mc-Si Wafers Research Articles

Generation Mechanism of Inhomogeneous Minority Carrier Lifetime Distribution in High Quality mc-Si Wafers and the Impacts on Electrical Performance of Wafers and Solar Cells

To find out the causation of inhomogeneous minority carrier lifetime distribution in high quality multicrystalline silicon (mc-Si) wafers, impurities and lattice defects were systematically studied by means of Fourier transform infrared (FTIR) spectroscopy and metallography. Inhomogeneously distributed oxygen impurity and dislocations were demonstrated to be key leading factors, and the restriction mechanism was discussed. Scattering process caused by ionized impurities and dislocations decreased carrier mobility, while carrier concentration was not significantly affected. Measurements showed that resistivity was higher and more dispersive in low lifetime area. Solar cells were fabricated with these wafers. Cells' efficiency of inhomogeneous ones exhibited averagely 0.27% lower than the regular ones in absolute terms. Recombination centers and leakage loss induced by dislocations and impurities led to the reduction in shunt resistors and open-circuit voltage, and then affected the performance of cells.

Comparison of phosphorus gettering effect in faceted dendrite and small grain of multicrystalline silicon wafers grown by floating cast method

We attempt to clarify the effect of phosphorus diffusion gettering (PDG) on the differences in microstructures of multicrystalline silicon (mc-Si) wafers. From the results of the floating cast method, the obtained microstructure of mc-Si was determined to contain unique microstructures, consisting of large faceted dendrites and small grains with low and high dislocation densities. The PDG efficiency was evaluated through the change in minority carrier lifetime. As a result, a stronger positive PDG effect was found for large faceted dendrites with low dislocation density, attributed to a small number of defects that can act as recombination centers of carriers. Lifetime improvement was also found in dislocation regions possibly owing to the redistribution of interstitial metals. These results suggest the importance of controlling the microstructure of mc-Si, which could strongly affect the PDG efficiency and existence of incorporated impurities.

Impact of structural defect density on gettering of transition metal impurities during phosphorus emitter diffusion in multi-crystalline silicon solar cell processing

The impact of structural defect density on gettering of transition metal impurities during phosphorous emitter diffusion has been investigated using a pair of multi-crystalline silicon (mc-Si) wafers. Chromium (Cr) impurities incorporated during growth were identified by deep level transient spectroscopy (DLTS) and used to evaluate the gettering efficiency. The Cr impurity concentration in the low defect density region of mc-Si wafers was reduced from ~3.5 × 1013 cm−3 to ~1.7 × 1012 cm−3 after phosphorous diffusion gettering (PDG), while for the high defect density region, there is no appreciable variation in the Cr concentration which only changed from ~3.0 to ~2.2 × 1012 cm−3 following PDG. It was concluded that the gettering process is not effective for highly defective regions of mc-Si wafers due to the ineffective impurity release from structural defects during the PDG process. Open image in new window

Structural and optical properties of ZnO nanoparticles deposited on porous silicon for mc-Si passivation

In this paper, we demonstrate the potential application of ultrathin ZnO film combined with porous Si treatment for multi-crystalline silicon (mc-Si) surface passivation. ZnO films were grown on porous silicon (PS) and glass substrate by spray pyrolysis method using zinc nitrate precursor with different molar concentrations varying from 0.05 to 0.25 M. The effects of molar concentration on the structural, optical and electronic properties of ZnO film were discussed. This method has been successfully used to achieve both antireflection and passivation properties of the mc-Si-treated samples. As a consequence, the effective minority carrier lifetime increases from 1 to 90 μs at a minority carrier density (Δn) of 3 × 1013 cm−3. The proposed treatment demonstrates also a significant decrease in the reflectivity of ZnO/PS-treated mc-Si as compared to untreated one. The reflectivity decreases from 32 % for untreated mc-Si wafers to 9 % for ZnO/PS-treated ones. The prepared cells give relatively high short-circuit currents (I sc) which leads to the recorded high conversion efficiency. An optimum Zn concentration is required to optimise the solar cell efficiency. This simple and cost-effective passivation method leads to an increase of mc-Si solar cells efficiency to 12 %.

