Interstitial iron concentrations across multicrystalline silicon wafers via photoluminescence imaging

We present high‐resolution images of the lateral distribution of interstitial iron across wafers from various positions along the length of a directionally solidified multicrystalline silicon ingot. Iron images were taken on wafers in the as‐cut state and also after two different phosphorus gettering steps performed at 845°C for 30 min, one with an additional anneal at 600°C for 5 h (referred to as extended gettering). The iron images were obtained by taking calibrated photoluminescence (PL) images of the low injection carrier lifetimes, before and after dissociation of iron–boron pairs via strong illumination. The iron images clearly reveal the internal gettering of iron during ingot cooling to grain boundaries and dislocation clusters, resulting in much lower dissolved iron concentrations at those features. In contrast, the PL images of gettered wafers exhibit a reversed distribution of dissolved iron compared to the as‐cut wafers, in other words, with higher interstitial iron concentrations at the grain boundaries than within the grains, most probably owing to the precipitated iron at the grain boundaries partly dissolving during the high‐temperature gettering process. Phosphorus gettering was found to result in a significant reduction of interstitial iron both inside the grains and at grain boundaries. The extended gettering resulted in a further significant reduction in all parts of the wafers and along all sections of the ingot. Copyright © 2011 John Wiley & Sons, Ltd.     

High‐efficiency solar cells on phosphorus gettered multicrystalline silicon substrates

Measurements of the dislocation density are compared with locally resolved measurements of carrier lifetime for p‐type multicrystalline silicon. A correlation between dislocation density and carrier recombination was found: high carrier lifetimes (>100 µs) were only measured in areas with low dislocation density (<10⁵ cm⁻²), in areas of high dislocation density (>10⁶ cm⁻²) relatively low lifetimes (<20 µs) were observed. In order to remove mobile impurities from the silicon, a phosphorus diffusion gettering process was applied. An increase of the carrier lifetime by about a factor of three was observed in lowly dislocated regions whereas in highly dislocated areas no gettering efficiency was observed. To test the effectiveness of the gettering in a solar cell manufacturing process, five different multicrystalline silicon materials from four manufacturers were phosphorus gettered. Base resistivity varied between 0·5 and 5 Ω cm for the boron‐ and gallium‐doped p‐type wafers which were used in this study. The high‐efficiency solar cell structure, which has led to the highest conversion efficiencies of multicrystalline silicon solar cells to date, was used to fabricate numerous solar cells with aperture areas of 1 and 4 cm². Efficiencies in the 20% range were achieved for all materials with an average value of 18%. Best efficiencies for 1 cm² (20·3%) and 4 cm² (19·8%) cells were achieved on 0·6 and 1·5 Ω cm, respectively. This proves that multicrystalline silicon of very different material specification can yield very high efficiencies if an appropriate cell process is applied. Copyright © 2006 John Wiley & Sons, Ltd.     

太阳电池用多晶硅锭红外检测中阴影的表征分析

研究了多晶硅铸锭过程中,硅锭内部红外探伤测试中显示黑斑位置晶体缺陷的杂质组成,并根据杂质成分推导了此种缺陷的形成机理和条件.采用光致发光(PL)技术、扫描电子显微镜(SEM)和X射线能谱仪(EDS)对杂质进行了表征与分析.结果显示,形成阴影的夹杂在晶界中存在的形态主要为针状或薄片状,其组成成分主要为C,N和Si元素.而Si3N4的出现可能有两个原因:一是Si3N4涂层脱落而沉积在晶界中;二是溶解在液相中的N局部过饱和.此外,结晶过程中,SiC也随之成核并生长,在晶界上形成夹杂物,同时伴随着微缺陷的增加.据此提出了去除多晶硅锭内部阴影的几点措施.     

