Gettering in multicrystalline silicon wafers with screen‐printed emitters

In this paper, the gettering potential of phosphorus dopant pastes used in single‐sided screen‐printing processes is investigated including the consequences for essential solar cell parameters. These results are supported by minority carrier lifetime measurements with the quasi‐steady‐state photoconductance method and certified by the analysis of the recombination current density in solar cells on mc‐Si wafers. Copyright © 2011 John Wiley & Sons, Ltd.     

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.     

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.     

Effect of P‐induced gettering on extended defects in n‐type multicrystalline silicon

The electrical properties and the minority charge carrier recombination behaviour of grain boundaries (GBs) and intragrain dislocations in different n‐type multicrystalline silicon (mc‐Si) ingots were systematically studied through microwave‐detected PhotoConductance Decay (µW‐PCD), Electron Beam Induced Current (EBIC) and PhotoLuminescence (PL) spectroscopy on as‐grown samples and on samples submitted to P‐diffusion step. It was confirmed that the overall quality of n‐type mc‐Si is high, indicating that n‐type‐Si is a valid source for photovoltaic applications. As expected, the average lifetime increases after the P‐diffusion process, which induces impurity gettering effects at the external surfaces, like in the case of p‐type samples, but an evident local increase of electrical activity of some GBs after that process was also observed using the EBIC mapping technique. Apparently, a redistribution of impurities occurs at the processing temperature and impurities are captured at the deepest sinks. In fact, while all GBs act as heterogeneous segregation/precipitation sites, some of them will compete with the external surfaces sinks, partly vanishing the effect of P‐gettering. Last but not least, it was experimentally demonstrated that the average lifetime values measured with the µW‐PCD technique well correlate with the recombination activity of GBs measured with the EBIC technique, showing the extreme importance of GBs on the effective lifetime of this material. Copyright © 2007 John Wiley & Sons, Ltd.     

Influence of temperature during phosphorus emitter diffusion from a spray‐on source in multicrystalline silicon solar cell processing

We have investigated the influence of diffusion temperature during phosphorus emitter diffusion from a spray‐on source on the performance of screen‐printed multicrystalline silicon solar cells. Because of the dual diffusion mechanism present at high concentration in‐diffusion of phosphorus in silicon, applying lower diffusion temperatures for a longer duration results in significantly enhanced penetration of the low concentration tail relative to the high concentration region. Moreover, we show that the sheet resistance of in‐diffused emitters from a high concentration source depends primarily on the extension of the high concentration region, thus significantly different emitter profiles can be manufactured without altering the sheet resistance considerably. Because of the enhanced tail penetration, emitters of a specified sheet resistance diffused at reduced temperatures can result in higher fill factors of screen‐printed solar cells due to diminution of Schottky type shunts. Furthermore, emitters diffused at lower temperatures for longer durations can yield a higher gettering efficiency, resulting in increased bulk recombination lifetime, and thus a higher internal quantum efficiency at long wavelengths. The deeper tail extension of low temperature emitters, however, causes increased absorption within the highly recombinative emitter, resulting in current losses due to a lower internal quantum efficiency at short wavelengths. Copyright © 2006 John Wiley & Sons, Ltd.     

Improvement of multicrystalline silicon solar cells by a low temperature anneal after emitter diffusion

The influence of an annealing step at about 500 °C after emitter diffusion of multicrystalline solar cells is investigated. Neighboring wafers from a silicon ingot were processed using different annealing durations and temperatures. The efficiency of the cells was measured and detailed light beam induced current measurements were performed. These show that mainly areas with high contents of precipitates near the crucible walls are affected by the anneal. An efficiency increase from 14.5 to 15.4% by a 2 h anneal at 500 °C was observed. The effect seems to be more likely external than internal gettering. Copyright © 2010 John Wiley & Sons, Ltd.     

Development of high‐performance multicrystalline silicon for photovoltaic industry

The low cost and high quality of multicrystalline silicon (mc‐Si) based on directional solidification has become the main stream in photovoltaic (PV) industry. The mc‐Si quality affects directly the conversion efficiency of solar cells, and thus, it is crucial to the cost of PV electricity. With the breakthrough of crystal growth technology, the so‐called high‐performance mc‐Si has increased about 1% in solar cell efficiency from 16.6% in 2011 to 17.6% in 2012 based on the whole ingot performance. In this paper, we report our development of this high‐performance mc‐Si. The key ideas behind this technology for defect control are discussed. With the high‐performance mc‐Si, we have achieved an average efficiency of near 17.8% and an open‐circuit voltage (V_oc) of 633 mV in production. The distribution of cell efficiency was rather narrow, and low‐efficiency cells (<17%) were also very few. The power of the 60‐cell module using the high‐efficiency cells could reach 261 W as well. Copyright © 2013 John Wiley & Sons, Ltd.     

