Multicrystalline Si Wafers Research Articles

Photocatalytic activity of Cr-doped TiO2 nanoparticles deposited on porous multicrystalline silicon films

This work deals with the deposition of Cr-doped TiO2 thin films on porous silicon (PS) prepared from electrochemical anodization of multicrystalline (mc-Si) Si wafers. The effect of Cr doping on the properties of the TiO2-Cr/PS/Si samples has been investigated by means of X-ray diffraction (XRD), atomic force microcopy (AFM), photoluminescence, lifetime, and laser beam-induced current (LBIC) measurements. The photocatalytic activity is carried out on TiO2-Cr/PS/Si samples. It was found that the TiO2-Cr/PS/mc-Si type structure degrades an organic pollutant (amido black) under ultraviolet (UV) light. A noticeable degradation of the pollutant is obtained for a Cr doping of 2 at. %. This result is discussed in light of LBIC and photoluminescence measurements.

Nanostructuring of c-Si surface by F2-based atmospheric pressure dry texturing process

A novel atmospheric pressure dry texture process is investigated in order to create nanostructures at the c-Si surface. The texture process uses diluted molecular fluorine (F2) as the process gas. F2 is partially dissociated at an elevated temperature before it is delivered to the c-Si wafer. Thermal activation of fluorine occurs on Si wafer surface in a dissociative chemisorption process leading to the removal of Si in the form of volatile SiFx species. The etching process can be controlled to form nanostructures with different aspect ratios and surface reflection values. In this work, we dry textured multicrystalline (mc) Si wafers to reach weighted surface reflection ∼12% in the wavelength range of 250–1200 nm. Nanotextured mc Si wafers were used to prepare p-type Al-BSF solar cells. The fabricated nanostructured cells show a gain in short circuit current (Jsc) of ∼0.5 mA/cm2 and reached a conversion efficiency of 17.3%.

Study of surface morphology and optical characterization of crystalline and multi-crystalline silicon surface textured in highly diluted alkaline solutions

In this work, alkaline texturing of (100) crystalline Si and multicrystalline Si wafers in diluted KOH solution leading to pyramidal structures is studied as a function of the etching temperature. The surface morphology is investigated using Atomic Force Microscopy and Scanning Electron Microscopy and the surface reflectance is measured by spectrophotometry in the wavelength range 200-1200 nm. It is found that etching in diluted 1% KOH solution leads to incomplete surface texturing when the etching temperature is equal to 70oC. The optimum etching temperature is found to be in the range 80-85oC which results in a minimum surface reflectance for crystalline silicon covered with an antireflection coating of 0.8%, with a uniform distribution over a wider wavelength range for samples that received a saw damage removal in 30% KOH solution prior to texturing. On the other hand, the optimum etching temperature shifts to the higher range 85-95oC for multicrystalline silicon surface with a minimum reflectance of 4.6% with ARC.

Effects of phosphorus diffusion gettering on minority carrier lifetimes of single-crystalline, multi-crystalline and UMG silicon wafer

Phosphorus diffusion gettering, which can effectively reduce the transition-metal impurities in the bulk of Si wafer and enhance the minority carrier lifetime (MCLT), is a well-known process to improve the performances of solar cells. Especially, the appropriate gettering process is further required for manufacturing solar cells using an upgraded metallurgical-grade silicon (UMG Si) wafer. In this work, an improvement in the MCLT of the UMG Si wafer including the single-crystalline and multi-crystalline Si wafer after phosphorus diffusion gettering was confirmed by using the quasi-steady state photo-conductivity (QSSPC) measurement and the microwave photo-conductance decay (μW-PCD) method. The experimental results were compared with the MCLT variations calculated through the simulation of the Fe distributions in the Si wafers. It was also observed that the efficiency of the UMG Si solar cell increased by 0.53% due to the two-step gettering process.

Cell Performance Prediction based on the Wafer Quality

Among many other parameters, wafer quality plays an important role in determining the efficiency of a solar cell and the power output of a module. Researchers have spent a lot of efforts in developing new casting methods to produce low-cost high-quality multicrystalline Si wafers. However, multicrystalline Si ingot technology is, still at this stage, a crystallization process that is difficult to control and produces silicon wafers with a wide range of quality. There is a rising need to develop in-line techniques to accurately assess wafer quality. In this paper, a new wafer characterization method is presented and the results of research to find the correlation between wafer quality and cell performance are reported. Based on the analysis of photoluminescence (PL) of as-cut wafers, and taking the cell process into account, a new method to predict the cell efficiency is demonstrated. The objective of the proposed method is to be able to sort, with a simple in-line technique, multicrystalline as-cut wafers for the reason that high- quality wafers are more suitable for high efficiency processing.

