Multicrystalline Si Wafers Research Articles

Novel technique to study the wet chemical etching response of multi-crystalline silicon wafers

The current work aimed to demonstrate the application of a technique where white light interferometry (WLI) and Laue X-ray crystallography scanner characterisation were combined to study the chemical etching response of diamond cut multi-crystalline Si (mc-Si) wafers. Using this technique, the effect of different texturing additives (isopropyl alcohol, natrium hypochlorite) was evaluated by examining the topography of the mc-Si surfaces before and after etching. The etching responses of monocrystalline Si wafers of (100), (110) and (111) orientations were used as reference for comparison with the multi-crystalline wafers investigated. The texturing results illustrated the influence of different crystal-orientations on the etching rate. It was revealed that for the mc-Si wafers, the etching speed of the different crystal grain-planes is increasing with their crystallographic similarity with the main (hkl) planes (100, 110,111). The comparison of isopropyl alcohol (IPA) and sodium hypochlorite (NaOCl) additives to KOH solutions showed that NaOCl additive is favourable for the polishing of mc-Si wafers, while IPA can be used as polishing only for crystal grains close to the (111) orientation.

Phase transformation pre-treatment of diamond wire-sawn multi-crystalline silicon wafers for metal-assisted chemical etching of solar cells

Phase transformation by the heat treatment, prior to metal-assisted chemical etching (MACE) of solar cells, has been proposed and studied, as a pre-treatment approach for diamond wire sawn (DWS) Si wafers. The results of the study show that the phase transformation pre-treatment can modify the microstructure of amorphous Si surface layers of DWS multi-crystalline (mc) Si wafers. The modified surface layer proved to be more reactive to the chemical etching solution, which was the primary factor for the improvement in the treated solar cells. A significant improvement on the surface uniformity and morphology of the textured Si wafer was achieved, subsequent to the pre-treatment, during the MACE texturization process. These solar cells based on the treated DWS mc-Si wafers displayed a gain of about 0.18% (absolute value), in the overall photoelectric conversion efficiency (η), when compared to the solar cells of the same batch, for the untreated wafers, on the same production line. It is hence, proven that this pre-treatment method, when used as a supplementary technique to the wet-chemical texturization process, has great potential as an effective application technology for solar cells based on DWS mc-Si wafers.

Effects of surface morphology on Ag crystallite formation in screen-printed multi-crystalline Si solar cells

The electrode collects electrons produced by a solar cell. Ag crystallites generated during front electrode formation play an important role in transferring electrons from the emitter to the Ag finger. Previously, the formation mechanism of Ag crystallites and the various factors influencing Ag crystallite formation have been studied, including the effect of surface morphology on Ag crystallite formation. The glass coverage varies with the texture of the Si surface, and thus the degree of exposure of the Si surface varies correspondingly. When the Si surface is exposed, direct contact is achieved, and contact resistance is improved. However, the effect of surface morphology on the Ag crystallites formed on the Si surface under the glass layer has not yet been studied. The effect of surface morphology on Ag crystallite formation is presented in this work. Two multi-crystalline Si wafers with different surface morphologies were compared. The first multi-crystalline Si wafer was textured using metal-catalyzed chem ical etching (MCCE). The second multi-crystalline Si wafer was textured by MCCE followed by reactive ion etching. Wafers with sharper textures had lower series resistances, higher fill factors, and a large amount of Ag crystallites. Ag crystallites primarily formed at the tips of the texture. To analyze the effect of surface morphology, the surface electron concentration was confirmed by simulation. Electron concentration at the tip was proportional to the sharpness of the tip. Therefore, when the firing process is performed at the same temperature, wafers with sharper tips are expected to exhibit better electrode characteristics.

