Light Trapping In Solar Cells Research Articles

Numerical simulation of light absorption enhancement in microcrystalline silicon solar cells with Al nanoparticle arrays

Metal nanoparticles with low cost and high performance have good potential applications in light-trapping of solar cells. In this paper, a three-dimensional model is proposed to simulate the light absorption of microcrystalline silicon (μc-Si:H) thin film solar cells. The effects of spherical and hemispherical Al nanoparticle arrays located on the front surfaces of solar cells are investigated, and the particle radius and array period are optimized by the finite element method. The results show that the optimal Al nanoparticle arrays can enhance broadband absorption in thin film solar cells. For spherical particle arrays, the key parameter that influences light absorption in solar cells is period/radius ratio (P/R) or particle surface coverage. When P/R=4-5, the optimum integrated absorption enhancement (Eabs) is over 20% under AM1.5 illumination compared with the solar cell without nanoparticles. The value of Eabs is small and decreases with the increase of P/R when P/R>5, and Eabs is less than zero when P/RP=500 nm and R=120 nm, the spectral absorption rate as a function of wavelength shows broadband absorption including four distinct peaks, which are attributed to quadrupole plasmon resonance mode, dipole resonance mode and waveguide mode respectively according to the electric field distribution in the solar cell. For hemispherical particle arrays, the maximum value of Eabs is 24.5%, which is higher than that of the solar cell with optimized spherical particle arrays. This is due to the high coupling efficiencies of the particles, so that most of the scattered light is directly coupled into the substrate. However, the value of Eabs is very sensitive to the hemispherical particle radius. As the radius decreases, the scattering cross-section and scattering efficiency of the particle decrease dramatically. As the radius increases, the dipole plasmon resonance wavelength rapidly shifts towards longer wavelength (red shift). Both of these are detrimental to absorption enhancement of solar cells. Thus we conclude that spherical Al particle arrays are more preferable in actually fabricating the light-trapping of solar cells.

Thermodynamics of light management in photovoltaic devices

This paper elaborates a comprehensive theory of the thermodynamics of light management in solar cells explicitly considering imperfect light trapping, parasitic absorption and nonradiative recombination losses. A quantitative description of the entropic losses that reduce the open-circuit voltage and the energy conversion efficiency from the radiative limit towards realistic situations is presented. The theory embraces the fundamental limits for idealized solar cell devices given by the Yablonovitch limit and the Shockley-Queisser limit. We discriminate between reversible and irreversible entropic loss processes for four fundamental light management concepts: (i) conventional light trapping as an integral part of the device, (ii) geometric concentration of incident light, (iii) angular restriction of incoming and outgoing light, and (iv) light concentration by luminescent solar collectors. Based on this discrimination, a comprehensive discussion of the interplay between the loss processes and light management is presented. As part of this analysis, a new figure of merit for efficient light trapping in solar cells is introduced as well as an example of a deterministic light trapping concept which induces almost optimal light trapping.

Patterning Submicrometer Thick Inorganic Nanoparticle Films by Solution Process and Application for Light Trapping in Solar Cells

We present a low-cost fabrication process to deposit patterned inorganic nanoparticle films with submicrometer thickness and in turn to build higher dimensional structures through sequential multilayer deposition. Oxide nanoparticle films including semiconductors, dielectrics, and conductors have been patterned by moulding or imprinting from their solvent–suspension/paste using polydimethylsiloxane stamps. The easily controlled film thickness and good duplication fidelity with high resolution allows one to fabricate various layered structures, such as double layer and multilayer structures with minimized residual materials between them to finally define quasi-3D structures. Our experiment shows that colloidal suspension of materials can readily be patterned by stamping techniques with similar quality as compared to well-developed thermal or UV imprinting using solvent-free molecule-based materials. The usability of the fabricated structure is further demonstrated by integration of a 2-D grating on dye sensitized solar cell for improved power conversion efficiency.

Enhanced light trapping in solar cells with a meta-mirror following generalized Snell's law.

