Reactive Ion Etching Texturing Research Articles

Dry Etching Using Atmospheric Plasma for Crystalline Silicon Solar Cells

Reactive Ion Etching (RIE) and wet etching are employed in existing texturing processes to fabricate solar cells. Laser etching is used for particular purposes such as selective etching for grooves. However, such processes require a higher level of cost and longer processing time and those factors affect the unit cost of each process of fabricating solar cells. As a way to reduce the unit cost of this process of making solar cells, an atmospheric plasma source will be employed in this study for the texturing of crystalline silicon wafers. In this study, we produced the atmospheric plasma source and examined its basic properties. Then, using the prepared atmospheric plasma source, we performed the texturing process of crystalline silicon wafers. The results obtained from texturing processes employing the atmospheric plasma source and employing RIE were examined and compared with each other. The average reflectance of the specimens obtained from the atmospheric plasma texturing process was 7.88 %, while that of specimens obtained from the texturing process employing RIE was 8.04 %. Surface morphologies of textured wafers were examined and measured through Scanning Electron Microscopy (SEM) and similar shapes of reactive ion etched wafers were found. The Power Conversion Efficiencies (PCE) of the solar cells manufactured through each process were 16.97 % (atmospheric plasma texturing) and 16.29% (RIE texturing).

Cell-to-Module Variation of Optical and Photovoltaic Properties for Monocrystalline Silicon Solar Cells with Different Texturing Approaches

The power output and efficiency of solar cells are affected by the encapsulation, a fact that is often overlooked when the optical performance of different types of wafers is compared. In this work a full understanding of the impact of textured monocrystalline silicon wafers typically applied in the industry on the cell-to-module (CTM) variation of reflectivity, current–voltage (IV), and quantum efficiency (QE) characteristics, has been developed. Comparing to the acid and reactive ion etching (RIE), texturization of silicon surface with alkali (KOH) treatment was found to have significantly reduced reflection by 7.02%, enhanced short-circuit current (Isc) by 9.325 A, the maximum power (Pmax) by 4.770 W and external quantum efficiency (EQE) by 83.76%. The internal quantum efficiency (IQE) spectra indicates that recombination within the nanoporous surface from RIE texture evidently limits its blue IQE and then affects the Isc. After encapsulation, the CTM variation from reduced reflection and IQE results in a coupling effect of short circuit current gain. However, such current gain leads to a CTM power reduction due to the increased resistance (Rs) loss of I2Rs. Compared to alkali and RIE texturing, the acid texture approach can give the minimum CTM power loss, although obtained absolute power is the smallest. Further photoelectric conversion efficiency improvements are possible with better texturing structures, while the coupling CTM gain can be increased by optimizing encapsulation process.

Solar cell fabrication using two-step textured quasi-single crystalline silicon by alkaline and RIE texturing processes

After alkaline chemical texturing process, SF6/O2 reactive ion etching (RIE) was used for maskless textured Si surface independently from crystal orientation. Additional RIE texturing on alkaline textured Si surface increased the light receiving areas and decreased the surface reflectance by random pyramidal structures with a width of hundreds of nanometers. The weighted average reflectance (WAR) of the textured Si surface by two-step texturing processes was achieved as low as 9.6% in the 300–1200 nm wavelength range. Compared to the alkaline texturing, the alkaline and RIE texturing of front Si surfaces decreased the WAR by 6%. By means of two-step texturing processes for quasi-single crystalline silicon (QSC-Si) surfaces, performance parameters of the solar cell such as conversion efficiency, short circuit current density (Jsc), open circuit voltage (Voc) and fill factor (FF) were examined in 18.2%, 36.6 mA/cm2, 624 mV and 79.7%, respectively.

Improvement of conversion efficiency of multi-crystalline silicon solar cells using reactive ion etching with surface pre-etching

In this study, a large-area (156×156mm2) sub-wavelength antireflection structure has been fabricated on multi-crystalline Si substrates to reduce their surface reflectivity. A reactive ion etching (RIE) system was used to fabricate nanostructures on the multi-crystalline silicon surface. Reactive gases, comprised of chlorine (Cl2), sulfur hexafluoride (SF6) and oxygen (O2), were activated to fabricate nanoscale pyramids by radio frequency plasma. The pre-etching of poly-Si substrates using acidic and alkaline solutions was studied for creating the microstructures, which may affect the subsequent formation of nano-structures by RIE. Both the solar reflectance and the effective carrier lifetimes of multi-crystalline Si surface are measured to understand the effects of two pre-etching solutions. The pre-etching of silicon wafers using the acidic solution before RIE was found to improve the solar cell efficiency better than that using the alkaline solution. The absolute efficiency of solar cells with acidic pre-etching and RIE texturing can be improved by 0.64% over the traditional conventional cells.

