Search

Metallic impurities in the silicon wafer bulk are one of the major efficiency-limiting factors in silicon solar cells. Gettering can be used to significantly lower the bulk metal concentrations. Aluminum oxide thin films from plasma-enhanced atomic layer deposition (PE-ALD) have been reported to getter iron from silicon wafers. However, its gettering mechanism and kinetics remain unclear. In this study, by experimentally monitoring the kinetics of iron reduction in the silicon wafer bulk, aluminum oxide gettering of iron is shown to be caused by a segregation mechanism. Fitting the experimental iron reduction kinetics by the simulation of a segregation gettering process based on various diffusion scenarios suggests that the gettering kinetics is limited by both the diffusivities of iron in the silicon wafer bulk and in the aluminum oxide film. The activation energy of the segregation gettering process (negative meaning exothermic reaction) is estimated to be −0.47 ± 0.16 eV for the investigated as-deposited PE-ALD aluminum oxide film at 550–900 °C, and −0.35 ± 0.06 eV at 400–900 °C for the same film after a 400 °C forming gas anneal (FGA), i.e., after activating the passivation effect of the film. Capacitance–voltage measurements of the films indicate a higher surface defect density in the as-deposited films as compared to the FGA-activated films, which suggests a possible correlation between the surface defect density and gettering. [ABSTRACT FROM AUTHOR]

The research results present the influence of nickel impurities introduced by diffusion into monocrystalline silicon on the characteristics of solar cells (SCs). It is established that doping with nickel atoms makes it possible to increase the lifetime of the MCCs in the material by up to a factor of two and the efficiency of SCs by 20–25%. It is shown that the distribution of nickel clusters in the volume of the material is almost uniform, and their size does not exceed 0.5 μm. The concentration of clusters in the volume is ~1011–1013 cm–3; and in the near-surface layer, ~1013–1015 cm–3. The physical mechanisms of the influence of the bulk and near-surface clusters of nickel atoms on the efficiency of silicon SCs are revealed. It is experimentally established that the decisive role in increasing their efficiency is played by the processes of gettering of recombination-active technological impurities by nickel clusters, occurring in the nickel-enriched front surface region of the SCs. [ABSTRACT FROM AUTHOR]

In addition to excellent surface passivation and carrier selectivity, the structure based on the heavily doped polysilicon layer on an ultrathin silicon oxide interlayer also demonstrates strong impurity gettering effects. Herein, the gettering strength of a range of phosphorus‐ or boron‐doped polysilicon films from different fabrication techniques is assessed and compared. Iron, one of the most common metallic impurities in silicon, is used as a tracer impurity to quantify the gettering strength (segregation coefficient). A comparison of the experimental results to the literature, combined with measurements of the electrically active and inactive dopant concentrations, enables us to suggest the main gettering mechanisms in different polysilicon films. The differences in the segregation coefficients of the phosphorus‐doped polysilicon films for iron are within one order of magnitude, in spite of their different combinations of gettering mechanisms. On the other hand, boron‐doped polysilicon films show a large variation in their gettering effects, although the predominant gettering mechanisms are all attributed to electrically inactive boron, according to the current understanding of the gettering mechanisms from the literature. Finally, the impact of different polysilicon gettering effects on the efficiency of tunnel oxide‐passivated contact (TOPCon) cells is simulated and discussed. [ABSTRACT FROM AUTHOR]

High-performance multicrystalline silicon (HP mc-Si) from directional solidification has become the mainstream industrial material for fabricating mc-Si based solar cells for photovoltaic applications. Transition metal impurities are inherently contained in HP mc-Si during ingot growth, and they are one of the major efficiency-limiting drawbacks. In this work, we investigate the gettering of transition metals (Cu, Ni, Fe, and Cr) in HP mc-Si wafers along an industrial-standard p-type HP mc-Si ingot, via examining the metal concentration and distribution in the near-surface gettering layers using secondary ion mass spectrometry. We applied both conventional phosphorus diffusion gettering and the recently developed silicon nitride (from plasma-enhanced chemical vapour deposition) gettering techniques. Both techniques are shown to remove significant quantities of metals from the silicon wafer bulk to the surface gettering layers. Improvements in the bulk minority carrier lifetimes throughout the ingot height are also observed by lifetime measurements and spatially-resolved photoluminescence imaging. The gettered Cu and Ni concentrations, as well as the as-grown dissolved Fe concentrations in the silicon wafer bulk, along the HP mc-Si ingot height are shown to follow a similar concentration profile as the metals in conventional mc-Si ingots. [ABSTRACT FROM AUTHOR]