Comparison of the characteristics of solar cells fabricated from multicrystalline silicon with those fabricated from silicon obtained by the monolike technology

In order to raise the efficiency of solar cells and reduce the cost of their production, a new process for obtaining silicon ingots based on the so-called moonlike technology is developed. New technologies, which use “solar-grade” silicon, make it possible to fabricate solar cells at a lower cost with a higher efficiency of solar-energy conversion. It is exactly for this reason that the “monolike” process was tested and optimized by us for Kazakhstan solar-grade silicon. The aim of this study is a comparison of the characteristics of solar cells fabricated from “monolike” silicon with those of solar cells obtained on the basis of multicrystalline silicon grown by oriented crystallization. For our study, ingots of multicrystalline silicon are grown on an industrial scale and through the use of Kazakhstan-sourced silicon; solar cells are fabricated and the characteristics of the obtained silicon ingots and solar cells are studied.     

Elimination of light‐induced degradation with gallium‐doped multicrystalline silicon wafers

Lifetime stability of gallium‐doped multicrystalline silicon wafers has been evaluated under illumination. Quality and stability of the Ga‐doped multicrystalline silicon wafers were intensively studied by means of quasi‐steady‐state photocondcutance lifetime measurement. Results show that as‐grown Ga‐doped multicrystalline silicon wafers have high lifetimes, and no significant degradation was observed under illumination. The Ga‐doped multicrystalline silicon wafers are a promising material for future photovoltaics. Copyright © 2003 John Wiley & Sons, Ltd.     

Grain boundary segregation in multicrystalline silicon: correlative characterization by EBSD,EBIC, and atom probe tomography

This study aims to better understand the influence of crystallographic structure and impurity decoration on the recombination activity at grain boundaries in multicrystalline silicon. A sample of the upper part of a multicrystalline silicon ingot with intentional addition of iron and copper has been investigated. Correlative electron‐beam‐induced current, electron backscatter diffraction, and atom probe tomography data for different types of grain boundaries are presented. For a symmetric coherent Σ3 twin boundary, with very low recombination activity, no impurities are detected. In case of a non‐coherent (random) high‐angle grain boundary and higher order twins with pronounced recombination activity, carbon and oxygen impurities are observed to decorate the interface. Copper contamination is detected for the boundary with the highest recombination activity in this study, a random high‐angle grain boundary located in the vicinity of a triple junction. The 3D atom probe tomography study presented here is the first direct atomic scale identification and quantification of impurities decorating grain boundaries in multicrystalline silicon. The observed deviations in chemical decoration and induced current could be directly linked with different crystallographic structures of silicon grain boundaries. Hence, the current work establishes a direct correlation between grain boundary structure, atomic scale segregation information, and electrical activity. It can help to identify interface–property relationships for silicon interfaces that enable grain boundary engineering in multicrystalline silicon. Copyright © 2015 John Wiley & Sons, Ltd.     

Imaging of Metal Impurities in Silicon by Luminescence Spectroscopy and Synchrotron Techniques

Metals are detrimental to silicon solar cells in two ways: (i) they typically introduce defect levels in the bandgap, leading to enhanced carrier recombination and thus to lower voltage in solar cells; and (ii) they may, in the form of precipitates, contribute to the formation of shunts and reverse breakdown sites. This paper provides a review on techniques to access the spatial distribution of recombination sites for multicrystalline silicon. Methods to detect metal precipitates as well as, in the case of iron, dissolved point defects are presented. These methods are applied to clarify the distribution of iron after high-temperature processes and the identification of breakdown sites.     

Directed crystallization of multicrystalline silicon under weak melt convection and gas exchange

For the first time, the multicrystalline silicon is grown via directed crystallization using a heater submerged into the melt. The interaction of the heater casing material with molten silicon is studied on a model heater in the form of a graphite plate coated with a special structure SiC protective layer. During the crystallization, the plate has been kept on the melt surface and has almost completely overlaid the melt mirror, which favored a significant decrease in the rate of gas exchange between the melt and the atmosphere in the furnace. Marangoni convection was not observed in the absence of the free surface of the melt and the crystal was grown on the reduced melt convection, especially at the final stages of crystallization, when the thickness of the melt layer was inferior to the cross size of the crucible. The crystal is established to have a strongly pronounced columnar structure. The measured specific resistance varies over the ingot height from 1 to 1.3 Ω cm, and the lifetime of minority carriers is about 3.7 μs. The carbon and oxygen content in the ingot has been measured via FTIR spectroscopy, and the carbon concentration through the ingot height is shown to strongly differ from the linear dependence typical of directed crystallization.     

