Improved Light Trapping Research Articles

Diffuse scattering from hemispherical nanoparticles at the air–silicon interface

There has been much recent interest in the application of plasmonics to improve the efficiency of silicon solar cells. In this paper we use finite difference time domain calculations to investigate the placement of hemispherical gold nanoparticles on the rear surface of a silicon solar cell. The results indicate that nanoparticles protruding into the silicon, rather than into air, have a larger scattering efficiency and diffuse scattering into the semiconductor. This finding could lead to improved light trapping within a thin silicon solar cell device.

Accounting for Localized Defects in the Optoelectronic Design of Thin-Film Solar Cells

Controlled nanostructuring of thin-film solar cells offers a promising route toward increased efficiency through improved light trapping. Many such light trapping designs involve structuring of the active region itself. Optimization of these designs is aided by the use of computer simulations that account for both the optics and electronics of the device. We describe such a simulation-based approach that accounts for experimental tradeoffs between high-aspect ratio structuring and electronic material quality. Our model explicitly accounts for localized regions of degraded material quality that is induced by light trapping structures in n-i-p a-Si:H solar cells. We find that the geometry of the defects couples to the geometry of light absorption profiles in the active region and that this coupling impacts the spectral response of the device. Our approach yields insights into the nanoscale device physics that is associated with localized geometry-induced defects and provides a framework for full optoelectronic optimization.

Improved dye sensitized solar cell performance in larger cell size by using TiO2 nanotubes

Typical dye sensitized solar cells (DSSCs) exhibit a severe reduction of power conversion efficiency when the cell size is increased. In order to cope with this issue, we have investigated the use of anodized TiO2 nanotubes on Ti foil in combination with the standard TiO2 nanoparticle paste coated anode structure. The presence of nanotubes in the anode structure enabled a significant mitigation of the size-dependent deterioration of the DSSC performance, with a trend of much milder decrease of the efficiency as a function of the cell dimension up to 9 cm2. The observed improvement is partly attributed to the elimination of fluorine-doped tin oxide glass in the anode structure, as well as the enhanced charge collection via the nanotube coated Ti substrate, resulting from enhanced mechanical and electrical connections and possibly improved light trapping. The introduction of TiO2 nanotubes on the Ti foil substrate led to a substantial improvement of the Jsc current density.

Quantification of Optical Deflection by Laser-structured ZnO:Al

A novel camera-based measurement setup fully quantifies the transmitted radiation deflected by textured front electrodes for thin-film solar cells. The conventional measurement of angular intensity distribution only analyzes one polar plane. The new setup examines the optical scattering and/or diffraction over the complete hemisphere. The hemispheric intensity distribution of laser-structured ZnO:Al proves reduced specular transmission and, hence, improved light trapping.

Surface morphology control of Transparent Conducting Oxide layers for improved light trapping using wafer grating and feedback control

This work first introduces a kinetic Monte-Carlo simulation model for a two-species thin film deposition process on a grated (patterned) wafer, and then utilizes a model predictive controller, which manipulates a spatially distributed deposition rate, to produce thin films whose surface morphology has a structure that improves light trapping. This approach to thin film surface morphology control can be applied to many deposition processes, and in particular, in the context of Transparent Conducting Oxide (TCO) thin film manufacturing processes used in thin film solar cells where it is desirable to produce thin films with precisely tailored surface morphology. Specifically, a two species thin film deposition process involving atom adsorption, surface relaxation and surface migration is considered and is modeled on a large-scale lattice (lattice size=40,000) via kinetic Monte-Carlo methods and aggregate surface roughness and slope are used to describe the surface morphology. Subsequently, multiple sets of simulations are carried out to understand the process dynamics dependence on wafer grating parameters, such as magnitude and period of grating, and other process parameters, such as temperature and deposition rate. From these simulations, it is concluded that a spatially distributed deposition rate profile is necessary to be utilized across the lattice in order to induce desirable surface morphology at light wavelength spatial scales that lead to desired thin film solar cell performance. Then, an Edwards–Wilkinson-type equation is utilized to predict the surface evolution and forms the basis for the design of a predictive feedback controller. The model parameters of the Edwards–Wilkinson equation are identified from kinetic Monte-Carlo open-loop simulations. Analytical solutions of the expected surface roughness and surface slope at the visible light wavelength spatial scale are obtained by solving the Edwards–Wilkinson equation and are used in the predictive controller formulation and in the control action calculation. The controller is applied to the kinetic Monte-Carlo simulation of the deposition process taking place on a sinusoidal grated wafer. Simulation results demonstrate that the proposed controller, wafer grating and patterned actuator design successfully regulate aggregate surface roughness and slope to desirable set-point values at the end of the deposition.

