Coatings For Silicon Solar Cells Research Articles

Selection of materials and optimization of antireflection coatings for silicon solar cells using Sentaurus TCAD

Selection of materials and optimization of antireflection coatings for silicon solar cells using Sentaurus TCAD

Porous Silicon Antireflective Coatings for Silicon Solar Cells

This study presents a numerical investigation of the reflectivity of a Single Anti-Reflective Layer (SARL) and a stack of antireflective layers made of porous silicon. The stack consists of a certain number of periods, and each period contains two layers with different porosity. The simulations were conducted using the well-known Stratified Medium Theory (SMT) framework and the effect of porosity was studied. The optimal value was determined at 60% for the SARL and 65/55% for the stack of 12 periods and 6 layers. The angle of incidence was found to have more influence on the stack reflection than on the SARL reflection. The results of this investigation show that porous silicon can be used as an effective anti-reflective coating for silicon solar cells.

Guaranteed global optimization of thin-film optical systems

A parallel deterministic global optimization algorithm for thin-film multilayer optical coatings is developed. This algorithm enables locating a global solution to an optimization problem in this class to within a user-specified tolerance. More specifically, the algorithm is a parallel branch-and-bound method with applicable bounds on the merit function computed using Taylor models. This study is the first one, to the best of our knowledge, to attempt guaranteed global optimization of this important class of problems, thereby providing an overview and an assessment of the current state of such techniques in this domain. As a proof of concept on a small scale, the method is illustrated numerically and experimentally in the context of antireflection coatings for silicon solar cells—we design and fabricate a three-layer dielectric stack on silicon that exhibits an average reflectance of (2.53 ± 0.10)%, weighted over a broad range of incident angles and the solar spectrum. The practicality of our approach is assessed by comparing its computational cost relative to traditional stochastic global optimization techniques which provide no guarantees on their solutions. While our method is observed to be significantly more computationally expensive, we demonstrate via our proof of concept that it is already feasible to optimize sufficiently simple practical problems at a reasonable cost, given the current accessibility of cloud computing resources. Ongoing advances in distributed computing are likely to bring more design problems within the reach of deterministic global optimization methods, yielding rigorous guaranteed solutions in the presence of practical manufacturing constraints.

Simultaneously enhanced solar absorption and radiative cooling with thin silica micro-grating coatings for silicon solar cells

Recently, the idea of radiative cooling by dissipating infrared thermal energy to the cold space through the atmospheric window, especially from 8 to 13 μm in wavelength, has become an attractive way to cool down outdoor devices. Here we show that thin silica (SiO2) micro-gratings as solar-transparent and radiatively cooling coatings for silicon solar cells. The well-designed silica micro-gratings were fabricated with plasma-enhanced chemical vapor deposition, photolithography, and reactive ion etching processes. Spectrometric measurements showed that the SiO2 micro-gratings atop doped silicon wafer could remarkably enhance the infrared emittance up to 100% within the atmospheric window and increase the solar absorptance with anti-reflection. Numerical modeling confirmed the measured optical and radiative properties and elucidated the underlying physical mechanisms for the anti-reflection in the solar spectrum and enhanced infrared thermal emission. The radiative cooling performance calculated based on a heat transfer model signified that by enhancing the radiative heat dissipation to the space, the grating structure could increase the radiative cooling power when the structure temperature is higher than 45 °C, and reduce the stagnation temperature by up to 20 °C depending on convective heat transfer coefficients. Furthermore, an outdoor field test has been conducted to experimentally demonstrate the cooling performance of the silica micro-gratings, where the grating covered sample showed a lower temperature than the bare silicon sample did under direct sunlight.

New design graded refractive index antireflection coatings for silicon solar cells

Reflectance spectrum of nanoporous silicon dioxide (SiO2) double layer was calculated by using the matrix method. The results were compared with the corresponding spectrum of silicon oxynitride (SiO[Formula: see text]N[Formula: see text])–porous silicon (PS) double layer which deposited on nanostructured black silicon coatings. The nanoporous silicon dioxide (SiO2) double layer deposited on nanostructure black silicon antireflection coating presents a lower reflectance in a broad range of solar spectrum. This research outcome may find a wide application in solar cell industry.