Phosphorus gettering using tubular diffusion furnaces was performed on n-type cast monocrystalline silicon wafers to assess its impact on wafer quality and the conversion efficiency of heterojunction solar cells. A comprehensive analysis of temperature, duration, and cooling rate in the diffusion process was conducted. The optimal parameters were identified at a temperature of 850 °C for 30 min, with a cooling rate of 5 °C/min down to 500 °C. This gettering process effectively reduced interstitial iron concentration, improved minority carrier lifetime, and increased the average efficiency of HJT cells by an absolute 0.4 %, with a 1.1 % increase for those made from bottom wafers with high metal concentration. [ABSTRACT FROM AUTHOR]

This study reports on the electronic properties of industrial phosphorus-doped n-type silicon ingots for photovoltaic applications grown using the Recharged Czochralski method. The electronic quality is assessed via carrier lifetime measurements, both directly on the ingots and on passivated wafers, and via implied open-circuit (i V O C), and implied maximum power point (i V M P P) voltages. The wafers are studied in the as-grown state, and after various high temperature steps, including Tabula Rasa , phosphorus diffusion gettering, and boron diffusion. The material exhibited very high bulk quality, with bulk lifetimes up to 8 ms at an injection level of 5 × 10 14 cm − 3 , and with i V O C (1-sun) values up to 750 mV , prior to any high temperature processing. A Tabula Rasa step did not significantly improve the wafer quality, indicating a low presence of oxygen-related defects in this material, consistent with the low interstitial oxygen content of below 5 × 10 17 cm − 3 . However, phosphorus diffusion gettering improved the wafer quality, especially towards the tail end of each ingot, and at lower injection levels near maximum power point. Phosphorus diffusion gettering increased the i V O C (1-sun) of the wafers by around 5 mV , approaching the Auger limit. Additionally, a boron diffusion step had minimal impact on the bulk lifetimes. Overall, our findings suggest that these RCz-grown n-type wafers exhibit very high quality, approaching the Auger limit near open-circuit, and are well-suited for high-efficiency solar cells without the need for additional high-temperature processing. • Assessing the material quality of n-type RCz industrial ingots, both directly on the ingot and on passivated wafers. • Bulk lifetimes of up to 8 ms and i V O C (1-sun) values up to 750 mV. • Low interstitial oxygen content of below 5 × 10 17 cm − 3 , low presence of oxygen-related defects. • Improvement of the wafer quality after gettering, particularly towards the bottom of each ingot, nearing the Auger limit. • No significant degradation after boron diffusion. [ABSTRACT FROM AUTHOR]

Polycrystalline‐silicon/oxide (poly‐Si/SiOx) passivating contacts for high efficiency solar cells exhibit excellent surface passivation, carrier selectivity, and impurity gettering effects. However, the ultrathin SiOx interlayer can act as a diffusion barrier for metal impurities and this potentially slows down the overall gettering rate of the poly‐Si/SiOx structures. Herein, the factors that determine the blocking effects of the SiOx interlayers are identified and investigated by examining two general types of the SiOx interlayers: 1.3 nm ultrathin tunneling SiOx with negligible pinholes and 2.5 nm SiOx with thermally created pinholes. Iron is used as tracer impurity in silicon to quantify the gettering rate. By fitting the experimental gettering kinetics by a diffusion‐limited segregation gettering model, the blocking effects of the SiOx interlayers are quantified by a transport parameter. Both the oxide stoichiometry and pinhole density affect the effective transport of iron through SiOx interlayers. The oxide stoichiometry depends strongly on the oxidation method, while the pinhole density is affected by the activation temperature, doping concentration, doping technique, and possibly the dopant type as well. To enable a fast gettering process during typical high‐temperature formation of the poly‐Si/SiOx structures, a SiOx interlayer that is less stoichiometric or with a higher pinhole density is preferred. [ABSTRACT FROM AUTHOR]

In this work, we integrate defect engineering methods of gettering and hydrogenation into silicon heterojunction solar cells fabricated using low‐lifetime commercial‐grade p‐type Czochralski‐grown monocrystalline and high‐performance multicrystalline wafers. We independently assess the impact of gettering on the removal of bulk impurities such as iron as well as the impact of hydrogenation on the passivation of grain boundaries and B‐O defects. Furthermore, we report for the first time the susceptibility of heterojunction devices to light‐ and elevated temperature–induced degradation and investigate the onset of such degradation during device fabrication. Lastly, we demonstrate solar cells with independently verified 1‐sun open‐circuit voltages of 707 and 702 mV on monocrystalline and multicrystalline silicon wafers, respectively, with a starting bulk minority‐carrier lifetime below 40 microseconds. These remarkably high open‐circuit voltages reveal the potential of inexpensive low‐lifetime p‐type silicon wafers for making devices with efficiencies without needing to shift towards n‐type substrates. [ABSTRACT FROM AUTHOR]

