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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.

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. The current world energy generation system is unsustainable, insufficient, cost-ineffective, and environmentally unfriendly. A number of alternative energy production from renewable sources such as solar, wind, hydroelectric, tidal, bioenergy, and geothermal have been extensively explored.

The renewable energy sources are free and abundantly available and most important do not harm the environment. Solar energy is one of the promising alternatives to replacing fossil fuel among other energy sources because it has the potential to meet future energy demands at low cost with no detrimental effects to the environment.

There are different technologies to harvest solar energy, and typical examples include solar electric photovoltaic , solar thermal conversion, and solar fuel technologies [ 1 ]. Photovoltaic energy conversion, which converts light energy directly into electricity without any intermediate stage, has already demonstrated its success and widespread applications for solar energy utilization. The photovoltaic market has shown very significant yearly growth rates and the total global installed solar photovoltaic PV capacity had grown to over GW by the end of [ 2 ].

A projected additional GW of PV capacity is expected to be installed by , driven by greater cost reduction and higher demand. In order to meet the requirements of the global energy demand using photovoltaics, further conversion efficiency improvements and reductions in production costs are necessary.

The use of advanced nanophotonic light trapping approaches can contribute to both objectives simultaneously. Enhancing the optical absorption also allows for decreasing the active absorber layer thickness, which in turn decreases the production costs through the use of significantly less materials. In addition, light trapping can enhance solar cell efficiency as thinner devices offer improved photogenerated charge carrier collection with less constraints on the diffusion lengths, potentially a higher open-circuit voltage, and improved stability through texturing material encapsulation [ 12 , 13 , 14 ].

In addition, incident light reaching the active absorber layer in solar cells can be interfered by dust particles in typical terrestrial environments and thus decreases the power conversion efficiency. Therefore, nanophotonic structures with a self-cleaning capability coating the solar cell are becoming necessary for sustainability and improved performance of the solar cells in typical terrestrial environments [ 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 ].

Minimizing optical losses such as reflections from front surfaces, preventing light from entering the solar cell active material, and poor absorption due to the transmission, particularly in thin film solar cells, have long been the main challenge in increasing the conversion efficiency. Light trapping structures are needed to maximize the optical path length of sunlight into solar cells through multiple passes and reduce reflections as they act as an antireflection coating in order to enhance the overall efficiency.

Typically, a thicker active layer can improve the absorption of more sunlight. However, the optical thickness of the active absorber layer can be increased several times by the use of light trapping structures in a solar cell while its physical thickness remains unchanged. In general, light trapping techniques have been utilized in the development of high performance and low-cost solar cells by enhancing light absorption without requiring thicker active layers.

Such micron-scale features are not beneficial for thin film solar cells in which the active absorber layer is just a couple of microns or even several hundred nanometers in thicknesses.

In addition, micron-scale features require deep etching and are known to introduce defects in the material [ 27 ]. Therefore, nanostructures are needed in order to apply light trapping in thin films and emerging low-cost solar cells. The use of nanoscale surface structures for improving light absorption of thin film solar cells is a promising method compared with the traditional micro-sized surface texturing for crystalline silicon solar cells [ 28 , 29 ].

This is because of the reduced etching depths required to form the nanoscale features and consequently decrease the level of damage to the substrates [ 30 ]. Furthermore, reflections are reduced over a wide range of wavelength in sub-wavelength nanophotonic structures.

It has also been theoretically illustrated that nanophotonic structures can achieve optical path length enhancement beyond the Yablonovitch conventional limit [ 31 ]. The light trapping in thin film solar cells can be achieved using various nanostructures. The most widely recognized approaches for light trapping in thin film solar cells can be listed as periodic grating structures [ 32 , 33 , 34 , 35 ], photonic crystal structures [ 36 , 37 , 38 , 39 ], nanowires [ 40 , 41 , 42 ], random scattering surfaces [ 43 , 44 ], and plasmonic structures [ 45 , 46 , 47 ].

