells enhanced by nanoparticles have attracted much attention of the scientific community. To improve the performance of such cells, a systematic study on the influence of the nanoparticle material on the efficiency of the enhancement is performed. Based on optimization of the nanoparticle array parameters, the role of dispersion and dissipation of the nanoparticle material is discussed and analyzed with respect to optical absorption of the photoactive layer. Finally, it is demonstrated that the use of dielectric nanoparticles can lead to similar and even higher enhancements compared to that of metal nanoparticles.

Abstarct:We analyze the localized surface plasmon resonance spectra of periodic square lattice arrays of gold nano-disks, and we describe numerically and experimentally the effect of disorder on resonance width, spectrum, and EM field enhancement in increasingly randomized patterns. The periodic structure shows a narrower and stronger extinction peak, conversely we observe an increase of up to (1−2)×102 times enhancement as the disorder is gradually introduced. This allows for simpler, lower resolution fabrication, cost-effective in light harvesting for solar cell and sensing applications. We show that dipole-dipole interactions contribute to diffract light parallel to the surface as a mean of long-range coupling between the nano-disks.

Abstract: Overall performance of a thin film solar cell is determined by the efficiency of converting photons to electrons through light absorption, carrier generation, and carrier collection. Recently, photon management has emerged as a powerful tool to further boost this conversion efficiency. Here we propose a novel nanograting solar cell design that achieves enhanced broadband light absorption and carrier generation in conjunction with the reduced use of active and non-earth-abundant materials. A test using this design for the short circuit current density in CuInxGa(1-x)Se2 (CIGS) thin film solar cells shows up to 250% enhancement when compared to the bare thin film cells. In addition, placing metal strips on top of the nanograting to act as the top electrode reduces the use of non-earth-abundant materials that is normally used as the transparent conducting materials. This novel solar cell design has the potential to become a new solar cell platform technology for various thin film solar cell systems.

Abstract:If the active layer of efficient solar cells could be made 100 times thinner than in today’s thin film devices, their economic competitiveness would greatly benefit. However, conventional solar cell materials do not have the optical capability to allow for such thickness reductions without a substantial loss of light absorption. To address this challenge, the use of plasmon resonances in metal nanostructures to trap light and create charge carriers in a nearby semiconductor material is an interesting opportunity. In this Perspective, recent progress with regards to ultrathin (∼10 nm) plasmonic nanocomposites is reviewed. Their optimal internal geometry for plasmon near-field induced absorption is discussed, and a zero thickness effective medium representation is used to optimize stacks including an Al back reflector for photovoltaics. This shows that high conversion efficiencies (>20%) are possible even when taking surface scattering effects and thin passivating layers inserted between the metal and semiconductor into account.

Abstract:Freely propagating sunlight can be diffractively coupled and transformed into several guided whispering gallery modes within an array of wavelength scale dielectric spheres. Incident optical power is then transferred to the thin-film cell by leaky mode coupling into a thin solar cell absorber layer and significantly enhances its efficiency by increasing the fraction of incident light absorbed.

Abstract: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.

Abstarct:The integration of nanophotonic and plasmonic structures with solar cells offers the ability to control and confine light in nanoscale dimensions. These nanostructures can be used to couple incident sunlight into both localized and guided modes, enhancing absorption while reducing the quantity of material. Here we use electromagnetic modeling to study the resonances in a solar cell containing both plasmonic metal back contacts and nanostructured semiconductor top contacts, identify the local and guided modes contributing to enhanced absorption, and optimize the design. We then study the role of the different interfaces and show that Al is a viable plasmonic back contact material.

 - use of EM modeling to study resonances in solar cell with both plasmonic metal back contacts light trapping and photonic front surface light trapping'.
 - simulation used to understand the difference between localized and guided modes contribution to enhanced absorption in realistic 3D solar cells.
 - studying the role of texturing on different interfaces, and predicting broad-band response.
 - study proved that Al is a viable plasmonic back contact material, with a potential for higher photocurrents than Ag.
 - EM modeling and cell optimization done using experimental cross sections and atomic force microscopy (AFM) data as a guide to model realistic cell architectures.
 - FDTD method which does not require assumptions of periodicity and regularity was considered to offer more flexibility compared to other methods by the researchers.
 - experimental and simulated parameters showed a high degree of agreement allowing the researchers to optimize their nanostructure designs.
 - localized resonances within the semiconductor - key role is in enhancing absorption on the blue side of the spectrum.
 - metal patterns on the back contact used to couple red light into waveguides modes of the cell.
 Researchers' conclusion: Ideal light trapping geometry is composed of separately optimized front and back nanopatterns. 

Abstract:The impact of controlled nanopatterning on the Ag back contact of an n-i-p a-Si:H solar cell was investigated experimentally and through electromagnetic simulation. Compared to a similar reference cell with a flat back contact, we demonstrate an efficiency increase from 4.5% to 6.2%, with a 26% increase in short circuit current density. Spectral response measurements show the majority of the improvement between 600 and 800 nm, with no reduction in photocurrent at wavelengths shorter than 600 nm. Optimization of the pattern aspect ratio using electromagnetic simulation predicts absorption enhancements over 50% at 660 nm.


Abstract:We demonstrate that subwavelength scatterers can couple sunlight into guided modes in thin film Si and GaAs plasmonic solar cells whose back interface is coated with a corrugated metal film. Using numerical simulations, we find that incoupling of sunlight is remarkably insensitive to incident angle, and that the spectral features of the coupling efficiency originate from several different resonant phenomena. The incoupling cross section can be spectrally tuned and enhanced through modification of the scatterer shape, semiconductor film thickness, and materials choice. We demonstrate that, for example, a single 100 nm wide groove under a 200 nm Si thin film can enhance absorption by a factor of 2.5 over a 10 μm area for the portion of the solar spectrum near the Si band gap. These findings show promise for the design of ultrathin solar cells that exhibit enhanced absorption.

Abstract:Nanostructured light trapping has emerged as a promising route toward improved efficiency in solar cells. We use coupled optical and electrical modeling to guide optimization of such nanostructures. We study thin-film n-i-p a-Si:H devices and demonstrate that nanostructures can be tailored to minimize absorption in the doped a-Si:H, improving carrier collection efficiency. This suggests a method for device optimization in which optical design not only maximizes absorption, but also ensures resulting carriers are efficiently collected.

Experiment Based Articles[edit]

Abstract:Noble metal nanostructures can enhance absorption in thin-film solar cells by simultaneously taking advantage of i) high near-fields surrounding the nanostructures close to their surface plasmon resonance frequency and ii) coupling to waveguide modes. We develop basic design rules for the realization of broadband absorption enhancements for such structures. Following this approach, we can attain a 43% enhancement in the short circuit current as compared to a cell without metallic structures. It is suggested that 3-dimensional nanoparticle arrays with even larger boosts in short circuit current can also be generated using the presented framework.


Abstract:Crystalline Si wires, grown by the vapor–liquid–solid (VLS) process, have emerged as promising candidate materials for low-cost, thin-film photovoltaics. Here, we demonstrate VLS-grown Si microwires that have suitable electrical properties for high-performance photovoltaic applications, including long minority-carrier diffusion lengths (Ln ≫ 30 µm) and low surface recombination velocities (S ≪ 70 cm·s−1). Single-wire radial p–n junction solar cells were fabricated with amorphous silicon and silicon nitride surface coatings, achieving up to 9.0% apparent photovoltaic efficiency, and exhibiting up to [similar]600 mV open-circuit voltage with over 80% fill factor. Projective single-wire measurements and optoelectronic simulations suggest that large-area Si wire-array solar cells have the potential to exceed 17% energy-conversion efficiency, offering a promising route toward cost-effective crystalline Si photovoltaics.

 -Double plasmonic nanostructure investigated for absorption enhancement in OPVs.
 -Structure design; period array of metal nanodiscs on one side of the active layer and a thin metal nanohole array on the other side.  
 -Total photon absorption enhancement factor >125% for molecular OPVs based on double heterojunction of an electron donor/hole transporter & electron acceptor/transoprter.
 -Tunability achieved by varying structural parameters of either layer .
 -Spectral range of 500 - 800 nm for nanostructure periods range of 140-300 nm were investigated.
 -Claim of total photon absorptivity enhancement by up to 128% compared to the reference cell without plasmonic nanostructures. 
 Conclusion: Article does not quantify the Jsc enhancement resulting from the enhanced absorptivity.

Abstract: Resonant plasmonic and metamaterial structures allow for control of fundamental optical processes such as absorption, emission and refraction at the nanoscale. Considerable recent research has focused on energy absorption processes, and plasmonic nanostructures have been shown to enhance the performance of photovoltaic and thermophotovoltaic cells. Although reducing metallic losses is a widely sought goal in nanophotonics, the design of nanostructured ‘ black ’ super absorbers from materials comprising only lossless dielectric materials and highly reflective noble metals represents a new research direction. Here we demonstrate an ultra-thin (260 nm) plasmonic super absorber consisting of a metal – insulator – metal stack with a nanostructured top silver film composed of crossed trapezoidal arrays. Our super absorber yields broadband and polarization-independent resonant light absorption over the entire visible spectrum (400 – 700 nm) with an average measured absorption of 0.71 and simulated absorption of 0.85. Proposed nanostructured absorbers open a path to realize ultrathin black metamaterials based on resonant absorption.

