Light Trapping Scheme Research Articles

Large-Area Nanosphere Gratings for Light Trapping and Reduced Surface Losses in Thin Solar Cells

Light trapping in thin silicon solar cells demands radically different fabrication approaches to standard commercial cells. Weaker optical absorption and increased sensitivity to surface recombination requires light trapping to be achieved over a broader spectral range and, ideally, without texturing the absorber itself. Nano-scale light trapping structures allow the strongest scattering to be tuned to wavelengths, where oblique scattering into the absorber is needed most. Furthermore, applying these structures “externally,” i.e., on a well-passivated planar silicon surface, reduces the surface area and permits optimal electronic conditions to be maintained. Despite these advantages, the challenges of balancing efficiency gain, cost, and lithographic fidelity have prevented the commercial use of nano-scale light trapping schemes. Here, we demonstrate the use of nanosphere lithography for producing high-quality and cost-effective nano-scale light trapping structures suitable for incorporation in thin solar cells. We have successfully fabricated large-area and uniform metal nanospheregrating structures, with embedded dielectric nanospheres, on 30 μ m thick c-Si pseudo cells and measured their effectiveness for light trapping. Comparison between simulations and the fabricated pseudo cells’ characteristics highlighted key challenges in fabricating uniform structures, including the impact of air gaps within non-conformal coatings and minor changes in the geometry. Optical characterization via absorption spectroscopy and both spectral and spatially resolved photoluminescence showed a clear enhancement in the short-circuit current density of up to 4.33 mA/cm2 in comparison with a planar 30 μ m thick device and a 3.7 times absorptance enhancement close to the bandgap of Si.

Toward Efficient Charge Collection and Light Absorption: A Perspective of Light Trapping for Advanced Photoelectrodes

Sun light driven water splitting is a promising method to produce hydrogen in a clean and sustainable way. The photoelectrochemical (PEC) cell is highly desirable among various solar water splitting techniques as it is potentially robust, cost-effective, and durable. However, arguably the biggest problem to date for sustainable photoelectrodes currently under intensive study is that the carrier diffusion length is always much shorter than the optical length, thus seriously limiting the cell’s power conversion efficiency. Light trapping strategies could enable multiple passes of light through the absorber to enhance the absorption while still keeping the absorber thickness below the transport limitation. Herein we briefly review recent developments of light management strategies in PEC electrodes by focusing on the relevant concepts, materials, and structures. In addition, current shortcomings and outlooks will be discussed in the hope that more attention of the research community will be drawn to this pro...

Spectral splitting for an InGaP/GaAs parallel junction solar cell.

In this paper, a design of a diffractive optical element to split the solar spectrum into two separate parts for a laterally arrayed InGaP/GaAs solar cell is presented. Optical simulation is done by using the three-dimensional finite-difference time-domain method and the results are demonstrated to evaluate the optical performance of the designed structure. Anti-reflection coating for the designed splitter is also put forth. In addition to the optical analysis, electrical simulations are performed and current density-voltage and power density-voltage curves are presented in order to explain the electrical performance of the InGaP/GaAs solar cell by implementing the designed spectrum splitter. The results of the electrical simulations show that the designed InGaP/GaAs solar cell's best efficiency is 34.7% under unconcentrated sunlight. Further improvement is feasible if the parameters of the spectral splitting structure are optimized by the incorporation of non-intuitive optimization algorithms. Lastly, light trapping strategies can also be considered to enhance the efficiency of the solar cells.

Ultrathin c-Si solar cells based on microcavity light trapping scheme

Ultrathin c-Si solar cells with a new light trapping scheme, which is a double-microcavity light trapping structure, are optimized and analyzed. The results show that by optimizing the geometric parameters of each light trapping unit, the light trapping structure can effectively improve the light absorption efficiencies of ultrathin crystalline silicon cells with different thicknesses. The thinner the active layer of a crystalline silicon solar cell, the more obvious the effect of the light trapping structure to enhance the light absorption. When the active layer thickness of a crystalline silicon cell is 3 μm, the battery efficiency can reach 16.3193%.