SF6/O2혼합가스에 의한 실리콘 웨이퍼의 표면 텍스쳐링 특성

The optical losses associated with the reflectance of incident radiation are among the most important factors limiting the efficiency of a solar cell. Therefore, photovoltaic cells normally require special surface structures or materials, which can reduce reflectance. In this study, nano-scale textured structures with anti-reflection properties were successfully formed on silicon. The surface of sicon wafer was etched by the inductively coupled plasma process using the gaseous mixture of . We demonstrate that the reflection characteristic has significantly reduced by ~0% compared with the flat surface. As a result, the power efficiency of the nano-scale textured silicon solar cell were enhanced up to 20%, which can be ascribed primarily to the improved light trapping in the proposed nano-scale texturing.

Development of Nanopatterned Fluorine-Doped Tin Oxide Electrodes for Dye-Sensitized Solar Cells with Improved Light Trapping

Transparent conductors (TCs) are an important component of optoelectronic devices and nanoscale engineering of TCs is important for optimization of the device performance through improved light trapping. In this work, patterned periodic arrays of nanopillars and nanolines of pitch size of ~700 nm were created on fluorine-doped tin oxide (FTO) using nanoimprint lithography and reactive ion etching using environmentally friendly gases. The patterned FTO exhibits enhanced light trapping as compared to the unpatterned FTO, which agrees well with simulations based on Finite-Difference Time-Domain method for up to a distance of 4 μm. Dye sensitized solar cells (DSSCs) fabricated on the patterned FTO exhibited improved performance (fill factor and power conversion efficiency), which can be attributed to enhanced light absorption in the range 400-650 nm. Further, electrochemical impedance measurements revealed lower recombination resistance for the patterned FTO/TiO(2) electrode compared to the unpatterned FTO electrode/TiO(2) electrode as a result of better light capturing properties of patterned FTO. The direct fabrication of nanopatterns on TCs developed in the present study is expected to be a viable scheme for achieving improved performance in many other optoelectronic devices.

Modeling and control of Transparent Conducting Oxide layer surface morphology for improved light trapping

This work first introduces a kinetic Monte–Carlo simulation model for a two species thin film deposition process and demonstrates the use of feedback control, coupled with a suitable actuator design, in manufacturing thin films whose surface morphology has a structure that improves light trapping. This work is relevant in the context of a Transparent Conducting Oxide (TCO) thin film layer manufacturing used in thin film solar cells where it is desirable to produce thin films with precisely tailored surface morphology. Specifically, a two species thin film deposition process involving atom adsorption, surface relaxation and surface migration is initially considered and is modeled using a large-lattice (lattice size=40,000) kinetic Monte–Carlo simulation. Subsequently, thin film surface morphology characteristics like roughness and slope are computed with respect to different characteristic length scales ranging from atomic to the ones corresponding to visible light wavelength and it is found that a patterned actuator design is needed to induce thin film surface roughness and slope at visible light wavelength spatial scales, that lead to desired thin film solar cell performance. Then, an Edwards–Wilkinson type equation is used to model the surface evolution at the visible light wavelength spatial scale and form the basis for the design of a predictive feedback controller whose objective is to manipulate the deposition rate across the spatial domain of the process. The model parameters of the Edwards–Wilkinson equation are estimated from kinetic Monte–Carlo simulations and their dependence on the deposition rate is used in the formulation of the predictive controller to predict the influence of the control action on the surface roughness and slope throughout the thin film growth process. Analytical solutions of the expected surface roughness and surface slope at the visible light wavelength spatial scale are obtained by solving the Edwards–Wilkinson equation and are used in the predictive controller formulation and in the control action calculation. The controller is applied to the large-lattice kinetic Monte–Carlo simulation. Simulation results demonstrate that the proposed controller and patterned actuator design successfully regulate aggregate surface roughness and slope to set-point values at the end of the deposition.