 - polarization - dependent experiment was carried first.
 - Ultrathin (260 nm) plasmonic structure consisting of both a nano structure Ag film and a metal-insulator-metal stack is proposed in the article.
 - Broadband and polarization-independent resonant light absorption over the entire visible spectrum (400–700 nm) reported.
 - Average measured absorption of 0.71 and simulated absorption of 0.85 were achieved.
 - Proposed broadband absorber consists of a three-layer metal – insulator – metal (MIM) thin-film laminate where only the top metal layer is patterned. 
 - Two MIM absorbers structural geometries employed;  metal stripe, and trapezoid arrays.Periodicity along the x axis restricted to 300 nm.
 - Top and bottom Ag layers of the MIM stack were 100 nm thick, and while the middle SiO2 layer thickness was kept at 60 nm.
 - Layer thicknesses kept constant for all the MIM absorber structures investigated.
 - Metal stripes of two different widths, w = 60 nm and w = 120 nm, were fabricated using electron beam (e-beam) lithography.
 - Two different crossed gratings with the crossed wire widths of w = 60 nm and w = 120 nm were studied (polarization - independent).
 - Such a symmetric configuration was observed to yields the same optical responses for both TM and TE polarizations.
 - Simulations performed in both experiments to validate experimental results.
 - Additional simulations were performed with 1° increments in polarization angle for the 120-nm-wide crossed grating(investigating polarization-independence.
 - Combination of trapezoid arrays to enable broad spectral extinction, and a crossed symmetric arrangement for polarization-independent resonant response.
 - Extinction spectra for two different polarization angles, θ = 0 ° and 45 °  were also investigated.
 - T and R were measured using an inverted optical microscope, and extinction defined by 1- T – R, since T = 0 the extinction reduces to 1- R. 
 - Optical measurements performed at normal incidence for both Tm and TE polarizations.
 - the resonance peak observed at 488 nm for the 60-nm-wide metallic stripe array, and maximum extinction value = 0.65.
 - the 120-nm-wide stripe recorded a maximum extinction value of  0.51 at 507 nm.  
 - Maximum extinction was measured to be 0.77 at 504 nm for the trapezoid nanostructure.
 - Fishnet gratings of 120 nm width, four different resonant peaks observed in the extinction spectra, at 450, 532, 578 and 668 nm.
 - For the 60-nm-wide fishnet structures the resonant peaks reduce to two distinct peaks at 477 and 617 nm.
 - Electromagnetic simulations were performed with ideal crossed gratings, results shown to agree with the experimental data in terms of the spectral shapes. 
 - A much broader and flatter extinction peak with a maximum value of 0.9 was observed, extinction between 400 and 700 nm is found to be 0.71.

Comment

 - Other teams have reported absorption  > 0.9 (Jiaming et al, 2011) over the same bandwidth.
 - Bai et al, 2011, also reported photon absorptivity of up to 128% for organic solar cell in the 500 – 800 nm range.
 - Theoretical values of absorption greater than 99.9% have also been reported as early as 2008.   
 

Design of a plasmonic back reflector for silicon nanowire decorated solar cells [28]

Abstract:This Letter presents a crystalline silicon thin film solar cell model with Si nanowire arrays surface decoration and metallic nanostructure patterns on the back reflector. The nanostructured Ag back reflector can significantly enhance the bsorption in the near-infrared spectrum. Furthermore, by inserting a ZnO:Al layer between the silicon substrate and nanostructured Ag back reflector, the absorption loss in the Ag back reflector can be clearly depressed, contributing to a maximum Jsc of 28.4 mA∕cm2. A photocurrent enhancement of 22% is achieved compared with a SiNW solar cell with a planar Ag back reflector.

Abstract:Six-particle and eight-particle common-gap plasmonic nanoantennas are utilized to obtain a broadband spectral response when illuminated with circular and elliptical polarization. Due to the insensitivity of dipole antennas to circular polarization, the resonant structures are brought together around the common-gap to expand the spectrum of the whole system. Their ability to focus light at different frequencies is demonstrated. The spectral response is manipulated by geometrical parameters and the strength of the spectral peaks is tailored through the ellipticity of the elliptically polarized light

Abstract:We report on the design, fabrication, and measurement of ultrathin film a-Si:H solar cells with nanostructured plasmonic back contacts, which demonstrate enhanced short circuit current densities compared to cells having flat or randomly textured back contacts. The primary photocurrent enhancement occurs in the spectral range from 550 nm to 800 nm. We use angle-resolved photocurrent spectroscopy to confirm that the enhanced absorption is due to coupling to guided modes supported by the cell. Full-field electromagnetic simulation of the absorption in the active a-Si:H layer agrees well with the experimental results. Furthermore, the nanopatterns were fabricated via an inexpensive, scalable, and precise nanopatterning method. These results should guide design of optimized, non-random nanostructured back reflectors for thin film solar cells.

 Objective: Investigating the effect of nanostructured plasmonic back contacts in ultrathin film a-Si:H solar cells on short circuit current densities.
 Cells with a flat / randomly textured back contacts were used as refence.
 Fabrication challenges include; precise control of shape, size and spacing of nanoparticles in a cost-effective way.
 researchers claim to have used an inexpensive, scalable and precise nanopatterning method to overcome the above challenges.
 Conclusions: Photocurrent enhancement reported for given cell thickness with nano-engineered plasmonic back contacts.
 The highest efficiency(η = 6.6%)recorded among cells with the 340 nm thick a-Si:H layer for a plasmonic scatterer pitch of 500 nm and a diameter of 250 nm.

ABSTRACT: Nanophotonic structures have attracted attention for light trapping in solar cells with the potential to manage and direct light absorption on the nanoscale. While both randomly textured and nanophotonic structures have been investigated, the relationship between photocurrent and the spatial correlations of random or designed surfaces has been unclear. Here we systematically design pseudorandom arrays of nanostructures based on their power spectral density, and correlate the spatial frequencies with measured and simulated photocurrent. The integrated cell design consists of a patterned plasmonic back reflector and a nanostructured semiconductor top interface, which gives broadband and isotropic photocurrent enhancement.

Abstract: We report on the design, fabrication and measurement of ultra-thin film Silicon On Insulator (SOI) Schottky photo-detector cells with nanostructured plasmonic arrays, demonstrating broadband enhanced photocurrent generation using aperiodic golden angle spiral geometry. Both golden angle spiral and periodic arrays of various center-to-center particle spacing were investigated to optimize the photocurrent enhancement. The primary photocurrent enhancement region is designed for the spectral range 600nm-950nm, where photon absorption in Si is inherently poor. We demonstrate that cells coupled to spiral arrays exhibit higher photocurrent enhancement compared to optimized periodic gratings structures. The findings are supported through coupled-dipole numerical simulations of radiation diagrams and finite difference time domain simulations of enhanced absorption in Si thin-films.

 - Objective: Enhancing broadband photocurrent generation using aperiodic golden angel spiral geometry for the 600 - 950 nm specral range.
 - Cell design: 300nm thick reflecting Al film deposited on Si chip followed by a 50 nm layer of a-Si. Cylindrical Au nanoparticle array fabricated on top of a-Si layer.
 - Spectrally integrated photocurrent enhancedment of 31% reported as measured from the photodetector coupled to optimized GA spiral geometry.

Abstarct:Here we discuss the design, fabrication, and simulation of ultrathin film n-i-p a-Si:H solar cells incorporating light trapping plasmonic back reflectors which exceed the performance of n-i-p cells on randomly textured Asahi substrates. The periodic patterns are made via an inexpensive and scalable nanoimprint method, and are structured directly into the metallic back contact. Compared to reference cells with randomly textured back contacts and flat back contacts, the patterned cells exhibit higher short-circuit current densities and improved overall efficiencies than either reference case. Angle-resolved photocurrent measurements confirm that the enhanced photocurrents are due to coupling to waveguide modes of the cell. Electromagnetic modeling is shown to agree well with measurements, and used to understand further details of the device.

 Paper report similar results already reported in other papers LIGHT TRAPPING IN THIN FILM PLASMONIC SOLAR CELLS,
 Simulations of solar cell absorption enhancement using resonant modes of a nanosphere arrayandLight trapping in ultrathin plasmonic solar cells.
 Objective was to investigate effect of a plasmonic back reflector as opposed to plasmonic back contact already reported.
 No much value addition from this paper since results are similar to those reported in Light trapping in ultrathin plasmonic solar cells paper.
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Abstract:Plasmonic nanostructures have been recently investigated as a possible way to improve absorption of light in solar cells. The strong interaction of small metal nanostructures with light allows control over the propagation of light at the nanoscale and thus the design of ultrathin solar cells in which light is trapped in the active layer and efficiently absorbed. In this paper we review some of our recent work in the field of plasmonics for improved solar cells. We have investigated two possible ways of integrating metal nanoparticles in a solar cell. First, a layer of Ag nanoparticles that improves the standard antireflection coating used for crystalline and amorphous silicon solar cells has been designed and fabricated. Second, regular and random arrays of metal nanostructures have been designed to couple light in waveguide modes of thin semiconductor layers. Using a large-scale, relative inexpensive nano-imprint technique, we have designed a back-contact light trapping surface for a-Si:H solar cells which show enhanced efficiency over standard randomly textured cells.

R. E. I. Schropp, H. A. Atwater, A. Polman, (2010).[36]

ABSTRACT: Advanced light management in thin-film solar cells is becoming increasingly important to reduce semiconductor layer thicknesses (and thus costs) while still absorbing the full solar spectrum in the cell. Here, we discuss how the excitation of surface plasmon resonances in metal nanoparticles can serve to enhance the trapping of light in thin-film solar cells. We discuss three geometries, with particles either at the top or back of the cell, or embedded in the solar cell, and compare these different designs. We demonstrate the effectiveness of the plasmonic light trapping concept by presenting experimental data on ultra-thin n-i-p a-Si:H solar cells with plasmonic back reflectors. We show that using a properly designed scattering structure, the performance of cells on randomly textured Asahi substrates can be surpassed. The periodic patterns are made via an inexpensive and scalable nanoimprint method. Electromagnetic modeling is shown to agree well with measurements, and used to understand further details of the device.

 Three light trapping geometries were investigated. All designs using controlled scattering and / or optical near field enhancement due to excitations of SPs
 Resonant wavelength tuning achivied through modification of size, shape, metal choice and surrounding material. 160 nm and 340 nm thick cells were designed.
 Flat cell used as reference cell. 
 Jsc increased by 30-50%,for all the cells investigated.
 Highest efficiency recorded for a cell with 275 nm diameter Ag nanoparticles with a 500 nm pitch. Jsc = 11.8 mA/cm^2 Vs 10.8 mA/cm^2 (best Asahi cell).
 Voc and FF were similar for all cells (0.84-0.89 V and 4.8 - 6.6 respectively).'
 Conclusion: Thinnner cells have higher Voc due to reduced bulk recombination, and strong reduction of photodegradation,
 more work is necessary to determine the best patterndesign for optimal scattering.
  

Abstract:We propose a design that increases significantly the absorption of a thin layer of absorbing material such as amorphous silicon. This is achieved by patterning a one-dimensional photonic crystal (1DPC) in this layer. Indeed, by coupling the incident light into slow Bloch modes of the 1DPC, we can control the photon lifetime and then, enhance the absorption integrated over the whole solar spectrum. Optimal parameters of the 1DPC maximize the integrated absorption in the wavelength range of interest, up to 45% in both S and P polarization states instead of 33% for the unpatterned, 100 nm thick amorphous silicon layer. Moreover, the absorption is tolerant with respect to fabrication errors, and remains relatively stable if the angle of incidence is changed.