High efficiency light trapping scheme used for ultrathin c-Si solar cells

Ultrathin c-Si solar cells with their active layer thicknesses ranging from a few micrometers to several tens of micrometers will play a very important role in reducing the material costs of industrial c-Si solar cells. This work presents a new high efficiency light trapping scheme (HE-LTS) with two cavities for ultrathin c-Si solar cells. By analyzing the variations of the important optical parameters of each light trapping unit with its geometric parameters, the light trapping performances of the HE-LTS have been optimized. The results show that the light trapping abilities of the HE-LTS can exceed obviously that of Lambertian LTS in a broad range of geometric parameters for both TE and TM modes; the photocurrent densities of an ultrathin c-Si solar cell with the optimal HE-LTS can reach 36.65 mA/cm2 and 40.47 mA/cm2 for the TE and TM modes, respectively.

Multilayer Polypyrrole Nanosheets with Self-Organized Surface Structures for Flexible and Efficient Solar-Thermal Energy Conversion.

Converting solar energy into concentrated heat is very appealing for various applications. Polypyrrole (PPy) is known to possess excellent photothermal property with low thermal conductivity, and thus is an ideal candidate for solar-thermal energy conversion. However, solar-thermal materials based on PPy or other conducting polymers still exhibit limited energy conversion efficiency due to the lack of effective light-trapping schemes. Here, it is demonstrated that multilayer PPy nanosheets with spontaneously formed surface structures such as wrinkles and ridges via sequential polymerization on paper substrates can dramatically enhance broadband and wide-angle light absorption across the full solar spectrum, leading to an impressive solar-thermal conversion efficiency of 95.33%. The intriguing solar-thermal properties and structural features of multilayer PPy nanosheets can be used for solar heating and photoactuators. Meanwhile, when used for solar steam generation, the measured efficiency could achieve ≈92% under one sun irradiation. The hierarchically multilayer structure is mechanically flexible and robust, holding great potential for practical solar energy utilization. This study provides a simple and straightforward approach toward engineering light-weight and thermally insulating polymers into efficient solar-thermal materials for emerging solar energy-related applications.

Asymmetric metasurface structures for light absorption enhancement in thin film silicon solar cell

Thin-film silicon solar cells with light-trapping structures can enhance light absorption within the semiconductor absorber layer. Metasurfaces, consisting of single-layer of planar structures, can be realized inexpensively by means of new nanopatterning techniques. Here we propose an asymmetric metasurface light trapping scheme that enables broadband absorption enhancement within c-Si thin film. Our numerical results demonstrate that the proposed metasurfaces exhibit better light trapping performance than the inverted pyramid that has been widely studied in recent years. Our work illustrates the impressive promise for metasurface structures used in thin film silicon solar cells.

Ergodic Light Propagation in a Half‐Cylinder Photonic Plate for Optimal Absorption in Perovskite Solar Cells

AbstractIn an optical medium slab, a randomization of the interfaces will lead to an ergodic light propagation, implying a kind of light trapping that sets an upper limit for a broadband absorption enhancement factor. However, in thin film solar cells where such light trapping is most needed, it is not straightforward how the implementation of a surface randomization should be done. In addition, such randomization may also severely limit the electronic device performance. Here, a cylindrical periodic corrugation, which is named half‐cylinder photonic plate, is introduced at the light entering interface which leads to an ergodic light propagation. Advantage of such ergodicity is taken to enhance light absorption in a perovskite cell, physically separated by a planar glass substrate from such half‐cylinder photonic plate. Theoretically and experimentally it is demonstrated that the enhancement light absorption factor measured is close to the maximum possible for light trapping schemes using one interface periodic corrugations. This upper limit results from the ergodic light propagation that is achieved with a half‐cylinder photonic plate made from an optical transparent material with a low refractive index that minimizes light rejection by reflectivity.