Variation of back reflector morphology in n–i–p microcrystalline silicon thin film solar cells using texture-etched ZnO

Surface morphology of ZnO layers was varied by changing etching time and statistically analyzed in terms of the root mean square roughness and the distribution of surface feature (crater) size. The texture-etched ZnO covered with Ag and ZnO layers was applied into n–i–p μc-Si:H solar cells as back reflectors. The effects of the back reflector morphologies on solar cell performances were investigated. The correlation between the back reflector morphology and the light trapping properties of solar cells was investigated, including the effect of crater size on the short circuit current density (JSC). We find that craters with the lateral size between 900nm and 2500nm are beneficial for light trapping and are suggested to be mainly responsible for the change in JSC in the solar cells. An improved light trapping results in an increase in JSC by 4.5mA/cm2 (25%) relative to a flat back reflector.

Broadband scattering of the solar spectrum by spherical metal nanoparticles

ABSTRACTMetal nanoparticles offer the possibility of improved light trapping in solar cells, but careful design is required to maximise scattering and minimise parasitic absorption across the wavelength range of interest. We present an analysis of the broadband scattering and absorption characteristics of spherical metal nanoparticles, optimized for either crystalline silicon (c‐Si) or amorphous silicon (a‐Si:H) solar cells. A random two‐dimensional array of optimally sized Ag spheres can scatter over 97% of the AM1.5 spectrum from 400 to 1100 nm. Larger particles are required for c‐Si devices than a‐Si:H due to the increased spectral range, with optimum particle sizes ranging from 60 nm for a‐Si:H to 116 nm for c‐Si. Positioning the particles at the rear of the solar cell decreases absorption losses because these principally occur at short wavelengths. Increasing the refractive index of the surrounding medium beyond the optimum value, which is 1.0 for a‐Si:H and 1.6 for c‐Si, shifts absorption to longer wavelengths and decreases scattering at short wavelengths. Ag nanoparticles scatter more of the solar spectrum than Au, Cu or Al nanoparticles. Of these other metals, Al can only be considered for a‐Si:H applications due to high absorption in the near‐infrared, whereas Au and Cu can only be considered for the rear of c‐Si devices due to high absorption in the ultraviolet (UV) and visible. In general, we demonstrate the importance of considering the broadband optical properties of metal nanoparticles for photovoltaic applications. Copyright © 2012 John Wiley & Sons, Ltd.

Simulation and control of aggregate surface morphology in a two-stage thin film deposition process for improved light trapping

This work focuses on the development of a model predictive control algorithm to simultaneously regulate the aggregate surface slope and roughness of a thin film growth process to optimize thin film light reflectance and transmittance. Specifically, a two-stage thin film deposition process, which involves two microscopic processes: an adsorption process and a migration process, is modeled based on a one-dimensional solid-on-solid square lattice. The first stage of this process utilizes a uniform deposition rate profile to control the thickness of the thin film and the second stage of the process utilizes a spatially distributed deposition rate profile to control the surface morphology of the thin film. Kinetic Monte Carlo (kMC) methods are used to simulate this two-stage thin film deposition process. To characterize the surface morphology and to evaluate the light trapping efficiency of the thin film, aggregate surface roughness and slope corresponding to length scale of visible light are introduced as the root-mean squares of the aggregate surface height profile and aggregate surface slope profile. An Edwards–Wilkinson (EW)-type equation with appropriately computed parameters is used to describe the dynamics of the surface height profile and predict the evolution of the aggregate root-mean-square (RMS) roughness and aggregate RMS slope. A model predictive control algorithm is then developed on the basis of the EW equation model to regulate the aggregate RMS slope and the aggregate RMS roughness at desired levels. Closed-loop simulation results demonstrate the effectiveness of the proposed model predictive control algorithm in successfully regulating the aggregate RMS slope and the aggregate RMS roughness at desired levels that optimize thin film light reflectance and transmittance.

Micromorph tandem solar cells: optimization of the microcrystalline silicon bottom cell in a single chamber system

We report on the development of single chamber deposition of microcrystalline and micromorph tandem solar cells directly onto low-cost glass substrates. The cells have pin single-junction or pin/pin double-junction structures on glass substrates coated with a transparent conductive oxide layer such as SnO2 or ZnO. By controlling boron and phosphorus contaminations, a single-junction microcrystalline silicon cell with a conversion efficiency of 7.47% is achieved with an i-layer thickness of 1.2 μm. In tandem devices, by thickness optimization of the microcrystalline silicon bottom solar cell, we obtained an initial conversion efficiency of 9.91% with an aluminum (Al) back reflector without a dielectric layer. In order to enhance the performance of the tandem solar cells, an improved light trapping structure with a ZnO/Al back reflector is used. As a result, a tandem solar cell with 11.04% of initial conversion efficiency has been obtained.

