Trapping In Thin-film Solar Cells Research Articles

Titanium Nitride as an Alternative Plasmonic Material for Plasmonic Enhancement in Organic Photovoltaics

This paper investigates TiN for its potential to enhance light-harvesting efficiency as an alternative material to Au for nanoscale plasmonic light trapping in thin-film solar cells. Using nanosphere lithography (NSL), plasmonic arrays of both Au and TiN are fabricated and characterized. Later, the fabricated TiN and Au arrays are integrated into a thin-film organic photovoltaic (OPV) device with a PBDB-T:ITIC-M bulk heterojunction (BHJ) active layer. A comparative study between these Au and TiN nanostructured arrays evaluates their fabrication process and plasmonic response, highlighting the advantages and disadvantages of TiN compared to a conventional plasmonic material such as Au. The effect of the fabricated arrays when integrated into an OPV is presented and compared to understand the viability of TiN. As one of the first experimental studies utilizing TiN arrays for the plasmonic enhancement of photovoltaics, the results offer valuable insight that can guide future applications and decisions in design.

Developing an optimized metasurface for light trapping in thin-film solar cells using a deep neural network and a genetic algorithm

In this paper, using a deep neural network and a genetic algorithm, an optimized digital metasurface is designed to trap sunlight in thin-film solar cells. The deep neural network is trained using full-wave numerical simulation results as the training dataset, and it is designed to predict the electromagnetic response of thin-film solar cells whose active layers are shaped as a digital metasurface. The developed neural network can predict the results much faster than full-wave solvers and therefore can be used for optimization purposes. Using the results generated by the trained neural network, an evolutionary procedure based on the genetic algorithm is developed to find the optimum structure for the digital metasurface, which provides the highest short circuit current inside the thin-film solar cell. The performance of the resultant optimum design is validated using full-wave numerical simulation illustrating a short circuit current of 15.39 m A / c m 2 and 13.30 m A / c m 2 for TE and TM polarization of the incident light, respectively. The resultant short circuit current is 2.47 and 2.13 times higher than a simple thin-film solar cell with the same amount of silicon inside, for TE and TM polarization of the incident light, respectively. To have a more comprehensive comparison, the designed optimum structure is compared with several standard shapes for the metasurface, such as star and plus sign. This comparison showed that the optimum structure provides a short circuit current which is much higher than the current achieved by standard shapes.

Complex trapezoid grating for light trapping in thin-film solar cells: super-fine structure

The research of the optimal surface structure has attracted considerable interest because of its potential application in light trapping in thin-film solar cells (TFSCs). In this paper, a super-fine structure named complex trapezoid grating is proposed to improve the optical absorption comparing to the conventional simple trapezoid grating in a-Si:H TFSCs. The numerical calculation by utilizing rigorous coupled-wave analysis (RCWA) is conducted to obtain the optical absorption of the structured surface. The results demonstrate that, compared to a planar slab, the optimized-simple trapezoid grating shows 97% enhancement of power conversion efficiency η while the complex trapezoid grating shows 131% enhancement. Obviously, the complex trapezoid grating exhibits a better performance than the simple grating, which is due to the perfect antireflective effect and microcavity resonance effect. The angular response of the optical absorption in a-Si:H TFSCs was also investigated. The results further indicate that it is a better way to select the complex trapezoid grating in improving the optical absorption of silicon-based TFSCs.

Design and analysis of light trapping in thin-film gallium arsenide solar cells using an efficient hybrid nanostructure

Light trapping in thin-film solar cells is important for improving efficiency and reducing cost. We propose a hybrid nanostructure based on the anodic aluminum oxide grating and Si 3 N 4 double-layer antireflection coatings combined with Ag nanoparticles to achieve advanced light trapping property in gallium arsenide (GaAs) solar cells with 500-nm thickness. The finite-element method is used to study the relationship between geometrical parameters of hybrid nanostructure and optical characteristics of thin-film GaAs solar cells. The light trapping ability is systematically studied by COMSOL multiphysics. The simulation results show that the hybrid nanostructure can highly increase the light absorption in the wavelengths from 300 to 860 nm. The average absorption in 500-nm-thick GaAs layer is 96.7%. The short circuit current density in 500-nm-thick GaAs layer is 30.2 mA / cm 2 , representing a 58.9% enhancement compared with that ( 19.0 mA / cm 2 ) of the reference cell. Research paves the way for designing highly efficient light trapping structures in thin-film GaAs solar cells.