Wet Texturing of a Multi-Crystalline Si-Wafer Using the Phenol-Formaldehyde Resin Mask

In the process of multi-crystalline silicon etched by the Metal Assisted Chemical Etching (MACE) method, due to its too small and uneven microstructures, heavy doping often occurs, which usually requires mixed acid (HNO3+HF) treatment, but this will ultimately affect the efficiency of crystalline silicon solar cells. In this article, phenol-formaldehyde (PF) resin, a main component of the positive photoresist, and water were dissolved in ethanol to form a PF resin-water-ethanol solution (PWEs) toward wet etching of multi-crystalline Si wafer, aim to prepared the texture structure with uniform distribution and reasonable size. A liquid film of PWEs was prepared on the Si wafer using dip-coating method. After the ethanol volatilizing, a mask of the resin was formed on the wafer through phase separation between the resin and water. A net like or a particle array mask was formed on the wafer through controlling the water/resin ratio, pulling speed and drying speed of the liquid film. With appropriate parameters above, a desirable particle array mask of PF resin was prepared. The size of particles and their spaces in this array were about $1{\mu }\text{m}$ and $2~{\mu }\text{m}$ respectively. With assistance of this particle array as a mask, a better textured surface of the Si wafer was prepared through MACE combined with etching of HNO3-HF mixed solution. The minimum reflectance of the wafer with the textured surface was as low as 14.2 %.

Forming submicron in micron texture on the diamond‐wire‐sawn mc‐Si wafer by introducing artificial defects

AbstractBased on a traditional acid etch system (i.e., HNO3/HF), a complex texture comprising microscale and submicroscale structures was produced on the surface of a diamond‐wire‐sawn (DWS) multicrystalline Si (mc‐Si) wafer, upon whose surface it is typically difficult to form an effective texture for suppressing the reflection of incident light. Immersing the as‐cut wafer into an HF/HNO3/AgNO3 solution introduced a large number of artificial defects onto the wafer surface. A subsequent HNO3/HF etch induced a micron texture expanded from the original DWS‐induced damage as well as a submicron texture converted from the artificial defects. The multiscale textured DWS exhibited a reflectivity of ~19%, which is much lower than the reflectivity after only an HNO3/HF etch (~28%). Therefore, the solar cell performance was improved owing primarily to improved optical antireflection and surface passivation. The method is simple and can be easily scaled up into the in‐line texture process.

Surface structure chemical transfer method to fabricate low reflectance multi-crystalline Si wafers produced with fixed abrasive machining technology

Conventional acidic etching cannot form low reflectance surfaces from multi-crystalline silicon (multi-Si) wafers sawn by fixed abrasive (FXA) machining technology, which makes it difficult to replace the time- and cost-consuming free abrasive (FRA) machining method. In the present work, a nanocrystalline Si (nc-Si) layer is formed by use of the surface structure chemical transfer (SSCT) method, and the layer is used as a starting point of alkaline etching to fabricate low reflectance submicron texture on FXA multi-Si wafers. Although the nc-Si layer cannot be passivated by deposition of a silicon nitride (SiN) layer, the submicron textured surface formed from the nc-Si layer by alkaline etching can effectively be passivated by the SiN layer. Using the developed method, the SiN passivated submicron textured FXA multi-Si wafers possess both high minority carrier lifetime and lower reflectance than that of acidic textured FRA multi-Si wafers. The excellent passivation effect of the SiN layer on the low reflectance textured surface is attributed to the low interface state density of 1.2×1011 cm-2eV-1.

A Phenomenological Model of the Screen-Printed, Silver Paste Contact to Si Substrate

A phenomenological model of screen-printed silver contact to an n-doped, p-type multi-crystalline Si wafer, based on extensive electrical, morphological, and compositional evaluations, has been developed. Rapid and quasi steady state heating configurations over broad (150–925°C) temperature ranges were investigated. Conventional rapid thermal annealing (RTA) with a conveyor belt was used for a rapid and custom-designed three-zone quartz tube furnace (QTF) for slow temperature variations. Lowest contact resistivity at 0.15 mΩ cm2 was observed in RTA horizontal configuration which was 25 times smaller than the same in QTF. RTA contact resistivity measurements revealed a minimum at 870°C while linear reduction in contact resistance was observed for the QTF configuration. The silver/silicon contact was based on three physical mechanisms: (1) migration of Si into glass and silver regions of the paste, (2) intermixing of silver and silicon (nano- and micrometer scale), and (3) epitaxial growth of silver/silicon crystallites. Experimental evidence of silicon migration was supported through extensive phosphorous concentration measurements from silicon and silver/silicon regions. The glass film with a colloidal distribution of randomly-distributed silver/silicon crystallites leads to lower contact resistance. Rapid temperature fluctuations facilitate development of Ag/Si crystallites. The higher contact resistance in quasi steady state thermal configuration was attributed to glass films with reduced density of Ag/Si crystallites. This disadvantage may be eliminated through post-contact, forming gas annealing at lower temperatures.

