Finite-difference Time-domain Simulations Research Articles

Sponge-shaped Au nanoparticles: a stand-alone metallic photocatalyst for driving the light-induced CO2 reduction reaction

Coinage metal nanoparticles (NPs) enable plasmonic catalysis by generating hot carriers that drive chemical reactions. Making NPs porous enhances the adsorption of reactant molecules. We present a dewetting and dealloying strategy to fabricate porous gold nanoparticles (Au-Sponge) and compare their CO2 photoreduction activity with respect to the conventional gold nanoisland (Au-Island) morphology. Porous gold nanoparticles exhibit an unusually broad and red-shifted plasmon resonance which is in agreement with the results of finite difference time domain (FDTD) simulations. The key insight of this work is that the multi-step reduction of CO2 driven by short-lived hot carriers generated by the d → s interband transition proceeds extremely quickly as evidenced by the generation of methane. A 3.8-fold enhancement in the photocatalytic performance is observed for the Au-Sponge in comparison to the Au-Island. Electrochemical cyclic voltammetry measurements confirm the 2.5-fold increase in the surface area and roughness factor of the Au-Sponge sample due to its porous nature. Our results indicate that the product yield is limited by the amount of surface adsorbates i.e. reactant-limited. Isotope-labeled mass spectrometry using 13CO2 was used to confirm that the reaction product (13CH4) originated from CO2 photoreduction. We present the plasmon-mediated photocatalytic transformation of 4-aminothiophenol (PATP) into p,p'-dimercaptoazobenzene (DMAB) using Au-Sponge and Au-Island samples.

Ultra-dense Green InGaN/GaN Nanoscale Pixels with High Luminescence Stability and Uniformity for Near-Eye Displays.

Ultra-dense (>4,000 pixels per inch) and highly stable full-color III-nitride nanoscale pixels are crucial for near-eye display technologies like virtual and augmented-reality glasses. In this context, InGaN-based long wavelength green microscale light-emitting diodes face major bottlenecks, such as low efficiency and inadequate wavelength stability. These challenges are associated with the presence of both nonradiative surface defects and the strain induced quantum-confined Stark effect. Herein, we report nanoscale pixelation of green InGaN/GaN LEDs incorporating strain-engineered ultra-dense nanowire (NW) arrays, corresponding to ∼36,000 pixels per inch. The NW pixel arrays exhibit a stable peak wavelength emission at ∼500 nm for over 3 orders of magnitude of injection current densities (from ∼4 A/cm2 to ∼1 kA/cm2). The observed wavelength stability enhancement (a reduced blue-shift of just ∼4 nm) directly results from the suppressed built-in electric field achieved by strain relaxation of the axial multi quantum wells in the NWs. Finite difference time domain simulations show that emission of the pixel array is significantly increased owing to the enhanced spontaneous emission rate (characterized by a high Purcell factor of ≈2) of the ultra-dense NWs. We have demonstrated top-down NWs, where each NW (diameter ranging down to 200 nm) shows excellent uniformity and light output characteristics in direct contrast to bottom-up grown NW heterostructures. The results of this study establish a viable route for realizing nanoscale pixels with high luminescence stability and wafer-scale uniformity with high (>20%) indium composition InGaN/GaN LED heterostructures, for next-generation near-eye displays.

Simulation-Guided Preparation of Copper Chalcogenide Nanoparticle-Based Transparent Photothermal Coating with Enhanced Deicing Performance.