Monocrystalline-like silicon solar cells fabricated by wet and dry texturing processes for improving light-trapping effect

Surface texturing techniques of crystalline silicon (Si) wafers are an effective and permanent method for increases in the conversion efficiency of solar cells by reducing reflection losses and improving light trapping effects compared to antireflection coatings such as hydrogenated silicon nitride (SiNx:H) thin films. In this study, monocrystalline-like Si surfaces were textured by acidic solutions followed by reactive ion etching (RIE). Also, we investigated the optical and electrical properties of Si surfaces prepared by different texturing processes. By using the acidic and RIE texturing processes for monocrystalline-like Si surfaces, we achieved open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF) and conversion efficiency as high as 633 mV, 36.7 mA/cm2, 77.5% and 18%, respectively, in a solar cell with an active surface area of 156 × 156 mm2 (243 cm2). Compared to the conventional acidic texturing, the acidic and RIE texturing of front Si surfaces improved the solar cell conversion efficiency of 0.9%. Therefore, the results indicate that large-area monocrystalline-like Si solar cells with high efficiency can be fabricated by standard wet chemical and additional RIE texturing processes.

Investigation of surface features for 17.2% efficiency multi-crystalline silicon solar cells

Maskless reactive ion etching (RIE) texturing using a gas mixture of sulfur hexafluoride–oxygen (SF6/O2) and sulfur hexafluoride–oxygen–chlorine (SF6/O2/Cl2) was investigated to reveal the proper shape in surface features for higher efficiency multi-crystalline silicon (mc-Si) solar cells; hence, needle-like and round-top cone (RT cone) shapes were formed by RIE texturing with SF6/O2 gas, and pyramid and inverted pyramid shapes by RIE texturing with SF6/O2/Cl2 gas. RIE-textured mc-Si solar cells were fabricated on these surface features except for an inverted pyramid structure in the industrial production line. Performances of cells with RT cone and pyramid shapes were enhanced, whereas those with a needle-like cone were degraded, compared to the reference cells. Among these cells, those with RT cones represented the highest efficiency at 17.22%. By considering diode characteristics and electroluminescence images of fabricated solar cells, the proper shape for surface features was intimately related to control of the formation of a stable emitter layer as well as the reduction of surface reflectance.

Improvement of conversion efficiency of multicrystalline silicon solar cells by incorporating reactive ion etching texturing

The reactive ion etching in combination with acidic etching (acidic+RIE) is applied to form the front surface texturing of 156×156mm2 multicrystalline silicon (mc-Si) wafers in order to improve the cell efficiency. The scanning electron microscope (SEM) analyses indicate that the RIE process produces dense nanoscale ridge-like structures based on the acidic textured surfaces, and these structures generate an excellent antireflection effect. The matching processes including the post-cleaning, the phosphorus diffusion, and the deposition of silicon nitride (SiNX) antireflection coating are optimized. The acidic+RIE textured surfaces in combination with high sheet resistance emitters result in a remarkable enhancement in short wavelength response and then improve the short circuit current density (JSC) significantly. The absolute conversion efficiency of acidic+RIE textured solar cells is improved 0.51% on average compared to the acidic textured solar cells in mass production, and a maximum full-area cell efficiency of 18.49% is achieved on the mc-Si solar cell with a conventional cell structure.

A comparative study of wet and dry texturing processes of c-Si wafers for the fabrication of solar cells

A comparative study of texturing c-Si wafers using wet and dry processes has been performed in this work; our aim was to produce pyramid-like structures on c-Si surfaces with low reflectance values, employing any of both processes. For wet texturing solutions consisting of potassium hydroxide (KOH) or sodium hydroxide (NaOH), combined with deionized water (DI H2O) and isopropyl alcohol (IPA) were used, while for dry texturing sulfur hexafluoride/oxygen (SF6/O2) plasmas in a standard Reactive Ion etching (RIE) were employed. Our results demonstrated that RIE texturing produces highly textured c-Si surfaces, with pyramid-like structures, providing even lower reflectance values than those obtained with wet processes, which are currently used in the solar cells manufacturing industry.