Nickel assisted chemical etching for multi-crystalline Si solar cell texturing: a low cost single step alternative to existing methods

The texturing of silicon surfaces is a well-known method of reducing the reflection from the surface of crystalline Si solar cell devices. With the utilization of diamond wires in recent advances in wafer slicing technology, surface texturing for the multi-crystalline Si wafers by the traditional acid-based texturing technique has become difficult. Metal-Assisted Etching (MAE) has been shown to be a promising and low-cost alternative to the traditional acid-based isotropic texturing. This paper reports, for the first time, a new single-step Ni-assisted etching technique to obtain nano-scale porous structures, that is, black silicon on multi-crystalline wafers. We observed lower reflection results in comparison with standard isotropic texturing using a standard acid solution. The structural and optical properties of the surface were identified through reflection measurements and scanning electron microscopy imaging. As a final step, the optimized texturing process was applied to multi-crystalline solar cell devices and showed promising results regarding cell performance parameters.

Investigated performance improvement of the micro-pressure sandblast-treated multi-crystalline Si wafer sliced using diamond wire sawing

In this work, the textured surface of the multi-crystalline silicon (mc-Si) wafers sliced by the diamond wire sawing (DWS) technique was formed using the X-Y axis micro-pressure sandblast treatment and the acidic texturing process. From the scanning electron microscopy (SEM) images, the X-Y axis micro-pressure sandblast treated DWS-sliced mc-Si wafers could effectively remove the saw marks caused by the fixed diamond abrasives. Furthermore, the textured surface of the acidic-textured DWS-sliced mc-Si wafers was also observed in the SEM images. Besides, using the X-Y axis micro-pressure sandblast treatment, the average reflectivity of the DWS-sliced mc-Si wafers was reduced from 28.94% to 22.28%. Consequently, the short-circuit current of 8.70 A and the power conversion efficiency of 17.92% for the DWS-sliced mc-Si solar cells with micro-pressure sandblast treatment were respectively better than 8.59 A and 17.35% in comparison with the DWS-sliced mc-Si solar cells without micro-pressure sandblast treatment.

Temperature dependent photoluminescence imaging calibrated by photoconductance measurements

In this paper we present a novel method for measuring temperature dependent lifetime images with high spatial resolution using photoconductance calibrated photoluminescence (PL) imaging. In order to achieve this, PL images are recorded at various temperatures by implementing a temperature stage into a commercial, steady state PL imaging setup. Carrier lifetime images are then calculated from the detected PL intensity, based on quasi-steady state photoconductance calibration measurements performed at the same temperatures that are used for the PL images. By analysing the carrier lifetime data as a function of injection level and temperature, this method allows for in-depth, spatially resolved defect characterization of Si wafers. Such temperature dependent lifetime data are also useful as input for device simulations of the temperature coefficient of solar cell efficiency. The uses of the method are illustrated with different examples based on commercial high performance multicrystalline Si wafers.

Flash spectral imaging for optical metrology of solar cells

Flash spectral imaging of full area (156 mm by 156 mm) silicon solar wafers and cells is realized in a setup integrating pseudo-monochromatic LEDs over the wavelength range of 370 to 1050 nm and a high-resolution monochrome camera. The captured information allows the computation of sample reflectance as a function of wavelength and coordinates, thereby constituting a spectral reflectance map. The derived values match that obtained from monochromator-based measurements. Optical inspection is then based on the characteristic reflectance of surface features at optimally contrasting wavelengths. The technique reveals otherwise hidden stains and anti-reflection coating (ARC) non-uniformities, and enable more selective visualization of grains in multicrystalline Si wafers. Optical contrast enhancement of metallization significantly improves accuracy of metal detection. The high effective resolution of the monochrome camera also allows fine metallization patterns to be measured. The rapid succession of flash-and-image-capture at each wavelength makes the reported optical metrology technique amenable in photovoltaic manufacturing for solar wafers/cells sorting, monitoring and optimization of processes.