Stable and efficient multi-crystalline n+p silicon photocathode for H2 production with pyramid-like surface nanostructure and thin Al2O3 protective layer

When a Si photocathode is used in a photoelectrochemical cell for H2 production, an open nanostructure capable of enhanced light absorption, low surface recombination, and being fully protected by thin protective layer is highly desirable. Here, we explored a highly stable and efficient multi-crystalline (mc) n+p silicon photocathode. A pyramid-like surface nanostructure on mc-Si wafer was fulfilled through a two-step metal-catalyzed chemical etching process, and then a n+p junction photocathode protected by a thin Al2O3 layer was constructed. The photocathode exhibits a high stability of continuous photoelectrochemical H2 production for above 100 h after a thin layer of Al2O3 is coated on its surface, and its energy conversion efficiency can be up to 6.8% after Pt loading, due to the lowered surface light reflection, increased surface area and minority carrier life time on the electrode surface.

Influence of germanium doping on the performance of high-performance multi-crystalline silicon

The effect of germanium (Ge) doping on the performance of high-performance multi-crystalline silicon (mc-Si) has been investigated in this work. The Ge doping in the mc-Si ingot could reduce the concentration of FeB complexes and dislocation density, resulting in the improvement of minority carrier lifetime. Due to this reduction in dislocation density, the mechanical strength of the mc-Si wafers was enhanced by Ge doping. The preliminary experimental results showed that the average conversion efficiency of Ge-doped mc-Si solar cells was higher than that of normal undoped mc-Si solar cells under the same solar cell fabrication processes. Consequently, we propose that Ge doping is beneficial for the fabrication of higher performance of mc-Si solar cells.

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.

Enhanced visible light photocatalytic properties of TiO2 thin films on the textured multicrystalline silicon wafers

TiO2 thin films were grown on solar-grade (SoG) multicrystalline silicon (mc-Si) wafers to improve the visible light photocatalytic property.

TiO2/porous silicon nanocomposite passivation coating for mc-Si wafers

This work reports on passivation effect of solar-grade multicrystalline Si (mc-Si) using a TiO2/porous silicon double coating. For this purpose, TiO2 nanoparticles were deposited onto porous silicon (PS)/mc-Si using pulsed laser ablation of titanium target. The structural and optoelectronic properties of the TiO2/PS treated mc-Si substrates were investigated by X-ray diffraction, Raman spectroscopy, optical spectrometry, photo-conductance and photoluminescence (PL). It was found that the minority carrier lifetime (τeff) of the mc-Si wafer could enhance at a nominal thickness of the TiO2 film (nanoparticle sizes). This was attributed to a surface passivation of the mc-Si wafer via TiO2-passivation of the PS film, whose PL intensity improves consequently. An optimal TiO2 thickness of 80 nm was found to give the highest PL intensity and an enhancement of the minority carrier lifetime from 5 µs for untreated mc-Si wafer to about 391 µs for a TiO2/PS treated wafers.

Passivation study of multi-crystalline silicon wafer with i-a-Si:H layer deposited by HWCVD

Heterojunction solar cells (a-Si:H/c-Si) are interface dominated devices and hence suppression of charge carrier recombination at the a-Si:H/c-Si interface is a very important aspect. Till now, all efforts and results have been obtained on mono-crystalline silicon. It would be very important from the technological point of view if similar high efficiency devices are realized on cheaper mc-Si wafers. The aim of this work is to study the passivation effect on mc-Si wafer by i-a-Si:H thin layer. The i-a-Si:H thin passivation layer is deposited on mc-Si wafer by Hot-Wire CVD at various filament temperature and silane flow rates. The passivation effect has been studied by effective carrier lifetime measurement and implied open circuit voltage.

Passivating Silicon Grain Boundaries with Small Polar Molecules for Photovoltaics

Grain boundaries (GBs) play a major role in determining the device performance of in particular polycrystalline thin-film solar cells. Hydrogen has traditionally been applied to passivate defects at GBs. However, hydrogenated films are subject to light-induced degradation effects. In this study, we took a novel approach to passivating GBs in multicrystalline silicon (mc-Si) wafers with small polar molecules. We found an excellent correlation between the grain misorientation, electrical resistance across GBs, and passivation effectiveness. In particular, the charge transport across GBs was greatly enhanced after the wafers were properly treated in our polar molecule solutions; the sheet resistance can be reduced by up to more than one order for large-angle random GBs. The results were explained to be due to the effective charge neutralization and passivation of polar molecules on localized charge states at GBs. These findings may help us achieve high-quality materials at low cost for high-efficiency solar cells by enhancing carrier transport and minimizing carrier recombination.

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%.