In this investigation, transparent photothermal coatings utilizing plasmonic copper chalcogenide (Cu2-xS) nanoparticles were designed and fabricated for the deicing of glass surfaces. Cu2-xS nanoparticles, chosen for their high near-infrared (NIR) absorption and efficient photothermal conversion, were analyzed via finite difference time domain (FDTD) simulations to optimize nanoparticle morphology, thus avoiding costly trial-and-error synthesis. FDTD simulations determined that Cu2-xS nanorods (Cu-NRs) with an optimal aspect ratio of 2.2 had superior NIR absorption. Guided by FDTD simulations, the composite coating composed of Cu-NRs in clear acrylic resin paint was brush-coated to glass, achieving 62.4% visual transmittance and over 95% NIR absorbance. Photothermal conversion tests exhibited a significant temperature increase, with the coating reaching 65 °C under NIR irradiation within 6 min. The dynamic deicing process of ice beads on the coating at -20 °C completed within 220s, in contrast to the frozen state on glass coated with clear acrylic resin paint. Furthermore, heat transfer simulations in COMSOL illustrated melting initiation at the ice-coating interface and subsequent progression through the ice layer. This simulation-driven synthesis method and photothermal testing offer a design framework for the fabrication of photothermal deicing coatings with applications for automobiles, buildings, and aircraft in cold environments.

Silk Fabric Functionalized by Nanosilver Enabling the Wearable Sensing for Biomechanics and Biomolecules.

Integrating biomechanical and biomolecular sensing mechanisms into wearable devices is a formidable challenge and key to acquiring personalized health management. To address this, we have developed an innovative multifunctional sensor enabled by plasma functionalized silk fabric, which possesses multimodal sensing capabilities for biomechanics and biomolecules. A seed-mediated in situ growth method was employed to coat silver nanoparticles (AgNPs) onto silk fibers, resulting in silk fibers functionalized with AgNPs (SFs@Ag) that exhibit both piezoresistive response and localized surface plasmon resonance effects. The SFs@Ag membrane enables accurate detection of mechanical pressure and specific biomolecules during wearable sensing, offering a versatile solution for comprehensive personalized health monitoring. Additionally, a machine learning algorithm has been established to specifically recognize muscle strain signals, potentially extending to the diagnosis and monitoring of neuromuscular disorders such as amyotrophic lateral sclerosis (ALS). Unlike electromyography, which detects large muscles in clinical medicine, sensing data for tiny muscles enhance our understanding of muscle coordination using the SFs@Ag sensor. This detection model provides feasibility for the early detection and prevention of neuromuscular diseases. Beyond muscle stress and strain sensing, biomolecular detection is a critical addition to achieving effective health management. In this study, we developed highly sensitive surface-enhanced Raman scattering (SERS) detection for wearable health monitoring. Finite-difference time-domain numerical simulations ware utilized to analyze the efficacy of the SFs@Ag sensor for wearable SERS sensing of biomolecules. Based on the specific SERS spectra, automatic extraction of signals of sweat molecules was also achieved. In summary, the SFs@Ag sensor bridges the gap between biomechanical and biomolecular sensing in wearable applications, providing significant value for personalized health management.

Ag@ZIF-8 core-shell nanocomposites as sensitive and stable SERS substrates: Synthesis, optimization, and application in pollutant detection

Although surface-enhanced Raman scattering (SERS) is a powerful spectral analysis technique, its practical application is restricted due to the poor chemical stability of some noble metal nanoparticles. In this work, Ag@ZIF-8 nanocomposites with different ZIF-8 core-shell thicknesses were prepared by coating AgNP surfaces with polyvinylpyrrolidone and adjusting the dosage of zinc nitrate and 2-methylimidazole. Several characterizations have confirmed that the Ag@ZIF-8 nanocomposites was successfully prepared. Based on transmission electron microscopy images and SERS spectral intensity, it was determined that Ag@ZIF-8 nanocomposites with a ZIF-8 shell thickness of 30 nm exhibited optimal SERS performance. As a result of evaluating the uniformity and stability of the composite with 4-ATP as the probe molecule, it showed a relative standard deviation of 6.91 % and a decrease in SERS intensity of 4.5 % after 28 days. Based on the results obtained with Ag@ZIF-8 nanocomposites as the SERS substrate, the detection limits of 4-mercaptopyridine and thiram were as low as 5.87×10−9 M and 6.61×10−8 M, respectively. Furthermore, the finite-difference time-domain simulation reveals an enhancement factor of 2.6×106, which is in good agreement with the experimental value of 1.8×106, confirming its effectiveness as a SERS substrate.