HF/HNO3 Etching of the Saw Damage

Wet chemical etching of multicrystalline Si wafer in mixtures of hydrofluoric (HF) and nitric acid (HNO3) combines the removal of the saw damaged silicon lattice on top of the wafer with the creation of a certain surface morphology (texture) on the surface of the underlying bulk silicon. However, it is yet unclear how the kinetics of silicon etching, the constitution of the saw damage, and the resulting surface morphology are related to each other. As a first step to answer this question the etching of the saw damage was studied at three different levels: (i) The development of the surface morphology was studied with increasing etch depth. Regions of key morphological features were identified and parameters describing the morphological development were derived from the surface topography. (ii) The etch rate was studied as parameter that is very sensitive to changes regarding the bonding state of the etched silicon atoms. (iii) The surface chemistry of HF/HNO3 etched Si wafer was monitored by diffuse reflectance Fourier-transform infrared spectroscopy (DRIFT) in order to identify processes that are related to changes in the surface termination of silicon. By the combination of these three approaches a detailed picture from the etching of the saw damage, the development of the texture, and the interaction between structure and etch rate can be drawn.

Investigation of defect luminescence from multicrystalline Si wafer solar cells using X‐ray fluorescence and luminescence imaging

AbstractMulticrystalline silicon wafer solar cells reveal performance‐ reducing defects by luminescence. X‐ray fluorescence spectra are used to investigate the elemental constituents from regions of solar cells yielding reverse‐bias or sub‐bandgap luminescence from defects. It is found that a higher concentration of metals is present in regions yielding reverse‐bias electroluminescence than in regions yielding sub‐bandgap electroluminescence. This suggests, dislocations do not create strong breakdown currents in the absence of impurity precipitates.magnified image(a) Topographies of sub‐bandgap (red) and reverse‐bias (blue) luminescence from defects in a multicrystalline Si wafer solar cell. (b) Their distinct X‐ray spectra indicate highest concentrations of metals in the blue regions.(© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Characteristics of gettering process in multicrystalline Si wafers with combined porous Si/Al getters

Characteristics of gettering process in multicrystalline Si wafers with combined porous Si/Al getters

Water‐based sonochemical cleaning in the manufacturing of high‐efficiency photovoltaic silicon wafers

AbstractWe found that organic particle contaminants are effectively removed during ultrasonic cleaning in distilled water, as evidenced by the disappearance of organic‐related absorption peaks and remarkable shortening of the surface photovoltage decay transients (SPV). SPV decay transient mappings obtained with a 100‐μm spatial resolution (see abstract figure) show that, in multicrystalline Si wafers, the resulting effect on the decay time is markedly non‐uniform compared with that observed in crystalline Si, implying the existence of a mechanism associated with the grain boundaries. The data are tentatively interpreted in terms of the wafer hydrogenization, which most probably occur through the grain boundaries. It is assumed that elevated temperatures inside a cavitating bubble may lead to water and bubble gas decomposition, accompanied by a subsequent trapping of the decomposed particulates at the silicon surface. These particulates can then be predominantly incorporated into the grain boundary regions. We suggest that this can be in part due to hydrogen molecules decomposed in water which are mobile in silicon (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Correlation between solar cell efficiency and minority carrier lifetime for batch processed multicrystalline Si wafers

Among the key parameters that ultimately control the performance of a solar cell, the “Minority carrier lifetime” plays the most crucial role. In the process chain involved in conversion of a wafer to a solar cell, each process contributes to change in the carrier lifetime. Hence performance of a particular process can be quantified based on the measure of carrier lifetime before and after the specific process. In an industrial production line, measure of carrier lifetime can be used for real time diagnosis. This paper deals with how to model the overall performance of a production line based on minority carrier lifetime of the wafers used to fabricate the cell. We have shown in this paper that the processing of wafers which were segregated based on their minority carrier lifetime results in increase in yield of the production line. Segregated processing also resulted in improvement of electrical performance of the batch processed solar cells.

