Surface Of Silicon Solar Cells Research Articles

Double AAO nanogratings for broad spectrum absorption enhancement in thin film Si solar cells

Abstract In this paper, we presented a double light trapping structure containing anodic aluminum oxide (AAO) nanogratings on both the surface and rear of thin film crystalline silicon (c-Si) solar cell. Finite difference time domain (FDTD) method was used to study the light absorption enhancement of this design. Simulation results show that, the double light trapping structure can highly increase the light absorption in the wavelengths from 280 to 1100 nm. The relative enhancement of short-circuit current density (Jsc) in this situation is 137%. Besides, the double light trapping structure can highly reduce the influence of incident angle on light absorption. The relative position of AAO pore on the surface and rear has very little influence on light trapping.

Ultralow Interface State Density Achieved by Light-Induced Anodization of Aluminum on Silicon Solar Cell Surfaces

A light-induced anodization (LIA) method, which uses the light-induced current of a silicon solar cell to anodize aluminum and form an anodic aluminum oxide (AAO) layer that can reduce recombination at a p-type surface of the cell, is reported. This method can result in anodic oxides with uniform properties over the surfaces of industrially sized silicon wafers since the current flows through the wafer rather than in the surface aluminum layer, as it does when the aluminum is directly electrically contacted. Uniform AAO layers can be formed within 4 min on p-type silicon surfaces by anodizing 300-nm-thick aluminum layers, and when formed over a thin interfacial silicon dioxide layer, the interface state density at the p-type silicon wafer interface can be reduced to values as low as 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> ·eV <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and a fixed charge density of 2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> . Photoconductance carrier lifetime measurements indicate that these AAO dielectric stack layers are capable of passivating silicon surfaces to a level similar to that provided by silicon nitride layers, which are deposited using plasma-enhanced chemical vapor deposition, although at potentially a much-reduced cost. The high-quality surface passivation, rapid room temperature processing, and large-area uniformity of LIA make it a promising method for producing passivation materials that can further improve efficiency and potentially reduce the cost of photovoltaic solar cells and other p-n junction devices.

In-Situ Fabrication of a Self-Aligned Selective Emitter Silicon Solar Cell Using the Gold Top Contacts To Facilitate the Synthesis of a Nanostructured Black Silicon Antireflective Layer Instead of an External Metal Nanoparticle Catalyst.

Silicon solar cells with nanopore-type black silicon (b-Si) antireflection (AR) layers and self-aligned selective emitter (SE) are reported in which the b-Si structure is prepared without the traditional addition of a nanoparticle (NP) catalyst. The contact-assisted chemical etching (CACE) method is reported here for the first time, in which the metal top contacts on silicon solar cell surfaces function as the catalysts for b-Si fabrication and the whole etching process can be done in minutes at room temperature. The CACE method is based on the metal-assisted chemical etching (MACE) solution but without or metal precursor in the Si etchant (HF:H2O2:H2O), and the Au top contacts, or catalysts, are not removed from the solar cell surface after the etching. The effects of etching time, HF and H2O2 concentration, and the HF:H2O2 ratio on the b-Si morphology, surface reflectivity, and solar cell efficiency have been investigated. Higher [HF] and [H2O2] with longer etching time cause collapse of the b-Si nanoporous structure and penetration of the p-n junctions, which are detrimental to the solar cell efficiency. The b-Si solar cell fabricated with the HF:H2O2:H2O volume ratio of 3:3:20 and a 3 min etch time shows the highest efficiency 8.99% along with a decrease of reflectivity from 36.1% to 12.6% compared to that of the nonetched Si solar cell.

Effective light trapping by hybrid nanostructure for crystalline silicon solar cells

Ag nanoparticles (NPs), as etching catalyst, are applied onto the pyramid textured surface of the semi-finished single crystalline silicon solar cell by spraying a solution containing AgNO3 and sodium citrate. Upon chemical etching, nanopits are formed with the Ag NPs staying inside of the nanopits. Complete solar cells with and without the Ag-assisted etching are fabricated to study the effects of the hybrid nanostructure. It is found that the optical reflection is effectively reduced by the nanostructure and the incident light is harvested more effectively for enhanced external quantum efficiency (EQE) by as much as 17% for the cells before the SiNx anti-reflection coating, and the EQE can be further enhanced by applying an electric bias 1V during the EQE measurement. It is believed that when the solar cell with the Ag NPs is illuminated, the local surface plasmon resonance (LSPR) is induced at the specific wavelength regions around 405 and 810nm. For ultraviolet light, the LSPR effect dominates the increases of the EQE while the effective medium effect is also believed to be responsible for the effective light trapping.