Triple plasmon induced transparency based on multilayer graphene metamaterials

In this study, a novel multilayer terahertz metamaterial with four graphene sub-structures is designed to achieve dynamically tunable triple plasmon-induced transparency (PIT) due to interference among bright-bright modes. Coupled mode theory (CMT) and finite-difference time-domain (FDTD) simulations show a high degree of consistency in their results. Interestingly, enhanced transmission dips are observed when various graphene structures couple with the two longitudinal graphene strips at the bottom layer. Based on the dynamic modulation characteristics of graphene, the metamaterial demonstrates seven-band optical switching capabilities, achieving modulation depths (MD) of 94.6%, 88.1%, 90.8%, 90.8%, 90.4%, 86.6%, and 87.5%. Additionally, the proposed graphene metamaterial also shows excellent slow-light effects, with a group index reaching up to 1034. This proposed terahertz metamaterial holds significant theoretical implications for the development of dynamically integrated terahertz optoelectronic devices.

Purcell gain equalized zero-mode waveguide

We refresh the design of zero-mode waveguides (ZMWs) by introducing metamaterials that makes the zeroth order resonant mode existence. Of particular importance, the resulting electromagnetic field exhibits nearly constant distribution but not a trivial solution of Maxwell’s equation, showing great advantage to equalize the excitation rate of molecules throughout the waveguides. A closed form expression for the wave impedance is derived which is verified by the finite-difference time-domain simulations. Benefitted from the cavity Purcell effect which is lacking in existing ZMWs, fluorescence amplification and lifetime reduction are simultaneously enhanced. A practical design where the excitation volume reduced down to sub-zeptoliter and the fluorescence lifetime shortened to picosecond scale is illustrated. This result makes single molecule real time (SMRT) sensing of biochemical reactions at biophysically relevant concentrations (~ μM) possible, combining off-the-shelf ultrafast lasers.

A triple plasmon-induced transparency terahertz sensor based on graphene metamaterials

A metamaterial-based terahertz sensor design is presented in this paper. The metamaterial unit structure comprises a graphene cross and three sets of graphene strips, which collectively achieve a triple plasmon-induced transparency (PIT) in both polarization directions. The theoretical results based on the Lorentz resonance theoretical model are in good agreement with finite difference time domain (FDTD) simulations. The sensor is polarization-independent and changes in the angle of incidence between 0° and 10° have a negligible impact on the sensing performance. Furthermore, the sensor exhibits a notable slow-light effect, with a maximum sensitivity of 1.65 THz/RIU and a FOM value of 4.74/RIU for the triple PIT, which is superior to that of comparable devices.

Plasmonic perovskite photodetector with high photocurrent and low dark current mediated by Au NR/PEIE hybrid layer

Hybrid organic-inorganic perovskite photodetectors have gained significant attention due to their superior potential for optoelectronic applications, offering various advantages such as low-cost processing, high charge carrier mobility, and lightweight properties. However, these perovskite photodetectors exhibit relatively low absorption in the near-infrared (NIR) range, which limits their potential applications. Here, to address this challenge, the integration of gold nanorods (Au NR) utilizing localized surface plasmon resonance (LSPR) effects in the NIR range has been developed, leading to enhanced light absorption in the active region and higher photocurrent generation. Additionally, ∼7.9 nm of thin polyethyleneimine ethoxylated (PEIE) interlayers were incorporated into the Au NR photodetectors, suppressing dark current by blocking charge injection. As a result, the synergistic effect of the Au NR/PEIE hybrid layer has led to a high-performance photodetector with a responsivity of 0.360 A/W and a detectivity of 1.81 × 1010 Jones, demonstrating a noticeable enhancement compared to the control device. Finite-difference time-domain (FDTD) simulations, morphological characterizations, and photoluminescence studies further support the mechanism for enhancing the performance of the device. We believe that our plasmon-enhanced protocol holds strong potential as a promising platform for perovskite optoelectronic devices.