Investigation of Optical and Dielectric Constants of Organic-Inorganic CH3NH3PbI3 Perovskite Thin Films

CH3NH3PbI3 thin films with thicknesses in the region of 230-365 nm were prepared. The films were characterized by scanning electron microscope (SEM), x-ray diffraction (XRD), ultraviolet-visible (UV-Vis) absorption and transmittance spectra. Based on the Kramers-Kronig relation and the transmittance spectra, the optical constants of the CH3NH3PbI3 thin films were investigated. Our results show that the absorption coefficient, extinction coefficient, refractive indices, real and imaginary components of the dielectric constant of the CH3NH3PbI3films are basically independent on the thicknesses of the films, which indicate our results are reliable. At 500 nm, above parameters of the films are ~1.38 × 105 cm-1, 0.55, 2.70, 7.01, and 3.13, respectively. The band gap is 1.595 eV. The refractive index of CH3NH3PbI3thin films is 2.84 at 633 nm, which imply that CH3NH3PbI3-based solar cells are ideal antireflection coatings for silicon solar cells (SCs) and are applicable to be top solar cell in monolithic silicon-based tandem solar cells for beyond the efficiency limit of Si SCs. Our results are meaningful for designing optoelectronic devices related to the perovskite thin-films.

Pulsed laser deposition of refractive-index-graded broadband antireflection coatings for silicon solar cells

This paper reports the design, fabrication and characterization of broadband antireflection coatings for silicon-based solar cells, using a graded-index technique and a novel dual-beam pulsed laser deposition method. A graded refractive index was realized by mixing two different optical materials, silicon and soda-lime glass, in a way that the composition of the mixture changes gradually from silicon (that of the solar cell) to glass (that of the outside protective glass) along the coating thickness direction following pre-designed refractive-index profiles. In this study, two different refractive-index profiles were fabricated, linear and Southwell profiles, on silicon substrates. In order to emulate a protective cover glass for solar-cells, coatings with an additional 400nm-thick outer glass layer were also fabricated. The antireflection coatings showed reflectance of 2.2–5.5% (mostly around 4%) for a wavelength range of 400–1000nm, and for the Southwell profile with a top glass layer, the reflectance was found to be between 2.2% and 4%, which is believed to be close to the theoretically best antireflection performance of a solar cell with a cover glass. Finite-difference time-domain simulations were also performed and the numerical results were in line with the measurement results. Because these coatings are based on silicon and glass and can be fabricated by the one-step pulsed laser deposition procedure, it can simplify the solar-cell structure considerably. The fabricated antireflection coatings may be also used for many other optical applications, such as thin-film optical attenuator and some optical sensors, and the presented pulsed laser deposition method is capable of fabricating a variety of high-performance optical coatings, which cannot be fabricated by other methods.

Anti-reflection coatings for silicon solar cells from hydrogenated diamond like carbon

Aiming towards a specific application as antireflection coatings (ARC) in Si solar cells, the growth of hydrogenated diamond like carbon (HDLC) films, by RF magnetron sputtering, has been optimized through comprehensive optical and structural studies. Various physical properties of the films e.g., (ID/IG) ratio in the Raman spectra, percentage of sp3 hybridization in XPS spectra, H-content in the network, etc., have been correlated with different ARC application properties e.g., transmittance, reflectance, optical band gap, refractive index, surface roughness, etc. The ARC properties have been optimized on unheated substrates, through systematic variations of RF power, gas flow rate, gas pressure and finally controlled introduction of hydrogen to the DLC network at its most favorable plasma parameters. The optimum HDLC films possess (T700)max∼95.8%, (R700)min∼3.87%, (n700)min∼1.62 along with simultaneous (Eg)max∼2.53eV and ∼75.6% of sp3 hybridization in the C-network, corresponding to a bonded H-content of ∼23at.%. Encouraging improvements in the ARC properties over the optimized DLC film were obtained with the controlled addition of hydrogen, and the optimum HDLC films appear quite promising for applications in Si solar cells. Systematic materials development has been performed through comprehensive understanding of the parameter space and its optimization, as elaborately discussed.