Suppression of surface and Auger recombination by formation and control of radial junction in silicon microwire solar cells

Black silicon (b-Si) with nanotextures is a promising light-trapping scheme for potentially achieving high conversion efficiency at reduced manufacturing cost in crystalline-silicon solar cells. However, the inherently high aspect-ratio and tiny feature size of the nanostructures are subject to severe surface (large surface areas) and Auger recombination (worse doping profile). These will abate the cost values of b-Si since one has to adopt a comprise strategy of applying shallow nanotextures with antireflection and passivation layers. Here, we show that silicon microwire solar cells featuring well-defined radial junctions can extensively suppress both surface and Auger recombination by providing excellent all-around electrical field. The radially doped silicon micropillar devices even show an internal quantum efficiency as good as that of planar substrate and their measured minority carrier lifetimes become nearly independent of total surface area. A great reduction in short-circuit current density loss was further identified as the junction abruptly changed from a fully diffused to a core-shell configuration, manifesting the powerful effectiveness of radial p-n+ junction on the suppression of Auger recombination. Furthermore, silicon microwire solar cell with a radial junction demonstrates 37% increase in efficiency compared with the reference cell, suggesting a feasible strategy towards high-efficiency solar devices.

Performance Enhancement of Thin Film Solar Cell Using Two-Dimensional Plasmonic Grating in Rear Electrode

An effective scheme is proposed and investigated to improve the performance of thin film solar cells using a two-dimensional (2-D) plasmonic grating structure at the rear electrode or back metal contact. The proposed light trapping scheme replaces the conventional flat rear electrode by a corrugated electrode with a square mesh of periodic grooves forming a 2-D plasmonic grating. The grating surface diffracts the light and increases the path for reflected light. Further, the energy of surface plasmons excited inside the grooves gets coupled to the absorber layer. An efficient light trapping scheme and surface plasmons resonance conjointly increase the absorption in the absorber layer of solar cell, which results as enhanced efficiency. Maximum contribution of light path enhancement and surface plasmons is achieved by optimizing the periodicity, width, and depth of the grooves. The proposed design is compared with the conventional simple solar cell as well as with the solar cell having the rear electrode with 1-D plasmonic grating structure to demonstrate the effectiveness of the scheme. Finally, the optimized design is proposed which exhibits 92% absorption, 43% enhanced light path and more than 200% increase in the short circuit current as well as conversion efficiency.

A leaf-inspired photon management scheme using optically tuned bilayer nanoparticles for ultra-thin and highly efficient photovoltaic devices

We present a leaf-inspired biomimetic omnidirectional photon management scheme for ultrathin flexible graphene silicon Schottky junction solar cell. An all-dielectric approach comprising lossless spheroidal silica and titania nanoparticle bilayers is used for mimicking the two essential light trapping mechanisms of a leaf - focusing and waveguiding, and scattering. The ratio of the nanoparticle diameters of the two optically tuned layers plays a crucial role in confining the incident light through whispering gallery modes and subsequent forward scattering into the substrate via strong leaky channels. The scheme does not employ any nanostructuring of the silicon substrate, thereby preventing the optical gain from being offset by recombination losses, completely decoupling the optical and electrical performances of the device. The light-trapping scheme shows ultralow broadband reflection of only 10.3% and causes a 30% increase in efficiency compared to a bare graphene/silicon solar cell. An efficiency of ~9% is obtained for solar cell with 20 µm thick n-silicon absorber and doped bilayer graphene, resulting in highest (1.89) watt/gram utilization of silicon among all graphene/silicon solar cells. The light-trapping nanoparticle-embellished solar cell retains its characteristics for >103 bending cycles for a bend radius as low as 3 mm, demonstrating its flexibility, durability and reliability.

Light Trapping Effect in Perovskite Solar Cells by the Addition of Ag Nanoparticles, Using Textured Substrates.

In this contribution, the efficiencies of perovskite solar cells have been further enhanced, based on optical optimization studies. The photovoltaic devices with textured perovskite film can be obtained and a power conversion efficiency (PCE) of the textured fluorine-doped tin oxide (FTO)/Ag nanoparticles (NPs) embedded in c-TiO2/m-TiO2/CH3NH3PbI3/Spiro-OMeTAD/Au showed 33.7% enhancement, and a maximum of up to 14.01% was achieved. The efficiency enhancement can be attributed to the light trapping effect caused by the textured FTO and the incorporated Ag NPs, which can enhance scattering to extend the optical pathway in the photoactive layer of the solar cell. Interestingly, aside from enhanced light absorption, the charge transport characteristics of the devices can be improved by optimizing Ag NPs loading levels, which is due to the localized surface plasmon resonance (LSPR) from the incorporated Ag NPs. This light trapping strategy helps to provide an appropriated management for optical optimization of perovskite solar cells.