Fabrication of Silicon Pyramid-Nanocolumn Structures with Lowest Reflectance by Reactive Ion Etching Method

We have developed nanosized structures on a silicon surface by the reactive ion etching method. The purpose of this work is to fabricate pyramid-nanocolumn structures for crystalline silicon solar cells using a reactive ion etching (RIE) system with a metal mesh and to simulate surface texturing structures using the PC1D program. The reflectance of the structures was lower than that of only-pyramid or only-nanocolumn structure arrays. We formed micropyramids by general anisotropic saw damage removal (SDR) etching using potassium hydroxide (KOH) solution, as well as nanocolumn structures by RIE with a metal mesh and SF6/O2 gas (35/30 sccm). We studied the fundamentals and morphologies of structures (micropyramids, nanocolumn structures, and pyramid-nanocolumn structures). Owing to anisotropic SDR etching in KOH solution for a single-crystalline silicon wafer, the whole wafer surface is covered with pyramid structures with a size range of 1–5 µm. After RIE texturing, nanocolumn structures with a size range of 20–50 nm were formed on the pyramid structures. Owing to binary surface texturing with pyramid and nanocolumn structures, spectra with weighted average reflectances below 3% in the wavelength range from 350 to 1100 nm were obtained under optimized condition. In particular, the average reflectance was 1.16% in the wavelength range of 400 to 1000 nm without an antireflection coating (ARC). The combination of the pyramid and nanostructures has a very low reflectance. Therefore, ARC is not required in the solar cell process in the case of using pyramid-nanocolumn structures.

Random reactive ion etching texturing techniques for application of multicrystalline silicon solar cells

To maximize the conversion efficiency of solar cells by improving the trapping of incident light, surface texturing of multicrystalline silicon wafers is a more permanent and effective solution to decrease reflections compared with antireflection coatings. In general, many techniques of wet chemical surface texturing depend on crystal orientation. Also, these processes place a limitation on the growth of solar production capacity due to consumption of the large amount of material, de-ionized water and chemicals. Reactive ion etching (RIE) is therefore useful for mc-Si solar cells with the random grain orientation. In this study, we have fabricated mc-Si solar cells with a large-area (156mm×156mm) by a random RIE texturing method using SF6/O2 plasma chemistry. A large amount of silicon loss, which is due to saw damage removal and texturing process of mc-Si wafers, have dramatically decreased by applying RIE texturing process. As a result of the optimum RIE process, the best mc-Si solar cell showed conversion efficiency, fill factor, short circuit current density, and open circuit voltages as high as 17.4%, 80%, 35mA/cm2 and 620mV, respectively. This study demonstrates that it is possible to apply the thin mc-Si Solar cells fabrication for low cost and high efficiency in conventional industrial production line.

Fabrication and Characteristics of mc-Si Solar Cells with RIE-Textured Surface

Reactive ion etching (RIE) texturing is well-known as an effective method to form the surface structure on a multi-crystalline (mc-Si) wafer that has grains with randomly oriented crystallites. The saw damage removal (SDR) process using HF/HNO3/D.I (HND) solution was employed in this work, since the etching rate of RIE dry etching was lower than that of wet etching. The surface morphology on mc-Si surface was formed by RIE using a gas flow ratio of SF6:O2 = 1:1.22. The control of RF power and working pressure could etch the mc-Si surface of the 15.6 × 15.6 cm2 area uniformly during RIE texturing process. The surface morphologies textured for 5 and 10 min were needle-like structures and sharp grass-like structures, respectively. Solar cells with the needle-like structure had higher values for open circuit voltage (Voc), short circuit current (Isc), fill factor (FF), and efficiency, despite higher reflectance compared to those with the sharp grass-like structure. The cell textured for 10 min was expected to have non-homogeneous emitter layer as the dark I-V curves of the cells textured for 5 and 10 min were compared.

Industrial 6 Inch Multicrystalline Silicon Solar Cells Fabricated Using Reactive Ion Etching with Efficiency Exceeding 18%

To obtain a lower reflectance, we applied a maskless plasma texturing technique by reactive ion etching (RIE) to acid-textured multicrystalline silicon (mc-Si) wafer. RIE texturing produced a deep and narrow textured surface with an excellent low reflectance. Owing to plasma-induced damage, unless the RIE-textured surface is subjected to proper damage removal etching (DRE), it shows drops in open circuit voltage (Voc) and fill factor (FF). RIE-textured samples with proper DRE showed sufficiently higher short circuit current (Isc) than acid-textured samples without a drop in Voc. In this study, we applied RIE texturing under optimized DRE condition to the selective emitter structure. In comparison with the acid-textured solar cells, RIE-textured solar cells have absolute gains in Isc above 200 mA. We successfully fabricated a 6-in. mc-Si solar cell with a conversion efficiency exceeding 18% by applying selective emitter technology with RIE texturing.