Study of concentric iridescent ring around the laser-induced pits on the solar cell surface

The laser-induced damage on the surface of monocrystalline silicon (m-Si) solar cells and GaAs/Gesingle heterojunction solar cells are investigated. The solar cells were irradiated by a continuous wave laser at the wavelength of 532nm. Concentric iridescent ring appeared on the damaged surfaces when observed with an optical microscope (OM) of broad spectrum. The damaged surface film was characterized by X-ray photoelectron spectroscopy (XPS) and the Contour Meter. The laser-induced temperature in silicon was calculated. The formation mechanism of the film and the cause of the concentric iridescent ring were analyzed.

Study on modification of single-walled carbon nanotubes on the surface of monocrystalline silicon solar cells.

Modification of single-walled carbon nanotubes (SWNTs) on the surface of monocrystalline silicon solar cells was investigated. The modification was realized by dropping a well-distributed mixture of SWNTs and ethanol with different dosages on the surface of monocrystalline silicon solar cells in the same effective area. The experimental results showed that the increasing rates of conversion efficiency, short-circuit current, and fill factor were 4.37%, 2.18%, and 2.11%, respectively; the open circuit voltage and series resistance decreased by 0.11% and 9.37% compared with the bare solar cell without an antireflection (AR) layer, when the modification reached the best state by dropping a 0.5 mL mixture solution with a concentration of 0.08 g/L. With the energy-band diagrams of the heterojunction and p-n junction, the principles of the modification of SWNTs on monocrystalline silicon solar cells and the reasons for the change of electrical parameters were analyzed theoretically. Through experiments and theoretical analyses, the modification of SWNTs on solar cells is a potential and effective way to improve the performance of solar cells.

Enhancing the efficiency of solar cells by down shifting YAG: Ce3+ phosphors

Ce3+-doped yttrium aluminum garnet (YAG:Ce3+) is a luminescent down shifting material, and can convert ultraviolet (UV) and blue lights into yellow light. In this paper, the YAG:Ce3+ phosphors are applied on the surface of a monocrystalline silicon solar cell. Although the number of total photons after the down shifting conversion does not increase, the quantum efficiency of the solar cell in short wavelength range has been improved. The reason is that the UV and blue photons can hardly generate carriers in the depletion region of the p–n junction because most of them are absorbed before getting there, while the yellow photons emitted from the YAG:Ce3+ phosphors can generate electron–hole pairs in the region. Experimental results show that the down shifting phosphors have enhanced the energy conversion efficiency of the solar cell from 15.30% to 15.46%. Although the efficiency increase is small, the down-shifting phosphors provide a possible way to reduce the energy loss in the short wavelength range.

External quantum efficiency response of thin silicon solar cell based on plasmonic scattering of indium and silver nanoparticles.

This study characterized the plasmonic scattering effects of indium nanoparticles (In NPs) on the front surface and silver nanoparticles (Ag NPs) on the rear surface of a thin silicon solar cell according to external quantum efficiency (EQE) and photovoltaic current–voltage. The EQE response indicates that, at wavelengths of 300 to 800 nm, the ratio of the number of photo-carriers collected to the number of incident photons shining on a thin Si solar cell was enhanced by the In NPs, and at wavelengths of 1,000 to 1,200 nm, by the Ag NPs. These results demonstrate the effectiveness of combining the broadband plasmonic scattering of two metals in enhancing the overall photovoltaic performance of a thin silicon solar cell. Short-circuit current was increased by 31.88% (from 2.98 to 3.93 mA) and conversion efficiency was increased by 32.72% (from 9.81% to 13.02%), compared to bare thin Si solar cells.