Novel imaging techniques for dislocation‐related D1‐photo‐luminescence of multicrystalline Si wafers – two different approaches

AbstractHere we report on two different approaches, which allow rapid photoluminescence imaging of dislocation‐related D1, and band‐to‐band radiation on multicrystalline solar cell wafers at room temperature. We compared a new light emitting diode based method with the recently introduced pulsed laser based method. With both techniques we obtained spatially resolved images from 15.6 × 15.6 cm2 wafers originating from different stages of the standard solar cell production process, showing that these techniques are highly interesting for inline quality control. Both methods provide homogeneous illumination over the whole sample area. With a resolution around 300 μm, and a total recording time of ∼45 s, full D1 images could be captured by the new diode technique. The most promising fact was that surface artifacts did not lead to any noticeable disturbance in the D1 image. For acid textured wafers, the pulsed laser based method was clearly in advantage over the continuous diode technique. Possible explanations are discussed in the text. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

H-initiated extended defects from plasma treatment: Comparison between c-Si and mc-Si

In this study n- and p-type polished Czochralski-grown Si (Cz-Si) and p-type polished and as-cut multi-crystalline (mc) Si wafers have been directly H plasma treated in a plasma enhanced chemical vapour system in order to study H subsurface defect formation. Raman spectroscopy, secondary ion mass spectroscopy, scanning electron microscopy and transmission electron microscopy have been used to characterise the samples. In polished Cz-Si wafers, H induced defects were only observed up to 1 μm below the surface [1,2], while in similarly treated mc samples H induced defects were observed on grain boundaries and dislocations up to several μm below the surface [3]. It is also established that the distribution of H in the subsurface regions of the Cz-Si substrates after hydrogenation as well as the formation of structural defects depend on the type of doping. Evolution of Si-Hx bonds in hydrogenated Cz Si samples starts at 400 °C, while evolution of H initiated structural defects starts at 600 °C, when SiHx bonds are mostly dissolved.

Rapid dislocation‐related D1‐photoluminescence imaging of multicrystalline Si wafers at room temperature

AbstractHere we report on a novel method, which allows rapid photoluminescence imaging of band‐to‐band and dislocation‐related radiation, D1, on multicrystalline silicon wafers at room temperature. We demonstrate spatially resolved 5.0 × 5.0 cm2 D1‐images, with a resolution of ∼120 µm, within a total recording time of 550 ms. The method provides homogeneous illumination over the whole sample area. Comparison with results from a conventional photoluminescence mapping technique demonstrates the potential of this new method for application as an in‐line wafer characterization technique.

Simultaneous gettering and emitter formation in multicrystalline-Si wafers by annealing phosphorus doped amorphous silicon compounds

We demonstrate gettering of metal impurities in p-type multicrystalline silicon (mc-Si) wafers by annealing wafers that are surface passivated by double layers of amorphous silicon-based compounds acting as gettering sites. As inner layer we use a phosphorus-doped amorphous silicon-carbon-nitrogen alloy, providing surface passivation and acting as dopant source for the emitter formation during subsequent anneal. The outer layer is silicon nitride with antireflective properties. Anneals are done at 750, 800, and 850 °C for 30 and 60 min. The gettering effect is as good as for a conventional POCl3 diffusion followed by extended gettering at low temperature, and it is weakly influenced by the temperature step chosen. In the range explored, the sheet resistances of the emitters and the junction depths lay between 3000 to 60 Ω/sq. and 100–300 nm, respectively.

Light-Induced Plating of Screen-Printed Multi-Crystalline Silicon Solar Cells

Light-induced plating (LIP) has been used to evaluate solar cells screen-printed on 28 mm × 28 mm multi-crystalline Si (mc-Si) wafers from experimental silver thick-film pastes A and B. Cells were fabricated from paste A using screens with 60, 80, and 100 μm wide line openings. After firing at optimal temperature and measuring electrical performance, the cells were plated using the LIP process. Cell fill factor and efficiency increased noticeably while cell series resistance decreased. The observed efficiency increase ranged from 0.4% to 1.0% and was due to improved contact resistivity and decreased conductor line resistance. The mechanism for improved performance was elucidated through conductivity measurements and microstructural investigation using advanced scanning electron microscopy. Surface morphology of the silver conductor line obtained from the LIP process is reported. Cells from paste B were printed with a 100 μm wide opening screen. The efficiency of cells printed with paste B using the 100 μm screen followed by the LIP process did not show an efficiency improvement. It is shown that an optimally designed, state-of-the-art paste (paste B) can deliver sufficiently low series resistance and achieve high cell efficiency without using the LIP process.