Fabrication and characterization of monocrystalline-like silicon solar cells

Due to the necessity of associating high-purity monocrystalline silicon and lower-cost multicrystalline silicon wafers, the monocrystalline-like silicon technique is used as a promising method for solar-cell fabrication. The grain orientation of this material is composed of both 〈111〉 and 〈100〉 orientations. As a result of a combination for properties on the basis of two types of materials, silicon surface texturing was carried out by using alkaline (NaOH) chemical and reactive ion etching (RIE) texturing. In this research, we investigated the optical and the electrical properties of silicon surfaces prepared by using different texturing processes. By using the alkaline and RIE texturing processes, we obtained a fill factor of 78.6% with a conversion efficiency of 17.2% for a monocrystalline-like silicon wafer with an active surface area of 243 cm2. Compared to alkaline texturing, this conversion efficiency shows an increase of 0.5%. Thus, large-area monocrystalline-like silicon solar cells with high efficiency and low cost can be fabricated.

17.6% Conversion Efficiency Multicrystalline Silicon Solar Cells Using the Reactive Ion Etching with the Damage Removal Etching

For lower reflectance, we applied a maskless plasma texturing technique using reactive ion etching (RIE) on acidic-textured multicrystalline silicon (mc-Si) wafer. RIE texturing had a deep and narrow textured surface and showed excellent low reflectance. Due to plasma-induced damage, unless the RIE-textured surfaces have the proper damage removal etching (DRE), they have a drop inVocand FF. RIE texturing with a proper DRE had sufficiently higher short circuit current(Isc)than acidic-textured samples without a drop in open circuit voltage(Voc). And in order to improve efficiency of mc-Si solar cell, we applied RIE texturing with optimized DRE condition to selective emitter structure. In comparison with the acidic-textured solar cells, RIE-textured solar cells have above 200 mA absolute gain in Isc. And optimized RIE samples with a DRE by HNO3/HF mixture showed 17.6% conversion efficiency, which were made using an industrial screen printing process with selective emitter structure.

공정가스와 RF 주파수에 따른 웨이퍼 표면 텍스쳐 처리 공정에서 저반사율에 관한 연구

일반적으로 결정질 실리콘 태양전지에서 표면에 텍스쳐링(texturing)하는 것은 알칼리 또는 산성 같은 화학용액을 사용하고 있다. 그러나 실리콘 부족으로 실리콘의 양의 감소로 인하여 웨이퍼 두께가 감소하고 있는 추세에 일반적으로 사용하고 있는 습식 텍스쳐링 방법에서 화학용액에 의한 많은 양의 실리콘이 소모되고 있어 웨이퍼의 파손이 심각한 문제에 직면하고 있다. 그리하여 습식 텍스쳐링 방법보다는 플라즈마로 텍스쳐링할 수 있는 건식 텍스쳐링 방법인 RIE (reactive ion etching) 기법이 대두되고 있다. 그리고 습식 텍스쳐링으로는 결정질 실리콘 태양전지의 반사율을 10% 이하로는 낮출 수가 없다. 다결정 실리콘 웨이퍼 표면에 텍스쳐링을 하기 위하여 125 mm 웨이퍼 144개를 수용할 수 있는 대규모 플라즈마 RIE 장비를 개발하였다. 반사율을 4% 이하로 낮추기 위하여 공정가스는 <TEX>$Cl_2$</TEX>, <TEX>$SF_6$</TEX>, <TEX>$O_2$</TEX>를 기반으로 RIE 텍스쳐링을 하였고 텍스쳐링의 모양은 공정가스, 공정시간, RF 주파수 등에 의해 조절이 가능하였다. 본 연구에서 RIE 공정을 통하여 16.1%의 변환효율을 얻었으며, RF 주파수가 텍스쳐링의 모양에 미치는 영향을 살펴보았다. Conventional surface texturing in crystalline silicon solar cell have been use wet texturing by Alkali or Acid solution. But conventional wet texturing has the serious issue of wafer breakage by large consumption of wafer in wet solution and can not obtain the reflectance below 10% in multi crystalline silicon. Therefore it is focusing on RIE texturing, one method of dry etching. We developed large scale plasma RIE (Reactive Ion Etching) equipment which can accommodate 144 wafers (125 mm) in tray in order to provide surface texturing on the silicon wafer surface. Reflectance was controllable from 3% to 20% in crystalline silicon depending on the texture shape and height. We have achieved excellent reflectance below 4% on the weighted average (300~1,100 nm) in multi crystalline silicon using plasma texturing with gas mixture ratio such as <TEX>$SF_6$</TEX>, <TEX>$Cl_2$</TEX>, and <TEX>$O_2$</TEX>. The texture shape and height on the silicon wafer surface have an effect on gas chemistry, etching time, RF frequency, and so on. Excellent conversion efficiency of 16.1% is obtained in multi crystalline silicon by RIE process. In order to know the influence of RF frequency with 2 MHz and 13.56 MHz, texturing shape and conversion efficiency are compared and discussed mutually using RIE technology.

