About the origin of low wafer performance and crystal defect generation on seed‐cast growth of industrial mono‐like silicon ingots

The era of the seed‐cast grown monocrystalline‐based silicon ingots is coming. Mono‐like, pseudomono or quasimono wafers are product labels that can be nowadays found in the market, as a critical innovation for the photovoltaic industry. They integrate some of the most favorable features of the conventional silicon substrates for solar cells, so far, such as the high solar cell efficiency offered by the monocrystalline Czochralski‐Si (Cz‐Si) wafers and the lower cost, high productivity and full square‐shape that characterize the well‐known multicrystalline casting growth method. Nevertheless, this innovative crystal growth approach still faces a number of mass scale problems that need to be resolved, in order to gain a deep, 100% reliable and worldwide market: (i) extended defects formation during the growth process; (ii) optimization of the seed recycling; and (iii) parts of the ingots giving low solar cells performance, which directly affect the production costs and yield of this approach. Therefore, this paper presents a series of casting crystal growth experiments and characterization studies from ingots, wafers and cells manufactured in an industrial approach, showing the main sources of crystal defect formation, impurity enrichment and potential consequences at solar cell level. The previously mentioned technological drawbacks are directly addressed, proposing industrial actions to pave the way of this new wafer technology to high efficiency solar cells. Copyright © 2012 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.     

Impurity gettering of polycrystalline solar cells fabricated from refined metallurgical-grade silicon

A damage-gettering technique is described which reduces the impurity content in grown crystals and enhances cell performance of diffused solar cells. Crystalline ingots were Czochralski-grown from an acid-leached metallurgical-grade source. Damage gettering was performed by preparing a mechanically damaged layer on the wafer back surface and subsequent annealing. Optimum annealing conditions were investigated as a function of ambient gas species, temperature, and time. In an O2ambient, the fill factor of the cells degraded to 0.25, while cell performance was greatly improved by annealing in N2. Conversion efficiency tends to increase with annealing time at higher temperatures. Maximum conversion efficiencies attained for mono- and polycrystalline solar cells fabricated from MG-Si are 9.8 and 7.7 percent, respectively. Light current-voltage characteristics and the leakage-current variations with depth were analyzed. It was found that impurity gettering begins at the wafer surfaces and proceeds gradually into the bulk regions.     

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     

Double-layered silicon nitride antireflection coatings for multicrystalline silicon solar cells

Silicon nitride coating possesses both optical antireflection and electrical passivation effects for crystalline silicon solar cells. In this work, we employed a 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. Double-layered silicon nitride coating provides a lower optical reflection and better surface passivation than those of single-layered silicon nitride. Details for optimizing the double-layered silicon nitride coating are presented. In order to get statistical conclusions, we fabricated a large number of multicrystalline silicon solar cells using the production line for both the double-layered and single-layered cell types. It was statistically demonstrated that the double-layered silicon nitride coating provided a consistent enhancement in the photovoltaic performance of multicrystalline silicon solar cells over those of the single-layered silicon nitride coating.     

Investigation of bulk and solar cell properties of ingots cast from compensated solar grade silicon

In this work, the effect of the concurrent presence of B and P on bulk and solar cell properties of directionally solidified multicrystalline ingots from commercially compensated solar grade silicon (SoG‐Si) feedstock produced by Elkem Solar was investigated. The initial B and P content prior to the directional solidification experiment was 1260 and 762 ppba, respectively. Two reference ingots have been solidified in a silica crucible from 100% electronic grade silicon (EG‐Si) feedstock, with 332 ppba of boron added. All ingots have been cast under similar process parameters. The resistivity measurements by Four Point Probe (FPP) are in good agreement with the net dopant content, i.e., N_A − N_D for p‐type material, measured by Glow Discharge Mass Spectrometer (GDMS). Bulk lifetime measurements show a decrease in the values compared to the EG reference. Lifetime distributions show the highest values of 13 and 19 µs at approximately half ingot height, compared to 30 and 44 µs in the reference ingots. This decrease can be due to the concurrent effect of compensation and of other impurities present in the ingot. However, the content of several transition metals measured by GDMS at half ingot height was not significantly higher than that of the reference ingots. Oxygen content as measured by Fourier Transform Infra‐Red (FTIR) spectroscopy shows no significant difference compared to the references. Solar cells made from the compensated ingots and processed under standard process conditions show efficiency values up to 15.5% and fill factor values up to 78%, comparable to conventional multicrystalline silicon cells. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

Recovery of silicon from kerf loss slurry waste for photovoltaic applications

The constantly rising price of silicon feedstock has been the most important factor preventing photovoltaic (PV) energy from reaching grid parity. On the other hand, large amount of silicon gets wasted during slicing. We report a promising approach to recycle kerf loss silicon from cutting slurry waste for solar cell applications. Silicon carbide (SiC) and metal impurities were successfully removed by chemical/ physical processing from the slurry waste to recover solar grade silicon. The effects of centrifugation using heavy fluids and high‐temperature treatment in the removal of SiC particles are discussed in detail. Ingots from the recycled silicon were grown by using directional solidification. The average resistivity and minority carrier lifetime of the grown crystals were found to be about 0·7 Ω cm and 1·02 µs, respectively, which were close to the original sawing silicon ingots. Solar cells using multi‐crystalline wafers of recovered silicon were fabricated and the best energy conversion efficiency was found to be 12·6% comparable to those from the high‐purity silicon. Copyright © 2008 John Wiley & Sons, Ltd.     

