Ag纳米粒子等离子体共振增强太阳能电池吸收

针对薄膜太阳能电池硅薄膜层吸收效率较低的问题,提出了运用金属纳米粒子局域表面等离子体共振(LSPR)增强太阳能电池的吸收效率,采用时域有限差分(FDTD)法,模拟计算了太阳能电池中不同厚度的硅薄膜层吸收特性,分析了不同几何参数的矩形Ag纳米粒子与Ag背反射膜对增强太阳能电池吸收效率的影响作用。计算结果表明,硅薄膜层厚度为500nm的太阳能电池具有较高的吸收效率,通过调整Ag纳米粒子的相关参数,有效地降低了太阳电池硅薄膜表面的反射损耗,取得最大吸收增强因子为1.35。Ag背反射膜有效地降低了Ag纳米粒子硅薄膜结构的透射损耗,其最大的吸收增强因子达到1.42。     

Photocurrent enhancement in thin film amorphous silicon solar cells with silver nanoparticles

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. Copyright © 2010 John Wiley & Sons, Ltd.     

Nanoparticle‐enhanced light trapping in thin‐film silicon solar cells

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 plasmon‐enhanced light‐trapping scheme for self‐assembled Ag metal nanoparticles is presented, including a comparison of rear‐located 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 back‐surface 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 short‐circuit current enhancement of 38%. Copyright © 2011 John Wiley & Sons, Ltd.     

Light trapping with titanium dioxide diffraction gratings fabricated by nanoimprinting

Dielectric scattering structures are a promising way of trapping light in solar cells. Titanium dioxide is a particularly attractive candidate material because of its high refractive index and ability to be deposited on a finished solar cell. Here, we present an experimental demonstration of photocurrent enhancement in thin film recrystallised silicon solar cells using TiO₂ pillar arrays fabricated on the rear of the cells using nanoimprint lithography. A short circuit current enhancement of 19% is measured experimentally, and excellent agreement with numerical simulations is obtained. We show numerically that by replacing the Ag capping present on the cells with a detached rear Ag back reflector, the enhancement could reach 37%. Copyright © 2012 John Wiley & Sons, Ltd.     

Design of Highly Efficient Light-Trapping Structures for Thin-Film Crystalline Silicon Solar Cells

We present a design optimization of a highly efficient light-trapping structure to significantly increase the efficiency of thin-film crystalline silicon solar cells. The structure consists of an antireflection (AR) coating, a silicon active layer, and a back reflector that combines a diffractive reflection grating with a distributed Bragg reflector. We have demonstrated that with careful design optimization, the presented light-trapping structure can lead to a remarkable cell-efficiency enhancement for the cells with very thin silicon active layers (typically 2.0-10.0 mum) due to the significantly enhanced absorption in the wavelength range of 800-1100 nm. On the other hand, less enhancement has been predicted for much thicker cells (i.e.,>100 mum) due to the limited absorption increase in this wavelength range. According to our simulation, the overall cell efficiency can be doubled for a 2.0-mum-thick cell with light-trapping structure. It is found that the improvement is mainly contributed by the optimized AR coating and diffraction grating with the corresponding relative improvements of 36% and 54%, respectively. The simulation results show that the absolute cell efficiency of a 2.0-mum-thick cell with the optimal light-trapping structure can be as large as 12%.     

Improved micromorph solar cells by means of mixed‐phase n‐doped silicon oxide layers