A Front Surface Optimization Study for Photovoltaic Application

In this paper, we presented a possible front surface optical enhancement of Si solar cell by optimizing the Antireflection (AR) and light trapping (LT) schemes. Conventional plasma enhanced chemical vapor deposition (PECVD) and in house hot wire chemical vapor deposition (HWCVD) tool was used to deposit Silicon Nitride (SiNX) layer and optimized at 668nm wavelength. This was followed by surface texturing of random pyramids to further enhance the broadband reflectance of the front surface. Broadband reflectance measurement using integrating sphere method showed achieved weighted average reflectance (WAR) value of as low as 1.8% and 1.5%, when 85nm SiNX was deposited on top of random pyramids structure using HWCVD and PECVD methods, respectively.

Impact of silicon wafer thickness on photovoltaic performance of crystalline silicon heterojunction solar cells

The impact of Si wafer thickness on the photovoltaic performance of hydrogenated amorphous silicon/crystalline silicon (a-Si:H/c-Si) heterojunction solar cells was examined from the optical and electrical points of view. Optical characterization of c-Si wafers of various thicknesses showed that a realistic light-trapping scheme, i.e., pyramidally textured Si wafers with a dielectric antireflection coating and a back reflector, realizes an efficient quasi-Lambertian light absorption enhancement, even for very thin wafers. This indicates that high photocurrent densities are achievable by using the realistic light-trapping scheme, assuming that the parasitic absorption loss is minimized. The potentials of open-circuit voltage (VOC) and the fill factor (FF) of thin c-Si cells were investigated using thin c-Si wafers passivated with intrinsic/doped amorphous silicon film stacks. It was experimentally confirmed that the implied VOC increases steadily with decreasing wafer thickness down to 30 µm, while the implied FF weakly depends on the thickness. As a result of the trade-off between light absorption and implied VOC, a high implied efficiency is expected for a wide range of wafer thicknesses. The VOC increase by thinning the wafer was also experimentally confirmed in an a-Si:H/c-Si heterojunction cell with a thickness below 60 µm, resulting in a conversion efficiency of 21.0%.

Accurate Calculation of the Absorptance Enhances Efficiency Limit of Crystalline Silicon Solar Cells With Lambertian Light Trapping

The widely accepted limiting efficiency for crystalline silicon solar cells with Lambertian light trapping under 1 sun was previously calculated to be 29.43% for a 110-μm-thick device by using the commonly applied weak absorption approximation for light trapping. However, the short-circuit current density increases by 0.17 mA/cm2 when modeling the optical absorptance of an ideal Lambertian light trapping scheme exactly. The resulting new 1-sun efficiency limit is 29.56% and holds for a cell that is 98.1 μm in thickness.

Progress and limitations of thin-film silicon solar cells

The major issues of thin-film silicon solar cells have been the light-induced metastability of hydrogenated amorphous silicon (a-Si:H) and the weak infrared light absorption of hydrogenated microcrystalline silicon (μc-Si:H). In order to overcome these challenges, we have developed a novel deposition process for a-Si:H and an improved light trapping scheme for μc-Si:H light absorbers, leading to four record independently-confirmed conversion efficiencies with single-junction and multijunction device structures. In this article, we review some technological progresses that led to the notable improvement in thin-film silicon solar cells, and address some limitations imposed by the absorber layer quality as well as the cell design.