In this article, we review some of the recent developments in the design and implementation of nanostructures for light trapping in solar cells.

This includes geometric engineering of the solar cell and the implementation of photonic and plasmonic nanostructures. The distinct advantages and challenges of these strategies are also discussed. The periodic structures incorporated into a solar cell surface can contribute to both reducing reflection and enhancing the optical path length of light. However, certainly both impacts cannot be utilized simultaneously, depending on the place front or back side of the cell surface , type, and size of the surface structure.

Figure 1 illustrates the optical impacts of textured surfaces. Schematic illustration of the optical effects induced for a specified wavelength by periodically textured surfaces of changing unique frequency. The textured surface shape heavily affects the intensity spreading of these higher diffraction orders. The optical path length within the active volume of solar cells can be enhanced owing to these effects linked to a change of propagation direction.

Another effect is that various reflections can happen geometrically in these large textured surfaces. Thus, the overall reflection can be additionally decreased at the front surface.

The response in terms of transmission and reflection properties of the textured surface depends strongly on wavelength due to such effects.

Both reduction in reflection and significant enhancement in optical path length can be achieved by proper engineering of structure geometry. The effective medium theories can be used to explain the optical properties of periodically textured surfaces with structure sizes much smaller than the incident light wavelengths [ 48 ]. The theory suggests that the light does not pass through these structures and thus behave as an effective refractive index medium. These small size structures affect reflections and transmission but do not cause any light guidance impact as there is no change in light propagation direction.

Thus, it is possible to achieve a very effective and broadband antireflection effect with these structures. One, two, or three dimensional 1D, 2D, or 3D periodic nanostructures or gratings are promising for achieving light trapping in solar cells and hence enhancing their efficiencies.

Figure 2 illustrates a schematic diagram of photonic nanostructures used in several configurations to improve solar cell performance. Schematic illustration of nanophotonic structures used for enhancing solar cell performance: a 1D Bragg stacks, b 2D gratings, c photonic crystal, and d nanowires. Reprinted adapted with permission from [ 49 ]. Copyright AIP Publishing.

The optical path length of light in the active absorber layer can be doubled by using optimized 1D dielectric gratings or Bragg stacks as back reflectors. The reflection can be reduced from the illuminated surface of the solar cells or light can be trapped inside the active absorber layer using single or bi-periodic dielectric structures [ 49 ].

Two-dimensional sub-wavelength gratings are even more promising than one-dimensional gratings since the reflectivity does not depend on the polarization of the incident light [ 50 ]. In tandem solar cells, 3D periodic nanophotonic structures or photonic crystals can be employed as vastly efficient omnidirectional reflectors [ 39 ].

A variety of nanophotonic structures, such as nanocones [ 51 , 52 , 53 , 54 ], nanorods [ 55 , 56 , 57 ], nanopillars [ 58 , 59 , 60 ], nanowells [ 61 , 62 , 63 ], nanopyramids [ 64 , 65 , 66 , 67 , 68 , 69 , 70 ], and nanospheres [ 71 , 72 ], have been extensively studied for enhancing the performance of the solar cells.

The photonic nanostructures themselves can be dielectric, metallic, or the absorber layer itself [ 71 , 73 , 74 ]. Nanostructures can be used at the front surface of the solar cell to offer an efficient pathway to couple the incoming light into the absorber layer and thus reduce reflection. For example, a light trapping element that consists of a periodic nanoisland structure formed on the front surface of thin film silicon fabricated by polystyrene colloidal lithography Figure 3 was demonstrated [ 75 ].

In this design, the nanoisland shape not only exhibited gradual refractive index matching for antireflection but also enhanced the light trapping through diffraction of incident light in a periodic structure. Here, careful engineering of the dimensions of the nanoisland structures can assist in further improving the flow of light into the absorber layer. Furthermore, the nanoisland structures contribute to the enhancement of the photocurrent densities at large angles of incidence, as compared to conventional antireflection coatings.