Gold nanoparticles created by rapid thermal annealing process applied to thin film solar cell

In this work, the formation of gold nanoparticles (Au NPs) was studied. A gold thin film was first deposited by E-beam Evaporation method followed by rapid thermal annealing (RTA) to create nanoparticles with many different sizes and shapes. The localized surface plasmon resonance (LSPR) effect was also determined. By proper engineering of these metallo-dielectric structures, light can be concentrated and “folded” into a thin semiconductor layer, thereby the absorption can be increased. This approach provides a new development for improving light trapping in thin-film solar cells (TFSCs).

Light trapping in thin-film solar cells measured by Raman spectroscopy

In this study, Raman spectroscopy is used as a tool to determine the light-trapping capability of textured ZnO front electrodes implemented in microcrystalline silicon (μc-Si:H) solar cells. Microcrystalline silicon films deposited on superstrates of various roughnesses are characterized by Raman micro-spectroscopy at excitation wavelengths of 442 nm, 514 nm, 633 nm, and 785 nm, respectively. The way to measure quantitatively and with a high level of reproducibility the Raman intensity is described in details. By varying the superstrate texture and with it the light trapping in the μc-Si:H absorber layer, we find significant differences in the absolute Raman intensity measured in the near infrared wavelength region (where light trapping is relevant). A good agreement between the absolute Raman intensity and the external quantum efficiency of the μc-Si:H solar cells is obtained, demonstrating the validity of the introduced method. Applications to thin-film solar cells, in general, and other optoelectronic devices are discussed.

Enhancing the driving field for plasmonic nanoparticles in thin-film solar cells.

The scattering cross-section of a plasmonic nanoparticle is proportional to the intensity of the electric field that drives the plasmon resonance. In this work we determine the driving field pattern throughout a complete thin-film silicon solar cell. Our simulations reveal that by tuning of the thicknesses of silicon and transparent conductive oxide layers the driving field intensity experienced by an embedded plasmonic nanoparticle can be enhanced up to a factor of 14. This new insight opens the route towards more efficient plasmonic light trapping in thin-film solar cells.

Disorder improves nanophotonic light trapping in thin-film solar cells

We present a systematic experimental study on the impact of disorder in advanced nanophotonic light-trapping concepts of thin-film solar cells. Thin-film solar cells made of hydrogenated amorphous silicon were prepared on imprint-textured glass superstrates. For periodically textured superstrates of periods below 500 nm, the nanophotonic light-trapping effect is already superior to state-of-the-art randomly textured front contacts. The nanophotonic light-trapping effect can be associated to light coupling to leaky waveguide modes causing resonances in the external quantum efficiency of only a few nanometer widths for wavelengths longer than 500 nm. With increasing disorder of the nanotextured front contact, these resonances broaden and their relative altitude decreases. Moreover, overall the external quantum efficiency, i.e., the light-trapping effect, increases incrementally with increasing disorder. Thereby, our study is a systematic experimental proof that disorder is conceptually an advantage for nanophotonic light-trapping concepts employing grating couplers in thin-film solar cells. The result is relevant for the large field of research on nanophotonic light trapping in thin-film solar cells which currently investigates and prototypes a number of new concepts including disordered periodic and quasi periodic textures.

Light management through guided-mode resonances in thin-film silicon solar cells

We theoretically explain and experimentally demonstrate light trapping in thin-film solar cells through guided-mode resonance (GMR) effects. Resonant field enhancement and propagation path elongation lead to enhanced solar absorption. We fabricate nanopatterned solar cells containing embedded 300-nm period, one-dimensional gratings. The grating pattern is fabricated on a glass substrate using laser interference lithography followed by a transparent conducting oxide coating as a top contact. A ∼320-nm thick p-i-n hydrogenated amorphous silicon solar cell is deposited over the patterned substrate followed by bottom contact deposition. We measure optical and electrical properties of the resonant solar cells. Compared to a planar reference solar cell, around 35% integrated absorption enhancement is observed over the 450 to 750-nm wavelength range. This light-management method results in enhanced short-circuit current density of 14.8 mA/cm 2 , which is a ∼40% improvement over planar solar cells. Our experimental demonstration proves the potential of simple and well-designed GMR features in thin-film solar cells.

ITO distributed Bragg reflectors fabricated at low temperature for light-trapping in thin-film solar cells

Abstract A high performance distributed Bragg reflector (DBR) fabricated using the single indium tin oxide (ITO) material is proposed for light trapping in thin-film solar cells. The large index contrast of the ITO DBRs was obtained by depositing alternating layers of dense ITO films and porous ITO films. The high refractive index of the dense ITO films was achieved by long-throw radio-frequency magnetron sputtering technique at room temperature. On the other hand, the porous ITO films with low refractive index were formed by applying supercritical CO 2 (SCCO 2 ) treatment at 60 °C on gel-coated ITO thin films. Porous structures were obtained with the SCCO 2 treatment to volatilize or extract organic matters from the films. The index contrast of the ITO bilayers was higher than 0.5 at blue and green spectral ranges. In addition, small deviations on the optical thickness of the ITO bilayers were observed during the DBR stacking processes. For the DBR comprising 4 periods ITO bilayers, the reflectance and sheet resistance of 87.9% and 35 Ω/□ were achieved at 426 nm.