SiC sedimentation and carbon migration in mc-Si by election beam melting with slow cooling pattern

The development of photovoltaic industry demands great amount of multicrystalline silicon. Carbon and SiC in silicon need to be contained in a limited amount since they can cause great adverse affect to solar cells. The behavior of carbon and its precipitation SiC in silicon by electron beam melting (EBM) with a slow cooling pattern was investigated in this study. SiC is found to sedimentate to ingot bottom after EBM. The presence of Si₃N₄ can be heterogeneous nucleation agent for SiC to nucleate continually and both of them precipitate to the ingot bottom. The comprehensive effect of slow solidification condition, temperature gradient and melt convection causes the sedimentation of SiC. It is also found that oxygen plays an important role on the migration of the dissolved carbon. The formation of carbon-oxygen complexes tend to migrate to ingot top since oxygen can transfer from silicon melt to vacuum environment during EBM.     

SHORT COMMUNICATION: ACCELERATED PUBLICATION: Multicrystalline silicon solar cells exceeding 20% efficiency

This paper presents the first conversion efficiency above 20% for a multicrystalline silicon solar cell. The application of wet oxidation for rear surface passivation significantly reduces the process temperature and therefore prevents the degradation of minority‐carrier lifetime. The excellent optical properties of the dielectrically passivated rear surface in combination with a plasma textured front surface result in a superior light trapping and allow the use of substrates below 100 μm thickness. A simplified process scheme with laser‐fired rear contacts leads to conversion efficiencies of 20·3% for multicrystalline and 21·2% for monocrystalline silicon solar cells on small device areas (1 cm²). Copyright © 2004 John Wiley & Sons, Ltd.     

Impact of Metal Contamination in Silicon Solar Cells

The impact of the transition metals iron, chromium, nickel, titanium and copper on solar‐cell performance is investigated. Each impurity is intentionally added to the silicon feedstock used to grow p‐type, directionally solidified, multicrystalline silicon ingots. A state‐of‐the‐art screen‐print solar‐cell process is applied to this material. Impurities like iron, chromium and titanium cause a reduction in the diffusion length. Nickel does not reduce the diffusion length significantly, but strongly affects the emitter recombination, reducing the solar‐cell performance significantly. Copper has the peculiarity of impacting both base‐bulk recombination as well as emitter recombination. Two models based on the Scheil distribution of impurities are derived to fit the degradation along the ingot. Solar‐cell performances are modelled as a function of base‐bulk recombination and emitter‐bulk recombination. The model fits the experimental data very well and is also successfully validated. Unexpectedly, the distribution of impurities along the ingot, due to segregation phenomena (Scheil distribution), leaves its finger‐print even at the end of the solar‐cell process. A measure of impurity impact is defined as the level of impurity that causes a degradation in cell performance of less than 2% up to 90% of the ingot height. The advantage of this impurity‐impact metric is that it comprises the different impurities’ physical characters in one single parameter, which is easy to compare.     