A good light trapping scheme is necessary to improve the performance of amorphous/microcrystalline silicon tandem cells. This is generally achieved by using a highly reflective transparent conducting oxide/metal back contact plus an intermediate reflector between the component cells. In this work, the use of doped silicon oxide as alternative n‐layer in micromorph solar cells is proposed as a means to obtain high current values using a simple Ag back contact and no extra reflector between the component cells n‐doped silicon oxide layers with a wide range of optical and electrical properties have been prepared. The influence of different deposition regimes on the material properties has been studied. The main findings are the following: (i) when carbon dioxide is added to the gas mixture, sufficiently high hydrogen dilution is necessary to widen the transition region from highly conductive microcrystalline‐like films to amorphous material characterized by low electrical conductivity; (ii) lower refractive index values are found with lower deposition pressure. Optimal n‐doped silicon oxide layers have been used in both component cells of micromorph devices, adopting a simple Ag back contact. Higher current values for both cells are obtained in comparison with the values obtained using standard n‐doped microcrystalline silicon, whereas similar values of fill factor and open circuit voltage are measured. The current enhancement is particularly evident for the bottom cell, as revealed by the increased spectral response in the red/infrared region. The results prove the high potential of n‐doped silicon oxide as ideal reflector for thin‐film silicon solar cells. Copyright © 2011 John Wiley & Sons, Ltd.     

Silver versus white sheet as a back reflector for microcrystalline silicon solar cells deposited on LPCVD‐ZnO electrodes of various textures

We compare the performance of two back reflector designs on the optoelectrical properties of microcrystalline silicon solar cells. The first one consists of a 5‐µm‐thick low‐pressure chemical vapor deposition (LPCVD)‐ZnO electrode combined with a white sheet; the second one incorporates an Ag reflector deposited on a thin LPCVD‐ZnO layer (with thickness below 200 nm). For this latter design, the optical loss in the nano‐rough Ag reflector can be strongly reduced by smoothing the surface of the thin underlying ZnO layer, by means of an Ar‐plasma treatment. Because of its superior lateral conductivity, the thin‐ZnO/Ag back reflector design provides a higher fill factor than the dielectric back reflector design. When decreasing the roughness of the front electrode with respect to our standard front LPCVD‐ZnO layer, the electrical cell performance is improved; in addition, the implementation of the thin‐ZnO/Ag back reflector leads to a significant relative gain in light trapping. Applying this newly optimized combination of front and back electrodes, the conversion efficiency is improved from 8.9% up to 9.4%, for cells with an active‐layer thickness of only 1.1 µm. We thereby highlight the necessity to optimize simultaneously the front and back electrodes. Copyright © 2014 John Wiley & Sons, Ltd.     

Design principles for light trapping in thin silicon films with embedded dielectric nanoparticles

We investigate the light‐trapping effects of dielectric nanoparticles embedded within the active semiconductor layer of a thin‐film solar cell. The baseline model consists of a 1·0 µm slab of crystalline silicon on an aluminum back contact topped with a 75 nm Si₃N₄ anti‐reflective coating. Using finite‐difference time‐domain simulations, we calculate the absorption gain due to a periodic array of SiO₂ nanospheres with characteristic depth, diameter, and pitch. Under optimal conditions, absorption gain due to embedded spheres can reach as high as 23·4% relative to the baseline geometry. Using Au‐core/SiO₂‐shell nanoparticles, it is even possible to reach 30%. We then infer a series of design principles from our data that include trade‐offs between broadband scattering efficiency, poor absorption at long wavelengths, and semiconductor displacement. We also find that the optimal spacing between particles is approximately 400 nm. Above this distance, each scatterer acts in near isolation from any neighboring particles, and absorption gain is approximately linear with area coverage. Such gains are also expected for disordered as well as ordered arrays. These results demonstrate the potential of embedded dielectric nanoparticles as a tool for enhancing carrier generation in thin silicon solar cells. Copyright © 2011 John Wiley & Sons, Ltd.     

Enhanced light trapping in solar cells using snow globe coating

A novel method, snow globe coating, is found to show significant enhancement of the short circuit current J_SC (35%) when applied as a scattering back reflector for polycrystalline silicon thin‐film solar cells. The coating is formed from high refractive index titania particles without containing binder and gives close to 100% reflectance for wavelengths above 400 nm. Snow globe coating is a physicochemical coating method executable in pH neutral media. The mild conditions of this process make this method applicable to many different types of solar cells. Copyright © 2012 John Wiley & Sons, Ltd.     