Surface passivation of multicrystalline silicon wafers by porous silicon combined with an ultrathin nanoparticles aluminum coating film

In this paper, we demonstrate that we may efficiently improve surface passivation of multi-crystalline silicon (mc-Si) while combining formation of porous silicon (PS) and deposition of ultrathin aluminum (Al) film. Aluminum Nanoparticles were deposited by thermal evaporation onto PS formed on mc-Si wafers. Optoelectronic properties of Al/PS/mc-Si and Al/mc-Si treated samples were investigated before and after annealing in the 400–700 °C temperature range. The surface passivation effectiveness was pointed out based on minority carrier lifetime and photoluminescence measurements. It was found that, at a minority carrier density Δn = 1015 cm−3, the effective minority carrier lifetime increases from 1.5 μs (for the bare mc-Si wafer) to about 6 and 14 μs before and after thermal annealing, respectively. FTIR analyses show strong correlation between the minority carrier lifetime values and hydrogen and Al passivation. Major beneficial effect of the co-presence of Al and Al–O on the optoelectronic properties is also demonstrated. The reflectivity of Al/PS treated mc-Si decrease significantly at 500 nm as compared to untreated mc-Si (from 31 % for untreated mc-Si wafers to 8 % for Al/PS treated ones), which is due to the roughly ordered structure and to the Al nanoparticles.

Effect of damage removal etch (DRE) on plasma textured, multi-crystalline solar cells

In the present work, a self-masked, dry, plasma texturing process for multi crystalline silicon (mc-Si) wafers has been developed that results in a higher cell performance than that with un-textured wafers. Plasma textured samples prepared have low levels (∼4%) of reflectance. Plasma damage of textured wafers has been eliminated by a damage removal etch (DRE). The improvement in efficiency of mc-Si solar cells up to 15.1% has been attributed to complete suppression of reflectivity (4–5%) in a broad spectral range (350–800nm) leading to black silicon surface. Also, DRE on plasma textured wafers has been found to result in reduced surface damage compared to cells without DRE leading to higher cell efficiencies.

A study of mottling phenomenon on textured multicrystalline silicon wafers and its potential effects on solar cell performance

The mottling phenomenon refers to the appearance of irregular dark patterns on HF–HNO3 acid etching textured multicrystalline silicon (mc-Si) wafers. The mottles have been identified as clusters of dislocation etch pits. In the acidic texturization, conditions favoring light reflectivity reduction usually lead to enhanced mottling. In a belief of adverse effect of the mottles to cell performance, light reflectivity reduction is more or less compromised in industry to avoid the mottling. The present study aims to identify whether appearance of the mottles alone really adversely affects the wafers minority carrier lifetime. Both serial examinations of an acid etched mc-Si wafer sample and parallel examinations of neighboring pairs of mc-Si wafer samples etched in acids of different HF/HNO3 ratios were carried out. The results show that development of the mottling, i.e., growth of dislocation etch pits, does not deteriorate mc-Si wafers in their minority carriers lifetime; rather it even slightly increases the lifetimes. Light reflectivity measurement and modeling show that the mottles can contribute to reduction of light reflectivity, and ~3% relative reduction of reflectivity is expected for multicrystalline silicon wafers of ordinary level of dislocation density. Removal of the defected zone surrounding a dislocation by the etching is postulated as a reason for the observed mottling-enhancement of the lifetime. It is further postulated that, in texturization of mc-Si wafers for cell production, instead of compromising light reflectivity reduction to avoid the mottling, it may be better to pursue lower light reflectivity, allowing some extent of the mottling. Meanwhile, more attention should be paid to compatibility of the cone-shaped dislocation etch-pit with grid printing of solar cells.

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.

Impact of Iron Precipitates on Carrier Lifetime in As-Grown and Phosphorus-Gettered Multicrystalline Silicon Wafers in Model and Experiment

The impact of iron point defects on the measured injection-dependent minority carrier lifetime in silicon after different processing steps (described by the Shockley-Read-Hall equation) is well known. However, in some parts of multicrystalline silicon (mc-Si) (used for solar cells), a large share of the iron atoms is precipitated. In this study, we simulate realistic iron precipitate distributions in mc-Si after crystallization, as well as after phosphorus diffusion gettering within grains by employing the Fokker-Planck equations. Taking the Schottky contact between metallic precipitates and semiconductor into account, in a second step, we analyze the effect of recombination at iron precipitates on carrier lifetime by means of finite-element simulations. The results are compared with experimental injection-dependent lifetime measurements on a p-type mc-Si wafer before and after phosphorus diffusion. Our simulations show that in the low-lifetime edge region close to the crucible wall, a considerable share of the carrier recombination can be attributed to iron precipitates in both the as-grown and in the phosphorus-diffused state. In addition, the simulated injection dependences of iron precipitates and iron interstitials differ significantly, with the precipitates influencing the carrier lifetime especially in the mid- to high-injection range, which is supported by carrier lifetime measurements.