다양한 실리콘 웨이퍼 제조를 위한 와이어 전기 방전가공

Wire electrical discharge machining (WEDM) process was evaluated to slice Silicon (Si) for various applications. Specifically, various Si workpieces with various resistances, such as single and multi crystalline Si bricks and wafers were used. As conventional slicing processes, such as slurry-on or diamond-on wire slicing, are based on mechanical abrasions between Si and abrasive, there is a limitation to decrease the wafer thickness as well as kerf-loss. Especially, when the wafer thickness is less than <TEX>$150{\mu}m$</TEX>, wafer breakage increases dramatically during the slicing process. Single crystalline P-type Si bricks and wafers were successively sliced with considerable slicing speed regardless of its growth direction. Also, typical defects, such as microcracks, craters, microholes, and debris, were introduced when Si was sliced by electrical discharge. Also, it was found that defect type is also dependent on resistance of Si. Consequently, this study confirmed the feasibility of slicing single crystalline Si by WEDM.

A new single-component low-cost emitter etch-back process for silicon wafer solar cells

A way of achieving lightly doped emitter is a combination of a heavy emitter diffusion and emitter etch back, which has an added advantage of phosphorous diffusion gettering. However, this chemical emitter etch-back process must fulfil some critical requirements, e.g. cost-effectiveness, near-conformal Si etching even after deep emitter etch back, controlled Si etch rate, post-etch clean Si surface and lowest safety issues in chemical handling and drainage. In this work, we report a new low-cost (less than 1 US Cents/wafer), single-chemical, non-acidic, high-throughput emitter etch-back process for tube-diffused emitters for crystalline Si wafers. This process uses only sodium hypochlorite solution at 80 °C as the Si etchant. This process is versatile with its applications on phosphorous and boron tube-diffused monocrystalline Si and phosphorous tube-diffused multicrystalline Si wafers. The preparation, usage and drainage of this highly diluted solution are easy and safe. The Si etching process leads to excellent spatial uniformity over large-area Si wafers (243 cm2). With deep etch back resulting in a change of sheet resistance by ~60 Ω/sq, the standard deviation value changes by only 2.7%. High surface conformity in the etch-back surface is evident from reflectance studies. Quasi-steady-state photoconductance and photoluminescence imaging are used to demonstrate improved electrical parameters of the etch-back wafers.

21.63% industrial screen-printed multicrystalline Si solar cell

In this paper, we demonstrate industrially feasible large-area solar cells achieving energy conversion efficiency up to 21.63% on p-type boron doped multicrystalline Si wafers. Advanced light trapping, passivation and hydrogenation technology are used to achieve excellent light absorption with very low surface recombination velocity. The bulk lifetime of the multi-crystalline Si wafers used for the fabrication exceeds 500 μs after optimized gettering and hydrogenation processes. The high bulk lifetime and excellent surface passivation enable Voc to exceed 670 mV. The metallization process is carried out by screen printing and firing in a conventional belt furnace. Detailed performance parameters and quantum efficiency of the cells will be illustrated in the paper. In addition, free energy loss analysis and cell simulation are also performed using the control parameters measured during cell fabrication processes.

Pre-wetted treatment for optimized multicrystalline Si wafer texturing and its solar cell efficiency improvement

We report a simple way to optimize texturing of multi-crystalline Si (mc-Si) wafer by pre-wetted surface in the wet etching. In the range of 300–1200 nm, a better texturing surface was achieved with average reflectivity 40 mA) and efficiency (0.1% absolute) has been achieved compared with control cell without deterioration of other performance parameters (such as open circuit voltage and fill factor). Internal quantum efficiency measurement also proves the lower front surface recombination velocity of pre-wetted based solar cell. We believe that the improved wetting surface property of multi-crystalline Si prior to etching is important for optimizing texturing process, and pre-wetted treatment is easy integrated for industrial mass production.

Direct correlation of microstructure and device performance of liquid phase crystallized Si thin film solar cells on glass

Si thin films on glass grown by liquid phase crystallization (LPC) exhibit large grains resembling those in multicrystalline Si wafers. The present work gives direct insight into how planar defects in LPC‐Si thin films influence the device performance of the corresponding solar cells by acquiring electron‐backscatter diffraction maps and measuring solar cell parameters on the same identical positions. By this approach, it was possible to demonstrate how low scanning velocities of the laser line during the crystallization lead to lower densities of grain boundaries, to improved charge‐carrier diffusion lengths, and hence to improved device performances. magnified imageOrientation‐distribution map acquired by electron backscatter diffraction on a Si thin film crystallized via liquid phase crystallization.