Investigation on Antireflection Coatings for Silicon Solar Cells

Thin-film anti-reflecting coatings can greatly reduce the optical loss by making use of phase changes, and the reflectivity depends on the refractive index of materials. TiO2ZnS and Si3N4coatings are suitable for using as single layer anti-reflecting coating on bare silicon surface, while the MgF2/ZnS and SiO2/TiO2double-layer anti-reflecting coating result in a minimum reflectance lower than 0.5% over broad spectral regions, with an average reflectance of approximately 2.25% between 400 and 1100 nm on the non-textured Si substrate. The short circuit current of silicon solar cells has significant improvement after depositing anti-reflecting coatings, and it increases the efficiency of the Si solar cells.

Silicon carbon nitride films as passivation and antireflective coatings for silicon solar cells

A new method to obtain antireflective and passivation layers for p-type silicon solar cells is presented. Hydrogenated silicon carbon nitride (SiCxNy:H) films are obtained by low frequency plasma enhanced chemical vapor deposition (PECVD), using ammonia (NH3) and tetramethylsilane (TMS) as precursors; the influence of the deposition temperature and gas ratio on the film composition and properties is studied. It can be observed that the chemical composition of the film as well as its optical and passivation properties vary strongly with the gas ratio and temperature used. The optimal conditions with respect to the refractive index, the absorption coefficient and the minority carrier lifetime after rapid thermal annealing are determined. These conditions lead to an approximate stoichiometry of SiC0.6N0.8:H thin film including nearly 5×1021cm−3 hydrogen bonds.

Theoretical Analysis of Spectral Selective Transmission Coatings for Solar Energy PV System

Spectral selective transmission coatings can adjust the spectrum of incident solar radiation to reduce the waste heat generation in solar cells and to improve solar photovoltaic conversion efficiency. The ideal spectral transmissivity of the coatings should match the spectral response of solar cells, which means the ideal spectral transmissivity should be equal to 1.0 in the range of the spectral response and 0 in the other spectrum. The reflection performance of the three kinds of spectral selective transmission coatings for silicon solar cells are designed and analyzed. The results indicated that the ZnS/Na3AlF6 coating systems have a wider reflected infrared (IR) region than the TiO2/SiO2 coating systems, but lower transmissivity in the wavelength range of 0.5 μm to 1.1 μm. Furthermore, an Nb2O3/SiO2 coating system is proposed and optimized, which has 31 layers with a smaller total thickness of 2.675 μm. The radiative properties including reflectivity and transmissivity of spectral selective transmission coatings are investigated and theoretically analyzed.

Sol–gel derived hydrogen annealed ZnO:Al films for silicon solar cell application

A study of the effect of hydrogen annealing on antireflection and surface passivation properties of sol–gel derived aluminum rich zinc oxide (AZO) films coated on silicon wafers has been done in 300–600°C temperature range. The minority carrier lifetime in silicon wafers coated with AZO films was measured using microwave photoconductive decay (μ-PCD) technique. After annealing of AZO coated silicon wafer in hydrogen ambient for 30min between 400 and 600°C the effective minority carrier lifetime τeff improved above the initial value of ∼16μs and attained a maximal value of ∼71μs for annealing at 500°C. It may be that above 400°C the molecular hydrogen gets dissociated into atomic hydrogen in presence of Al induced defects at the AZO coated silicon surface and passivates them by formation of Si–H–Al complex. The annealing in air in the same temperature range did not affect the lifetime. AZO films annealed at 500°C in hydrogen or air show high transmittance in 400–1200nm wavelength range and are suitable as AR coatings for silicon solar cells. We have applied a hydrogen annealed AZO antireflection coating on n+ front surface and an equally thick hydrogen annealed AZO coating on the p-back surface of an n+-p multi crystalline silicon solar cell. The observed improvement in Jsc, Voc values of the cell established that while AZO film on front acted as a good AR coating the AZO film on the back surface passivated the back surface effectively.