Large area fabrication of vertical silicon nanowire arrays by silver-assisted single-step chemical etching and their formation kinetics

Vertically aligned silicon nanowire (SiNW) arrays have been fabricated over a large area using a silver-assisted single-step electroless wet chemical etching (EWCE) method, which involves the etching of silicon wafers in aqueous hydrofluoric acid (HF) and silver nitrate (AgNO3) solution. A comprehensive systematic investigation on the influence of different parameters, such as the etching time (up to 15 h), solution temperature (10–80 °C), AgNO3 (5–200 mM) and HF (2–22 M) concentrations, and properties of the multi-crystalline silicon (mc-Si) wafers, is presented to establish a relationship of these parameters with the SiNW morphology. A linear dependence of the NW length on the etch time is obtained even at higher temperature (10–50 °C). The activation energy for the formation of SiNWs on Si(100) has been found to be equal to ∼0.51 eV . It has been shown for the first time that the surface area of the Si wafer exposed to the etching solution is an important parameter in determining the etching kinetics in the single-step process. Our results establish that single-step EWCE offers a wide range of parameters by means of which high quality vertical SiNWs can be produced in a very simple and controlled manner. A mechanism for explaining the influence of various parameters on the evolution of the NW structure is discussed. Furthermore, the SiNW arrays have extremely low reflectance (as low as <3% for Si(100) NWs and <12% for mc-Si NWs) compared to ∼35% for the polished surface in the 350–1000 nm wavelength range. The remarkably low reflection surface of SiNW arrays has great potential for use as an effective light absorber material in novel photovoltaic architectures, and other optoelectronic and photonic devices.

18.5% efficient AlOx/SiNy rear passivated industrial multicrystalline silicon solar cells

Abstract Due to the trend toward thinner and higher efficient crystalline silicon solar cells, excellent rear surface passivation and internal optical reflectance have become more and more important. Aluminum oxide (AlOx) capped with silicon nitride (SiNy), which is considered as one of the most promising candidates to achieve superior rear passivation and internal reflectance, has to date been mostly used for the rear side of p-type monocrystalline silicon (mono-Si) solar cells. In this paper, we have optimized rear AlOx/SiNy stacks deposited by industrial plasma enhanced chemical vapor deposition (PECVD) for multicrystalline silicon (mc-Si) passivated emitter and rear cells (PERC). Sufficient passivation activation effect from industrial fast-firing process and SiNy deposition process have been demonstrated, so the samples were not subjected to additional thermal treatment process in the cell fabrication flow. For rear AlOx/SiNy stack, it is shown that when PECVD AlOx is thicker than 40 nm, apparent blisters in fired AlOx deteriorate the cell performance, and the appropriate SiNy capping is N-rich SiNy with thickness of at least 180 nm. After process optimization with the least additional process steps, independently confirmed efficiency of 18.5% for Pluto-PERC with PECVD AlOx/SiNy rear passivation on standard 156 mm × 156 mm p-type mc-Si wafers has been achieved.

Laue scanner: A new method for determination of grain orientations and grain boundary types of multicrystalline silicon on a full wafer scale

The grain structure, especially the grain orientation, number and type of grain boundaries, influences many of the properties of multicrystalline (mc) silicon (Si) wafers used for manufacturing solar cells. In order to increase the efficiency of silicon solar cells, the relation between material defects, such as dislocations, impurities and precipitates, and the grain structure must be understood. Currently no measurement system exists that can determine the grain orientation on a full mc Si wafer with an area of 156×156mm2. The most widely used measuring method is electron backscatter diffraction (EBSD). However, it is only possible to measure small sample sizes by EBSD depending on the scanning electron microscopy stage, and moreover a time-consuming surface preparation is needed. In this paper an innovative fast characterization method is presented that allows the orientation measurement of as-cut and polished Si samples up to 380×400mm2. This method, known as the Laue scanner, is based on X-ray diffraction. Experimental and theoretical considerations, limits of the system and example results of possible tasks are shown and discussed.