Diffractive Interference Design Using Front and Rear Surface Metal and Dielectric Nanoparticle Arrays for Photocurrent Enhancement in Thin Crystalline Silicon Solar Cells

Using an interference design model we were able to quickly optimize the nanophotonic control afforded by metal and dielectric nanoparticle arrays to enhance the absorption and photocurrent in a thin crystalline silicon solar cell, which were simulated via full field electromagnetic FEM calculations. Fabry–Perot interference fringes introduced by placing structures on the front and rear surfaces are shown to lead to significant enhanced light trapping in the absorbing silicon layer. We report a 46.7% increase in the estimated short-circuit photocurrent over the solar spectrum (AM 1.5G) range of 300–1100 nm for a 1 μm thick crystalline silicon solar cell with metal and dielectric nanoparticle arrays designed using interference modeling. The enhancement was achieved by placing nanoparticle arrays on the front and rear surface of the cell, where the particles on the front surface are dielectric (Si) and those on the rear surface are metal (Al). Additionally, an estimated photocurrent enhancement of 29.4% was obtained with just the aluminum nanoparticle array on the rear surface of the cell. Our study of nanoparticle arrays of metal (aluminum) and dielectrics (silicon, titanium oxide, aluminum oxide, and silicon nitride) involved different radius, pitch, and position on the front and rear surface of silicon solar cells with antireflection coatings. Improved light trapping and absorption thanks to the arrays are unambiguously shown. The rear surface array is shown to broadly reduce the reflectance of the cell via multiple Fabry–Perot resonances due to the interference of the incident and diffracted field from the array of particles. The front surface array broadly reduces the reflectance due to the excitation of local resonances and preferential scattering into the substrate. Together with local resonance excitation, these two combined effects are significant for the design of thin-film solar cells with light-trapping properties to increase the absorption and improve cell performance.

Design of Anodic Aluminum Oxide Rear Surface Plasmonic Heterostructures for Light Trapping in Thin Silicon Solar Cells

A metal-dielectric heterostructure that provides the combined capability of light trapping and surface passivation is reported. The light-trapping layer employs a porous aluminum anodic oxide (AAO) with metal nanoparticles formed in the pores on the rear surface of a thin crystalline silicon solar cell. Numerical finite-difference time domain (FDTD) simulations were performed to determine the pore diameter and spacing that would result in optimal light trapping for this metal-dielectric heterostructure. For a 2.5-μm-thick crystalline silicon device, the optimal pore diameter and spacing were determined to be ~250 and ~450 nm, respectively. These conditions resulted in an enhancement of the simulated photocurrent by ~12.6% compared with a device in which the heterostructure was replaced with a homogenous aluminum oxide layer. Simulations also confirmed that the thickness of an underlying dielectric layer should be minimized to 10-20 nm, with the AAO barrier layer being maintained as thin as possible. Finally, it was shown that replacement of silver by aluminum in the pores resulted in a reduction in the photocurrent of 6.3% and would necessitate much larger pore spacing that is difficult to achieve experimentally and would result in thicker AAO barrier layers, which are undesirable.

Optimum Process of Texturing on Silicon Solar Cell

Silicon solar cell surface texture is effective to reduce reflectance and improve light collection. Alkaline etchants is a conventional process of monocrystalline silicon texturing. Isopropyl alcohol (IPA) as buffer solution is expensive and high consumption, which increased the cost of solar cell. In reserch, It has been found that a cheaper surfactant as buffer solution decrease silicon surface interfacial force and increase etchant infiltrating of silicon wafer, so a large number of small and uniform texture appeared on the silicon surface. Pyramids size of about 3μm formed after etching of 15min. A new process to reduce the production cost and form the high quality pyramids texturing has been developed by the investigation.

REDUCTION OF REFLECTION LOSSES IN SOLAR CELL BY USING TIAL2 AND ZR ANTI REFLECTIVE COATING

Direct energy conversion from sunlight to electricity is obtained through solar cells. The Efficiency of solar cells is about 23.89% in the laboratory and 13.76%. More than 30% of incident light is reflected back from the surface of single crystalline Silicon (Si) solar cells because of the high refractive index of Silicon material. The initial objective of this project is to minimize the reflection losses in solar cells using an Anti-Reflection coating (ARC). In solar cell applications, a single layer thin film Anti-Reflective coating is often used. But such a Single Layer Anti-Reflective coating (SLARC) and Double layer Anti-Reflective coating reduces reflectivity only in a limited range. Triple Layer Anti-Reflective coatings (TLARC) are so widely utilized to improve conversion efficiencies and current density of silicon solar cells. Such Triple layer anti reflective coating can eliminate the need for a mechanical tracking device for proper optical alignment of the solar cell with respect to incident sunlight.