Light trapping in thin-film solar cells with randomly rough and hybrid textures

We study light-trapping in thin-film silicon solar cells with rough interfaces. We consider solar cells made of different materials (c-Si and μc-Si) to investigate the role of size and nature (direct/indirect) of the energy band gap in light trapping. By means of rigorous calculations we demonstrate that the Lambertian Limit of absorption can be obtained in a structure with an optimized rough interface. We gain insight into the light trapping mechanisms by analysing the optical properties of rough interfaces in terms of Angular Intensity Distribution (AID) and haze. Finally, we show the benefits of merging ordered and disordered photonic structures for light trapping by studying a hybrid interface, which is a combination of a rough interface and a diffraction grating. This approach gives a significant absorption enhancement for a roughness with a modest size of spatial features, assuring good electrical properties of the interface. All the structures presented in this work are compatible with present-day technologies, giving recent progress in fabrication of thin monocrystalline silicon films and nanoimprint lithography.

Light trapping in thin-film solar cells via scattering by nanostructured antireflection coatings

The use of nanostructured TiO2 layers fabricated on thin-film solar cells to provide, simultaneously, both antireflection functionality and light trapping via scattering of long-wavelength photons into guided optical modes is demonstrated and analyzed in thin-film quantum-well solar cells. Nanosphere lithography is used for fabrication of periodic arrays of subwavelength-scale TiO2 structures, and separation of active device layers from their epitaxial growth substrate and integration with the nanostructured TiO2 layer enables increased optical absorption via coupling to both Fabry-Perot resonances and guided lateral propagation modes in the semiconductor. The nanostructured TiO2 layer is shown to act as a graded-index coating at optical wavelengths and simultaneously to scatter incident light into guided optical modes within the device. The dependence of these effects on angle of incidence is also analyzed.

Dual gratings for enhanced light trapping in thin-film solar cells by a layer-transfer technique

Thin film solar cells benefit significantly from the enhanced light trapping offered by photonic nanostructures. The thin film is typically patterned on one side only due to technological constraints. The ability to independently pattern both sides of the thin film increases the degrees of freedom available to the designer, as different functions can be combined, such as the reduction of surface reflection and the excitation of quasiguided modes for enhanced light absorption. Here, we demonstrate a technique based on simple layer transfer that allows us to independently pattern both sides of the thin film leading to enhanced light trapping. We used a 400 nm thin film of amorphous hydrogenated silicon and two simple 2D gratings for this proof-of-principle demonstration. Since the technique imposes no restrictions on the design parameters, any type of structure can be made.

Back Cover: Solar cell efficiency enhancement via light trapping in printable resonant dielectric nanosphere arrays (Phys. Status Solidi A 2/2013)

Resonant dielectric structures are a promising platform for addressing the key challenge of light trapping in thin-film solar cells. Grandidier et al. (pp. 255–260) experimentally and theoretically demonstrate efficiency enhancements in solar cells from dielectric nanosphere arrays. Two distinct amorphous silicon photovoltaic architectures were improved using this versatile light-trapping platform. In one structure, the colloidal monolayer couples light into the absorber in the near-field acting as a photonic crystal light-trapping element. In the other, it acts in the far-field as a graded index antireflection coating to further improve a cell which already included a state-of-the-art random light-trapping texture to achieve a conversion efficiency over 11%.

Engineering Gaussian disorder at rough interfaces for light trapping in thin-film solar cells

A theoretical study of randomly rough interfaces to obtain light trapping in thin-film silicon solar cells is presented. Roughness is modeled as a surface with Gaussian disorder, described using the root mean square of height and the lateral correlation length as statistical parameters. The model is shown to describe commonly used rough substrates. Rigorous calculations, with short-circuit current density as a figure of merit, lead to an optimization of disorder parameters and to a significant absorption enhancement. The understanding and optimization of disorder is believed to be of general interest for various realizations of thin-film solar cells.