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.     

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.     

A 19.8% efficient honeycomb multicrystalline silicon solar cellwith improved light trapping

This paper reports a substantially improved efficiency for a multicrystalline silicon solar cell of 19.8%. This is the highest ever reported efficiency for a multicrystalline silicon cell. The improved multicrystalline cell performance results from enshrouding cell surfaces in thermally grown oxide to reduce their detrimental electronic activity and from isotropic etching to form a hexagonally-symmetric “honeycomb” surface texture. This texture, largely of inverted hemispheres, reduces reflection loss and improves absorption of infrared light by effectively acting as a randomizer. Results of a ray tracing model are presented, with the notable finding that up to 90% of infrared light is trapped in the substrate after the first two passes, compared with only 65% for the well known inverted pyramid structure. These optical features are considered to contribute to an exceptionally high short-circuit current density of 38.1 mA/cm². A further improvement is expected by using under-etched wells for these honeycomb cells     

Thermal oxidation for crystalline silicon solar cells exceeding 19% efficiency applying industrially feasible process technology

Thermal oxides are commonly used for the surface passivation of high‐efficiency silicon solar cells from mono‐ and multicrystalline silicon and have led to the highest conversion efficiencies reported so far. In order to improve the cost‐effectiveness of the oxidation process, a wet oxidation in steam ambience is applied and experimentally compared to a standard dry oxidation. The processes yield identical physical properties of the oxide. The front contact is created using a screen‐printing process of a hotmelt silver paste in combination with light‐induced silver plating. The contact formation on the front requires a short high‐temperature firing process, therefore the thermal stability of the rear surface passivation is very important. The surface recombination velocity of the fired oxide is experimentally determined to be below S ≤ 38 cm/s after annealing with a thin layer of evaporated aluminium on top. Monocrystalline solar cells are produced and 19·3% efficiency is obtained as best value on 4 cm² cell area. Simulations show the potential of the developed process to approach 20% efficiency. Copyright © 2008 John Wiley & Sons, Ltd.     

A Comparison of Bulk Lifetime, Efficiency, and Light-Induced Degradation in Boron- and Gallium-Doped Cast mc-Si Solar Cells

High-efficiency boron- and gallium-doped multicrystalline silicon (mc-Si) cells were fabricated and compared in this paper. The quality of three different boron-doped mc-Si ingots and one gallium-doped mc-Si ingot was investigated and compared by means of lifetime measurements and solar cell efficiencies. Untextured screen printed 4-cm² cell efficiencies in excess of 16% were achieved in this paper when the lifetime after gettering and hydrogenation exceeded 100 mus. This was true for most wafers from top, middle, and bottom regions of the boron-doped ingots. Lifetimes in excess of 300 mus were achieved from the middle region of some boron- and gallium-doped mc-Si ingots. High efficiencies in excess of 16.7% were attained from the middle region of most ingots investigated in this paper regardless of gallium or boron dopant. Light-induced degradation in efficiency (2%-3% relative) was observed in some of the boron-doped mc-Si wafers in which oxygen concentration was high (15 ppm). In contrast, gallium-doped solar cells were found to be very stable under illumination irrespective of their location in the ingot. Device characterization and modeling were performed to show that the combined effect of large variation in resistivity and lifetime along the gallium-doped mc-Si ingots results in variation in the cell efficiency from different regions of the gallium-doped ingots. Design rules were established to determine the optimum thickness of the solar cell for extracting maximum efficiency when the bulk lifetime and resistivity vary along the length of the ingot for a better utilization of the whole ingot     

Characterization of novel mono- and bifacially activesemi-transparent crystalline silicon solar cells

This paper presents the latest cell results for semi-transparent mono- as well as bifacially active POWER (Polycrystalline Wafer Engineering Result) solar cells of different cell sizes on Cz and multicrystalline silicon substrates. Top efficiencies of 10.4% for monofacial and 12.9% for bifacial cells are reported. Attention has been paid to apply a fully industrially compatible production process. It uses dicing saw based mechanical texturization of the front and rear side of the silicon wafer and screen printing metallization. In the POWER solar cell concept, perpendicular grooves on the front and rear side create holes with a variable diameter at their crossing points. This results in a partial optical transparency of the solar cell. In this study, holes of 200 μm diameter lead to a transparency of 16-18% on average for the total cell area. The cell characteristics for the different cell types are compared by means of illuminated and dark current-voltage (I-V), spectral response, and Laser Beam Induced Current (LBIC) measurements. While bifacial POWER cells need a more elaborate production process, they reveal better I-V characteristics and a higher efficiency as compared to monofacial cells. This is mainly explained by a better surface passivation due to an active emitter and a passivating silicon nitride ARC both on the front and rear surface     