Silica nanospheres as back surface reflectors for crystalline silicon thin‐film solar cells

In this paper, fabrication of a non‐continuous silicon dioxide layer from a silica nanosphere solution followed by the deposition of an aluminium film is shown to be a low‐cost, low‐thermal‐budget method of forming a high‐quality back surface reflector (BSR) on crystalline silicon (c‐Si) thin‐film solar cells. The silica nanosphere layer has randomly spaced openings which can be used for metal‐silicon contact areas. Using glass/SiN/p⁺nn⁺ c‐Si thin‐film solar cells on glass as test vehicle, the internal quantum efficiency (IQE) at long wavelengths (>900 nm) is experimentally demonstrated to more than double by the implementation of this BSR, compared to the baseline case of a full‐area Al film as BSR. The improved optical performance of the silica nanosphere/aluminium BSR is due to reduced parasitic absorption in the Al film. Copyright © 2007 John Wiley & Sons, Ltd.     

Absorption enhancement of silicon solar cell with Ag nanoparticles by surface plasmons resonance

The absorption enhancements of silicon layer in silicon solar cells with three kinds of Ag nanoparticles including sphere, cylinder and cuboid are studied by the finite difference time domain （FDTD） method, respectively. The results show that the light absorption of silicon is significantly improved due to the localized surface plasmon （LSP） reso- nance. The relations of the absorption enhancement with the parameters of nanoparticles are thoroughly analyzed. The optimal absorption enhancement can be achieved by adjusting the relevant parameters. Among the three types of Ag nanoparticles, i.e., sphere, cylinder and cuboid, the silicon with the cubical Ag nanopaticles shows the most efficient absorption enhancement at optimal conditions, its maximum absorption enhancement factor is 1.35, and that with the spherical Ag nanopaticles gets the lowest absorption enhancement. The work is useful for the further theoretical study and design for the plasmonic thin-film solar cell.     

Design of Ag nanograting for broadband absorption enhancement in amorphous silicon thin film solar cells

Light trapping is one of the key issues to improve the light absorption and increase the efficiency of thin film solar cells. The effects of the triangular Ag nanograting on the absorption of amorphous silicon solar cells were investigated by a numerical simulation based on the finite element method. The light absorption under different angle and area of the grating has been calculated. Furthermore, the light absorption with different incident angle has been calculated. The optimization results show that the absorption of the solar cell with triangular Ag nanograting structure and anti-reflection film is enhanced up to 96% under AM1.5 illumination in the 300–800 nm wavelength range compared with the reference cell. The physical mechanisms of absorption enhancement in different wavelength range have been discussed. Furthermore, the solar cell with the Ag nanograting is much less sensitive to the angle of incident light. These results are promising for the design of amorphous silicon thin film solar cells with enhanced performance.     

Investigation of the impact of the rear‐dielectric/silver back reflector design on the optical performance of thin‐film silicon solar cells by means of detached reflectors

Thin‐film silicon solar cells often rely on a metal back reflector separated from the silicon layers by a thin rear dielectric as a back reflector (BR) design. In this work, we aim to obtain a better insight into the influence of the rear‐dielectric/Ag BR design on the optical performance of hydrogenated microcrystalline silicon (µc‐Si:H) solar cells. To allow the application of a large variety of rear dielectrics combined with Ag BRs of diverse topographies, the solar cell is equipped with a local electrical contact scheme that enables the use of non‐conductive rear dielectrics such as air or transparent liquids of various refractive indices n. With this approach, detached Ag BRs having the desire surface texture can be placed behind the same solar cell, yielding a direct and precise evaluation of their impact on the optical cell performance. The experiments show that both the external quantum efficiency and the device absorptance are improved with decreasing n and increasing roughness of the BR. Calculations of the angular intensity distribution of the scattered light in the µc‐Si:H are presented. They allow for establishing a consistent picture of the light trapping in the solar cell. Copyright © 2013 John Wiley & Sons, Ltd.     