The periodic nanoisland structure on front surface of thin film silicon with an aluminum back-reflector. Reprinted adapted with permission from [ 75 ]. Copyright John Wiley and Sons. It has also been demonstrated that placement of a periodic array of resonant dielectric nanospheres on a top of an a-Si layer supporting whispering gallery modes significantly enhances the efficiency of a thin film a-Si solar cell [ 71 ]. Wavelength-scale resonant dielectric nanospheres can diffractively couple the incoming light from free space and also assist confined resonant modes.

In addition, a periodic array of dielectric nanospheres can lead to light coupling between the spheres due to whispering gallery resonances within the spheres [ 76 , 77 ]. The optical path length of light inside the active material can be enhanced due to the light coupling into resonant modes when dielectric nanospheres are close to the absorber material and thus significantly improving light absorption.

Another significant advantage of the nanosphere structure for solar cell application is its spherical shape that naturally admits light from large angles of incidence.

We reported [ 35 ] that significant improvements in photocurrent and power conversion efficiency were achieved for monocrystalline silicon solar cells with periodic inverted nanopyramid structures due to the reduction of reflections and entrapment of more incident light inside the active material. The periodic inverted nanopyramid structures were fabricated by UV nanoimprint lithography using Si master mold, which was fabricated by laser interference lithography and subsequent pattern transfer by combined reactive ion etching and KOH wet etching Figure 4.

The solar cell with periodic inverted nanopyramid structures showed an improvement in power conversion efficiency by SEM image of a the periodic inverted nanopyramid structures on Si master mold, b the periodic upright nanopyramid structures replica after the first imprint, and c the periodic inverted nanopyramid structures fabricated on the surface of the solar cells after the second imprint.

Reprinted adapted with permission from [ 35 ]. Copyright Elsevier. It has been reported that high refractive index dielectric nanostructures supporting Mie resonances can be utilized to achieve very efficient light trapping in solar cells [ 78 , 79 , 80 , 81 ].

The excitation of Mie resonance leads to strong forward scattering of incident light into the higher-index absorber layer due to the high optical density of states. For example, Spinelli et al. It was demonstrated theoretically that light trapping can be enhanced with silicon-on-insulator SOI wafers decorated with arrays of subwavelength light funnels LFs [ 82 , 83 ].

The mechanism behind the enhanced light absorption is the preferential forward scattering of light due to excitation of Mie modes in the arrays of light funnels. Various other types of nanostructures on the front surface, such as nanopillars [ 60 ], nanowells [ 61 ], nanowires [ 84 ], and nanocones [ 53 ], have gained substantial attention due to their outstanding ability to reduce optical reflection from properly engineering the surface structure to produce graded refractive index structures.

Various techniques can be used to fabricate nanostructures for light trapping such as nanosphere lithography NSL [ 85 ], colloidal lithography [ 75 ], electron beam lithography EBL [ 50 ], laser interference lithography LIL [ 35 ], and nanoimprint lithography NIL [ 86 ].

Nanostructure fabrication should be low-cost, and have high throughput, high fidelity, and be scalable for application in commercial photovoltaic technologies. NIL is one of the most cost efficient nanopatterning methods. Nanostructures can be utilized at the back surface of an absorber layer as high performance reflectors of light. A highly promising approach is to use a periodic array of nanostructured back reflectors photonic crystal to couple incident light into guided modes, propagating in the absorber plane [ 32 , 87 , 88 ].

Careful tuning of the shape and periodicities of the nanostructures offers a new degree of control across the polarization and angular distributions of the scattered light. This strategy is capable of significantly improving the optical path length within the absorber layer. A broad range of nanostructure shapes, dimensions, and periodicities have been investigated to optimize light trapping in thin film solar cells [ 89 , 90 , 91 ]. For example, systematic rigorous coupled wave analysis RCWA was performed to investigate the possible benefits of nanostructured double-side nanocone grating of an ultrathin c-Si solar cell, as shown in Figure 5 [ 91 ].