As the performance of photovoltaic cells approaches the Shockley-Queisser limit, appropriate schemes are needed to minimize the losses without compromising the current performance. In this paper we propose a planar absorber-mirror light trapping structure where a conventional mirror is replaced by a meta-mirror with asymmetric light scattering properties. The meta-mirror is tailored to have reflection in asymmetric modes that stay outside the escape cone of the dielectric, hence trapping light with unit probability. Ideally, the meta-mirror can be designed to have such light trapping for any angle of incidence onto the absorber-mirror structure. We illustrate the concept by using a simple gap-plasmon meta-mirror. Even though the response of the mirror is non-ideal with the unwanted scattering modes reducing the light absorption, we observe an order of magnitude enhancement compared to single pass absorption in the absorber. The bandwidth of the enhancement can be matched with the range of wavelengths close to the solar cell absorber band-edge where improved light absorption is required.

Optimization of the optical properties of nanostructured silicon surfaces for solar cell applications

Surface nanostructuration is an important challenge for the optimization of light trapping in solar cell. We present simulations on both the optical properties and the efficiency of micro pillars—MPs—or nanocones—NCs—silicon based solar cells together with measurements on their associated optical absorption. We address the simulation using the Finite Difference Time Domain method, well-adapted to deal with a periodic set of nanostructures. We study the effect of the period, the bottom diameter, the top diameter, and the height of the MPs or NCs on the efficiency, assuming that one absorbed photon induces one exciton. This allows us to give a kind of abacus involving all the geometrical parameters of the nanostructured surface with regard to the efficiency of the associated solar cell. We also show that for a given ratio of the diameter over the period, the best efficiency is obtained for small diameters. For small lengths, MPs are extended to NCs by changing the angle between the bottom surface and the vertical face of the MPs. The best efficiency is obtained for an angle of the order of 70°. Finally, nanostructures have been processed and allow comparing experimental results with simulations. In every case, a good agreement is found.

Reply to 'On the thermodynamics of light trapping in solar cells'

We are grateful to Rau and Kirchartz for their thoughtful remarks on our Commentary, and hope this dialog will generate clarity in the physics of high-efficiency photovoltaics. Our Commentary identified several new design approaches and architectures using photonic structures to increase photovoltaic efficiency. It was driven by thinking about the design of solar cells with (i) high radiative efficiency, (ii) 1 sun unconcentrated (C = 1) operation and (iii) employing a light-collimating angle restrictor element (as opposed to a geometrical concentrator).

On the thermodynamics of light trapping in solar cells

On the thermodynamics of light trapping in solar cells

Metamaterial band theory: fundamentals & applications

Remarkable progress has been made over the past decade in controlling light propagation and absorption in compact devices using nanophotonic structures and metamaterials. From sensing and modulation, to on-chip communication and light trapping for solar cells, new device applications and opportunities motivate the need for a rigorous understanding of the modal properties of metamaterials over a broad range of frequencies. In this review, we provide an overview of a metamaterial band theory we have developed that rigorously models the behavior of metamaterials made of dispersive materials such as metals. The theory extends traditional photonic band theory for periodic dielectric structures by coupling the mechanical motion of electrons in the metal directly to Maxwell’s equations. The solution for the band structures of metamaterials is then reduced to a standard matrix eigenvalue problem that nevertheless fully takes into account the dispersive properties of the constituent materials. As an application of the metamaterial band theory, we show that one can develop a perturbation formalism based on this theory to physically explain and predict the effect of dielectric refractive index modulation or metallic plasma frequency variation in metamaterials. Furthermore, the metamaterial band theory also provides an intuitive physical picture of the source of modal material loss, as well as a rigorous upper bound on the modal material loss rate of any plasmonic, metamaterial structure. This in turn places fundamental limits on the broadband operation of such devices for applications such as photodetection and absorption.

Using nanowires to enhance light trapping in solar cells

Using nanowires to enhance light trapping in solar cells

Light Trapping Textures Designed by Electromagnetic Optimization for Subwavelength Thick Solar Cells

Light trapping in solar cells allows for increased current and voltage, as well as reduced materials cost. It is known that in geometrical optics, a maximum 4n^2 absorption enhancement factor can be achieved by randomly texturing the surface of the solar cell, where n is the material refractive index. This ray-optics absorption enhancement limit only holds when the thickness of the solar cell is much greater than the optical wavelength. In sub-wavelength thin films, the fundamental questions remain unanswered: (1) what is the sub-wavelength absorption enhancement limit and (2) what surface texture realizes this optimal absorption enhancement? We turn to computational electromagnetic optimization in order to design nanoscale textures for light trapping in sub-wavelength thin films. For high-index thin films, in the weakly absorbing limit, our optimized surface textures yield an angle- and frequency-averaged enhancement factor ~39. They perform roughly 30% better than randomly textured structures, but they fall short of the ray optics enhancement limit of 4n^2 ~ 50.

Modeling combined coherent and incoherent scattering structures for light trapping in solar cells

Current structures for solar cells or LEDs often incorporate layers of various optical regimes, with a mixture of coherent, partially coherent or incoherent behavior. We developed a simple and efficient calculation method to study such combined solar cell structures with both wave and ray optics sections. These One-Pass Coherent calculations take wave effects into account where they matter the most, while avoiding a large computational domain to model rough structures. The method simulates a general diffuser by working directly with the reflected wavefronts, instead of using its geometry. We utilize this method to study thin film silicon solar cell structures with a grating on the front and a diffuser at the back. More absorption is obtained with the combined light trapping scheme of appropriate characteristics, compared with grating-only or diffuser-only counterparts. Finally, we report a significant effect of incoherence on the absorption of fairly thin (∼10 μm) cells. We demonstrate that partially incoherent light can be more efficiently absorbed than fully coherent light on average over a broad wavelength range. It turns out that the scarcity of guided modes for fully coherent light can hinder the grating enhancement, leading to a consistently better performance when light coherence is limited or lost.

Plasmon resonances and strong electric field enhancements in side-by-side tangent nanospheroid homodimers

The plasmon resonance and electric field enhancement in a side-by-side tangent nanospheroid homodimer (TNSHD) have been investigated theoretically by using DDA and FDTD methods, respectively. The simulation results indicate that this side-by-side TNSHD has its novel optical properties. We find that the plasmon resonance with a distinct Fano lineshape can be achieved and the electric field intensity can be enhanced strongly. The tunability of the Fano resonance could provide important applications in biosensing. The obtained electric field enhancement might open a promising pathway for surface-enhanced Raman scattering (SERS) and light trapping in solar cells.

Broadband Light Absorption with Multiple Surface Plasmon Polariton Waves Excited at the Interface of a Metallic Grating and Photonic Crystal

Light incident upon a periodically corrugated metal/dielectric interface can generate surface plasmon polariton (SPP) waves. This effect is used in many sensing applications. Similar metallodielectric nanostructures are used for light trapping in solar cells, but the gains are modest because SPP waves can be excited only at specific angles and with one linear polarization state of incident light. Here we report the optical absorptance of a metallic grating coupled to silicon oxide/oxynitride layers with a periodically varying refractive index, i.e., a 1D photonic crystal. These structures show a dramatic enhancement relative to those employing a homogeneous dielectric material. Multiple SPP waves can be activated, and both s- and p-polarized incident light can be efficiently trapped. Many SPP modes are weakly bound and display field enhancements that extend throughout the dielectric layers. These modes have significantly longer propagation lengths than the single SPP modes excited at the interface of a metallic grating and a uniform dielectric. These results suggest that metallic gratings coupled to photonic crystals could have utility for light trapping in photovoltaics, sensing, and other applications.

Light transmission through periodic sub-wavelength textured surface

Using the rigorous coupled-wave analysis method, we present a detailed study about the light transmission through periodic sub-wavelength textured surfaces. We investigate to what extent and under what conditions these textured surfaces are equivalent to a multilayer dielectric coating from the perspective of reversibility and azimuthal symmetry of the reflected and transmitted light. Our analyses show that this is completely determined by the relative size between the reciprocal lattice period of the textured surface structure and the wavevectors of the reflected and transmitted waves in the dielectric materials. It is found that these analyses can be used to determine the optimal surface texture which provides the best light trapping for solar cells in terms of the total internal reflection occurring in the high-index medium at incidence angles larger than the nominal critical angle. We also perform calculations for the antireflective characteristics of these sub-wavelength textured surfaces and find that an optimized surface texture can generally give quite small reflectance in a wide spectral and angle range, but for the current texture design its performance at higher incidence angles is usually poorer than an optimized multilayer AR coating with graded index along the thickness direction.

Light trapping in solar cells at the extreme coupling limit

We calculate the maximal absorption enhancement obtainable by guided mode excitation in a weakly absorbing dielectric slab over wide wavelength ranges. The slab mimics thin film silicon solar cells in the low absorption regime. We consider simultaneously wavelength-scale periodicity of the texture, small thickness of the film, modal properties of the guided waves and their confinement to the film. Also we investigate the effect of the incident angle on the absorption enhancement. Our calculations provide tighter bounds for the absorption enhancement but still significant improvement is possible. Our explanation of the absorption enhancement can help better exploitation of the guided modes in thin film devices.

Nanoimprint lithography of textures for light trapping in thin film silicon solar cells

AbstractNanoimprint lithography (NIL) is a versatile and commercially viable technology for fabrication of structures for light trapping in solar cells. We demonstrate the applicability of NIL in thin film silicon solar cells in substrate configuration, where NIL is used to fabricate a textured rear contact of the solar cells. We applied random structures, based on the natural texture of SnO/F grown by APCVD, and designed 2D periodic structures and show that for single junction µc‐Si cells these textured rear contacts lead to an increase of Jsc of more than 40% in comparison to cells with flat rear contacts. Cells on optimized periodic textures showed higher fill factors which can be attributed to reduced microcrack formation, leading to less shunting in comparison to cells on random textures.

Nanophotonic light trapping in solar cells

Nanophotonic light trapping for solar cells is an exciting field that has seen exponential growth in the last few years. There has been a growing appreciation for solar energy as a major solution to the world’s energy problems, and the need to reduce materials costs by the use of thinner solar cells. At the same time, we have the newly developed ability to fabricate controlled structures on the nanoscale quickly and cheaply, and the computational power to optimize the structures and extract physical insights. In this paper, we review the theory of nanophotonic light trapping, with experimental examples given where possible. We focus particularly on periodic structures, since this is where physical understanding is most developed, and where theory and experiment can be most directly compared. We also provide a discussion on the parasitic losses and electrical effects that need to be considered when designing nanophotonic solar cells.

Light trapping in solar cells: When does a Lambertian scatterer scatter Lambertianly?

We derive scaling laws for the Rayleigh-Sommerfeld formulation we recently developed to describe light scattering from nanotextured interfaces. These scaling laws provide precious intuition on how to link scattering from different interfaces. In particular, we answer the question how to obtain a Lambertian scatterer into silicon, starting from a Lambertian scatterer into air relevant to the development of light trapping schemes in thin-film silicon solar cells. We also define a Lambertionality factor which measures how close an arbitrary scatterer approaches Lambertian scattering and extend the fundamental 4n2 light trapping limit to arbitrary scattering distributions.

Finite-Difference Time-Domain Calculation of Light Scattering Efficiency for Ag Nanorings

Enhancement of light trapping in solar cells is becoming increasingly urgent for the development of next generation thin film solar cells. One of the possible candidates for increasing light trapping in thin film solar cells that has emerged recently is the use of scattering from metallic nanostructures. In this study, we have investigated the effects of the geometric parameters of Ag nanorings on the light scattering efficiency by using three dimensional Finite Different Time Domain (FDTD) calculations. We have found that the forward scattering of incident radiation from Ag nanorings strongly depends on the geometric parameters of the nanostructures such as diameter, height, etc. The forward scattering to substrate direction is increased as the outer diameter and height of the nanorings decrease. In particular, for nanorings larger than 200 nm, the inner diameter of Ag nanorings should be optimized to enhance the forward scattering efficiency. Light absorption and scattering efficiency calculations for the various nanoring arrays revealed that the periodicity of nanorings arrays also plays an important role in the absorption and the scattering efficiency enhancement. Light scattering efficiency calculations for nanoring arrays also revealed that enhancement of scattering efficiency could be utilized to enhance the light absorption through the forward scattering mechanism.

Laser machined macro and micro structures on glass for enhanced light trapping in solar cells

In order to increase the efficiency of solar cell modules it is necessary to make the optimum use of light incident upon them. Much research has been done on improving light absorption through front surface texturisation and light trapping schemes. Laser light is commonly used in industry for various applications including marking and texturisation. By controlling laser parameters, it is possible to tailor macro and micro structures in most materials. The CO2 laser used in this investigation emits radiation at 10.6 μm with the ability to pulse in the micro-second range. The laser was used to ablate grooved textures in the fused quartz material, used in this study as the light trapping medium, following which an analysis of the effects of the laser parameters on the texture geometry and surface morphology was performed through a combination of cross sectioning and scanning electron microscopy. Transmission through the textured glass was improved for most samples after acid etching. The light trapping effects of the best performing textures were analysed by investigating the effects on a silicon solar cell’s performance at varying angles of incidence. Results indicated a significant increase in light trapping when light was incident at acute angles. For an angle of incidence of 10∘ a relative increase in efficiency of up to 51 % was observed.