In silico full-angle high-dynamic range scattering of microscopic objects exploiting holotomography.

Accurate optical characterization of microscopic objects is crucial in academic research, product development, and clinical diagnosis. We present a method for obtaining full and high-dynamic range, angle-resolved light scattering attributes of microparticles, enabling distinction of variations in both overall morphology and detailed internal structures. This method overcomes previous limitations in observable scattering angles and dynamic range of signals through computationally assisted three-dimensional holotomography. This advancement is significant for particles spanning tens of wavelengths, such as human erythrocytes, which have historically posed measurement challenges due to faint side-scattering signals indicative of their complex interiors. Our technique addresses three key challenges in optical side-scattering analysis: limited observational angular range, reliance on simplified computational models, and low signal-to-noise ratios in both experimental and computational evaluations. We incorporate three-dimensional tomographic complex refractive index data from Fourier-transform light scattering into a tailored finite-difference time-domain simulation space. This approach facilitates precise near-to-far-field transformations. The process yields complete full-angle scattering phase functions, crucial for particles like Plasmodium falciparum-parasitized erythrocytes, predominantly involved in forward scattering. The resultant scattering data exhibit an extreme dynamic range exceeding 100 dB at various incident angles of a He-Ne laser. These findings have the potential to develop point-of-care, cost-effective, and rapid malaria diagnostic tools, inspiring further clinical and research applications in microparticle scattering.

Study of hybrid nanoscatterer for enhancing light efficiency of quantum dot-converted light-emitting diodes

Optical scatterer additives play a critical role in enhancing the light conversion efficiency and uniformity of quantum dot-converted light-emitting diodes (Qc-LEDs). This study investigates the impact of optical characteristics and morphology of nanoscatterer on the light conversion and extraction efficiency of Qc-LEDs. Various metal oxides and boron nitride nanoparticles and nanoplates with different diffraction index were selected to carry out the study. Finite-Difference Time-Domain (FDTD) simulation was employed to evaluate the scattering effect of various nanosphere and nanoplate scatterers on the optical performance of the Qc-LEDs. The simulation results revealed that the hybrid nanoscatterer integrates the forward-scattering from the nanosphere and backward-scattering of blue light from the nanoplate. Spectral analysis was conducted to examine the optical performance of Qc-LEDs with varying combinations and concentrations of nanoscatterers. TiO2 nanoparticles and Al2O3 nanoplates were found to be the best combination for maximal light conversion and extraction efficiency within Qc-LEDs. The results indicate an optimal light efficiency is obtained with the optimal ratio of 1:2:2 for quantum dots, TiO2 nanoparticles, and Al2O3 nanoplates. These findings reveal the relationship between optical properties, morphology, and light conversion efficiency in Qc-LEDs, highlighting the advantages of the hybrid nanoscatterers for improving optical performance of Qc-LEDs.

Femtosecond Terahertz Pulse Propagation Measurements and Simulations of Dewetting Kinetics in Real Time.

We report a terahertz time-domain study of dewetting kinetics on two time scales: femtoseconds and in real time (on the order of minutes). We recorded full electric field terahertz time domain signals with femtosecond time resolution during dewetting of water in cellulose. The femtosecond time-domain signals were analyzed with respect to the amplitude and signal emission times and how these two quantities changed over the course of dewetting kinetics. The femtosecond time-domain signals were modeled by a combination of finite-difference time-domain electromagnetic simulations of linear terahertz pulse propagation and an effective medium description of the dielectric permittivity. A logistic regression mechanism was incorporated into the electrodynamics simulations to account for the observed kinetics. In addition, analysis of the area-normalized, real-time time-dependent terahertz spectra was used to identify broad regions in the terahertz spectral range that correlated with the kinetic process. Real-time two-dimensional terahertz correlation maps were used to identify the pure rotational spectrum of water vapor, thus characterizing the evaporation part of the dewetting kinetics problem. We conclude with a kinetic model that accounts for our observations through a microscopic mechanism involving the interconversion among bulk water, water clusters, and individual water molecules in the gas phase. Overall, the approach presented here illustrates an application of femtosecond time-resolved experiments in the terahertz spectral range to the study of kinetic processes.