Reprinted adapted with permission from [ 91 ].

Unified Electromagnetic-Electronic Design of Light Trapping Silicon Solar Cells

Dielectric metamaterials with high refractive indices may have an incredible capability to manipulate the phase, amplitude, and polarization of the incident light. Combining the high refractive index and the excellent electrical characteristics of the hybrid organic-inorganic perovskites HOIPs , for the first time we experimentally demonstrate that metamaterial made of HOIPs can trap visible light and realize effective photon-to-electron conversion. The optical absorption is significantly enhanced at the visible regime compared to that of the flat HOIP film, which originates from the excited Mie resonances and transverse cavity modes with inhibited interface reflection. Our data point to the potential application of HOIP metamaterials for high-efficiency light trapping and photon-to-electron conversion. Mie resonances in dielectric metamaterials yield strong electric and magnetic resonances and allow substantial control over the light scattering amplitude and phase [ 1 ], [ 2 ], [ 3 ]. Similar to that in the metallic building blocks in plasmonic metamaterials [ 4 ], [ 5 ], [ 6 ], [ 7 ], the scattering properties of the dielectric resonators can be manipulated by varying the size, geometry, orientation, and material parameters of the resonators [ 8 ].

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Introduction to Light Trapping in Solar Cell and Photo-detector Devices

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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. 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.

This site contains links to the solar cell papers published by Prof. Alam's group. The papers have been organized in a way that makes self-study of these papers easier. A set of resources are available at the bottom of the page.

Atom-to-Farm Physics of Solar Cell

Plasmonics can be used to improve absorption in optoelectronic devices and has been intensively studied for solar cells and photodetectors. Graphene has recently emerged as a powerful plasmonic material. It shows significantly less loss compared to traditional plasmonic materials such as gold and silver and its plasmons can be tuned by changing the Fermi energy with chemical or electrical doping.

Metrics details. Light manipulation has drawn great attention in photodetectors towards the specific applications with broadband or spectra-selective enhancement in photo-responsivity or conversion efficiency. In this work, a broadband light regulation was realized in photodetectors with the improved spectra-selective photo-responsivity by the optimally fabricated dielectric microcavity arrays MCAs on the top of devices. Both experimental and theoretical results reveal that the light absorption enhancement in the cavities is responsible for the improved sensitivity in the detectors, which originated from the light confinement of the whispering-gallery-mode WGM resonances and the subsequent photon coupling into active layer through the leaky modes of resonances. In addition, the absorption enhancements in specific wavelength regions were controllably accomplished by manipulating the resonance properties through varying the effective optical length of the cavities. This work well demonstrated that the leaky modes of WGM resonant dielectric cavity arrays can effectively improve the light trapping and thus responsivity in broadband or selective spectra for photodetection and will enable future exploration of their applications in other photoelectric conversion devices.

Nanophotonics silicon solar cells: status and future challenges

Light management plays an important role in high-performance solar cells. Nanostructures that could effectively trap light offer great potential in improving the conversion efficiency of solar cells with much reduced material usage. Developing low-cost and large-scale nanostructures integratable with solar cells, thus, promises new solutions for high efficiency and low-cost solar energy harvesting. In this paper, we review the exciting progress in this field, in particular, in the market, dominating silicon solar cells and pointing out challenges and future trends. As a light-electricity conversion device, light absorption plays a primary role in determining the achievable efficiency of a solar cell.

A solar cell , or photovoltaic cell , is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect , which is a physical and chemical phenomenon. Individual solar cell devices are often the electrical building blocks of photovoltaic modules , known colloquially as solar panels. The common single junction silicon solar cell can produce a maximum open-circuit voltage of approximately 0.

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Seidiucahan 04.05.2021 at 05:58

New Approaches to Light Trapping in Solar Cell Devices discusses in detail the use of photonic and plasmonic effects for light trapping in solar cells.