On the application of thin films of silicon nanoparticles for increasing solar cell efficiency

The effect of thin films of silicon nanoparticles (nc-Si), deposited onto the front surface of single-crystal silicon solar cells, on their conversion efficiency is studied. The thin films are grown using non-luminescent silicon nanoparticles with an average diameter of 12 nm with SiO x (0 ≤ x ≤ 2) shells and silicon nanoparticles 2 nm in diameter with organic shells of octadecene, which exhibit photoluminescence in the red spectral region. It was found that nc-Si film deposition increases the solar-cell conversion efficiency by 12% with respect to the initial value. An analysis of the current-voltage characteristics and reflectance spectra of solar cells allows the conclusion that the increase in the conversion efficiency is controlled by the passivation of defects on the front surface of the solar cell by nanoparticles and a decrease in the light reflectance of this surface.

Characterization of Silicon Oxides Formed by Light-Induced Anodisation for Silicon Solar Cell Surface Passivation

ABSTRACTIn this paper we report the properties of the anodic silicon dioxide film formed using light-induced anodisation (LIA) method and its potential to be used as surface passivation layer of p-type silicon surfaces of silicon solar cells. The high uniformity of the formed oxide is possibly due to the uniform drift of the positive charge carriers in the silicon to the surface being anodised. The oxide grows at higher rate than that in nitric acid, an oxide layer with thickness of 18 nm can be formed by anodising for 10 min with 15 V bias in 0.5 M sulphuric acid. After annealing in oxygen and then forming gas at 400 °C for 30 min, an average effective carrier lifetime of 120 μs was measured by quasi-steady state photoluminance on 180 μm p-type 3-5 Ohm cm Cz silicon wafers, with a value of 110 μs being measured for the same wafers passivated by a thermally-grown oxide of the same thickness. The properties of the anodic silicon dioxide layers formed by LIA have been characterized by ellipsometry, x-ray photoelectron spectroscopy, quasi-steady state photoluminance and Fourier transform infrared spectroscopy.

Optimization of Laser Processes for Local Rear Contacting of Passivated Silicon Solar Cells

Laser Firing Contact (LFC) and Laser Doping (LD) have become potential alternatives to the Al BSF thermal processing conventionally used in p-type c-Si solar cell rear contacts. Optimized LFC and LD processes allow, not only the generation of efficient micro-contacts, but also the diffusion of p-type doping impurities reducing the surface recombination velocity due to the formation of a local back surface field (BSF). In this work, three different laser strategies to create ohmic micro-contacts are studied: 1) evaporated Aluminum LFC, 2) Aluminum foil LFC and 3) Aluminum oxide (Al2O3) LD. The laser source used was a pulsed Nd-YAG 1064nm laser working in the nanosecond regime. Laser parameters were explored to optimize the electrical behavior of the contacts and their carrier recombination rate. Optimized laser parameters lead to specific contact resistance in the 1.0 - 1.3 mΩ·cm2 range for all three strategies. From the point of view of carrier recombination, better results were obtained for Al2O3 LD, probably related to the lower energy pulse needed to create the contact. Next, the three proposed laser approaches were applied to the back surface of heterojunction silicon solar cells. Contact quality was not limiting any cell performance indicating that the contact quality is good enough to be applied in high-efficiency c-Si cell concepts. On the other hand, surface recombination velocity at the rear surface on the final devices also points out to Al2O3 LD as the best alternative.

Nanotexturing Process on Microtextured Surfaces of Silicon Solar Cells by SF&lt;SUB&gt;6&lt;/SUB&gt;/O&lt;SUB&gt;2&lt;/SUB&gt; Reactive Ion Etching

We investigated a nanotexturing process on the microtextured surface of single crystalline silicon solar cell by the reactive ion etching process in SF6/O2 mixed gas ambient. P-type Si wafer samples were prepared using a chemical wet etching process to address saw damage removal and achieve microtexturing. The microtextured wafers were further processed for nanotexturing by exposure to reactive ions within a circular tray of wafer carrier containing many small holes for uniform etching. As the dry etching times were increased to 2, 4 and finally to 8 min, surface structures were observed in a transition from nanoholes to nanorods, and a variation in wafer color from dark blue to black. The surface nanostructures showed a lowered photoreflectance and enhanced quantum efficiency within the visible light region with wavelengths of less than 679 nm. The nanohole structure etched for 2 min showed enhanced conversion efficiency when compared to the bare sample; however, the nanorod structure etched for 8 min exhibited the decreased efficiency with a reduced short circuit current, indicating that the surface nanostructural damage with the enlarged nanoperimetric surface area is sensitive to surface passivation from the surface recombination process.