Abstract:Here for the first time, we demonstrate novel nanodome solar cells, which have periodic nanoscale modulation for all layers from the bottom substrate, through the active absorber to the top transparent contact. These devices combine many nanophotonic effects to both efficiently reduce reflection and enhance absorption over a broad spectral range. Nanodome solar cells with only a 280 nm thick hydrogenated amorphous silicon (a-Si:H) layer can absorb 94% of the light with wavelengths of 400-800 nm, significantly higher than the 65% absorption of flat film devices. Because of the nearly complete absorption, a very large shortcircuit current of 17.5 mA/cm2 is achieved in our nanodome devices. Excitingly, the light management effects remain efficient over a wide range of incident angles, favorable for real environments with significant diffuse sunlight. We demonstrate nanodome devices with a power efficiency of 5.9%, which is 25% higher than the flat film control. The nanodome structure is not in principle limited to any specific material system and its fabrication is compatible with most solar manufacturing; hence it opens up exciting opportunities for a variety of photovoltaic devices to further improve performance, reduce materials usage, and relieve elemental abundance limitations. Lastly, our nanodome devices when modified with hydrophobic molecules present a nearly superhydrophobic surface and thus enable self-cleaning solar cells.

Abstract:Absorption enhancement in thin metal-backed solar cells caused by dipole scatterers embedded in the absorbing layer is studied using a semi-analytical approach. The method accounts for changes in the radiation rate produced by layers above and below the dipole, and treats incoherently the subsequent scattering of light in guided modes from other dipoles. We find large absorption enhancements for strongly coupled dipoles, exceeding the ergodic limit in some configurations involving lossless dipoles. An antireflection-coated 100-nm layer of a-Si:H on Ag absorbs up to 87% of incident above-gap light. Thin layers of both strong and weak absorbers show similar strongly enhanced absorption.

Abstract:We report three-dimensional modelling of plasmonic solar cells in which electromagnetic simulation is directly linked to carrier transport calculations. To date, descriptions of plasmonic solar cells have only involved electromagnetic modelling without realistic assumptions about carrier transport, and we found that this leads to considerable discrepancies in behaviour particularly for devices based on materials with low carrier mobility. Enhanced light absorption and improved electronic response arising from plasmonic nanoparticle arrays on the solar cell surface are observed, in good agreement with previous experiments. The complete three-dimensional modelling provides a means to design plasmonic solar cells accurately with a thorough understanding of the plasmonic interaction with a photovoltaic device.

Abstract:We show experimentally that there is asymmetry in photocurrent enhancement by Ag nanoparticle arrays located on the front or on the rear of solar cells. The scattering cross-section calculated for front- and rear-located nanoparticles can differ by up to a factor of 3.7, but the coupling efficiency remains the same. We attribute this to differences in the electric field strength and show that the normalized scattering cross-section of a front-located nanoparticle varies from two to eight depending on the intensity of the driving field. In addition, the scattering cross-section of rear-located particles can be increased fourfold using ultrathin spacer layers.

Abstract:Drop-coated high-refractive-index nanoparticles used as a back reflector for thin-film solar cells are non-absorbing Mie-scatterers that enhance light trapping. We present optical measurements and theory for this approach. A 40% enhancement of the photocurrent and efficiency of a 2.5 lm thick single-crystal Si solar cell on display glass is achieved by adding a back reflector of 270 nm rutile TiO2 nanoparticles.

Abstract: Currently, the performances of thin film solar cells are limited by poor light absorption and carrier collection. In this research, large, broadband, and polarization-insensitive light absorption enhancement was realized via integrating with unique metallic nanogratings. Through simulation, three possible mechanisms were identified to be responsible for such an enormous enhancement. A test for totaling the absorption over the solar spectrum shows an up to ∼30% broadband absorption enhancement when comparing to bare thin film cells.

Abstact:Plasmonics is a promising new approach to enhance the light trapping properties of thin-film solar cells. Metal nanoparticles support surface plasmon modes, which are used to couple light into the underlying optical modes of the semiconductor. Tuning the surface plasmon resonance can be used in order to enhance absorption in the wavelength region required. Excitation of surface plasmons is characterised by strong scattering and enhancement of the electric field around the vicinity of the metal nanoparticle. Photocurrent enhancements have been reported from both inorganic and organic solar cells due to either one of these mechanisms. This paper reviews recent progress in this area and also discusses the potential of surface plasmons in the third generation solar cells.

Abstract:Recently, thin film solar cells enhanced by nanoparticles have attracted much attention of the scientific community. To improve the performance of such cells, a systematic study on the influence of the nanoparticle material on the efficiency of the enhancement is performed. Based on optimization of the nanoparticle array parameters, the role of dispersion and dissipation of the nanoparticle material is discussed and analyzed with respect to optical absorption of the photoactive layer. Finally, it is demonstrated that the use of dielectric nanoparticles can lead to similar and even higher enhancements compared to that of metal nanoparticles.

Abstract:We present experimental results for photocurrent enhancements in thin c-Si solar cells due to light-trapping by selfassembled, random Ag nanoparticle arrays. The experimental geometry is chosen to maximise the enhancement provided by employing previously reported design considerations for plasmonic light-trapping. The particles are located on the rear of the cells, decoupling light-trapping and anti-reflection effects, and the scattering resonances of the particles are redshifted to target spectral regions which are poorly absorbed in Si, by over-coating with TiO2.We report a relative increase in photocurrent of 10% for 22mmSi cells due to light-trapping. Incorporation of a detached mirror behind the nanoparticles increases the photocurrent enhancement to 13% and improves the external quantum efficiency by a factor of 5.6 for weakly absorbed light

Abstract:Effective light management is imperative in maintaining high efficiencies as photovoltaic devices become thinner. We demonstrate a simple and effective method of enhancing light trapping in solar cells with thin absorber layers by tuning localized surface plasmons in arrays of Ag nanoparticles. By redshifting the surface plasmon resonances by up to 200 nm, through the modification of the local dielectric environment of the particles, we can increase the optical absorption in an underlying Si wafer fivefold at a wavelength of 1100 nm and enhance the external quantum efficiency of thin Si solar cells by a factor of 2.3 at this wavelength where transmission losses are prevalent. Additionally, by locating the nanoparticles on the rear of the solar cells, we can avoid absorption losses below the resonance wavelength due to interference effects, while still allowing long wavelength light to be coupled into the cell. Results from numerical simulations support the experimental findings and show that the fraction of light backscattered into the cell by nanoparticles located on the rear is comparable to the forward scattering effects of particles on the front. Using nanoparticle self-assembly methods and dielectrics commonly used in photovoltaic fabrication this technology is relevant for application to large-scale photovoltaic devices.

Abstract:An unusual almost flat broadband plasmonic absorption, ranging from 400 nm to well beyond 2500 nm, was observed in a 150 nm thin film of Ag nanoparticles embedded in a Teflon AF® matrix. The nanocomposites were synthesized by a simple single-step vapor-phase codeposition method. The Ag nanoparticles of various sizes and shapes, and thus various resonance frequencies, form a fractal percolating network. The broadband absorption, attributed to plasmon excitations within the nanoparticles, could be useful for multicolor applications in the visible and infrared wavelengths region.

Abstract:In this paper, we use the transfer matrix method to calculate the optical absorptance of vertically-aligned silicon nanowire (SiNW) arrays. For fixed filling ratio, significant optical absorption enhancement occurs when the lattice constant is increased from 100nm to 600nm. The enhancement arises from an increase in field concentration within the nanowire as well as excitation of guided resonance modes. We quantify the absorption enhancement in terms of ultimate efficiency. Results show that an optimized SiNW array with lattice constant of 600nm and wire diameter of 540nm has a 72.4% higher ultimate efficiency than a Si thin film of equal thickness. The enhancement effect can be maintained over a large range of incidence angles.


Abstract:On the basis of conformal transformation, a general strategy is proposed to design plasmonic nanostructures capable of an efficient harvesting of light over a broadband spectrum. The surface plasmon modes propagate toward the singularity of these structures where the group velocity vanishes and energy accumulates. A considerable field enhancement and confinement is thus expected. Radiation losses are also investigated when the structure dimension becomes comparable to the wavelength.

Abstract:Advanced photon management, involving both absorption enhancement and reflection reduction, is critical to all photovoltaic devices. Here we discuss a novel solar cell structure with an efficient photon management design. The centerpiece of the design is the nanocone structure, which is fabricated by a scalable low temperature process. With this design, devices with a very thin active layer can achieve near perfect absorption because of both efficient anti-reflection and absorption enhancement over a broadband of spectra and a wide range of angles of incidence. The device performance of this design is significantly superior to that of conventional devices. More excitingly, the design and process is in principle not limited to any specific materials; hence it opens up exciting opportunities for a variety of photovoltaic devices to further improve the performance, reduce materials usage, and relieve the abundance limitation.

Abstract:Localized plasmon resonances of spherical nanovoid arrays strongly enhance solar cell performance by a factor of 3.5 in external quantum efficiency at plasmonic resonances, and a four-fold enhancement in overall power conversion efficiency. Large area substrates of silver nanovoids are electrochemically templated through self-assembled colloidal spheres and organic solar cells fabricated on top. Our design represents a new class of plasmonic photovoltaic enhancement: that of localized plasmon-enhanced absorption within nanovoid structures. Angularly resolved spectra demonstrate strong localized Mie plasmon modes within the nanovoids. Theoretical modelling shows varied spatial dependence of light intensity within the void region suggesting a first possible route towards Third Generation plasmonic photovoltaics.

Abstract:The spatial dependence of absorption in a structured thin film solar cell is investigated through the rigorous coupled-wave analysis method. The investigated structure allows strong localized surface plasmon and surface plasmon polaritons, simultaneously. The absorptance of silver and amorphous silicon can be separately accounted for by calculating the time-averaged energy dissipation although only the absorption of amorphous silicon contributes to the photocurrent. In our studied case, the metallic material absorbs around 15%-20% of the total impinging sunlight while the active layer absorbs only ~50%.

Abstract:We theoretically study the angular response of thin-film organic solar cells with periodic Au back nanostrips. In particular, the equation of the generalized Lambert’s cosine law for arbitrary periodic nanostructure is formulated. We show that the periodic strip structure achieves wide-angle absorption enhancement compared with the planar nonstrip structure for both the s- and p-polarized light, which is mainly attributed to the resonant Wood’s anomalies and surface plasmon resonances, respectively. The work is important for designing and optimizing high-efficiency photovoltaic cells.

Abstarct:Plasmons are free-electron oscillations in a conductor that allow light to be manipulated at the nanoscale. The ability of plasmons to guide and confine light on subwavelength scales is opening up new design possibilities for solar cells.