Influence of PECVD SiOx and SiNx:H films on optical and passivation properties of antireflective porous silicon coatings for silicon solar cells

AbstractThe meso‐porous silicon layer (PS) is a promising material to reduce the surface reflectance of solar cells. PS layers were grown on the front surface n+ emitter of n+‐p mono‐crystalline Si junction. Single layers PS antireflection coating (ARC) achieved around 8% of effective reflectivity in the wavelength range between 400 to 1100 nm. To improve the stability and the passivation properties, and to reduce the reflectivity of PS ARC, the design of PECVD silicon oxide (SiOx) and silicon nitride (SiNx:H) layers were investigated. Optimised SiOx and SiNx layer deposited on PS decreased the effective reflectivity respectively to ∼3.8% and ∼7% between 400 and 1000 nm. Open circuit voltage (Voc) measurements were carried out on all the samples by suns‐Voc method and showed an improvement of 20% of the surface passivation quality brought by the nitride layer after rapid thermal anneal (RTA). Using the experimental reflectivity results and taking into account the surface passivation quality of the samples, numerical simulations predict an enhancement of the photogenerated current exceeding 49% for the PS/SiOx stacks (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Design of porous silicon/PECVD SiOx antireflection coatings for silicon solar cells

Abstract The meso-porous silicon (PS) has become an interesting material owing to its potential applications in many fields, including optoelectronics and photovoltaics. PS layers were grown on the front surface of the n + emitter of n + –p mono-crystalline Silicon junction. The thickness and the porosity of the PS layer were determined by an ellipsometer, as a function of time duration of anodization, and the variation law of the PS growth kinetics is established. Single layers PS antireflection coating (ARC) achieved around 9% of effective reflectivity in the wavelength range between 400 and 1000 nm on junction n + –p solar cells. To reduce the reflectivity and improve the stability and passivation properties of PS ARC, silicon oxide layers were deposited by PECVD on PS ARC. SiO x layers of thickness of 105 nm combined with PS layer led to 3.8% effective reflectivity. V oc measurements were carried out on all the samples by suns- V oc method and showed an improvement of the quality of the passivation brought by the oxide layer. Using the experimental reflectivity results and taking into account the passivation quality of the samples, the PC1D simulations predict an enhancement of the photogenerated current exceeding 44%.

Optimization of multilayer porous silicon antireflection coatings for silicon solar cells

Efficient antireflection coatings (ARC) improve the light collection and thereby increase the current output of solar cells. In this work, multilayered refractive index stacks optimized for antireflection, in bare air and within modules, are modeled. The relation between porous silicon (PS) etching parameters and PS structure is carefully investigated using spectroscopic ellipsometry, gravimetry, x-ray photoelectron spectroscopy, and scanning electron microscopy. The close relation between porosity and refractive index, modeled using the Bruggeman effective medium approximation, allows PS multilayers to be tailored to fabricate the optimized ARCs. Limits imposed by efficient application in photovoltaics, such as thickness restrictions and the angular distribution of incident light, are examined and accounted for. Low reflectance multilayer ARCs are fabricated with integrated reflectances of ∼3% in air and 1.4% under glass in the wavelength range 400–1100 nm.