Utilization of geometric light trapping in thin film silicon solar cells: simulations and experiments

In this study, we present a new light absorption enhancement method for p‐i‐n thin film silicon solar cells using pyramidal surface structures, larger than the wavelength of visible light. Calculations show a maximum possible current enhancement of 45% compared with cells on a flat substrate. We deposited amorphous silicon (a‐Si) thin film solar cells directly onto periodically pyramidal‐structured polycarbonate (PC) substrates, which show a significant increase (30%) in short‐circuit current over reference cells deposited on flat glass substrates. The current of the cells on our pyramidal structures on PC is only slightly lower than that of cells on Asahi U‐type TCO glass (Asahi Glass Co., Tokyo, Japan), but suffer from a somewhat lower open circuit voltage and fill factor. Because the used substrates have a locally flat surface area due to the fabrication process, we believe that the current enhancement in the cells on structured PC can be increased using larger or more closely spaced pyramids, which can have a smaller flat surface area. Copyright © 2012 John Wiley & Sons, Ltd.     

Light‐trapping and back surface structures for polycrystalline silicon solar cells

Light‐trapping in polycrystalline silicon solar cells is usually considered to be more difficult to implement than that in single crystal silicon solar cells due to the random crystallographic orientations in various grains. Furthermore, if minority carrier diffusion length is on the order of or less than solar cell thickness, which is the case of most cost‐effective polycrystalline silicon, the translation of optical gain, achieved from light‐trapping, into electrical gain will be rather limited, even with a perfect back surface passivation. In this work, geometrical light‐trapping structures are demonstrated using a simplified isotropic etching at polycrystalline silicon surfaces. Combined with a back surface reflector (BSR), an enhanced absorption in the long wavelength region is measured with a low parasitic absorption. Different light‐trapping structures are experimentally compared. To further examine the electrical gain from light‐trapping, a three‐terminal solar cell structure is used. This structure allows three different back surface configurations to be realized in a single device: unpassivated, passivated with a floating junction, and enhanced with a collecting junction. Results indicate that even with a relatively short minority‐carrier diffusion length the current collection in the long wavelength region can be significantly improved and the light‐trapping effect is enhanced as well. Copyright © 1999 John Wiley & Sons, Ltd.     

Theory and experiments on the back side reflectance of silicon wafer solar cells

New passivation layers for the back side of silicon solar cells have to show high performance in terms of electrical passivation as well as high internal reflectivity. This optical performance is often shown as values for the back side reflectance R_b which describes the rear internal reflection. In this paper, we investigate in detail the meaning of this single‐value parameter, its correct determination and the use in one‐dimensional simulations with PC1D. The free‐carrier‐absorption (FCA) as non‐carrier‐generating absorption channel is analyzed for solar cells with varying thickness. We apply the optical analysis to samples with different thickness, silicon oxide layer thickness, rear side topography as well as passivation layers (SiO₂, SiN_x, SiC and stack systems). Additionally, the optical influence of the laser‐fired contacts (LFC) process is experimentally investigated. Finally, we show that with correct parameters, the one‐dimensional simulation of very thin silicon solar cells can successfully be performed. Copyright © 2007 John Wiley & Sons, Ltd.     