 Localized plasmons have already demonstrated their potential to boost the performance of solar cells for cases when 
 traditional texturing may not be viable. For propagating plasmons, parasitic optical absorption in the metallic
 structure and strong optical absorption near high-recombination metal–semiconductor interfaces both remain significant 
 challenges. Nonetheless, plasmonics is opening up a new optical world at the subwavelength scale that is largely unexplored. 
 High-index, non-absorbing dielectrics and the emerging field of metamaterial plasmonics may both offer new photovoltaic 
 possibilities. The low cell thickness that is possible with plasmonics may not only deliver anticipated material savings but 
 also ultimately allow the successful implementation of advanced high-performance concepts, such as hotcarrier cells.

Abstract:We use a rigorous electromagnetic approach to analyze the fundamental limit of light-trapping enhancement in grating structures. This limit can exceed the bulk limit of 4n2, but has significant angular dependency. We explicitly show that 2D gratings provide more enhancement than 1D gratings. We also show the effects of the grating profile’s symmetry on the absorption enhancement limit. Numerical simulations are applied to support the theory. Our findings provide general guidance for the design of grating structures for light-trapping solar cells.

Abstract:Recent years have seen a renewed interest in the harvesting and conversion of solar energy. Among various technologies, the direct conversion of solar to chemical energy using photocatalysts has received significant attention. Although heterogeneous photocatalysts are almost exclusively semiconductors, it has been demonstrated recently that plasmonic nanostructures of noble metals (mainly silver and gold) also show significant promise. Here we review recent progress in using plasmonic metallic nanostructures in the field of photocatalysis. We focus on plasmon-enhanced water splitting on composite photocatalysts containing semiconductor and plasmonic-metal building blocks, and recently reported plasmon-mediated photocatalytic reactions on plasmonic nanostructures of noble metals. We also discuss the areas where major advancements are needed to move the field of plasmon-mediated photocatalysis forward.

Abstract:Dielectric roughness on the front surface enhances significantly solar cell efficiency by light trapping in the absorbing layer. However in plasmonic assisted thin silicon solar cells we show, by a detailed analysis of the various mechanisms, that front-surface plasmonic structures enhance the efficiency by a different mechanism—namely the effective broadband forward scattering into the silicon, while trapping (path length enhancement) is relatively small for these structures. The plasmonic local field enhancement contribution is even smaller in this configuration. Based on our study we optimized this “anti-reflection mechanism” by tuning the plasmonic structure such that the spectral location of the more efficiently scattering dipole and quadrupole resonances fit correctly the visible and NIR sun spectrum.

'Abstarct:Future generations of photoelectrodes for solar fuel generation must employ inexpensive, earth-abundant absorber materials in order to provide a large-scale source of clean energy. These materials tend to have poor electrical transport properties and exhibit carrier diffusion lengths which are significantly shorter than the absorption depth of light. As a result, many photoexcited carriers are generated too far from a reactive surface and recombine instead of participating in solar-to-fuel conversion. We demonstrate that plasmonic resonances in metallic nanostructures and multilayer interference effects can be engineered to strongly concentrate sunlight close to the electrode/liquid interface, precisely where the relevant reactions take place. On comparison of spectral features in the enhanced photocurrent spectra to full-field electromagnetic simulations, the contribution of surface plasmon excitations is verified. These results open the door to the optimization of a wide variety of photochemical processes by leveraging the rapid advances in the field of plasmonics.

Review Articles[edit]

Abstract:The emerging field of plasmonics has yielded methods for guiding and localizing light at the nano-scale, well below the scale of the wavelength of light in free space. Now plasmonics researchers are turning their attention to photovoltaics, where design approaches based on plasmonics can be used to improve absorption in photovoltaic devices, permitting a considerable reduction in the physical thickness of solar photovoltaic absorber layers, and yielding new options for solar-cell design. In this review, we survey recent advances at the intersection of plasmonics and photovoltaics and offer an outlook on the future of solar cells based on these principles.

 - Article is a complete review of recent advances at the intersection of plasmonics and photovoltaics and offer the outlook of the future of solar cells.
 - High efficiency photovoltaics devices only possible if we are able to construct optically thick but physically ultra thin PV devices.
 - Use of resonant scattering/concentration of light in nano arrays and/or light coupling into SPP and photonics modes propagating in the plane of active layer proposed.
 - Ultra-thin absorber layers with reduced bulk recombinations loses may enable the use of cheap and relatively impure materials. 
 - Plasmonic geometries in PVs has the potential for total light absorption in a single quantum well or QD layer.
 - Extreme light confinement through plasmonic nanostructures can enhance nonlinear effects such as up- and down- conversion / multiple carrier generation in cells.
 - Conclusion: Comprehensive Plasmonic solar-cell design, geometries and fabrication techniques are proposed in this review paper.

Abstract:This paper reviews the recent research progress in the incorporation of plasmonic nanostructures with photovoltaic devices and the potential for surface plasmon enhanced absorption. We first outline a variety of cell architectures incorporating metal nanostructures. We then review the experimental fabrication methods and measurements to date, as well as systematic theoretical studies of the optimal nanostructure shapes. Finally we discuss photovoltaic absorber materials that could benefi t from surface plasmon enhanced absorption.

Abstract:Photovoltaics is already a billion dollar industry. It is experiencing rapid growth as concerns over fuel supplies and carbon emissions mean that governments and individuals are increasingly prepared to ignore its current high costs. It will become truly mainstream when its costs are comparable to other energy sources. At the moment, it is around four times too expensive for competitive commercial production. Three generations of photovoltaics have been envisaged that will take solar power into the mainstream. Currently, photovoltaic production is 90% first-generation and is based on silicon wafers. These devices are reliable and durable, but half of the cost is the silicon wafer and efficiencies are limited to around 20%. A second generation of solar cells would use cheap semiconductor thin films deposited on low-cost substrates to produce devices of slightly lower efficiency. A number of thin-film device technologies account for around 5–6% of the current market. As second-generation technology reduces the cost of active material, the substrate will eventually be the cost limit and higher efficiency will be needed to maintain the cost-reduction trend. Third-generation devices will use new technologies to produce high-efficiency devices. Advances in nanotechnology, photonics, optical metamaterials, plasmonics and semiconducting polymer sciences offer the prospect of cost-competitive photovoltaics. It is reasonable to expect that cost reductions, a move to second-generation technologies and the implementation of new technologies and third-generation concepts can lead to fully cost-competitive solar energy in 10–15 years.

Abstract:The scattering from metal nanoparticles near their localized plasmon resonance is a promising way of increasing the light absorption in thin-film solar cells. Enhancements in photocurrent have been observed for a wide range of semiconductors and solar cell configurations. We review experimental and theoretical progress that has been made in recent years, describe the basic mechanisms at work, and provide an outlook on future prospects in this area.


Abstract:Recently plasmonic effects have gained tremendous interest in solar cell research because they are deemed to be able to dramatically boost the efficiency of thin-film solar cells. However, despite of the intensive efforts, the desired broadband enhancement, which is critical for real device performance improvement, has yet been achieved with simple fabrication and integration methods appreciated by the solar industry. We propose in this paper a novel idea of using nucleated silver nanoparticles to effectively scatter light in a broadband wavelength range to realize pronounced absorption enhancement in the silicon absorbing layer. Since it does not require critical patterning, experimentally these tailored nanoparticles were achieved by the simple, low-cost and upscalable wet chemical synthesis method and integrated before the back contact layer of the amorphous silicon thin-film solar cells. The solar cells incorporated with 200 nm nucleated silver nanoparticles at 10% coverage density clearly demonstrate a broadband absorption enhancement and significant superior performance including a 14.3% enhancement in the short-circuit photocurrent density and a 23% enhancement in the energy conversion efficiency, compared with the randomly textured reference cells without nanoparticles. Among the measured plasmonic solar cells the highest efficiency achieved was 8.1%. The significant enhancement is mainly attributed to the broadband light scattering arising from the integration of the tailored nucleated silver nanoparticles.

Abstract: Plasmonic light trapping in thin film solar cells is investigated using full-wave electromagnetic simulations. Light absorption in the semiconductor layer with three standard plasmonic solar cell geometries is compared to absorption in a flat layer. We identify near-field absorption enhancement due to the excitation of localized surface plasmons but find that it is not necessary for strong light trapping in these configurations: significant enhancements are also found if the real metal is replaced by a perfect conductor, where scattering is the only available enhancement mechanism. The absorption in a 60 nm thick organic semiconductor film is found to be enhanced by up to 19% using dispersed silver nanoparticles, and up to 13% using a nanostructured electrode. External in-scattering nanoparticles strongly limit semiconductor absorption via back-reflection.

ABSTRACT: We show that a planar structure, consisting of an ultrathin semiconducting layer topped with a solid nanoscopically perforated metallic film and then a dielectric interference film, can highly absorb (superabsorb) electromagnetic radiation in the entire visible range, and thus can become a platform for high-efficiency solar cells. The perforated metallic film and the ultrathin absorber in this broadband superabsorber form a metamaterial effective film, which negatively refracts light in this broad frequency range. Our quantitative simulations confirm that the superabsorption bandwidth is maximized at the checkerboard pattern of the perforations. These simulations show also that the energy conversion efficiency of a single-junction amorphous silicon solar cell based on our optimized structure can exceed 12%.

Abstract:We present the concept of a solar thermo-photovoltaic (STPV) collection system based on a large-area, nanoimprint-patterned film of plasmonic structures acting as an integrated solar absorber/narrow-band thermal emitter (SANTE). The SANTE film concept is based on integrating broad-band solar radiation absorption with selective narrow-band thermal IR radiation which can be efficiently coupled to a photovoltaic (PV) cell for power generation. By employing a low reflectivity refractory metal (e.g., tungsten) as a plasmonic material, we demonstrate that the absorption spectrum of the SANTE film can be designed to be broad-band in the visible range and narrow-band in the infrared range. A detailed balance calculation demonstrates that the total STPV system efficiency exceeds the Shockley–Queisser limit for emitter temperatures above Te = 1200 K, and achieves an efficiency as high as 41% for Te = 2300 K. Emitter temperatures in this range are shown to be achievable under modest sun concentrations (less than 1000 suns) due to the thermal insulation provided by the SANTE film. An experimental demonstration of the wide-angle, frequency-selective absorptivity is presented.