Origin of the low carrier lifetime edge zone in multicrystalline PV silicon

An investigation of impurities, crystal defects and microstructure has been performed on the edge zone, i.e. close to the crucible wall, which experiences reduced carrier lifetime in a directionally solidified multicrystalline p‐doped silicon ingot. The characterization methods applied have been QSSPC, FTIR, μW‐PCD, EBSD, CDI, PVScan, optical microscopy, FeB‐pair splitting and GDMS. The results of the minority carrier lifetime measurements have revealed strongly reduced values in the vicinity of the edge (< 1 μs). Increased values were obtained starting at 15–17 mm from the edge. Light elements analyses showed that the O, N and C concentrations, interstitially or in particles, did not increase in the edge zone, neither did the dislocation density. GDMS analyses detected traces of aluminium, iron, copper, titanium and chromium. The total iron concentration showed an increase towards the edge, though high concentrations were occasionally detected in the bulk. FeB pair analysis revealed large concentrations of Fe (∼1 × 10¹³ cm⁻³) in the vicinity of the edge with a distinctively decreasing trend moving away from the edge. The detected FeB‐concentrations are sufficient to account for the majority of the lifetime degradation close to the edge (0‐‐15 mm). In addition, Fe, in the form of FeB pairs, was extensively observed as object to internal gettering to high angle boundaries and dislocations. Fe, in the form of FeB pairs, is furthermore believed to originate from solid state diffusion from the crucible and coating. Copyright © 2008 John Wiley & Sons, Ltd.     

Weak light effect in multicrystalline silicon solar cells

Minority carrier trapping centers frequently exist in solar grade multicrystalline silicon, such trapping centers cause a drastic increase in photoconductance at carrier injection levels equal to and below the trap density, this phenomenon leads to higher open circuit voltage for multicrystalline silicon solar cells at illumination levels below about 0.2 suns compared to high performance crystalline silicon solar cells. In this paper, the open circuit voltage of multicrystalline silicon solar cells are investigated at low illumination levels, the experiments prove that some multicrystalline silicon solar cells which have higher trap density have higher open circuit voltage at weak illumination levels, and have lower efficiency, so a new method is presented to analyze quality of multicrystalline silicon by measuring open circuit voltage at weak illumination levels in-line, this makes cells manufacturers gain insight into the quality of multicrystalline silicon wafer from different multicrystalline silicon manufacturers easily with the same cell process before screenprinting and firing.     

Study of SiC and Si3N4 inclusions in industrial multicrystalline silicon ingots grown by directional solidification method

In directional solidification of multicrystalline silicon ingots for solar cells, the concentration of C and N impurities in the silicon melts increases with progression of solidification due to their relatively low segregation coefficients. In the case of supersaturation of C and N in silicon melts, SiC and Si₃N₄ inclusions are formed. In this work, a piece of multicrystalline silicon was selected from a central block, which was cut out from an industrial multicrystalline silicon ingot grown by directional solidification method. The distribution of SiC and Si₃N₄ inclusions from the top to bottom regions was systematically studied. It was found that majority of SiC and Si₃N₄ inclusions are present in the top region and the amount of inclusions decreases exponentially from the top surface down to the bulk of the ingot. Morphologies and characteristics of the SiC and Si₃N₄ inclusions were investigated. The presence of SiC and Si₃N₄ inclusions generates high density of dislocations in multicrystalline silicon, and sometimes can also introduce pores into multicrystalline silicon. The results of this work will be of practical interest to the photovoltaic industry.     

High-efficiency Multicrystalline Silicon Solar Cells using Standard High-temperature,Float-zoned Cell Processing

This paper reports recent results of fabricating multicrystalline silicon solar cells with the standard PERL (passivated emitter, rear locally-diffused) cell high-temperature processing sequence originally developed for float-zoned wafers. One of these multicrystalline silicon cells with a planar front surface demonstrated a 645-mV open-circuit voltage and 18.2% energy conversion efficiency tested at the National Renewable Energy Laboratory and Sandia National Laboratories under the 100 mW cm⁻² AM1.5 global spectrum at 25°C. This is the highest confirmed voltage and one of the highest confirmed conversion efficiencies ever reported to date for a multicrystalline silicon cell. Further optimization of the standard PERL processing and texturing of the cell surfaces is expected to improve the cell efficiency to over 19% in the near future. © 1997 John Wiley & Sons, Ltd.     