Impact of PECVD SiON stoichiometry and post-annealing on the silicon surface passivation

Hydrogenated silicon oxynitride (SiON) could be used in combination with silicon nitride (SiN) to create multi-layer antireflection coatings for silicon solar cells. It could also be used as a passivation layer, especially on the back side of the cell. This work deals with the passivation effect obtained on silicon surface by SiON layer deposited by Low Frequency Plasma Enhanced Chemical Vapour Deposition (LF-PECVD). SiON layers of different compositions have been deposited by varying the gas flow mixture (NH 3, SiH 4 and N 2O) in the reactor. Infrared and X-ray photon–electron spectroscopy were made to determine the chemical structure of SiON layer. Minority carrier lifetimes were measured by the photoconductance decay method (PCD) before and after a rapid thermal anneal. Effective lifetime, measured on 5 Ω cm FZ-silicon wafers, can reach up to several hundreds microseconds, depending on the stoichiometry of the SiON layer. Low oxygen content samples (close to SiN layer) exhibit a good surface passivation of 250 µs but after annealing, this value is critically reduced to 6 µs. The opposite situation is observed for oxygen-rich layers: the effective lifetime increases from 10 µs to 150 µs. These behaviours could be partly explained by the composition of SiON, the evolution of the main peaks values of the FTIR spectra and the disappearance of Si–H bonds with anneal.

Graded refraction index antireflection coatings based on silicon and titanium oxides

Thin films with a graded refraction index constituted from silicon and titanium oxides were deposited by plasma enhanced chemical vapor deposition using electron cyclotron resonance. A plasma of oxygen reacted with two precursors: the tetraethoxysilane (TEOS) and the titanium isopropoxide (TIPT). The automatic regulation of the precursor flows makes it possible to modify the chemical composition, and consequently the optical index, through the deposited films. To control the thickness, the refraction index and the growth kinetics, in situ spectroscopic ellipsometer was adapted to the reactor. The analysis of ex situ ellipsometric spectra measured at the end of each deposition allow to determine a refraction index profile and optical properties of the inhomogeneous deposited films. Measurements of reflectivity carried out in the ultraviolet-visible-near infrared range show that these films could be used as antireflective coatings for silicon solar cells: 3.7 % weighted average reflectivity between 300 and 1100 nm and 48 % improvement of the photo-generated current were obtained.

Design of porous silicon antireflection coatings for silicon solar cells

A porous silicon (PS) layer formed electrochemically in the outer part of the n + emitter of p-n + Si junctions can be used as an efficient antireflection coating (ARC). A two-step procedure is presented which can determine the electrochemical parameters leading to the formation of an optimized single-layer PS ARC. Single-layer PS ARCs achieving ≈7% effective reflectance between 400 and 1000 nm are obtained on shallow p–n + junction solar cells. To reduce the reflectance further, the design of double-layer ARCs based on PS is investigated. PS layers with different porosities can be realized in a single experiment by modulating the current density during the electrochemical process. It is shown theoretically and experimentally that such PS structures can lead to an effective reflectance below 3%.

Passivating TiO 2 coatings for silicon solar cells by pulsed laser deposition

Abstract. Pulsed laser deposition was used to deposit TiO2 anti-reflection coatings for silicon solar cells. We deposited smooth coatings with an optimal refractive index of 2.3 for use as anti-reflection coating. The introduction of passivating qualities was achieved by deposition in different gasses. The best result was obtained with deposition in a water vapour ambient. The plasma shape and the position of the substrate in the plasma appeared important for properties such as the smoothness, the thickness distribution and the passivating quality. An increase in the measured effective lifetime of up to 137% during modulated free carrier absorption measurements was observed.

Analytical design of antireflection coatings for silicon photovoltaic devices

Reflectivity and maximum short-circuit current for multilayer optical coating structures have been calculated analytically. This approach has been used to optimize antireflection coatings for silicon solar cells. Triple coating structures are indicated instead of the conventional two layer ones, especially when the goal is to minimize reflectivity in the UV range. The same approach was used to calculate the maximum short-circuit current for grooved surface solar cells. The results for flat and grooved surface solar cells were compared. This work shows that texturized cells need only one coating layer to achieve the same photocurrent obtained for flat surface cells with three layer coatings.