Effective passivation of silicon surface by AZO films: Application in bifacial solar cells

Aluminum rich zinc oxide (AZO) film was made on silicon substrates by spin coating using a precursor containing Al and Zn in the 20% atomic ratio. The films were annealed in air atmosphere and in hydrogen ambient. The minimum reflectance (Rλ) in air annealed (AZO:Air) samples was ∼2.5% in the vicinity (±50nm) of 750nm, however, the value was marginally lower (∼2%) in hydrogen annealed (AZO:H2) samples. The average Rλ over 300–1200nm range was 18.5% and 14.1% respectively. An additional XRD peak of H2Al10O16 at 2θ=51.5° corresponding to (121) reflection plane was observed in the AZO:H2 film. The AZO films were applied on the front and back surfaces of bifacial crystalline silicon (c-Si) solar cells where an improvement both in short circuit current density (Jsc) and open circuit voltage (Voc) was observed. The values of Jsc and Voc after AZO:Air were increased by 23% and ∼10mV respectively w.r.t without AZO layer cells which on subsequent annealing in H2 raised to 32% and ∼21mV. Jsc (AZO:Air) was used to calculate the expected enhancement in Voc which matched with the measured increase in Voc. The improvement in Jsc is due to reduced reflectance and, hence, the observed increase in Voc (AZO:Air) is attributed to it. On the other hand, increase in Voc after AZO:H2 step was much more and cannot be accounted only by Rλ, and therefore, the additional enhancement in Voc is attributed to back surface passivation that has occurred after annealing of the solar cells in hydrogen ambient. This attribution is substantiated by the internal and external quantum efficiencies (IQE and EQE) after back and front illumination of the solar cells and minority carrier lifetime measurements. The IQE data was used to calculate the diffusion length (Ln) of the base region where the value of Ln did not change after AZO:Air layer (=130μm for Cell #1) but its value substantially increased (=244μm) after the AZO:H2 treatment. This observation further reiterates our claim that the AZO film has surface passivation property.

Statistical Analysis of Surface Nanopatterned Thin Film Solar Cells Obtained by Inverse Optimization

This work is a statistical study of the broadband light absorption in thin film solar cells, enhanced by metallic surface nanotexturing. We consider optimum grating structures on the surface of amorphous silicon solar cells obtained by inverse optimization, and study the joint statistics of the resulting absorption enhancement/spectra in the presence of time and structural variants, such as fabrication error and year around changes in the solar irradiance, as well as the angle of incident. We adopt yearly data for solar irradiation at individual hours. In conjunction with the data for light absorption spectra at various incident angles and random samples of the fabrication error vector, we evaluate the real world performance of optimized solar cells. The resulting conclusions serve as a sensitivity/time analysis for better understanding the limits of performance and robustness of thin film cells and optimal light trapping mechanisms.

Self-patterned localized metal contacts for silicon solar cells

Abstract

Numerical simulations of rear point-contacted solar cells on 2.2 Ωcm p-type c-Si substrates

Rear surface of high-efficiency crystalline silicon solar cells is based on a combination of dielectric passivation and point-like contacts. In this work, we develop a 3D model for these devices based on 2.2 Ωcm p-type crystalline silicon substrates. We validate the model by comparison with experimental results allowing us to determine an optimum design for the rear pattern. Additionally, the 3D model results are compared with the ones deduced from a simpler and widely used 1D model. Although the maximum efficiency predicted by both models is approximately the same, large deviations are observed in open-circuit voltage and fill factor. 1D simulations overestimate open-circuit voltage because Dember and electrochemical potential drops are not taken into account. On the contrary, fill factor is underestimated because of higher ohmic losses along the base when 1D analytical model is used. These deviations are larger for relatively low-doped substrates, as the ones used in the experimental samples reported hereby, and poor passivated contacts. As a result, 1D models could mislead to too short optimum rear contact spacing. Copyright © 2013 John Wiley & Sons, Ltd.