Application of phosphorus diffusion gettering process on upgraded metallurgical grade Si wafers and solar cells

Although phosphorus (P) diffusion gettering process has been wildly used to improve the performance of Si solar cells in photovoltaic technology, it is a new attempt to apply P diffusion gettering process to upgraded metallurgical grade silicon (UMG-Si) wafers with the purity of 99.999%. In this paper, improvements on the electrical properties of UMG-Si wafers and solar cells were investigated with the application of P diffusion gettering process. To enhance the improvements, the gettering parameters were optimized on the aspects of gettering temperature, gettering duration and POCl 3 flow rate, respectively. As we expected, the electrical properties of both multicrystalline Si (multi-Si) and monocrystalline Si (mono-Si) wafers were significantly improved. The average minority carrier lifetime increased from 0.35 μs to nearly about 2.7 μs for multi-Si wafers and from 4.21 μs to 5.75 μs for mono-Si wafers, respectively. Accordingly, the average conversion efficiency of the UMG-Si solar cells increased from 5.69% to 7.03% for multi-Si solar cells (without surface texturization) and from 13.55% to 14.55% for mono-Si solar cells, respectively. The impurity concentrations of as-grown and P-gettered UMG-Si wafers were determined quantitively so that the mechanism of P diffusion gettering process on UMG-Si wafers and solar cells could be further understood. The results show that application of P diffusion gettering process has a great potential to improve the electrical properties of UMG-Si wafers and thus the conversion efficiencies of UMG-Si solar cells.

A new back surface passivation stack for thin crystalline silicon solar cells with screen-printed back contacts

In order to manufacture high-efficiency Si solar cells with a passivated rear surface and local contacts, it is necessary to develop both an excellent rear-passivation scheme compatible with screen-printing technology and a robust patterning technique for local contact formation. In this work, we have fabricated Si solar cells on ∼130 μm thick substrates using manufacturable processing, where rear side was passivated with a plasma-enhanced chemical vapor deposited SiO x /SiN x /SiO x N y stack and local back contacts using laser. As a result of both the rear surface passivation stack and the laser-fired local contacts, cell efficiencies of up to 17.6% on a 148.6 cm 2 Float-zone Si wafer and 17.2% for a 156.8 cm 2 multicrystalline Si wafer were achieved. PC-1D calculations revealed that the cells had a back surface recombination velocity (BSRV) of ∼400 cm/s and a back surface reflectance (BSR) of over 90%, as opposed to standard full Al-BSF cells having a BSRV of ∼800 cm/s and a 70% BSR. This result clearly indicates that the new technique of the passivation scheme and the patterning using laser developed in this study are promising for manufacturing high-efficiency PERC-type thin Si solar cells.

Damage-free reactive ion etch for high-efficiency large-area multi-crystalline silicon solar cells

Texturing of silicon (Si) wafer surface is a key to enhance light absorption and improve the solar cell performance. While alkaline texturing of single-crystalline Si (sc-Si) wafers was well established, no chemical solution has been successfully developed for multi-crystalline Si (mc-Si) wafers. Reactive-ion-etch (RIE) is a promising technique for effective texturing of both sc-Si and mc-Si wafers, regardless of crystallographic characteristics, and more suitable for thin wafers. However, due to the use of plasma source generated by high power, the wafer surface gets a physical damage during the processing, which requires an additional subsequent damage-removal wet processing. In this work, we developed a damage-free RIE texturing for mc-Si solar cells. An improved self-masking RIE texturing process, developed in this study, produced ∼0.7% absolute efficiency gain on 156×156 mm 2 mc-Si cells, where the gas ratio and the plasma power density were keys to mitigate the plasma-induced-damage during the RIE processing while maintaining decent surface reflectance. In the self-masking RIE texturing, a mixture of SF 6/Cl 2/O 2 gases was found to significantly affect the surface morphology uniformity and reflectance, where an optimal etch depth was found to be 200–400 nm. We achieved J sc gain of ∼1.3 mA/cm 2 while maintaining decent FFs of ∼0.78 without a V oc loss after optimization of firing conditions.

Cost-effective photovoltaics with silicon material

In this study, we have analysed the new emerging technologies in the field of photovoltaic (PV) and their impact on the finale cost of the solar cells. We have compared the 2 main groups of PV solar cells which are bulk silicon (Si) and thin-film. The aim of this project is the establishment of the technology roadmap for achieving Si solar cells at a competitive manufacturing cost. This goal can be achieved by using cheaper multicrystalline Si (mc-Si) wafers, thinner Si substrates (below 200 μm), better front and back surface passivation, improved screen printing, and surface texturing.

Technology development of high-quality n-type multicrystalline silicon for next-generation ultra-thin crystalline silicon solar cells

This paper, under the NEDO project, aims at technology development of high-quality, n-type multicrystalline Si wafers. The targets of the technology development are to realize a high minority-carrier lifetime of 1 ms and to apply for a commercial solidification furnace. In particular, real-time observation of the unidirectional solidification process, the effect of quartz crucible quality on the ingots, the quality improvement of n-type Si wafers and evolution to a commercial furnace will be described.