Abstract:In this paper we discuss on light management in silicon thin film solar cells, using photonic crystal (PhC) structures. We particularly focus on photovoltaic devices including amorphous silicon absorbers patterned as 2D PhCs. Physical principles and design rules leading to the optimized configuration of the patterned cell are discussed by means of optical simulations performed on realistic thin film solar cell stacks. Theoretically, a maximum increase of 40%rel in integrated absorption in the a-Si:H layer of the patterned cell is expected compared to the unpatterned case. Moreover, both simulation and optical characterization of the fabricated cells demonstrate the robustness of their optical properties with regard to the angle of incidence of the light and to the fabrication induced defects in the PhCs. Finally, the impact of surface recombination due to the generation of new free surfaces with higher defect densities is addressed. We demonstrate that patterning still induces a substantial increase in the conversion efficiency, with a reasonable surface recombination velocity.

Abstract:Three of central challenges in solar cells are high light coupling into solar cell, high light trapping and absorption in a sub-absorption-length-thick active layer, and replacement of the indium-tin-oxide (ITO) transparent electrode used in thin-film devices. Here, we report a proposal and the first experimental study and demonstration of a new ultra-thin high-efficiency organic solar cell (SC), termed “plasmonic cavity with subwavelength hole-array (PlaCSH) solar cell”, that offers a solution to all three issues with unprecedented performances. The ultrathin PlaCSH-SC is a thin plasmonic cavity that consists of a 30 nm thick front metal-mesh electrode with subwavelength hole-array (MESH) which replaces ITO, a thin (100 nm thick) back metal electrode, and in-between a polymer photovoltaic active layer (P3HT/PCBM) of 85 nm thick (1/3 average absorption-length). Experimentally, the PlaCSH-SCs have achieved (1) light coupling-efficiency/absorptance as high as 96% (average 90%), broad-band, and Omni acceptance (light coupling nearly independent of both light incident angle and polarization); (2) an external quantum efficiency of 69% for only 27% single-pass active layer absorptance; leading to (3) a 4.4% power conversion efficiency (PCE) at standard-solar-irradiation, which is 52% higher than the reference ITO-SC (identical structure and fabrication to PlaCSH-SC except MESH replaced by ITO), and also is among the highest PCE for the material system that was achievable previously only by using thick active materials and/or optimized polymer compositions and treatments. In harvesting scattered light, the Omni acceptance can increase PCE by additional 81% over ITO-SC, leading to a total 175% increase (i.e. 8% PCE). Furthermore, we found that (a) after formation of PlaCSH the light reflection and absorption by MESH are reduced by 2 to 6 fold from the values when it is alone; and (b) the sheet resistance of a 30 nm thick MESH is 2.2 ohm/sq or less–4.5 fold or more lower than the best reported value for a 100 nm thick ITO film, giving a lowest reflectance-sheet-resistance product. Finally, fabrication of PlaCSH has used nanoimprint on 4” wafer and is scalable to roll-to-roll manufacturing. The designs, fabrications, and findings are applicable to thin solar cells in other materials.

Abstract:The performances of thin film solar cells are considerably limited by the low light absorption. Plasmonic nanostructures have been introduced in the thin film solar cells as a possible solution around this issue in recent years. Here, we propose a solar cell design, in which an ultrathin Si film covered by a periodic array of Ag strips is placed on a metallic nanograting substrate. The simulation results demonstrate that the designed structure gives rise to 170% light absorption enhancement over the full solar spectrum with respect to the bared Si thin film. The excited multiple resonant modes, including optical waveguide modes within the Si layer, localized surface plasmon resonance (LSPR) of Ag stripes, and surface plasmon polaritons (SPP) arising from the bottom grating, and the coupling effect between LSPR and SPP modes through an optimization of the array periods are considered to contribute to the significant absorption enhancement. This plasmonic solar cell design paves a promising way to increase light absorption for thin film solar cell applications.


Abstract:Nanoplasmonics recently has emerged as a new frontier of photovoltaic research. Noble metal nanostructures that can concentrate and guide light have demonstrated great capability for dramatically improving the energy conversion efficiency of both laboratory and industrial solar cells, providing an innovative pathway potentially transforming the solar industry. However, to make the nanoplasmonic technology fully appreciated by the solar industry, key challenges need to be addressed; including the detrimental absorption of metals, broadband light trapping mechanisms, cost of plasmonic nanomaterials, simple and inexpensive fabrication and integration methods of the plasmonic nanostructures, which are scalable for full size manufacture. This article reviews the recent progress of plasmonic solar cells including the fundamental mechanisms, material fabrication, theoretical modelling and emerging directions with a distinct emphasis on solutions tackling the above-mentioned challenges for industrial relevant applications.

Abstract:We propose an efficient plasmonic structure consisting of metal strips and thin-film silicon for solar energy absorption. We numerically demonstrate the absorption enhancement in symmetrical structure based on the mode coupling between the localized plasmonic mode in Ag strip pair and the excited waveguide mode in silicon slab. Then we explore the method of symmetry-breaking to excite the dark modes that can further enhance the absorption ability. We compare our structure with bare thin-film Si solar cell, and results show that the integrated quantum efficiency is improved by nearly 90% in such thin geometry. It is a promising way for the solar cell.

Abstract:We have demonstrated a simple approach for developing a photovoltaic device consisting of semiconductor-insulator-semiconductor (SIS) heterojunction using surface plasmon polaritons (SPPs) generated in one of the semiconductors (Al:ZnO) and propagated through the dielectric barrier (SiO2) to other (Si). This robust architecture based on surface plasmon excitation within an SIS device that produces power based on spatial confinement of electron excitation through plasmon absorption in Al:ZnO in a broad spectrum of visible to infrared wavelengths enhancing the photovoltaic activities. This finding suggests a range of applications for photovoltaics, sensing, waveguides, and others using SPPs enhancement on semiconductors without using noble metals.

Abstract:We report on the ability of resonant plasmonic slits to efficiently concentrate electromagnetic energy into a nanoscale volume of absorbing material placed inside or directly behind the slit. This gives rise to extraordinary optical absorption characterized by an absorption enhancement factor that well exceeds the enhancements seen for extraordinary optical transmission through slits. A semianalytic Fabry–Perot model for the resonant absorption is developed and shown to quantitatively agree with full-field simulations. We show that absorption enhancements of nearly 1000% can be realized at 633 nm for slits in aluminum films filled with silicon. This effect can be utilized in a wide range of applications, including high-speed photodetectors, optical lithography and recording, and biosensors.

Abstract:Thin-film solar cells have the potential to significantly decrease the cost of photovoltaics. Light trapping is particularly critical in such thin-film crystalline silicon solar cells in order to increase light absorption and hence cell efficiency. In this article we investigate the suitability of localized surface plasmons on silver nanoparticles for enhancing the absorbance of silicon solar cells. We find that surface plasmons can increase the spectral response of thin-film cells over almost the entire solar spectrum. At wavelengths close to the band gap of Si we observe a significant enhancement of the absorption for both thin-film and wafer-based structures. We report a sevenfold enhancement for wafer-based cells at λ = 1200 nm and up to 16-fold enhancement at λ = 1050 nm for 1.25 μm thin silicon-on-insulator (SOI) cells, and compare the results with a theoretical dipole-waveguide model. We also report a close to 12-fold enhancement in the electroluminescence from ultrathin SOI light-emitting diodes and investigate the effect of varying the particle size on that enhancement.


Abstract:Light trapping structures in photovoltaics are essential to suppress reflection losses and increase conversion efficiency. For wafer silicon (Si) solar cells, this is commonly achieved by chemical texturing and the application of an antireflection coating. Such surfaces still show significant reflection losses that are ∼10%. Hence, for further reduction in reflection, new methods for light trapping need to be explored, which are effective for a broad solar spectral and angular range. In this paper, we explore an ultrafast laser texturing method that successfully reduces the reflection below 5% over a broad spectral and angular range and more importantly, is applicable to crystalline, multi-crystalline, thin film silicon and other materials. The optical properties of ultrafast laser textured silicon surfaces produced in a sulfur hexafluoride (SF6) gas ambient are evaluated by total reflection including scattering as a function of wavelength and angle of incidence. The optical results are further compared with other texturing schemes. This study also investigates the silicon bandgap modification induced by ultrafast texturing method. Finally, a comparison is made for the photovoltaic parameters of solar cells made of ultrafast laser textured surfaces, chemically textured surfaces, porous silicon surfaces, and etched silicon surfaces that result in nanowires for light trapping to understand impact of surface texturing on photovoltaic device performance.

Abstract:Currently, the performances of thin film solar cells are limited by poor light absorption and carrier collection. In this research, large, broadband, and polarization-insensitive light absorption enhancement was realized via integrating with unique metallic nanogratings. Through simulation, three possible mechanisms were identified to be responsible for such an enormous enhancement. A test for totaling the absorption over the solar spectrum shows an up to 30% broadband absorption enhancement when comparing to bare thin film cells

Summary:One of the most promising ways to enhance the localized light absorption and to improve the efficiency of extremely thin solar cells is to use plasmonic structures. Plasmons are embedded metal nanostructures which can localize incident light on a sub-micrometric scale enabling light concentration and trapping. The current research shows that the optical broadband absorption in thin-film solar cells can be enhanced due to the local field enhancement by surface plasmons, leading to lower recombination currents, higher open circuit voltages, higher conversion efficiencies and even completely new solar-cell designs. This review paper will present the current research on different thin cell designs; on both near-field light concentration close to the nanoparticles resonance and effective light trapping. Recent significant enhancements of light absorption as well as overall efficiency enhancements have been reported for different types of thin film cells (e.g. a-Si, organic, GaAs)..

Abstract:This paper investigates the improved photo-current response obtained by depositing Al nanoparticles on top of a Si diode. Well defined Al nanodiscs with a diameter and height of 100 nm are produced on the surface of a Si diode using electron-beam lithography, and the change in photo-current generation is characterized. A blue shift of the photo-current response is demonstrated, substantially improving the relation between gains and losses compared to what is typically observed in similar schemes using Ag nanoparticles. Enhanced photo-current response is observed in diodes with Al particles on the surface at all wavelengths larger than ≈465 nm, thereby minimizing the losses in the blue range usually reported with Ag nanoparticles on the surface.

Abstract:The excitation of surface plasmons on metallic nanoparticles has the potential to significantly improve the performance of solar cells, in particular thin-film structures. In this article, we investigate the effect of the dielectric spacer layer thickness on the photocurrent enhancement of 2 lm thick, thin-film poly-Si on glass solar cells, due to random arrays of self-assembled Ag nanoparticles deposited on the front or the rear of the cells. We report a strong asymmetry in the external quantum efficiency (EQE) of the cell for front and rear located particles for different spacer thicknesses, which is attributed to differences in the scattering behavior of the nanoparticles. We find that for random arrays, with spectrally broad scattering resonances, the strength of the driving field and the coupling efficiency are more important for light trapping than the resonance wavelength. For particles located on the front of the cells it is desirable to have a thin dielectric spacer layer to enhance the scattering from the Ag nanoparticles. Additionally, light trapping provided by the random sized particles on the front can overcome suppression of light transmitted in the visible wavelength regions for thin layers of Si, to result in overall EQE enhancements. However, for particles deposited on the rear it is more beneficial to have the particles as close to the Si substrate as possible to increase both the scattering and the coupling efficiency.