Preparation of Ga-doped ZnO films by pulsed dc magnetron sputtering with cylindrical rotating target for thin film solar cell applications

Applicability of Ga-doped ZnO (GZO) films for thin film solar cells (TFSCs) was investigated by preparing GZO films via pulsed dc magnetron sputtering (PDMS) with rotating target. The GZO films showed improved crystallinity and increasing degree of Ga doping with increasing thickness to a limit of 1000nm. The films also fulfilled requirements for the transparent electrodes of TFSCs in terms of electrical and optical properties. Moreover, the films exhibited good texturing potential based on etching studies with diluted HCl, which yielded an improved light trapping capability without significant degradation in electrical propreties. It is therefore suggested that the surface-textured GZO films prepared via PDMS and etching are promising candidates for indium-free transparent electrodes for TFSCs.

Diffractive gratings for crystalline silicon solar cells—optimum parameters and loss mechanisms

ABSTRACTIn this paper, we present guidelines for the design of backside gratings for crystalline silicon solar cells. We use a specially developed method based on a combination of rigorous 3D wave optical simulations and detailed semiconductor device modeling. We also present experimental results of fabricated structures. Simulation‐based optimizations of grating period Λ and depth d of a binary grating and calculations of the optical and electrical characteristics of solar cells with optimized gratings are shown. The investigated solar cell setup features a thickness of dbulk = 40 µm and a flat front surface. For this setup, we show a maximum increase in short‐circuit current density of ΔjSC = 1.8 mA/cm² corresponding to an efficiency enhancement of 1% absolute. Furthermore, we investigate different loss mechanisms: (i) an increased rear surface recombination velocity S0,b because of an altered surface caused by the introduction of the grating and (ii) absorption in the aluminum backside reflector. We analyze the trade‐off point between gain due to improved optical properties and loss due to corrupted electrical properties. We find that, increasing the efficiency by 1% absolute due to improved light trapping, the maximum tolerable recombination velocity is S0,b(max) = 5.2 × 103 cm/s. From simulations and measurements, we conclude that structuring of the aluminum backside reflector should be avoided because of parasitic absorption. Adding a dielectric buffer layer between silicon and the structured aluminum, absorption losses can be tuned. We find that for a planar reflector, the thickness of a SiO2 buffer layer should exceed = 120 nm. Copyright © 2011 John Wiley & Sons, Ltd.

Dynamics and control of aggregate thin film surface morphology for improved light trapping: Implementation on a large-lattice kinetic Monte Carlo model

This work demonstrates the use of feedback control, coupled with a suitable actuator design, in manufacturing thin films whose surface morphology is characterized by a desired visible light reflectance/transmittance level. The problem is particularly important in the context of thin film manufacturing for thin film solar cells where it is desirable to produce thin films with precisely tailored light trapping characteristics. Initially, a thin film deposition process involving atom adsorption and surface migration is considered and is modeled using a large-lattice (lattice size=40,000) kinetic Monte Carlo simulation. Subsequently, thin film surface morphology characteristics like roughness and slope are computed with respect to different characteristic length scales ranging from atomic to the ones corresponding to visible light wavelength and it is found that a patterned actuator design is needed to induce thin film surface roughness and slope at visible light wavelength spatial scales, which lead to desired thin film reflectance and transmittance levels. Then, an Edwards–Wilkinson-type equation (a second-order stochastic partial differential equation) is used to model the surface evolution at the visible light wavelength spatial scale and form the basis for the design of a feedback controller whose objective is to manipulate the deposition rate across the spatial domain of the process. The model parameters of the Edwards–Wilkinson equation are estimated from kinetic Monte Carlo simulations and their dependence on the deposition rate is used in the formulation of the predictive controller to predict the influence of the control action on the surface roughness and slope throughout the thin film growth process. Analytical solutions of the expected surface roughness and surface slope at the visible light wavelength spatial scale are obtained by solving the Edwards–Wilkinson equation and are used in the control action calculation. The cost function of the controller involves penalties on both surface roughness and slope from set-point values as well as constraints on the magnitude and rate of change of the control action. The controller is applied to the large-lattice kinetic Monte Carlo simulation. Simulation results demonstrate that the proposed controller and patterned actuator design successfully regulate aggregate surface roughness and slope to set-point values at the end of the deposition that yield desired levels of thin film reflectance and transmittance.