Wafer Surface Charge Reversal as a Method of Simplifying Nanosphere Lithography for Reactive Ion Etch Texturing of Solar Cells

A simplified nanosphere lithography process has been developed which allows fast and low-waste maskings of Si surfaces for subsequent reactive ion etching (RIE) texturing. Initially, a positive surface charge is applied to a wafer surface by dipping in a solution of aluminum nitrate. Dipping the positive-coated wafer into a solution of negatively charged silica beads (nanospheres) results in the spheres becoming electrostatically attracted to the wafer surface. These nanospheres form an etch mask for RIE. After RIE texturing, the reflection of the surface is reduced as effectively as any other nanosphere lithography method, while this batch process used for masking is much faster, making it more industrially relevant.

Improving the Anti-seizure Ability of SiC Seal in Water with RIE Texturing

The demand for higher pressure and higher speed of sealing systems is currently increasing due to stricter standards for permissible emissions. Silicon carbide (SiC) is thought to be a promising material for sealing systems operating in water, since the combination of SiC and water is both environmentally friendly and energy saving. The purpose of this study is to improve the anti-seizure ability of SiC seals working in water by means of surface texturing. The texture pattern of micro-pits evenly distributed in a square array is formed on one of the contact surfaces by reactive ion etching (RIE). Experiments which simulate the working condition of mechanical seals were carried out to evaluate the effect of micro-pits on the critical seizure load. It is found that micro-pit texturing is an effective way to stabilize friction, to reduce the friction coefficient, and to expand the low-friction range (μ < 0.05) of SiC seals working in water.

Reactive ion etching (RIE) as a method for texturing polycrystalline silicon solar cells

Reactive ion etching (RIE) has been applied and developed as a method for texturing polycrystalline silicon solar cells. Two structures (microgrooves and pyramids) were produced in this work. Reflectivity measurements between 400–1200 nm show an overall reflectance of 5.6% for pyramid and 7.9% for groove structures. These results are better than those using wet anisotropic etch techniques on single-crystal silicon. RIE texturing was performed on cast polycrystalline silicon and produced better reflection control than standard TiO 2 antireflection coatings. RIE texturing also changes the incidence angle of light into the silicon, this improves the response for long wavelengths which can be utilised in thin film, polycrystalline or amorphous silicon solar cells.

96/06196 15%-efficient multicrystalline-silicon photovoltaic modules: Cell processing and characterization

In this study, a large-area (156 × 156 mm2) sub-wavelength antireflection structure has been fabricated on multi-crystalline Si substrates to reduce their surface reflectivity. A reactive ion etching (RIE) system was used to fabricate nanostructures on the multi-crystalline silicon surface. Reactive gases, comprised of chlorine (Cl2), sulfur hexafluoride (SF6) and oxygen (O2), were activated to fabricate nanoscale pyramids by radio frequency plasma. The pre-etching of poly-Si substrates using acidic and alkaline solutions was studied for creating the microstructures, which may affect the subsequent formation of nano-structures by RIE. Both the solar reflectance and the effective carrier lifetimes of multi-crystalline Si surface are measured to understand the effects of two pre-etching solutions. The pre-etching of silicon wafers using the acidic solution before RIE was found to improve the solar cell efficiency better than that using the alkaline solution. The absolute efficiency of solar cells with acidic pre-etching and RIE texturing can be improved by 0.64% over the traditional conventional cells.