Design of an on-chip germanium cavity for room-temperature infrared lasing

Germanium (Ge) is one of the most promising material platforms to enable the realization of monolithically integrated laser on silicon because it is a group-IV material with a pseudo-direct-band structure that can be converted into direct-bandgap either through the application of tensile strain or via the tin (Sn) incorporation in Ge. The bandgap modification enhances the light emission efficiency of Ge, where lasing can also be observed if a suitable cavity preserving the strain can be realized. In fact, several different research groups have reported lasing from strained Ge and GeSn optical cavities, however they all report lasing at low temperatures and room-temperature lasing, which is the ultimate goal required for a fully integrated laser, has not been demonstrated yet. In this work, we design an on-chip germanium cavity that has all the ingredients combined to make the room-temperature lasing possible. The design includes a 4.6% uniaxially tensile strained Ge gain medium embedded in a Fabry-Perot like cavity composed of two distributed Bragg reflectors. 3-dimensional (3D) Finite Element Method (FEM) based strain simulations together with a proposed fabrication methodology provides a guideline for the realization of the structure. Furthermore, 3D Finite Difference Time Domain (FDTD) simulations demonstrate that the designed structure is suitable for the room-temperature lasing in a wavelength range of 2410–2570 nm. 3D FEM-based heat transfer simulations performed for the designed cavity verifies the eligibility of the room-temperature operation paving the way for a possible demonstration of on-chip laser that could take part in the fully integrated infrared systems for a variety of applications including biological and chemical sensing, as well as security such as alarm systems and free-space optical communications.

Plasmon-mediated photocatalytic conversion in Au or Ag nanorod aggregates by surface-enhanced Raman spectroscopy

Plasmonic nanoparticles (NPs) hold considerable potential as photocatalysts owing to their robust light–matter interactions across diverse electromagnetic wavelengths, which significantly influence the photophysical characteristics of the adjacent molecular entities. Despite the widespread use of noble-metal NPs in surface-enhanced Raman scattering (SERS) applications, little is known about the kinetics of nanoparticle aggregation and how it affects their configurations. This study investigates the plasmon-driven photochemical conversion of 4-nitrobenzenethiol (NBT) to 4,4′-dimercaptoazobenzene (DMAB) on Au and Ag nanorods (NRs) through SERS. Significantly, photoconversion phenomena were observed on Ag NRs but not on Au NRs upon laser excitation at 633 nm. Finite-difference time-domain simulations revealed the presence of stronger electromagnetic fields on Ag NRs than on Au NRs. The aspect ratios and gaps between individual NPs in dimer configurations were determined to elucidate their effects on electromagnetic fields. The Ag NR dimer with an end-to-end configuration, an aspect ratio of 3.3, and a 1-nm gap exhibited the highest enhancement factor of 1.05 × 1012. Our results demonstrate that the primary contribution from diverse configurations in NR aggregates is the end-to-end configuration. The proposed NP design with adjustable parameters is expected to advance research in plasmonics, sensing, and wireless communications. These findings also contribute to the understanding of plasmon-driven photochemical processes in metallic nanostructures.