Due to the low temperature processing constraint in silicon heterojunction (SHJ) solar cells, no defect engineering to improve silicon wafer quality is typically incorporated during cell fabrication. This means that high‐quality n‐type Cz wafers are required to produce high‐efficiency cells. Herein, recent demonstrations of incorporating defect engineering approaches, such as gettering and hydrogenation, into the SHJ process flow for both n‐type and p‐type wafers are surveyed. Defect engineering on wafers before cell fabrication can significantly improve the quality of low‐lifetime p‐type wafers, making them much more suitable for SHJ cells. Interestingly, these same approaches are also very effective in improving the cell performance in the n‐type wafers that are conventionally used in industry. Post‐cell illuminated annealing processes have been shown to eliminate boron–oxygen light‐induced degradation (LID) in p‐type wafers, leading to stable VOC exceeding 735 mV. New results indicate that the hydrogen naturally incorporated during SHJ processing is sufficient to passivate these defects. Similar illuminated annealing processes also lead to substantial improvements in n‐type SHJ cells. With these findings considered, it is demonstrated that a modified SHJ process flow, which includes defect engineering on a wafer level and post‐cell hydrogen passivation, is beneficial for SHJ production, regardless of the wafer type. [ABSTRACT FROM AUTHOR]

Czochralski‐grown gallium‐doped silicon wafers are now a mainstream substrate for commercial passivated emitter and rear cell (PERC) devices and allow retention of established processes while offering enhanced cell stability. We have assessed the carrier lifetime potential of such Czochralski‐grown wafers in dependence of resistivity, finding effective lifetimes well into the millisecond region without any gettering or hydrogenation processing, thus demonstrating one advantage over boron‐doped silicon. Second, the stability of gallium‐doped PERC cells are monitored under illumination (>3000 h in some cases) and anomalous behavior is detected. While some cells are stable, others exhibit a degradation then recovery, reminiscent of light and elevated temperature‐induced degradation (LeTID) observed in other silicon materials. Surprisingly, cells from one ingot exhibit LeTID‐like behavior when annealed at 300 °C but near stability when not annealed, but, for another ingot, the opposite is observed. Moreover, a stabilization process typically used to mitigate boron–oxygen degradation does not influence any cells that are studied. Secondary‐ion mass spectrometry of the PERC cells reveals significant concentrations of unintentionally incorporated boron in some cases. Nevertheless, even in the absence of mitigating light‐induced degradation, Ga‐doped silicon is still more stable than unstabilized B‐doped silicon under illumination. [ABSTRACT FROM AUTHOR]

This work presents results of a laboratory‐scale interdigitated back contact (IBC) solar cell with an independently measured efficiency of 25.0%, featuring open‐circuit voltage of 716 mV, short‐circuit current of 43.0 mA.cm−2 and fill factor of 81.0%. Notably, the high efficiency was achieved based on significant improvements resulting from the optimised cell structure, excellent SiO2‐SiNx‐SiOx (ONO) surface passivation, detailed bulk lifetime management strategy and improved random pyramid texturing. Experimental details and analysis of the individual improvements over prior work are presented in‐depth. [ABSTRACT FROM AUTHOR]

The article focuses on assessing the quality of industrial phosphorus-doped n-type silicon wafers grown via the Czochralski method along an ingot. Topics include evaluating the minority charge carrier lifetimes before and after phosphorous diffusion gettering, observing a significant increase in bulk lifetime indicating effective gettering and identifying iron as a limiting defect using experimental lifetime monitoring and simulation of gettering kinetics.

Abstract In this paper, the influence of an annealing process at 500 °C for 30 min. after emitter diffusion of multi-crystalline solar cells was investigated. Neighboring sister wafers from a silicon ingot were processed using light doped emitter (LDE) and low temperature annealing (LTA). Emitter profiles measured by Electrochemical Capacitance Voltage (ECV) showed that there is no obvious difference between LDE and LTA due to the lower annealing temperature. The lifetime and iron concentration measurement indicated that the gettering of LTA diffusion is much better than LDE. