Light Trapping In Solar Cells Research Articles

Design and fabrication of photonic crystal structures by single pulse laser interference lithography

Photonic crystal (PhC) structures formed by periodic surface nanostructuring have emerged as pivotal elements for controlling light-matter interactions. One important application is reducing losses due to the high surface reflectivity of semiconductor optoelectronic devices, such as enhancing light absorption in photovoltaic cells or improving light extraction in light-emitting diodes (LEDs). Although various methods for fabricating such structures have been documented, the utilization of single pulse laser interference lithography (LIL) using commercial photoresist and its subsequent effective use as an etch mask has not been previously reported. Rapid exposure of photoresists with single nanosecond pulses offers benefits for high throughput patterning and reduces the requirement for a stable optical platform. We have successfully employed single pulse LIL to fabricate antireflective PhC structures on GaAs substrates using a commercial photoresist. Exposure is performed with single 7 ns 355 nm pulses of relatively low energy (<10 mJ). High-quality nanohole arrays of pitch of approximately 365 nm are fabricated and depths up to 400 nm have been etched using inductively coupled plasma (ICP) through the exposed photoresist mask. Reflectivity analyses confirmed that these structures reduce the average reflectance of the GaAs to below 5 % across the 450 nm to 700 nm visible wavelength range. The fabrication of PhC structures using this approach has potential for low-cost wafer-level patterning to provide improved light extraction in LEDs and enhanced light trapping in solar cells.

Comparison of different approaches to texturing monocrystalline silicon wafers for solar cell applications

Texturing the surface of crystalline silicon wafers is a very important step in the production of high-efficiency solar cells. Alkaline texturing creates pyramids on the silicon surface, lowering surface reflectivity and improving light trapping in solar cells. This article provides a comparative evaluation of various wet texturing methods using alkaline solutions with or without additives commonly known as surfactants. One method uses sodium hydroxide (NaOH) and isopropyl alcohol (IPA) to create a surface with a height of about 4.5 μm by texturing for about 30 min, while the other method uses potassium hydroxide (KOH) and other additions known as additives. Texturing was performed using chemicals for only 15 min to create a surface shape with a height of approximately 3.5 μm. Additionally, the two solutions showed reflectance of 8.01 % or 12.1 % in 400–1100 nm, respectively. Both processes used alkaline etching at 80 °C for saw damage removal (SDR) before texturing. These processes have also been investigated in terms of removing potential organic contaminants from surfaces. Characterization techniques used throughout the investigation included optical microscopy, surface reflectance measurements, scanning electron microscopy (SEM), and electron dispersive spectroscopy (EDS). The purpose of this study is to confirm through experiments which texturing techniques are more suitable for mass production and to develop time- and cost-effective texturing techniques for industrial production of high-throughput, high-efficiency solar cells.

Anti‐Reflective Graded‐Index Metasurface with Correlated Disorder for Light Management in Planar Silicon Solar Cells

AbstractRecently, many research efforts have been dedicated to improving light coupling into solar cells and reducing optical losses. Promising candidates regarding scalability include direct nano‐structuring of the absorber layer, anti‐reflective (AR) coatings, or combining both, e.g., pyramidal textures with a conformal coating. However, many of these methods are either insufficient or infeasible for application in thin solar cells. Moreover, approaches based on directly texturing the silicon interface simultaneously strongly increase surface recombination, thus degrading the electronic properties of the solar cell. To circumvent these issues, conformal graded‐index metasurfaces with a correlated positional disorder for light trapping in solar cells are proposed and experimentally demonstrated in this contribution. When considered as a part of a prototypical solar cell geometry, a broadband reduction in reflection is observed that results in photocurrent enhancement. The combined consideration of disorder and conformal graded‐index layers outperforms structures containing only one of these components. The computational guidance toward optimized designs promises to adjust the framework to other settings. The possibility for large‐scale fabrication of the samples paves the way toward a future generation of supporting photonic structures in solar cells.

Examine the Transverse Magnetic Polarization Transmission of Solar Cell Models Based on Different Types of Perovskites and Nanoparticles

Background: Utilizing surface Plasmon in nanostructures can improve light trapping in solar cells. Method: A novel type of light-trapping structure incorporating nanoparticles of noble metals is under investigation to enhance the transmittance in perovskite solar cell modules (𝐶𝐻3𝑁𝐻3𝑃𝑏𝐼3, 𝐶𝐻3𝑁𝐻3𝑃𝑏𝐵𝑟3, 𝑎𝑛𝑑 𝑁𝐻3𝑃𝑏𝐼2𝐵𝑟). The transmission spectra of the suggested solar cell construction are computed and simulated using the transfer matrix method (TMM). Result: The best cell transmission came from the 𝐶𝐻3𝑁𝐻3𝑃𝑏𝐼3 perovskite cell which was doped by aluminum with 0.05 volume fractions with transmission values of more than 0.85 for each value of wavelength. Moreover, the worst cell is a cell with, 𝐶𝐻3𝑁𝐻3𝑃𝑏𝐵𝑟3 Perovskite and 0.35 gold nanoparticles volume fraction which has the lowest value of transmission up to 0.45 at 450nm wavelength; other proposed cells have transmission values between these two values which can be considered good values. Conclusion: As active layer thickness increases the transmission decreases for all wavelength spectrums, and increasing the doping volume fraction decreases the transmission.

Efficiency improvement of thin film solar cell using silver pyramids array and antireflective layer

In recent years, plasmonics has been widely employed to improve light trapping in solar cells. Silver nanospheres have been used in several research works to improve the capability of solar absorption. In this paper, we use silver pyramid-shaped nanoparticles, a noble plasmonic nanoparticle, inside thin-film silicon and InP solar cells to increase light absorption compared to previously published topologies. The proposed structure consists of a TiO2 pyramid structure placed at the top of the surface working as an anti-reflective layer, silicon/indium phosphate as an absorption layer, silver pyramid-shaped nanoparticles incorporated inside the absorption layer, and an aluminum reflecting layer at the bottom. In this research, we used finite difference time domain (FDTD) simulation to model the thin-film solar cell (TFSC). Optimizing the shape and placement of the silver pyramids, we have achieved an efficiency of 17.08% and 18.58% using silicon and InP as the absorbing layers respectively, which is significantly better than previously reported studies. The open-circuit voltages are 0.58 V and 0.92 V respectively, which is the highest among other configurations. To conclude, the findings of this study laid the foundation to create an efficient thin-film solar cell utilizing the light-trapping mechanism of noble plasmonic nanoparticles.

An efficient optical diffuser fabricated from fungal mycelium

Scattering of light from randomly textured materials is ubiquitous and of great interest in biology and for diverse applications—including filtering, light trapping in solar cells, and speckle photography. One attractive means to build these materials is by harnessing the complexity present in structures of biological origin. Here, we report on the development of a random phase diffuser based on intertwined filamentous cells (hyphae) of the fungus Trichoderma atroviride. A fungal colony (mycelium) is grown on the surface of a gel medium, and then removed, fixated, and dehydrated, resulting in a free-standing, two-dimensional random mesh (1 cm × 1 cm × 5 μm) composed of rigid hyphae separated by air gaps. A laser beam incident on the bioplate results in speckle patterns of nearly equal intensity in transmission and reflection. By modeling the bioplate as composed of optical phase elements and computing Fraunhofer diffraction, we recover the overall shape of the observed diffuse light spot. As the hyphal density composing the sample is increased, all optical power is in the speckle pattern, and approximate Lambertian transmissivity is reached. Altogether, our observations suggest that a planar fungal colony can scatter light efficiently by imparting a random phase. These results underscore the potential of a biological structure to develop optical elements and to use light scattering to evaluate morphology in complex structures—such as filamentous mycelia.

Light trapping in solar cells: simple design rules to maximize absorption

Solar cells can strongly benefit from optical strategies capable of providing the desired broadband absorption of sunlight and consequent high conversion efficiency. While many diffractive light-trapping structures prove high absorption enhancements, their industrial application rather depends on simplicity concerning the integration to the solar cell concept and the process technology. Here, we show how simple grating lines can perform as well as advanced light-trapping designs. We use a shallow and periodic grating as the basic element of a quasi-random structure, which is highly suitable for industrial mass production. Its checkerboard arrangement breaks the mirror symmetry and is shown, for instance, to enhance the bulk current of a 1 µm slab of crystalline silicon by 125%. We explain its excellent performance by drawing a direct link between a structure’s Fourier series and the implied photocurrent, derived from a large and diverse set of structures. Our design rule thus meets all relevant aspects of light-trapping for solar cells, clearing the way for simple, practical, and yet outstanding diffractive structures, with a potential impact beyond photonic applications.

Tailored disorder: a self-organized photonic contact for light trapping in silicon-based tandem solar cells.

We present a process development leading to efficient rear side light trapping structures with the purpose of enhancing the infrared response of a silicon-based tandem solar cell. To this end, we make use of phase separation effects of two immiscible polymers, polystyrene and poly(methyl methacrylate), resulting in a non-periodic polystyrene structure on silicon with a well-defined size distribution. Onto this pattern, we evaporate silver as a scattering rear side mirror and contact layer. Average feature sizes and periods can be tuned by varying material properties (e.g. molar weights or ratios of the polymers) as well as processing conditions during the spin coating. This way a favorable pseudo period of approx. 1 µm for these disordered structure features was realized and successfully implemented into a silicon solar cell. The structure shows a ring-shaped scattering distribution which is beneficial for light trapping in solar cells. External quantum efficiency measurements show that a gain in short circuit current density of 1.1 mA/cm2 compared to a planar reference can be achieved, which is in the same range as we achieved using nanoimprint lithography in a record triple-junction III/V on a silicon device.

The enhancement of near infrared light trapping in solar cells with backside crystalline silicon gratings: Realization and characterization investigation

The aim purpose of this work is to study the fundamental potential effect of circular crystalline silicon pillars gratings on the enhanced near infrared light trapping. Gratings with periods of 1.2, 1.5 and 1.8 μm and depth of 400 nm were located on the rear side of the wafers. The findings of this investigation clearly indicated that the gratings have an important effect on the enhancement of light. The measurements of absorption and reflectance spectra were carried out using an integrating sphere. It was clearly illustrated that the optimization was performed for grating with a period length of 1.2 μm. This type of grating enhanced around 75% the near infrared light trapping and thus the absorption is increased in the thin film. This phenomenon has a great role in solar cells applications to increase the efficiency.

Nanostructures for Light Trapping in Thin Film Solar Cells.

Thin film solar cells are one of the important candidates utilized to reduce the cost of photovoltaic production by minimizing the usage of active materials. However, low light absorption due to low absorption coefficient and/or insufficient active layer thickness can limit the performance of thin film solar cells. Increasing the absorption of light that can be converted into electrical current in thin film solar cells is crucial for enhancing the overall efficiency and in reducing the cost. Therefore, light trapping strategies play a significant role in achieving this goal. The main objectives of light trapping techniques are to decrease incident light reflection, increase the light absorption, and modify the optical response of the device for use in different applications. Nanostructures utilize key sets of approaches to achieve these objectives, including gradual refractive index matching, and coupling incident light into guided modes and localized plasmon resonances, as well as surface plasmon polariton modes. In this review, we discuss some of the recent developments in the design and implementation of nanostructures for light trapping in solar cells. These include the development of solar cells containing photonic and plasmonic nanostructures. The distinct benefits and challenges of these schemes are also explained and discussed.

Efficient enhancement of light trapping in the double-textured Al doped ZnO films with nanorod and crater structures

Double-textured Al doped ZnO (AZO) films with nanorod and crater structures were prepared by the simple hydrothermal growth of AZO nanorods on the single-textured AZO films with crater structures. The light trapping capabilities, morphological, structural and optoelectronic properties of double-textured AZO films prepared with different precursor concentrations were demonstrated systematically. Remarkable surface morphology evolution of the double-textured films depending on precursor concentration was observed. Especially, the relationship between surface morphologies and light trapping capabilities of the AZO films with nanorod and crater structures was discussed. The double-textured AZO films having nanorod and crater structures prepared with 0.01 M precursor concentration exhibited low sheet resistance, high visible total transmittance and haze value, which could be used as transparent front electrodes for enhancing the light trapping in solar cells.

Evolutionary design algorithm for optimal light trapping in solar cells

Light scattering at rough interfaces is a standard approach to enhance absorption of light in the absorber layers of solar cells due to light trapping. In this work, an automata optimization algorithm, based on the Constructal Theory, is proposed for the heuristic geometric design to enhance light trapping of silicon solar cell surface textures. Surface textures were investigated computationally for light absorption of a normally incident simulated solar spectrum, under constraints of transverse area of the texture. The photogenerated current density was calculated for textures to estimate light trapping. In the automata optimization, Elemental Constructals, consisting of dielectric material blocks, were assembled on top of the solar cell forming a texture. Three variations of the proposed automata algorithm were tested and their capabilities were determined. The best result for this optimization was a photocurrent J=17.48mA/cm2 for grid periodicity above the wavelength and J=18.31mA/cm2 for grid periodicity in the sub-wavelength range. The evolutionary algorithm was also applied to assemble a periodic-disordered texture from fixed shape gratings to better harness incident light. Different textures with fixed shapes of gratings had been investigated to determine optimal geometric parameters. Optimized triangular, trapezoidal, and rounded-tip textures achieved the best results in this phase and were implemented to assemble the periodic-disordered texture which reached J=19.75mA/cm2 for triangular gratings. Results found with this pristine evolutionary algorithm corroborate its efficiency in finding, practically, geometries that lead to greater light absorption with a minimum amount of dielectric materials.

Design of Light Trapping Solar Cell System by Using Zemax Program

Square micro lenses array have been designed (by using Zemax optical design program) to concentrate solar radiation into variable slits that reaching light to solar cell. This technique to increase the efficiency of solar system by trapping light due to internal reflection of light by mirrors that placed between upper and lower side of solar cell, therefore increasing optical path through the solar cell, and then increasing chance of photon absorption. The results show priority of solar system that have slit of (0.2 mm), and acceptance angle of (20°) that give acceptable efficiency of solar system.

(Invited) Light Management for Engineering Luminescence in Nanoscale Environments By Numerical Optimization

The spontaneous emission rate of fluorescencent species in general is affected by its environment. Geometrical structure and material composition of the environment can yield strongly increased rates. This effect is known as Purcell effect, and the magnitude of enhancement of the emission rate in a cavity is known as Purcell factor. The Purcell factor is proportional to the cavity mode's quality factor (Q factor) and inversely proportional to the modal volume. This implies that strongly resonant structures with small length scales (of the order of the wavelength of light) can greatly influence fluorescence. However, not only the emission rate can be controlled by a suitably chosen environment, but also the directional properties of the emitted light field can be engineered by placing the emitter in custom-made environments. In the regime of optical wave phenomena on nano-meter length scales (nano optics), Maxwell's equations for the 3D, vectorial electromagnetic field distribution can to be solved numerically in order to quantify the expected emission rate and far field radiation patterns. In order to obtain specific functionalities of optical devices, numerical optimization is performed. I.e., in numerical computations, structural parameters (layer thicknesses, structure dimensions, material composition, etc) are varied until a parameter set is reached which yields best performance for the specific functionality. This numerically optimized structure can then serve as design pattern for experimental realizations. We review our recent developments of finite-element method based and machine-learning inspired optimization algorithms [1]. We have applied these methods in order to optimize light extraction efficiency of quantum dots embedded in deterministically fabricated cavities [2] and in periodic nano-structures [3], as well as extraction efficiency from structured light emitting diodes (LED, OLED) [4]. We have also investigated possibilities to manipulate emission properties in chiral photonic structures [5], and to design vertical cavity lasers [6]. Further, in applications like light trapping in solar cells, for enhanced photovoltaic efficiency, the related problem of maximizing coupling of light of specific spectral properties into nanostructured solar cells can be treated [7]. In this contribution we will further present recent results on optimization of photon extraction efficiency from single photon sources for applications in quantum technology. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 675745 (MSCA-ITN-EID NOLOSS) and from the EMPIR programme co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme under grant agreement number 14IND13 (PhotInd). We further acknowledge funding by Einstein Foundation Berlin (ECMath-SE6) and by Freie Universität Berlin (Dahlem Research School). [1] P.-I. Schneider, et al. Global optimization of complex optical structures using Bayesian optimization based on Gaussian processes. Proc. SPIE 10335, 103350O (2017).[2] M. Gschrey, et al. Highly indistinguishable photons from deterministic quantum-dot microlenses utilizing three-dimensional in situ electron-beam lithography. Nat. Commun. 6, 7662 (2015).[3] C. Barth, et al. Increased fluorescence of PbS quantum dots in photonic crystals by excitation enhancement. Appl. Phys. Lett. 111, 031111 (2017).[4] L. Zschiedrich, et al. Numerical analysis of nanostructures for enhanced light extraction from OLEDs. Proc. SPIE 8641, 86410B (2013).[5] P. Gutsche, et al. Locally enhanced and tunable optical chirality in helical metamaterials. Photonics 3, 60 (2016).[6] N.N. Ledentsov, et al. Direct Evidence of the Leaky Emission in Oxide-Confined Vertical Cavity Lasers. IEEE J. Quant. Electron. 52, 2400207 (2016).[7] K. Jäger, et al. Simulations of Sinusoidal Nanotextures for Coupling Light into c-Si Thin-Film Solar Cells. Opt. Express 24, A569 (2016).

Broadband angle-independent antireflection coatings on nanostructured light trapping solar cells

Backscattering from nanostructured surfaces greatly diminishes the efficacy of light trapping solar cells. While the analytical design of broadband, angle-independent antireflection coatings on nanostructured surfaces proved inefficient, numerical optimization proves a viable alternative. Here, we numerically design and experimentally verify the performance of single and bilayer antireflection coatings on a 2D hexagonal diffractive light trapping pattern on crystalline silicon substrates. Three well-known antireflection coatings, aluminum oxide, silicon nitride, and silicon oxide, which also double as high-quality surface passivation materials, are studied in the 400--1000 nm band. By varying thickness and conformity, the optimal parameters that minimize the broadband total reflectance (specular and scattering) from the nanostructured surface are obtained. The design results in a single-layer antireflection coating with normal-angle wavelength-integrated reflectance below 4% and a bilayer antireflection coating demonstrating reflection down to 1.5%. We show experimentally an angle-averaged reflectance of $\ensuremath{\sim}5.2%$ up to 60\ifmmode^\circ\else\textdegree\fi{} incident angle from the optimized bilayer antireflection-coated nanostructured surface, paving the path toward practical implementation of the light trapping solar cells.

Comparison of metal/polymer back reflectors with half-sphere, blazed, and pyramid gratings for light trapping in III-V solar cells.

We report on the fabrication of diffraction gratings for application as back contact reflectors. The gratings are designed for thin-film solar cells incorporating absorbers with bandgap slightly lower than GaAs, i.e. InAs quantum dot or GaInNAs solar cells. Light trapping in the solar cells enables the increase of the absorption leading to higher short circuit current densities and higher efficiencies. We study metal/polymer back reflectors with half-sphere, blazed, and pyramid gratings, which were fabricated either by photolithography or by nanoimprint lithography. The gratings are compared in terms of the total and the specular reflectance, which determine their diffraction capabilities, i.e. the feature responsible for increasing the absorption. The pyramid grating showed the highest diffuse reflection of light compared to the half-sphere structure and the blazed grating. The diffraction efficiency measurements were in agreement with the numerical simulations. The validated model enables designing such metal/polymer back reflectors for other type of solar cells by refining the optimal dimensions of the gratings for different wavelength ranges.

Transfer the multiscale texture of crystalline Si onto thin-film micromorph cell by UV nanoimprint for light trapping

Surface texturing is of great significance in light trapping for solar cells. Herein, the multiscale texture, consisting of microscale pyramids and nanoscale porous arrangement, was fabricated on crystalline Si by KOH etching and Ag-assisted HF etching processes and subsequently replicated onto glass with high fidelity by UV nanoimprint method. Light trapping of the multiscale texture was studied by spectral (reflectance, haze ratio) characterizations. Results reveal the multiscale texture provides the broadband reflection reducing, the highlighted light scattering and the additional self-cleaning behaviors. Compared with bare cell, the multiscale textured micromorph cell achieves a 4% relative increase in power conversion efficiency. This surface texturing route paves a promising way for developing low-cost, large-scale and high-efficiency solar applications.

Laser fabrication of periodic arrays of microsquares on silicon for SERS application

Two dimensional periodic arrays of microsquares of different dimensions are fabricated by direct ns-laser writing on silicon substrate. The micro/nano structures pattern on the Si surface significantly minimizes the optical reflection by light trapping effect, but enhances the SERS signal intensity of analyte dye molecule Rhodamine B (RhB) and Methylene Blue (MB) in presence of gold nanoparticles. The dependence of surface roughness, optical reflection and SERS intensity on the laser pulse energy and line scan interval is investigated. The SEM images clearly show well-ordered features on the surface of patterned Si substrates and aggregation of micro/nanoparticles on the substrate surface. The optical reflection from the patterned surfaces reduces below 11% over a broad range of wavelengths from 300nm to 1200nm. The patterned surface can be used as a reproducible SERS substrate with a spatially uniform enhancement factor of 108. This fabrication method can be employed for preparation of multifunctional surfaces for light trapping in solar cells and for Raman signal amplification for versatile and reproducible SERS sensors.

Photonic materials and techniques for SERS and solar cell light trapping

Photonic materials and techniques for SERS and solar cell light trapping

Theory of solar cell light trapping through a nonequilibrium Green's function formulation of Maxwell's equations

We develop a theory of solar cell light trapping based on directly solving Maxwell's equations through a nonequilibrium Green's function formalism. This theory rigorously connects the maximum power absorbed by the solar cell to the density of states of the cell. With this theory we are able to reproduce all standard results in solar cell light trapping previously derived using approximate formalisms. Therefore our development places solar cell light trapping theory on a much firmer theoretical foundation. Moreover, here the maximum power formula is derived without the assumption of reciprocity, unlike previous theories on solar cell light trapping. Therefore, we prove that the upper bound of light trapping enhancement cannot be overcome with the use of nonreciprocal structures. As a numerical test, we simulate an absorber structure that consists of a nonreciprocal material, and show that the absorption enhancement factor is largely independent of nonreciprocity, in consistency with the theory.