Abstract:We investigate the influence of nanoparticle height on light trapping in thin-film solar cells covered with metal nanoparticles. We show that in taller nanoparticles the scattering cross-section is enhanced by resonant excitation of plasmonic standing waves. Tall nanoparticles have higher coupling efficiency when placed on the illuminated surface of the cell than on the rear of the cell due to their forward scattering nature. One of the major factors affecting the coupling efficiency of these particles is the phase shift of surface plasmon polaritons propagating along the nanoparticle due to reflection from the Ag/Si or Ag/air interface. The high scattering cross-sections of tall nanoparticles on the illuminated surface of the cell could be exploited for efficient light trapping by modifying the coupling efficiency of nanoparticles by engineering this phase shift. We demonstrate that the path length enhancement (with a nanoparticle of height 500 nm) at an incident wavelength of 700 nm can be increased from ~6 to ~16 by modifying the phase shift at the Ag/air interface by coating the surface of the nanoparticle with a layer of Si.

Abstract:Disk-shaped metal nanoparticles on high-index substrates can support resonant surface plasmon polariton (SPP) modes at the interface between the particle and the substrate. We demonstrate that this new conceptual model of nanoparticle scattering allows clear predictive abilities, beyond the dipole model. As would be expected from the nature of the mode, the SPP resonance is very sensitive to the area in contact with the substrate, and insensitive to particle height. We can employ this new understanding to minimise mode out-coupling and Ohmic losses in the particles. Taking into account optical losses due to parasitic absorption and outcoupling of scattered light, we estimate that an optimal array of nanoparticles on a 2 μm Si substrate can provide up to 71% of the enhancement in absorption achievable with an ideal Lambertian rear-reflector. This result compares to an estimate of 67% for conventional pyramid-type light trapping schemes.

Abstract:We provide a new physical interpretation of scattering from plasmonic nanoparticles on high-index substrates. We demonstrate the excitation of different types of resonant modes on disk-shaped, Ag nanoparticles. At short wavelengths, the resonances are localised at the top of the particle, while at longer wavelengths they are localised at the Ag/substrate interface. We attribute the long wavelength resonances to geometric resonances of surface plasmon polaritons (SPPs) at the Ag/substrate interface. We show that particles that support resonant SPP modes have enhanced scattering cross-sections when placed directly on a high-index substrate; up to 7.5 times larger than that of a dipole scatterer with an equivalent free-space resonance. This has implications for designing scattering nanostructures for light trapping solar cells.

Abstract:As a new method to improve the light trapping in solar cells, surface plasmon resonance (SPR) has attracted considerable attention because of its unique characteristics. Several studies have been reported on the photocurrent improvement of Si solar cells by surface plasmons, while little research has been done on III–V solar cells. In this work, we performed a systematic study of SPR on GaAs thin film solar cells with different sizes of Ag nanoparticles on the surface. The nanoparticles were fabricated by annealing E-beam evaporated Ag films in a N2 atmosphere. It was found that the surface plasmon resonance wavelength does not undergo a simple red-shift with increasing metal thickness. It depends on the shape of the metal nanoparticles and the interparticle spacing. It is necessary to optimize the particle size to obtain an optimum enhancement throughout the visible spectrum for solar cells. We found that the optimum thickness of the Ag film was 6 nm under our experimental conditions. Furthermore, from the calculation based on the external quantum efficiency data, the short circuit current density of a GaAs solar cell with 6 nm Ag film after annealing was increased by 14.2% over that of the untreated solar cell.

Abstract:A systematic investigation of the nanoparticle‐enhanced light trapping in thin‐film silicon solar cells is reported. The nanoparticles are fabricated by annealing a thin Ag film on the cell surface. An optimisation roadmap for the plasmonenhanced light‐trapping scheme for self‐assembled Ag metal nanoparticles is presented, including a comparison of rearlocated and front‐located nanoparticles, an optimisation of the precursor Ag film thickness, an investigation on different conditions of the nanoparticle dielectric environment and a combination of nanoparticles with other supplementary backsurface reflectors. Significant photocurrent enhancements have been achieved because of high scattering and coupling efficiency of the Ag nanoparticles into the silicon device. For the optimum light‐trapping scheme, a short‐circuit current enhancement of 27% due to Ag nanoparticles is achieved, increasing to 44% for a “nanoparticle/magnesium fluoride/ diffuse paint” back‐surface reflector structure. This is 6% higher compared with our previously reported plasmonic shortcircuit current enhancement of 38%.

Abstract:Hydrogenated amorphous Si (α-Si:H) is a promising material for photovoltaic applications due to its low cost, high abundance, long lifetime, and non-toxicity. We demonstrate a device designed to investigate the effect of nanostructured back reflectors on quantum efficiency in photovoltaic devices. We adopt a superstrate configuration so that we may use conventional industrial light trapping strategies for thin film solar cells as a reference for comparison. We controlled the nanostructure parameters via a wafer-scale self-assembly technique and systematically studied the relation between nanostructure size and photocurrent generation. The gain/loss transition at short wavelengths showed red-shifts with decreasing nanostructure scale. In the infrared region the nanostructured back reflector shows large photocurrent enhancement with a modified feature scale. This device geometry is a useful archetype for investigating absorption enhancement by nanostructures.

Abstract:These The paper reports a development and implementation of light trapping based on light scattering from plasmonic metal nanoparticles. The nanoparticles were formed on the surface of planar polycrystalline Si thin-film solar cells by annealing of a precursor Ag film. The light absorption by the cells and resulting photocurrent enhancement is maximised by optimising the design of the nanoparticle light-trapping scheme, which includes the nanoparticle size and location, the local dielectric environment, and an application of a supplementary back-surface reflector. A large photocurrent enhancement is achieved due to high scattering and coupling efficiencies of the incident light from Ag nanoparticles into the thin-film cells. For the optimum design comprising a “Si-film/nanoparticles/magnesium fluoride/diffuse white paint” structure short-circuit current enhancement of 44% is demonstrated for the cell fabricated by e-beam evaporation on 3 mm thick planar glass superstrate. The enhancement is further increased up to 50% when the developed light-trapping scheme is applied to the cell fabricated by PECVD on 1 mm thick planar glass superstrate.

Abstract:A new efficient plasmonic structure for solar energy absorption is designed. Numerical simulations demonstrate the absorption enhancement in a symmetrical structure based on the mode coupling between the localized plasmonic mode in an Ag strip pair and the excited waveguide mode in a silicon slab. Then the method of symmetry breaking is used to excite the dark modes that can further enhance the absorption ability. The new structure is compared with a bare thin-film Si solar cell, and results show that the integrated light harvest efficiency is greatly improved for the TM polarization in such thin geometry.

Abstract:The effect of the silver nanoparticle size distribution on the performance of plasmonic polycrystalline Si thin-film solar cells is studied. Monodisperse particle arrays are fabricated using nanoimprint lithography. Multidispersed particle arrays are fabricated using thermal evaporation followed by annealing. The short-circuit current enhancement for the cells without a back reflector is 24% and 18% with the multidisperse array and the monodispersed array, respectively. For the cells with a back reflector, the current enhancement increases to 34% and 30%, respectively, compared with 13% enhancement due to the reflector alone. Better performance of multidisperse Ag nanoparticle arrays is attributed to a broader scattering cross section of the array owing to a broad particle size distribution and a higher nanoparticle coverage.

Abstract:We review recent progress and future prospects for enhancement of solar cells using plasmonic resonances on metal nanoparticles.

Abstract:Coupling of light into a thin layer of high refractive index material by plasmonic nanoparticles has been widely studied for application in photovoltaic devices, such as thin-film solar cells. In numerous studies this coupling has been investigated through measurement of e.g. quantum efficiency or photocurrent enhancement. Here we present a direct optical measurement of light coupling into a waveguide by plasmonic nanoparticles. We investigate the coupling efficiency into the guided modes within the waveguide by illuminating the surface of a sample, consisting of a glass slide coated with a high refractive index planar waveguide and plasmonic nanoparticles, while directly measuring the intensity of the light emitted out of the waveguide edge. These experiments were complemented by transmittance and reflectance measurements. We show that the light coupling is strongly affected by thin-film interference, localized surface plasmon resonances of the nanoparticles and the illumination direction (front or rear)

Abstract:We numerically investigate the light trapping properties of two-dimensional diffraction gratings formed from silver disks or titanium dioxide pillars, placed on the rear of Si thin-film solar cells. In contrast to previous studies of front-surface gratings, we find that metal particles out-perform dielelectric ones when placed on the rear of the cell. By optimizing the grating geometry and the position of a planar reflector, we predict short circuit current enhancements of 45% and 67% respectively for the TiO2 and silver nanoparticles. Furthermore, we show that interference effects between the grating and reflector can significantly enhance, or suppress, the light trapping performance. This demonstrates the critical importance of optimizing the reflector as an integral part of the light trapping structure.

Abstract:Surface plasmon enhancement has been proposed as a way to achieve higher absorption for thin-film photovoltaics, where surface plasmon polariton(SPP) and localized surface plasmon (LSP) are shown to provide dense near field and far field light scattering. Here it is shown that controlled far-field light scattering can be achieved using successive coupling between surface plasmonic (SP) nano-particles. Through genetic algorithm (GA) optimization, energy transfer between discrete nano-particles (ETDNP) is identified, which enhances solar cell efficiency. The optimized energy transfer structure acts like lumped-element transmission line and can properly alter the direction of photon flow. Increased in-plane component of wavevector is thus achieved and photon path length is extended. In addition, Wood-Rayleigh anomaly, at which transmission minimum occurs, is avoided through GA optimization. Optimized energy transfer structure provides 46.95% improvement over baseline planar cell. It achieves larger angular scattering capability compared to conventional surface plasmon polariton back reflector structure and index-guided structure due to SP energy transfer through mode coupling. Via SP mediated energy transfer, an alternative way to control the light flow inside thin-film is proposed, which can be more efficient than conventional index-guided mode using total internal reflection (TIR).