Tolerance study of nanoparticle enhancement for thin-film silicon solar cells

The improved light trapping caused by incorporation of plasmonic nanoparticles makes it possible to significantly increase the performance of thin-film silicon solar cells. To investigate stability of this enhancement, a systematic study on random distribution of the nanoparticles is performed. Based on predictive numerical modeling, effects such as the influence of random nanoparticle sizes, shapes, and mutual positions are discussed and analyzed with respect to reproducibility of the enhancement by silver nanoparticles for thin-film hydrogenated amorphous silicon solar cells.

Controlling aggregate thin film surface morphology for improved light trapping using a patterned deposition rate profile

This work focuses on modeling and control of aggregate thin film surface morphology for improved light trapping using a patterned deposition rate profile. The dynamics of the evolution of the thin film surface height profile are modeled by an Edwards–Wilkinson-type equation (a second-order stochastic partial differential equation) in two spatial dimensions. The thin film surface morphology is characterized in terms of aggregate surface roughness and surface slope. These variables are computed with respect to appropriate visible light-relevant characteristic length scales and defined as the root-mean-squares of height deviation and slope of aggregate surface height profiles, respectively. Analytical solutions of the expected aggregate surface roughness and surface slope are obtained by solving the Edwards–Wilkinson equation and are used in the controller design. The model parameters of the Edwards–Wilkinson equation are estimated from kinetic Monte-Carlo simulations using a novel parameter estimation procedure. This parameter dependence on the deposition rate is used in the formulation of the predictive controller to predict the influence of the control action on the surface roughness and slope at the end of the growth process. The cost function of the controller involves penalties on both aggregate surface roughness and mean slope from set-point values as well as constraints on the magnitude and rate of change of the control action. The controller is applied to the two-dimensional Edwards–Wilkinson equation. Simulation results show that the proposed controller successfully regulates aggregate surface roughness and slope to set-point values at the end of the deposition that yield desired levels of thin film reflectance and transmittance.

Anti-reflective nano- and micro-structures on 4H-SiC for photodiodes

In this study, nano-scale honeycomb-shaped structures with anti-reflection properties were successfully formed on SiC. The surface of 4H-SiC wafer after a conventional photolithography process was etched by inductively coupled plasma. We demonstrate that the reflection characteristic of the fabricated photodiodes has significantly reduced by 55% compared with the reference devices. As a result, the optical response Iillumination/Idark of the 4H-SiC photodiodes were enhanced up to 178%, which can be ascribed primarily to the improved light trapping in the proposed nano-scale texturing.

Control of LPCVD ZnO growth modes for improved light trapping in thin film silicon solar cells

By controlling the deposition parameters of low pressure chemical vapor deposition (LPCVD) grown ZnO transparent conductive oxide (TCO), we show that it is possible to achieve a switch of crystallographic orientations and to obtain novel surface nanotexture. The new layers are (0 0 0 2) oriented and exhibit light scattering at larger angles than state-of-the-art TCO used in thin film silicon solar cells. This property translates into a strong enhancement of the top amorphous cell current in micromorph tandem devices.

Light trapping effect of patterned back surface reflectors in substrate-type single and tandem junction thin-film silicon solar cells

Light trapping is a key issue to boost the efficiency of thin-film Si solar cells including μc-Si:H. In this paper, effect of textured back reflectors on light trapping in μc-Si:H cells has been investigated with self-orderly patterned Al substrates obtained by anodic oxidation. With increase in the period of the patterned substrates from 0 to 1.1 μm, the short circuit current densities of 1-μm-thick μc-Si:H cells on the patterned substrates significantly increase from 18 to over 24 mA/cm 2, which is attributed to the improved light trapping in the infrared region. It has been clarified that this enhanced light-trapping effect in longer wavelengths is mainly attributed to the improved light scattering at the rear side by comparing μc-Si:H solar cells with polished and as-deposited front surfaces. The effectiveness of the patterned Al substrates has also been demonstrated in an a-Si:H/μc-Si:H tandem cell with a bottom cell thickness of 1 μm, showing a higher conversion efficiency than the reference cell.