Silicon nitride coating and crucible—effects of using upgraded materials in the casting of multicrystalline silicon ingots

A method for removal of metallic contaminants from commercial Si₃N₄‐powders has been developed. The method is based on acid leaching and shows promising results. Analyses show that the content of both intermetallic compunds as Fe, Ti and Al as well as O are significantly reduced. Clean, synthetic fused silica crucibles have been used along with normal sintered ones based on natural quartz. The crucibles were coated with normal, commercial silicon nitride powder and with purified Si₃N₄ in a cleanest possible environment. The crucibles were then used as vessels for directional solidification of multicrystalline silicon in a pilot scale furnace. The average lifetime of minority charge carriers in the cast silicon was determined by quasi steady state photoconductance (QSSPC) from the bottom to the top of the ingots. These varied in a systematic way, so that the materials cast in the pure environment had significantly higher values than the materials cast with conventional coating‐ and crucible materials. The maximum values for the lifetimes in the individual ingots varied from 7 to 135 µs. Copyright © 2007 John Wiley & Sons, Ltd.     

Determination of the bulk lifetime of bare multicrystalline silicon wafers

The determination of the bulk lifetime of bare multicrystalline silicon wafers without the need of surface passivation is a desirable goal. The implementation of an in‐line carrier lifetime analysis is only of benefit if the measurements can be done on bare unprocessed wafers and if the measured effective lifetime is clearly related to the bulk lifetime of the wafer. In this work, we present a detailed experimental study demonstrating the relationship between the effective carrier lifetime of unpassivated wafers and their bulk carrier lifetime. Numerical modelling is used to describe this relationship for different surface conditions taking into account the impact of a saw damage layers with poor electronic quality. Our results show that a prediction of the bulk lifetime from measurements on bare wafers is possible. Based on these results we suggest a simple procedure to implement the analysis for in‐line inspection. Copyright © 2010 John Wiley & Sons, Ltd.     

Influence of defect type on hydrogen passivation efficacy in multicrystalline silicon solar cells

We examine the effectiveness of hydrogen passivation as a function of defect type and microstructure at grain boundaries (GBs) in multicrystalline silicon. We analyze a solar cell with alternating mm‐wide bare and SiN_x‐coated stripes using laser‐beam‐induced current, electron backscatter diffraction, X‐ray fluorescence microscopy, and defect etching to correlate pre‐ and post‐hydrogenation recombination activity with GB character, density of iron‐silicide nanoprecipitates, and dislocations. A strong correlation was found between GB recombination activity and the nature/density of etch pits along the boundaries, while iron silicide precipitates above detection limits were found to play a less significant role. Copyright © 2010 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.     

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.     

Chemical natures and distributions of metal impurities in multicrystalline silicon materials

We present a comprehensive summary of our observations of metal‐rich particles in multicrystalline silicon (mc‐Si) solar cell materials from multiple vendors, including directionally‐solidified ingot‐grown, sheet, and ribbon, as well as multicrystalline float zone materials contaminated during growth. In each material, the elemental nature, chemical states, and distributions of metal‐rich particles are assessed by synchrotron‐based analytical x‐ray microprobe techniques. Certain universal physical principles appear to govern the behavior of metals in nearly all materials: (a) Two types of metal‐rich particles can be observed (metal silicide nanoprecipitates and metal‐rich inclusions up to tens of microns in size, frequently oxidized), (b) spatial distributions of individual elements strongly depend on their solubility and diffusivity, and (c) strong interactions exist between metals and certain types of structural defects. Differences in the distribution and elemental nature of metal contamination between different mc‐Si materials can largely be explained by variations in crystal growth parameters, structural defect types, and contamination sources. Copyright © 2006 John Wiley & Sons, Ltd.     