Abstract:Metal nanoparticles offer the possibility of improved light trapping in solar cells, but careful design is required to maximise scattering and minimise parasitic absorption across the wavelength range of interest. We present an analysis of the broadband scattering and absorption characteristics of spherical metal nanoparticles, optimized for either crystalline silicon (c-Si)or amorphous silicon (a-Si:H) solar cells. A random two-dimensional array of optimally sized Ag spheres can scatter over 97% of the AM1.5 spectrum from 400 to 1100 nm. Larger particles are required for c-Si devices than a-Si:H due to the increased spectral range, with optimum particle sizes ranging from 60 nm for a-Si:H to 116 nm for c-Si. Positioning the particles at the rear of the solar cell decreases absorption losses because these principally occur at short wavelengths. Increasing the refractive index of the surrounding medium beyond the optimum value, which is 1.0 for a-Si:H and 1.6 for c-Si, shifts absorption to longer wavelengths and decreases scattering at short wavelengths. Ag nanoparticles scatter more of the solar spectrum than Au, Cu or Al nanoparticles. Of these other metals, Al can only be considered for a-Si:H applications due to high absorption in the near-infrared, whereas Au and Cu can only be considered for the rear of c-Si devices due to high absorption in the ultraviolet (UV) and visible. In general, we demonstrate the importance of considering the broadband optical properties of metal nanoparticles for photovoltaic applications.

Abstract:Nanoplasmonic metal structures in the front end of thin film solar cells are able to enhance light absorption in the band gap region but result in absorption losses at short wavelengths. The particles are designed to resonate near the bandgap (long wavelengths) of the semiconductor, hence they appear electrically large at short wavelengths. Due to their apparently large size, they begin to shadow the semiconductor absorber layer. The shadow effect depends not just on electrical size but also the geometry of the metal nanoparticle. The geometry dependence of light absorption at short wavelengths for a top-coated thin film solar cell is studied in this paper by comparing the short wavelength absorption in thin film solar cells top-coated with metal nanospheres and nanohemispheres. The light absorption at short wavelengths is lower than that of the same model without any nanoparticle coating. Light incident on the metal structures experiences very weak forward scattering at short wavelengths and more light is reflected back into the air.

Abstract:A back reflector (BR) that can efficiently scatter weakly absorbed light is essential to obtain high-efficiency thin-film silicon solar cells. We present the design routes of plasmonic BR based on self-assembled silver nanoparticles (Ag NPs) for high-efficiency thin-film silicon solar cells. Both optical and electrical effects on solar cells are considered. The shape of Ag NPs, the thickness of ZnO:Al spacer layers, materials on top of Ag NPs, and nanoparticle size are crucial for the performance of plasmonic BR. Increased annealing temperature lead to the formation of more appropriate shapes (more spherical and regular shapes) for a good light scattering and, thus, increase the photocurrent. The ZnO:Al layer between the Ag NPs and the Ag planar film has an optical effect on solar cells, while the ZnO:Al layer between the Ag NPs and the doped a-Si:H has both optical and electrical influence on the device. Larger NPs have less parasitic absorption and can preferentially scatter light into larger angles, thus increasing the spectral response in the solar cell. However, for larger Ag NPs, the fill factor deteriorates due to the rougher surface in the plasmonic BR, indicating a compromise between light trapping and electrical performance. Following the design routes, we obtained 8.4% high-efficiency plasmonic a-Si:H solar cell.

Abstract:Metallic nanoparticles organized in regular arrays exhibit an extraordinary spectral feature that arises from electromagnetic coupling between localized surface plasmons and constructive interference from diffracted far-field radiation. A rapid semianalytical description of coupling between dipoles and scattering modes is applied to examine the influence of nanoparticle size, dielectric, and interparticle separation on the occurrence, resonant wavelength, and intensity of the extraordinary spectral feature. Introducing a dynamic polarizability that includes higher-order electric poles into the description accurately characterizes plasmon resonances of larger particles. Previously unrecognized patterns and periodic variations in the extraordinary feature were observed to result from modulations in polarizability, as well as from interference of scattered modes that were distinguishable for the first time using the rapid semianalytic solution. Streamlined rational design of metamaterials with optimum optical properties using the rapid semianalytic coupled dipole approximation is considered.

Abstract:Silver nanoparticles embedded in a dielectric material have strong scattering properties under light illumination, due to localized surface plasmons. This effect is a potential way to achieve light trapping in thin-film solar cells. In this paper we study light scattering properties of nanoparticles on glass and ZnO, and on silver coated with ZnO, which represent the back reflector of a solar cell. We find that large nanoparticles embedded in the dielectric at the back contact of amorphous silicon solar cells lead to a remarkable increase in short circuit current of 20% compared to co-deposited cells without nanoparticles. This increase is strongly correlated with the enhanced cell absorption in the long wavelengths and is attributed to localized surface plasmons. We also discuss the electrical properties of the cells.

Abstract:The simulation results of absorption enhancement in an amorphous-Si (a-Si) solar cell by depositing metal nanoparticles (NPs) on the device top and embedding metal NPs in a layer above the Al back-reflector are demonstrated. The absorption increase results from the near-field constructive interference of electromagnetic waves in the forward direction such that an increased amount of sunlight energy is distributed in the a-Si absorption layer. Among the three used metals of Al, Ag, and Au, Al NPs show the most efficient absorption enhancement. Between the two used NP geometries, Al nanocylinder (NC) are more effective in absorption enhancement than Al nanosphere (NS). Also, a random distribution of isolated metal NCs can lead to higher absorption enhancement, when compared with the cases of periodical metal NC distributions. Meanwhile, the fabrication of both top and bottom Al NCs in a solar cell results in further absorption enhancement. Misalignments between the top and bottom Al NCs do not significantly reduce the enhancement percentage. With a structure of vertically aligned top and bottom Al NCs, solar cell absorption can be increased by 52%.

Abstract:Enhancing the light absorption in ultrathin-film silicon solar cells is important for improving efficiency and reducing cost. We introduce a double-sided grating design, where the front and back surfaces of the cell are separately optimized for antireflection and light trapping, respectively. The optimized structure yields a photocurrent of 34.6 mA/cm2 at an equivalent thickness of 2 μm, close to the Yablonovitch limit. This approach is applicable to various thicknesses and is robust against metallic loss in the back reflector.

Abstract:Plasmonic metal nanoparticles are of great interest for light trapping in thin-film silicon solar cells. In this Letter, we demonstrate experimentally that a back reflector with plasmonic Ag nanoparticles can provide light-trapping performance comparable to state-of-the-art random textures in n-i-p amorphous silicon solar cells. This conclusion is based on the comparison to high performance n-i-p solar cell and state-of-the-art efficiency p-i-n solar cells deposited on the Asahi VU-type glass. With the plasmonic back reflector a gain of 2 mA/cm2 in short-circuit current density was obtained without any deterioration of open circuit voltage or fill factor compared to the solar cell on a flat back reflector. The excellent light trapping is a result of strong light scattering and low parasitic absorption of self-assembled Ag nanoparticles embedded in the back reflector. The plasmonic back reflector provides a high degree of light trapping with a haze in reflection greater than 80% throughout the wavelength range 520–1100 nm. The high performance of plasmonic back reflector is attributed to improvements in the self-assembly technique, which result in a lower surface coverage and fewer small and irregular nanoparticles.

Abstract:Silver nanoparticles can be used as light scattering elements for enhancing solar cell energy conversion efficiencies. The objective of our work is to gain more insight into the optical properties of silver nanoparticle films and their effect on the performance of solar cells. We use a common self-assembly technique to fabricate a range of silver island films on transparent substrates and measure their reflectance and transmittance for visible and near infrared light. We demonstrate that it is possible to represent silver island films by an effective medium with the same optical properties. The observed strong dependence on illumination side of the reflectance and absorptance, attributed to driving field effects, is reproduced very well. Thin-film silicon solar cells with embedded silver island films were fabricated and it was found that their performance is reduced due to parasitic absorption of light in the silver island film. Simulations of these solar cells, where the silver island film is represented as an effective medium layer, show a similar trend. This highlights the importance of minimizing parasitic absorption.

Abstract:The authors numerically investigate the absorption enhancement of an amorphous Si solar cell, in which a periodical one-dimensional nanowall or two-dimensional nanopillar structure of the Ag back-reflector is fabricated such that a dome-shaped grating geometry is formed after Si deposition and indium-tin-oxide coating. In this investigation, the effects of surface plasmon (SP) interaction in such a metal nanostructure are of major concern. Absorption enhancement in most of the solar spectral range of significant amorphous Si absorption (320-800 nm) is observed in a grating solar cell. In the short-wavelength range of high amorphous Si absorption, the weakly wavelength-dependent absorption enhancement is mainly caused by the broadband anti-reflection effect, which is produced through the surface nano-grating structures. In the long-wavelength range of diminishing amorphous Si absorption, the highly wavelength-sensitive absorption enhancement is mainly caused by Fabry-Perot resonance and SP interaction. The SP interaction includes the contributions of surface plasmon polariton and localized surface plasmon.

Abstract:A plasmonic-structure incorporated double layer of Au nanoparticles embedded in the transparent conducting oxide at the back-reflector of the hydrogenated amorphous silicon (a-Si:H) solar cells is demonstrated. These devices exhibit an increase of energy conversion efficiency of 18.4% and short-circuit current density of 9.8% while improving fill-factor and without sacrificing open-circuit voltage. The increase in photocurrent is correlated with the enhanced optical absorption in the cell, with improved optical-path-length by a factor of 7 at the wavelength of 800 nm, due to enhanced diffuse scattering of light through resonant plasmon excitations within Au nanoparticles. In addition to enhanced scattering, applying high-work-function Au nanoparticles can improve the work function match at TCO/a-Si:H interface.

Abstract:We fabricated amorphous silicon n-i-p solar cells with two types of nanopatterned back reflectors using stencil lithography. One reflector type has a plasmonic grating that is embedded in the ZnO layer; the other one has a metallic grating patterned on top of the Ag layer. From comparing the short-circuit current densities of the two device types, we conclude that light trapping through grating coupling is more efficient than coupling of light through the excitation of localized surface plasmons. The back reflectors were patterned with dot arrays by evaporation of Ag through millimeter-size stencil membranes. The stencils themselves were patterned by wafer-scale nanosphere lithography. The dot arrays have a periodicity of 428 nm and efficiently scatter light in the near-infrared wavelength range. Both back reflectors types lead to the same morphology for the silicon films. This allows us a fair comparison of the two light coupling mechanisms. We found a 14% and 19% short-circuit current density enhancement for the plasmonic and for the metallic grating, respectively. The external quantum efficiency gains between 550 and 650 nm show similar guided modes resonances for both device types, but the excitation is stronger for the device with the metallic grating.