Polycrystalline silicon solar cells from recrystallized plasma deposited thin films

Large grain polycrystalline silicon films are produced by a two step process involving plasma deposition of microcrystalline silicon films on a substrate, separation from the substrate, and subsequent grain enhancement of the silicon films. The effects of doping and substrate temperature during deposition on the solar cell conversion efficiency are investigated. Effects of ppm level molybdenum contamination from the substrate, and silicon microstructure after grain enhancement, on solar cell efficiency parameters are also investigated. Solar cells with efficiencies of up to 10.1% under AM1 illumination, were fabricated on these silicon films.     

丝网印刷制备黑硅太阳电池及其性能表征

采用纳米金颗粒催化腐蚀的方法在硅片表面制备得纳米多孔结构,实现了1.5%（300-1200 nm）的权重反射率。本文采用OPCl3扩散、丝网印刷制备前后电极及共烧等常规太阳电池工艺来制备黑硅太阳电池,对不同腐蚀深度及不同扩散方阻的黑硅太阳电池片的输出电性能进行了分析,并对制备工艺进行了优化,提高了电池的转换效率,实现了丝网印刷制备12.17%的黑硅太阳电池转换效率。     

The development of a purification technique of metallurgical silicon to silicon of the solar brand

The experimental results that show the possibility of obtaining silicon of the solar brand by the recrystallization of metallurgical silicon in fusible metals, e.g., tin, and growing monocrystalline silicon ingots from the obtained silicon scales by the Czochralski method are presented. The experiments on purifying fusible metal (tin) after ending a cycle to obtain silicon scales for the purpose of its repeated use were made. Tin purification was carried out by the method of vacuum decontamination of tin melt, its filtration, and then zone recrystallization. The qualitative and quantitative analysis of the initial materials (silicon and tin) and their structure after various stages of the technological process was carried out by the method of the roentgen-fluorescent analysis on the Elvax light device. The structural features of the obtained silicon scales were considered by means of raster electronic microscopy on the REMMA106I device. The conductivity type and the specific electric resistance of the obtained silicon monocrystalline ingot were measured by the fourprobe method on the PIUS-1UM-K device. It was shown that the grown monocrystalline silicon ingot has silicon content of at least 99.999% weight, n-type conductivity, and specific electric resistance of at least 2.0 Оhm сm. The described parameters correspond to the silicon the solar brand and confirm the possibility of obtaining it from metallurgical silicon by recrystallization in fusible metals, for example, tin.     

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.     

N‐type solar‐grade silicon purified via the metallurgical route: characterisation and fabrication of solar cells

This study focuses on the characterisation and the fabrication of solar cells using n‐type multicrystalline silicon purified via the metallurgical route. Electrical and chemical analyses were performed on wafers taken from several positions along the crystallised ingot. The impact of the fabrication processing steps was investigated via effective carrier lifetime measurements. Solar cells were processed, and their efficiencies were found to be dependent on the position of the wafer along the ingot height, that is, the wafer's resistivity. A maximum conversion efficiency of 15.0% was obtained on cells from the bottom part of the ingot. In this study, the minimum resistivity value of 0.4 Ω cm resistivity is given in order to reach adequate cell efficiency. Light‐soaking experiments were then performed on the fabricated cells. No significant variations of the cell performances were observed even after 110 h at 60 °C, meaning that the fabricated cells are stable under illumination. Copyright © 2012 John Wiley & Sons, Ltd.     

Impurity‐to‐efficiency simulator: predictive simulation of silicon solar cell performance based on iron content and distribution

We present a simulation tool that predicts solar cell efficiency based on iron content in as‐grown wafer and solar cell processing conditions. This “impurity‐to‐efficiency” (I2E) simulation tool consists of three serial components, which are independently and jointly validated using published experimental results: (1) a kinetic model that calculates changes in the distribution of iron and phosphorus atoms during annealing; (2) an electronic model that predicts depth‐dependent minority carrier lifetime based on iron distribution; and (3) a device simulator that predicts solar cell performance based on the minority carrier lifetime distribution throughout the wafer and the device architecture. The I2E model is demonstrated to be an effective predictor of cell performance for both single‐crystalline and multi‐crystalline silicon solar cells. We demonstrate the process optimization potential for the I2E simulator by analyzing efficiency improvements obtained using low‐temperature annealing, a processing concept that has been successfully applied to achieve higher solar cell efficiencies on Fe‐contaminated materials. Copyright © 2010 John Wiley & Sons, Ltd.