Recent progress in silicon-based solid-state solar cells

ABSTRACTSilicon (Si)-based solar cells constitute about 90% of the photovoltaic (PV) market, and a drastic reduction in module cost and significant improvement in PV performance have been observed since its first inception in 1941. This article aims to present the comprehensive review of prominent advancements enacted in Si solar cells after the year 2000. Monocrystalline Si solar cell has been the matured technology with the record efficiency (η) of 26.6% achieved so far. As the drive to push η around 30% Schokley–Quiesser limit is foreseeable in the near future, PV community is actively striving to fabricate efficient yet cost-effective devices. Polycrystalline Si solar cells contain small-sized grains, and efforts are underway to enhance the η beyond 21.9% by controlling the recombination at grain boundaries, optimising the passivated interfaces and deposition process. Thin-film amorphous Si technology proffers low-cost fabrication process and η of 13.6% has been recorded for a multijunction solar cell. Employment of sophisticated nanowire-based light trapping schemes and dopant-free carrier-selective layers along with the development of hybrid solar cells of organic and Si materials are among the emerging research trends for Si solar cells.

Superabsorbing Metasurfaces with Hybrid Ag–Au Nanostructures for Surface‐Enhanced Raman Spectroscopy Sensing of Drugs and Chemicals

AbstractReliability, shelf time, and uniformity are major challenges for most metallic nanostructures for surface‐enhanced Raman spectroscopy (SERS). Due to the randomness of the localized field supported by silver and gold nanopatterns in conventional structures, the quantitative analysis of the target in the practical application of SERS sensing is a challenge. Here, a superabsorbing metasurface with hybrid Ag–Au nanostructures is proposed. A two‐step process of deposition plus subsequent thermal annealing is developed to shrink the gap among the metallic nanoparticles with no top‐down lithography technology involved. Because of the light trapping strategy enabled by the hybrid Ag–Au metasurface structure, the excitation laser energy can be localized at the edges of the nanoparticles more efficiently, resulting in enhanced sensing resolution. Intriguingly, because more hot spots are excited over a given area with higher density of small nanoparticles, the spatial distribution of the localized field is more uniform, resulting in superior performance for potential quantitative sensing of drugs (i.e., cocaine) and chemicals (i.e., molecules with thiol groups in this report). Furthermore, the final coating of the second Au nanoparticle layer improves the reliability of the chip, which is demonstrated effective after 12 month shelf time in an ambient storage environment.

Giant enhancement of photoluminescence and tertiary emission in a chiral nematic by matching photonic band gap and excitation wavelength

We demonstrate that chiral nematic liquid crystal compositions incorporating fluorophores, wherein the photonic band gap (PBG) of the liquid crystal is matched with the excitation wavelength of the fluorophore results in (a) significant enhancement in the overall photoluminescence of the system and (b) eliciting secondary and tertiary emission such that a single low wavelength illumination can give emissions at multiple and highly desirable Red, Green and Blue wavelengths. The light-trapping strategy of matching the PBG with the excitation wavelength instead of the previously attempted protocol of matching with the emission wavelength is shown to be superior in terms of the magnitude of emission and consequently, the quantum yield of the device. Further, we show that by using an applied electric field, three different emission levels can be achieved, two of them stable even when the field is switched off; the magnitude of the field provides a powerful handle to control the emission level with an overall change of a factor of 2.The observed PL features are explained using radiative energy transfer mechanism and the short-range ordering of the fluorophores, as obtained from X-ray diffraction experiments.

On optical-absorption peaks in a nonhomogeneous thin-film solar cell with a two-dimensional periodically corrugated metallic backreflector

The rigorous coupled wave approach (RCWA) was implemented to investigate optical absorption in a triple-p-i-n-junction amorphous-silicon solar cell with a 2D metallic periodically corrugated backreflector (PCBR). Both total and useful absorptances were computed against the free-space wavelength $\lambda_o$ for both s- and p-polarized polarization states. The useful absorptance in each of the three p-i-n junctions was also computed for normal as well as oblique incidence. Furthermore, two canonical boundary-value problems were solved for the prediction of guided-wave modes (GWMs): surface-plasmon-polariton waves and waveguide modes. Use of the doubly periodic PCBR enhanced both useful and total absorptances in comparison to a planar backreflector. The predicted GWMs were correlated with the peaks of the total and useful absorptances. The excitation of GWMs was mostly confined to $\lambda_o < 700$ nm and enhanced absorption. As excitation of certain GWMs could be correlated with the total absorptance but not with the useful absorptance, the useful absorptance should be studied while devising light-trapping strategies.