Electromagnetic tomography measuring bench for estimating the density of materials

In the laboratory, the density of pavement cores (cylindrical samples of hot mix asphalt (HMA) material taken from roads) is assessed using an electromagnetic (EM) bench consisting of two ultra-wideband (UWB) Vivaldi antennas and a vector network analyser (VNA). The main objective is to replace the nuclear gauge system currently used in the laboratory as the standard method for this purpose. Firstly, specific antipodal Vivaldi antennas have been adapted from the literature. Their dimensions are 7 × 7 cm with a bandwidth [1.5–15 GHz]. Secondly, a tomographic approach is compared with an analytical solution and a Finite-Difference Time Domain (FDTD) simulation, based on a time-domain estimation of the dielectric under test (DUT) with a single transmitter/receiver configuration. A laboratory validation is presented and the adapted antennas as well as the time domain approach show acceptable results for assessing the dielectric constant on known materials. Finally, to show that the proposed EM bench is a promising non-ionizing solution, the density or equivalent compactness of HMA cylindrical samples is estimated and compared with nuclear gauge results.

Ultrasensitive detection of contaminants in milk using a novel NMS-Ag modified water-resistant paper substrate

Rapid and precise detection of harmful substances in food products is essential for ensuring public health and safety. This study introduces a novel surface-enhanced Raman spectroscopy (SERS) substrate, composed of a molybdenum disulfide‑silver nanocomposite, applied to flexible, water-resistant filter paper for detecting melamine and bisphenol A (BPA) in milk. Optimized molybdenum disulfide (NMS) nanoflowers (NFs) were synthesized through hydrothermal methods and high-temperature annealing, then modified with silver (Ag) nanoparticles to form the NMS-Ag nanocomposite (NMSA6). This substrate greatly enhances the Raman signal, achieving an enhancement factor of approximately 1.49 × 107 and a detection limit as low as 10−11 M for simultaneous multi-component analysis. Finite-difference time-domain (FDTD) simulations confirm the enhancement mechanism. The NMSA6 substrate demonstrates remarkably low detection limits for BPA and melamine, facilitating the analysis of various hazardous substances. These findings highlight the substrate's potential for highly sensitive, label-free detection, presenting a viable tool for food safety monitoring.

Impact of Graphene Oxide on SERS Enhancement of Arylated Gold Nanospheres: Mechanistic Insight.

The performance of gold nanospheres as substrates for surface-enhanced Raman spectroscopy (SERS) investigation has been compromised by their low adsorption efficiency, high colloidal dispersibility, and diminishing hot spots. However, gold nanosphere substrates modified using aryldiazonium gold(III) chemistry via durable gold-carbon bonds are promising for SERS enhancement due to their controlled organic layer density. In this study, arylated gold nanospheres AuNSs-COOH have shown SERS enhancement when incorporated into graphene oxide (GO) to form nanocomposites (NCs) labeled AuNSs-COOH/GO (AuNCs). Our investigation using X-ray photoelectron spectroscopy (XPS) surface analysis showed that the gold-aryl nanospheres reached their maximum SERS enhancement with an optimal coating. The evaluation included the Au 4f chemical environment and compact graphitic layers for the SERS substrate optimization. The fabricated AuNC substrates demonstrated superior efficiency and reproducibility. A broad linear range of 10-3-10-7 M 4-nitrophenol detection was obtained with exceptional repeatability, as evidenced by the relative standard deviation (RSD) of 9.32%. A detailed investigation of the energy profiles, particularly the valence band maximum (VBM) and band gap values of the substrate and analyte, depicted the electromagnetic (EM) and charge-transfer-induced enhancement and the role of GO inclusion in substrate efficiency in SERS enhancement mechanisms. The finite-difference time domain (FDTD) simulation results revealed that AuNCs incorporated with graphitic nanostructures exhibited the most substantial SERS effect through an EM field enhancement mechanism. This study demonstrated significant SERS enhancement using gold-aryl nanospheres when modified with GO, in contrast to the typical reliance on anisotropic nanostructures.