Defects in Multicrystalline Si Wafers Studied by Spectral Photoluminescence Imaging, Combined with EBSD and Dislocation Mapping

Defect related sub-band gap luminescence emissions due to Shockley-Read-Hall recombination in mc-Si wafers have been investigated with spectral photoluminescence imaging, combined with electron backscatter diffraction and dislocation mapping, for p- and n-type wafers, with and without intentionally introduced Fe. The well-known emission with energy 0.807eV (D1) is found to be correlated with heavily dislocated areas of the wafers with emissions emanating from the immediate vicinity of the defects. A less studied emission with energy centered around 0.7eV (D07) may be the product of two emissions and is found to exhibit very different characteristics in a boron-doped wafer intentionally contaminated with Fe than in the other samples. There is reason to believe that a radiative recombination pathway with characteristic photons with energy 0.694eV is present in this sample due to interstitial iron, Fei, while the D3/D4 (0.938eV/1.00eV) emission pair is related to the FeB complex.

Impact of the phosphorus emitter doping profile on metal contact recombination of silicon wafer solar cells

Metallisation of phosphorus-doped silicon (Si) surfaces using screen-printed silver (Ag) pastes is a well-established process and also a key process in the production of Si wafer solar cells. The metal-Si interface in a solar cell is a highly recombination-active region that impacts the device voltage. In this work, a facile test metallisation pattern with regions of varying front metal contact fractions is screen printed to create test cells on different phosphorus emitter doping profiles, all on commercially available multicrystalline Si wafers. On these and in conjunction with H-pattern cells and cells without front metallisation, intensity-dependent photoluminescence imaging is performed to extract both the metal-induced recombination saturation current densities and parameters related to wafer edge and peripheral recombination. It is observed that as the phosphorus emitter profile becomes lightly doped and shallower, the metallisation-induced recombination losses increase. The extracted metal recombination saturation current densities (J01-metal and J02-metal) on a 50Ω/sq phosphorus emitter surface were 1800fA/cm2 and 10nA/cm2 respectively when compared to values of 2500fA/cm2 and 120nA/cm2 respectively on a 130Ω/sq phosphorus emitter surface. As a consequence of high metal recombination current density values for the lightly doped emitters of this study (130Ω/sq), the solar cells fabricated from this group have a significant drop of 12mV in open-circuit voltage values after front grid metallisation.

Combined Thermography and Luminescence Imaging to Characterize the Spatial Performance of Multicrystalline Si Wafer Solar Cells

As-cut partially processed multicrystalline Si (mc-Si) wafers and complete mc-Si solar cells are characterized using photoluminescence imaging, defect mapping, and thermographic local current-voltage (I-V) analysis. Photoluminescence imaging applied to the partially processed mc-Si material after 1) RCA clean and a surface damage etch, 2) diffusion and phosphate silica glass removal, and 3) SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> :H coating reveals local performance enhancements over regions of the mc-Si wafer solar cell. A comprehensive imaging study including defect luminescence imaging, polarization imaging, breakdown imaging, and dark lock-in thermography is applied to the final device after metallization to obtain defect distributions and local spatially resolved I-V characteristics of the device. The analysis shows that substrate defects, reverse-bias, and subbandgap electroluminescence emission from various defects in the device correlate spatially with the determined electrical properties of the device.

MOCVD for solar cells, a transition towards a chamberless inline process

MOCVD has been associated with batch processing of III–V opto-electronic devices for decades, with epitaxial structures deposited on up to 200mm diameter wafers. Recent development in thin film PV has seen the gap in conversion efficiencies closing in on that of the commonly found multicrystalline Si wafer based PV. To further improve the conversion efficiency of thin film PV towards the theoretical limits of single junction solar cells requires a technique such as MOCVD with scalability potential. Preliminary results on the development of a chamberless inline process are reported for up to 15cm wide float glass, progressively coating each layer in the CdTe solar cell as the heated substrate passes under each coating head in turn and entirely at atmospheric pressure. Emphasis is made on ensuring that the chamberless coating heads can be operated safely using a combination of nitrogen curtain flows and a balanced exhaust pressure system. Results are also presented on the exclusion of oxygen and moisture from the coating area, achieved using the same gas flow isolation process. This paper also reviews the achievements made to-date in the transfer of the high efficiency batch MOCVD produced CdTe solar cell to the chamberless inline process demonstrating device quality thin films deposition.