Abstract:We investigate the use of silver nanoparticles as light scattering elements to improve light trapping in amorphous silicon solar cells. Simulations presented in literature show that these nanoparticles can scatter light very efficiently due to plasmon resonance. However, our previous experimental work showed that light trapping does not improve when silver nanoparticles, fabricated by annealing a thin silver film, are embedded in a-Si:H solar cells. To shed some light on this we investigate the optical properties of these nanoparticles in more detail. Silver nanoparticle films with a mass thickness ranging from 3 to 18 nm were fabricated, resulting in films with average particle sizes ranging from 20 to 120 nm. We found that in all cases less than 10% of the incident light is scattered. The undesired absorption of light by the silver nanoparticles is at least three times stronger than the desired light scattering effect. We tentatively attribute this to the wide size distribution and high surface coverage inherent to this particle fabrication technique. We use an effective medium approach to incorporate the experimentally obtained optical properties of the nanoparticle films into our opto-electrical device simulator. This allows us to use realistic optical properties in solar cell simulations. We focus on a solar cell design with the silver nanoparticles embedded in a transparent conductive oxide layer at the rear of the a-Si:H layer. The solar cell simulations show that light trapping does not improve as long as absorption dominates over scattering. The simulated quantum efficiency curves are in agreement with experimental results.

Abstract:Light trapping via plasmonic nanostructures has emerged as a novel method for guiding and confining light in nanoscale photovoltaics. In our design, the metal nanostructures are built directly into the back contact of an a-Si:H device, such that the large scattering cross section of the plasmonic particles couples incident sunlight into localized and guided modes overlapping with the a-Si:H layer. This enables the use of ultrathin absorbing layers, which are attractive for cost and stability as well as higher open circuit voltages. Here we show that electromagnetic simulation can be used to accurately simulate nanopatterned solar cells, including for randomly textured and non-periodic patterns. We also show that non-periodic arrangements of plasmonic nanostructures are promising for enhancing photocurrent in ultrathin film a-Si:H solar cells

Abstarct:Recently, thin-film solar cells enhanced with plasmonic nanoparticles have attracted much attention of the scientific community. To improve the performance of such cells, an optimization of the nanoparticle parameters such as size, surface coverage and material is performed. Based on this optimization, the role of surface plasmons is discussed and analyzed with respect to optical absorption of the photoactive layer in both the dipolar and multi-polar regimes of the forward scattering configuration.

Abstract:For the first time, we utilize the mechanical strain to tailor the locallized surface plasmon resonance of metallic nanoparticles on the top of a thin-film solar cell and improve the light trapping efficiency of corresponding plasmonic solar cell. A plasmonic solar cell is constructed by depositing metallic nanoparticle array on top of a thin-film solar cell to enhance the light trapping efficiency through activating surface plasmon. When the mechanical strain is applied on these metallic nanoparticles, their surface plasmon resonance will be tuned, which may eventually enhance the performance of underlying thin-film solar cell. Through numerical experiments, we demonstrate that 5% compressive strain could remarkably improve the absorption of the metal nanoparticle array on the top of thin-film solar cell by more than 24%.

Abstract:The application of a light trapping technique based on light scattering by metal nanoparticles through excitation of localized surface plasmons was studied. Our objective was to investigate experimentally the most suitable location for the metal nanoparticles inside hydrogenated amorphous silicon (a-Si:H) solar cells for efficient light scattering. Silver nanoparticles were embedded either at the front, inside, or at the rear of the a-Si:H absorber layer. Embedding the nanoparticles at the back of the absorber layer turned out to be most promising. We found that when the size of the particles was increased, the external quantum efficiency increased for wavelengths longer 600 nm. This was caused by increased light scattering and reduced parasitic absorption for the larger particles. Nonetheless, it proved challenging to enhance the performance of a-Si:H solar cells by embedding silver nanoparticles when state-of-the-art light trapping techniques are already applied.

Abstract:Photovoltaic (PV) power is a promising choice as low-cost, environmentally friendly energy source. Thin-film solar cell is proposed as an alternative to reduce the cost due to its very thin silicon absorbers and utilization of cheap substrates. However because of the thin thickness of thin film solar cell, the light absorption is poor. In order to increase the absorption efficiency, surface plasmonic effect is used to improve the optical absorption. In this paper, we firstly attempt to use alloy to increase the light absorption. The results show Al-Cu alloy gives a better performance of absorbing light especially in the long wavelength region. We also build a nano-hole / embedded particle structure which can increase the absorption of the long wavelength light as well. In the end, we recommend using alloy material and the hole-particle surface plasmonic structure with a good back reflector in order to have a higher absorption of light to achieve a better performance thin film solar cell.

Abstract:Resonant excitation of plasmons in metallic nanoparticles can cause a very strong enhancement in absorption and/or scattering by the nanoparticles. By tuning the positions of these resonances through varying the size and surface coverage of the nanoparticles, one can get the enhancement of solar cell performance due to improved optical absorption in the photoactive layer. Despite several experimental studies on metallic nanoparticle enhancement of solar cell performance, no systematic research has been carried out to find the optimum nanoparticle parameters, which enables the efficient optical absorption and light trapping in thin-film solar cells. This work highlights a systematic study performed on the optimization of the parameters of the silver nanoparticle array based on predictive numerical modeling to achieve better optical enhancement and light trapping inside a thin-film amorphous silicon solar cell.

Abstract:In this paper, the stack structure of hydrogenated amorphous silicon thin film solar cell for the enhanced light absorption was studied. The way to increase light absorption of PIN solar cell was investigated. As increasing of the light absorption, we can obtain higher efficiency of the thin film solar cell. We suggested a structure which achieves long light traveling path even with thin amorphous silicon layer.

Abstract:The surface plasmon polariton (SPP) is a novel approach for light trapping in solar cells. SPP enhanced nanocrystalline silicon thin film solar cells were studied in this work. Based on Mie's theory, the optical properties of Ag, Al, Cu, and Au nanoparticles were investigated approximately. The results show that the normalized scattering efficiency, scattering fraction and resonance frequency of SPP can be tuned effectively by dielectric environment, particle size and metal material. The bandgap of nanocrystalline silicon thin film were calculated based on the quantum confinement effect. To enhance the light absorption of solar cells, the resonance wavelength should be on the edge of the bandgap of absorbers, according to which the optimized matches of SPP mode and silicon grain size were discussed. As an example, 20 nm Cu particles can be used in nanocrystalline silicon solar cells which contain 5.7 nm silicon grains.

Abstract:The surface plasmon effect of gold (Au) nanoparticles in combination with the silicon nanohole (SiNH) arrays texture surface is studied via simulation for light absorption enhancement for the thin film silicon solar cells application. It is found that the ultimate efficiency of an optimized silicon nanohole (SiNH) arrays texture surface in combination with the surface and bottom-of-a-trench Au nanoparticle arrays described herein, is 39.67%, which compares favorably with the ultimate efficiency of 31.11% for an optimized silicon nanohole arrays texture surface without Au nanoparticles. Detailed balanced analysis is carried out for the limiting values of short circuit current density (JSC), open circuit voltage (VOC), and the power conversion efficiency (PCE) of the proposed solar cells geometries.

Abstract:In this paper, the localized surface plasmon effect of gold (Au) nanoparticles deposited on the silicon nanohole (SiNH) array textured surface is studied via simulation for better light trapping purpose for thin film silicon solar cells application. The ultimate efficiency of the optimized SiNH array textured surface is increased by 27.5% with the inclusion of Au nanoparticles on its surface. The highest ultimate efficiency of the proposed geometry herein (39.67 %) is still 12.8% greater than the single layer antireflection (AR) coating of Si3N4 on SiNH array textured surface without Au nanoparticles. The forward scattering of incident radiation by the Au nanoparticles near their localized plasmon resonance is responsible for the strong field enhancement in the substrate and hence the higher ultimate efficiency of the solar cell.

Abstract:Surface plasmons are collective oscillations of free electrons localized at surfaces of structures made of metals. Since the surface plasmons induce fluctuations of electric charge at surfaces, they are accompanied by electromagnetic oscillations. Electromagnetic fields associated with surface plasmons are localized at surfaces of metallic structures and significantly enhanced compared with the excitation field. These two characteristics are ingredients for making good use of surface plasmons in plasmonics. Plasmonics is a rapidly growing and well-established research field, which covers various aspects of surface plasmons towards realization of a variety of surface-plasmon-based devices. In this paper, after summarizing the fundamental aspects of surface plasmons propagating on planar metallic surfaces and localized at metallic nanoparticles, recent progress in plasmonic waveguides, plasmonic light-emitting devices and plasmonic solar cells is reviewed.

Plasmonic Structures Fabrication Techniques Articles[edit]

Abstract:We demonstrate the flexibility of UV nanoimprint lithography for effective light trapping in p–i–n a-Si:H/μc-Si:H tandem solar cells. A textured polymeric layer covered with pyramidal transparent conductive oxide structures is shown as an ideal system to promote front light scattering and thus enhanced photocurrent. The double structure incorporated into micromorph tandem thin film silicon solar cells is systematically investigated in order to find a relationship between interface morphology, optical properties and photovoltaic characteristics. To prevent the formation of defects during cell growth, a controllable smoothing of the imprinted texture is developed. Modules grown on polymer structures smoothed via multi-replication show excellent performance reaching a photocurrent of 12.6 mA/cm2 and an efficiency of 12.8%.

Abstract:Substrates with extremely low roughness to allow the growth of good-quality silicon material but that nevertheless present high light trapping properties are presented. In a first application, silver reflectors are used in single and tandem-junction amorphous silicon (a-Si:H) solar cells. High initial (stable) efficiencies of 10.4 % (8.1 %) for single-junction a-Si:H cells on glass and 11.1 % (9.2 %) for tandem-junction a-Si:H/a-Si:H cells on plastic are obtained. A second application better suited to multi-junction solar cells based on microcrystalline silicon (μc-Si:H) solar cells is presented: the substrate consists of rough zinc oxide (ZnO) grown on a flat silver reflector which is covered with a-Si:H; polishing of this structure yields an a-Si:H/ZnO interface that provides high light scattering even though the cell is deposited on a flat interface. We present results of ∼ 4-μm-thick μc-Si:H solar cells prepared on such substrates with high open-circuit voltages of 520 mV. A large relative efficiency gain of 20% is observed compared to a co-deposited cell grown directly on an optimized textured substrate.

References[edit]

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