Metal fingers on grain boundaries in multicrystalline silicon solar cells

We have developed a method of applying a net‐like finger grid to the front of multicrystalline (mc) silicon solar cells, which lies mainly on the grain boundaries (grain‐boundary‐oriented‐finger GBOF grid). This net has no busbars. It is drawn by a plotter, using screen printing paste dispensed through a fine tube. The power output of cells contacted in this manner has been tested in a statistical study of pairs and triplets of cells of size 100 × 100 mm² (Bayer) and 103 × 103 mm² (Eurosolare). In the pairs study, pairs of neighbouring wafers of the original ingot were processed into solar cells. One wafer received a GBOF grid, the other got the same grid rotated by 90°, and so had little coverage of grain boundaries. In the study of triplets the third wafer of each triplet was equipped with a standard H‐pattern of the same shading as the GBOF grid. In the pairs study, we find that under approximately standard conditions there is an 89% chance that the GBOF grid increases power output over cells with an identical, but 90° rotated, grid, the most probable increase being 2.6%. The triplets study shows that there is an 87% chance that the GBOF grid increases power output over cells with the standard H‐pattern, the most probable increase being 2.5%. Copyright © 2002 John Wiley & Sons, Ltd.     

Towards implementation of floating cast method for growing large‐scale high‐quality multicrystalline silicon ingot using designed double crucibles

We report on our attempt to scale up the floating cast method to grow high‐quality multicrystalline silicon ingot using specially designed double crucibles. The cross section of the grown ingot showed the large crystal grain size originating from initially formed dendrite crystals around the top of the melt. By using double crucibles, the residual melt was spontaneously removed from the inner crucible to the outer one at the final stage of the crystal growth. Performance of small‐scale solar cells revealed no significant changes between the top and the bottom part of the ingot except the edge of the bottom due to the non‐uniform removal of the melt. This suggests that strong contact of the ingot with the crucible was successfully avoided by the double crucibles. Therefore, the floating cast method combined with specially designed double crucibles is concluded to be feasible, which could be implemented to realize high‐quality multicrystalline silicon ingot for practical size wafers. Copyright © 2013 John Wiley & Sons, Ltd.     

冶金法n型多晶硅磷、硼吸杂实验的研究

研究了冶金法n型多晶硅片磷吸杂、硼吸杂效果及其机理。研究发现,经1 000℃/4h磷吸杂后,硅片平均少子寿命从1.21μs提高到11.98μs;经950℃/1h硼吸杂后,平均少子寿命从1.52μs提升到10.74μs。两种吸杂处理后,硅片电阻率从0.2Ω.cm提高到0.5Ω.cm。结果表明,两种吸杂工艺使得硅片表面形成的重磷、重硼扩散层对金属杂质有较好的吸杂作用,从而减少载流子的复合中心,改善多晶硅片的性能。     

n‐Type multicrystalline silicon wafers prepared from plasma torch refined upgraded metallurgical feedstock

n‐Type silicon wafers present some definite advantages for the photovoltaic industry, mainly due to the low capture cross sections of minority carriers for most metallic impurities. This peculiarity is beneficial for multicrystalline silicon (mc‐Si) wafers in which the interaction between crystallographic defects and impurities is the main source of recombination centres. Most importantly, this peculiarity could be of a great interest when mc‐Si ingots are produced directly from upgraded and purified metallurgical silicon feedstock. It is of a paramount importance to verify if the advantages of conventional n‐type silicon also characterizes n‐type wafers provided by a direct metallurgical route. It is found, in raw wafers, that minority carrier diffusion lengths are three times higher in n‐type than in p‐type wafers, when the wafers are cut from the same ingot, where the bottom is p‐type and the top is n‐type, due to the difference in the segregation coefficients of doping elements (boron and phosphorus). After different processing steps and gettering treatments the minority carrier diffusion lengths are always neatly larger in n‐type than in p‐type wafers The results confirm the interest for n‐type silicon. Copyright © 2009 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.     

Fundamental boron–oxygen‐related carrier lifetime limit in mono‐ and multicrystalline silicon

Boron‐doped crystalline silicon is the most relevant material in today's solar cell production. Following the trend towards higher efficiencies, silicon substrate materials with high carrier lifetimes are becoming more and more important. In silicon with sufficiently low metal impurity concentrations, the carrier lifetime is ultimately limited by a metastable boron–oxygen‐related defect, which forms under minority‐carrier injection. We have analysed 49 different Czochralski‐grown silicon materials of numerous suppliers with various boron and oxygen concentrations. On the basis of our measured lifetime data, we have derived a universal empirical parameterisation predicting the stable carrier lifetime from the boron and oxygen content in the crystalline silicon material. For multicrystalline silicon it is shown that the predicted carrier lifetime can be regarded as a fundamental upper limit. Copyright © 2005 John Wiley & Sons, Ltd.