Plasmon-enhanced sub-Debye-length nanogap photoelectrochemical cells for field-assisted electrolyte-free water splitting

This work studies pure water splitting (i.e., no additional electrolyte) using plasmonic sub-Debye-length nanogap photoelectrochemical cells (NPECs). With 60 nm Au gratings covered by 4 nm amorphous TiO2 under 638 nm normal incident illumination, we demonstrate up to 66 × higher photocurrent at 1.5 V than that of Au-gratings-only NPECs. In addition, we are able to efficiently drive pure water splitting at voltages as low as 1.3 V, which is very close to the theoretical thermodynamic potential threshold of 1.23 V. Our electromagnetic finite difference time domain (FDTD) simulation results (Lumerical Inc.) show a 240 × increase in the electric field intensity near the TiO2 layer compared to the incident electric field. We attribute our observations to 1) Hot spots generated by surface plasmonic resonance (SPR) on Au gratings, which greatly increase the number of electron-hole pairs; 2) The 60 nm gap distance between electrodes, which is smaller than the Debye length of pure water (∼220 nm) resulting in a much higher electric field by eliminating the shield effect and by coupling two half-reactions of water splitting together; and 3) The 4 nm amorphous TiO2, which improves hot carriers separation. Our NPEC is a promising device for efficient hydrogen production enabling the utilization of visible light.

Plasmonic structurally colored surfaces with metal film over microsphere lattices

Structural coloring through plasmonic nanostructures involves the manipulation of light interaction with nanostructured metallic surfaces to selectively reflect specific spectral ranges within the visible spectrum. Unlike conventional pigments that absorb light based on their chemical properties, plasmonic nanostructures achieve color through their physical configuration. This study investigates the potential of generating plasmonic structural colors by utilizing self-assembled colloidal microsphere lattices that are coated with thin metal films. Finite-Difference Time-Domain (FDTD) simulations are employed on realistic models to analyze the optical reflectance of dielectric microsphere lattices ranging in sizes from 300 to 700 nm, coated with varying thicknesses (20–200 nm) of different metals (Al, Ag, Au). The reflectance spectra are then translated into specific colors using a software algorithm, with the color representations plotted on CIE1931 diagrams. Furthermore, to validate the theoretical findings, two-dimensional polystyrene microsphere arrays coated with Au and Ag films are prepared, and their reflectance spectra and color images are examined. The resulting plasmonic colors can be adjusted by altering the sphere size, the type of metal used, and the thickness of the coating, demonstrating their potential for diverse color-based applications.

Light-trapping by wave interference in intermediate-thickness silicon solar cells

The power conversion efficiency of crystalline silicon (c − Si) solar cells have witnessed a 2.1% increase over the last 25 years due to improved carrier transport. Recently, the conversion efficiency of c − Si cell has reached 27.1% but falls well below the Shockley-Queisser limit as well as the statistical ray-optics based 29.43% limit. Further improvement of conversion efficiency requires reconsideration of traditional ray-trapping strategies for sunlight absorption. Wave-interference based light-trapping in photonic crystals (PhC) provides the opportunity to break the ray-optics based 4n 2 limit and offers the possibility of conversion efficiencies beyond 29.43% in c − Si cells. Using finite difference time domain simulations of Maxwell’s equations, we demonstrate photo-current densities above the 4n 2 limit in 50 − 300µm-thick inverted pyramid silicon PhCs, with lattice constant 3.1µm. Our 150µm-thick PhC design yields a maximum achievable photo-current density (MAPD) of 45.22mA/cm 2. We consider anti-reflection coatings and surface passivation consisting of SiO 2 − SiN x − Al 2 O 3 stacks. Our design optimization shows that a 80 − 120 − 150nm stack leads to slightly better solar light trapping in photonic crystal cells with thicknesses <50µm, whereas the 80 − 40 − 20nm stack performs better for cells with thicknesses >100µm. We show that replacing SiN x with SiC may improve the MAPD for PhC cells thinner than 100µm. For a fixed lattice constant of 3.1µm, we find no significant improvement in the solar absorption for 50 and 100µm-thick cells relative to a 15µm cell. A substantial improvement in the MAPD is observed for the 150µm cell, but there is practically no improvement in the solar light absorption beyond 150µm thickness.