Record efficiencies for EFG and string ribbon solar cells

Solar cells have been processed on Edge‐defined Film‐fed Growth and String Ribbon silicon. Based on a standard process developed for different types of multicrystalline materials including evaporation of contacts and photolithography for front‐contact formation, optimisations have been implemented to deal with the special needs of these low‐cost ribbon materials. Especially a remote plasma hydrogenation step and a change from evaporated to screen‐printed aluminium back‐surface‐field improved cell parameters drastically. Independently confirmed stable efficiencies of 16.7% (EFG) and 17.7% (String Ribbon) have been achieved on 4 cm² cells (full area). These values represent the highest stable efficiencies obtained for multicrystalline silicon ribbons so far. Copyright © 2003 John Wiley & Sons, Ltd.     

Performance enhancement of multicrystalline silicon solar cells and modules using double‐layered SiNx:H antireflection coatings

Silicon nitride coating deposited by the plasma‐enhanced chemical vapor deposition method is the most widely used antireflection coating for crystalline silicon solar cells. In this work, we employed double‐layered silicon nitride coating consisting of a top layer with a lower refractive index and a bottom layer (contacting the silicon wafer) with a higher refractive index for multicrystalline silicon solar cells. An optimization procedure was presented for maximizing the photovoltaic performance of the encapsulated solar cells or modules. The dependence of their photovoltaic properties on the thickness of silicon nitride coatings was carefully analyzed. Desirable thicknesses of the individual silicon nitride layers for the double‐layered coatings were calculated. In order to get statistical conclusions, we fabricated a large number of multicrystalline silicon solar cells using the standard production line for both the double‐layered and single‐layered antireflection coating types. On the cell level, the double‐layered silicon nitride antireflection coating resulted in an increase of 0.21%, absolute for the average conversion efficiency, and 1.8 mV and 0.11 mA/cm² for the average open‐circuit voltage and short‐circuit current density, respectively. On the module level, the cell to module power transfer factor was analyzed, and it was demonstrated that the double‐layered silicon nitride antireflection coating provided a consistent enhancement in the photovoltaic performance for multicrystalline silicon solar cell modules than the single‐layered silicon nitride coating. Copyright © 2015 John Wiley & Sons, Ltd.     

Use of porous silicon antireflection coating in multicrystallinesilicon solar cell processing

The latest results on the use of porous silicon (PS) as an antireflection coating (ARC) in simplified processing for multicrystalline silicon solar cells are presented. The optimization of a PS selective emitter formation results in a 14.1% efficiency multicrystalline (5×5 cm²) Si cell with evaporated contacts processed without texturization, surface passivation, or additional ARC deposition. Specific attention is given to the implementation of a PS ARC into an industrially compatible screen-printed solar cell process. Both the chemical and electrochemical PS ARC formation method are used in different solar cell processes, as well as on different multicrystalline silicon materials. Efficiencies between 12.1 and 13.2% are achieved on large-area (up to 164 cm² ) commercial Si solar cells     

Enhanced performance in the deteriorated area of multicrystalline silicon wafers by internal gettering

The deteriorated area of the multicrystalline silicon (mc‐Si) ingots grown by directional solidification, commonly known as the Red Zone, is usually removed before wafering. This area, characterized by poor minority carrier lifetime, is located on the sides, at the top, and the bottom of the mc‐Si ingots. In this study, the effect of internal gettering by oxygen precipitates and structural defects has been investigated on the bottom zone of a mc‐Si ingot. Nucleation and growth of oxygen precipitates as well as low temperature annealing were studied. Photoluminescence imaging, lifetime mapping, and interstitial iron measurements performed by μ‐PCD reveal a considerable reduction of the bottom Red Zone. An improvement of lifetime from below 1 µs to about 20 µs and a reduction of interstitial iron concentration from 1.32 × 10¹³ at/cm³ to 8.4 × 10¹⁰ at/cm³ are demonstrated in this paper. Copyright © 2013 John Wiley & Sons, Ltd.