The importance of light trapping in thin-film solar cells

For solar energy to provide a significant contribution to the energy needs of our society, the development of more efficient and less expensive photovoltaic cells is critical. Producing energy on a global scale—the so-called terawatt challenge—makes it essential to use a minimum amount of rare or costly photoactive elements. Photovoltaic cells based on thin semiconductor films are well placed to meet these criteria and have the potential to replace the dominant photovoltaic technology based on expensive bulk silicon wafers. To do this, their efficiency needs to be increased beyond current values, and the required thickness of the semiconductor film should be minimized as much as possible. However, ultra-thin semiconductor layers are less effective at absorbing sunlight, unless smart light-trapping techniques can be designed and implemented. Such techniques aim at increasing the optical path of light in the semiconductor, which enhances the absorption efficiency for the same material thickness. The ultimate limit to light trapping in thick semiconductor layers was determined by Yablonovitch and Cody1, 2 for the case of weak absorption, and was later generalized by Green3 to the case of arbitrary absorption. It is based on the concept of a Lambertian scatterer, i.e., a rough interface that randomizes the direction of propagation of incoming light when it enters the sample. This limit to light trapping is called the Lambertian limit. Such treatments assume a ray-optics approach and are rigorously valid only when the film thickness is much larger than the wavelength of visible light, which amounts to a few hundreds of nanometers. When the film thickness is less than this, a wave-optics approach becomes necessary and the light-trapping limit to absorption is not known. While it is generally agreed that wavelength-scale or nanophotonic structures should be employed, the optimal geometry for such structures is yet to be determined. A point of debate is whether ordered structures, Figure 1. Short-circuit current density (Jsc) under air mass 1.5 solar spectrum as a function of thickness for materials with (indirect bandgap) crystalline silicon (c-Si), and (direct bandgap) amorphous silicon (a-Si), cadmium telluride (CdTe), and copper indium gallium diselenide (CIGS). Solid lines refer to the single-pass case, while dashed lines indicate the Lambertian light-trapping limit.

Optimal design of light trapping in thin-film solar cells enhanced with graded SiN_x and SiO_xN_y structure

In this paper, a graded SiNx and SiOxNy structure is proposed as antireflection coatings deposited on top of amorphous silicon (α-Si) thin-film solar cell. The structural parameters are optimized by differential evolution in order to enhance the optical absorption of solar cells to the greatest degree. The optimal design result demonstrates that the nonlinear profile of dielectric constant is superior to the linear profile, and discrete multilayer graded antireflection coatings can outperform near continuously graded antireflection coatings. What's more, the electric field intensity distributions clearly demonstrate the proposed graded SiNx and SiOxNy structure can remarkably increase the magnitude of electric field of a-Si:H layer and hence, enhance the light trapping of a-Si:H thin-film solar cells in the whole visible and near-infrared spectrum. Finally, we have compared the optical absorption enhancements of proposed graded SiNx and SiOxNy structure with nanoparticles structure, and demonstrated that it can result in higher enhancements compared to the dielectric SiC and TiO2 nanoparticles. We have shown that the optimal graded SiNx and SiOxNy structure optimized by differential evolution can reach 33.31% enhancement which has exceeded the ideal limit of 32% of nanoparticles structure including plasmonic Ag nanoparticles, dielectric SiC and TiO2 nanoparticles.

Nano-patterned glass superstrates with different aspect ratios for enhanced light harvesting in a-Si:H thin film solar cells

Nano-patterned glass superstrates obtained via a large-area production approach are desirable for antireflection and light trapping in thin-film solar cells. The tapered nanostructures allow a graded refractive index profile between the glass and material interfaces, leading to suppressed surface reflection and increased forward diffraction of light. In this work, we investigate nanostructured glass patterns with different aspect ratios using scalable nanosphere lithography for hydrogenated amorphous silicon (a-Si:H) thin film solar cells. Compared to flat glass cell and Asahi U-type glass cell, enhancements in short-circuit current density (J(sc)) of 51.6% and 8%, respectively, were achieved for a moderate aspect ratio of 0.16. The measured external quantum efficiencies (EQE) spectra confirmed a broadband enhancement due to antireflection and light trapping properties.

Light trapping in amorphous silicon solar cells with periodic grating structures

We report on the design of amorphous silicon solar cells with the periodic grating structures. It is a combination of an anti-reflection structure and the metallic reflection grating. Optical coupling and light trapping in thin-film solar cells are studied numerically using the Rigorous Coupled Wave Analysis enhanced by the Modal Transmission Line theory. The impact of the structure parameters of the gratings is investigated. The results revealed that within the incident angles of − 40 ° to + 40 ° the reflectivity of the cell with a period of 0.5 μm, a filling factor of 0.1 and a groove depth of 0.4 μm is 4%–22.7% in the wavelength range of 0.3–0.6 μm and 1%–20.8% in the wavelength range of 0.6–0.84 μm, the absorption enhancement of the a-Si layer is 0.4%–10.8% and 20%–385%, respectively.

Coherent light trapping in thin-film photovoltaics

Abstract