双光栅结构薄膜太阳能电池的优化

为了提高单晶硅薄膜太阳能电池短路电流密度和转换效率, 采用在单晶硅薄膜太阳能电池正背面分别集成硅介质光栅和铝金属光栅的方法, 并利用有限时域差分法软件仿真研究了两种光栅的周期、厚度、占空比对单晶硅薄膜太阳能电池短路电流密度和光转换效率的影响。结果表明, 通过优化可得当正背面光栅都处于最优值时(介质光栅占空比F=0.8、介质光栅周期P=0.632μm、介质光栅厚度h_g=0.42μm; 金属光栅占空比F₁=0.9、金属光栅周期P=0.632μm、金属光栅厚度h_m=0.005μm), 短路电流密度可达35.15mA/cm2, 转换效率为43.35%;将最优光栅单晶硅薄膜太阳能电池与传统单晶硅薄膜太阳能电池对比, 无论是光程路径还是吸收效率, 光栅单晶硅薄膜太阳能电池都有显著的提高。这为以后制备高性能薄膜太阳能电池提供了理论指导。     

Study of thin-film GaAs solar cells with cylindrical Ag nanoparticles and distributed Bragg reflector

An efficient light-trapping structure,which consists of the periodic Ag nanoparticles and a distributed Bragg reflector（DBR）with high reflectivity,is presented for the thin-film gallium arsenide（GaAs）solar cells.The effects of both Ag nanoparticles and DBR on the optical absorption of GaAs solar cells are theoretically investigated by using finite-difference time-domain（FDTD）method.The optimization process of parameters for the solar cell with both structures is analyzed systematically.The great absorption enhancement in GaAs layer is demonstrated,especially in the wavelength region near the GaAs band gap.It is observed that the superposition of the two effects excited by Ag nanoparticles and DBR results in the obvious absorption enhancement.By using cylindrical Ag nanoparticles and DBR together,the maximum enhancement factor of the solar cell is obtained as 4.83 in the simulation.     

All‐Solution‐Processed Random Si Nanopyramids for Excellent Light Trapping in Ultrathin Solar Cells

Si nanopyramids have been suggested as one of the most promising Si nanostructures to realize high‐efficient ultrathin solar cells or photodetectors due to their low surface area enhancement and outstanding ability to enhance light absorption. However, the present techniques to fabricate Si nanopyramids are either complex or expensive. In parallel, disordered nanostructures are believed to be extremely effective to realize broadband light trapping for solar cells. Here, a simple and cost‐effective method is presented to form random Si nanopyramids based on an all‐solution process, the mechanism behind which is the successful transfer of the generation site of bubbles from Si surface to the introduced Ag nanoparticles so that OH^? can react with the entire Si surface to naturally form random and dense Si nucleus. For optical performance, it is experimentally demonstrated that the random Si nanopyramid textured ultrathin crystalline Si (c‐Si) can achieve light trapping approaching the Lambertian limit. Importantly, it is revealed, by numerical calculations, that random Si nanopyramids outperform periodic ones on broadband light absorption due to more excited optical resonance modes. The finding provides a new opportunity to improve the performance of ultrathin c‐Si solar cells with a simpler process and lower cost.     

Photonic crystal microcrystalline silicon solar cells

Enhancing the absorption of thin‐film microcrystalline silicon solar cells over a broadband range in order to improve the energy conversion efficiency is a very important challenge in the development of low cost and stable solar energy harvesting. Here, we demonstrate that a broadband enhancement of the absorption can be achieved by creating a large number of resonant modes associated with two‐dimensional photonic crystal band edges. We utilize higher‐order optical modes perpendicular to the silicon layer, as well as the band‐folding effect by employing photonic crystal superlattice structures. We establish a method to incorporate photonic crystal structures into thin‐film (~500 nm) microcrystalline silicon photovoltaic layers while suppressing undesired defects formed in the microcrystalline silicon. The fabricated solar cells exhibit 1.3 times increase of a short circuit current density (from 15.0 mA/cm² to 19.6 mA/cm²) by introducing the photonic crystal structure, and consequently the conversion efficiency increases from 5.6% to 6.8%. Moreover, we theoretically analyze the absorption characteristics in the fabricated cell structure, and reveal that the energy conversion efficiency can be increased beyond 9.5% in a structure less than 1/400 as thick as conventional crystalline silicon solar cells with an efficiency of 24%. © 2015 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons Ltd.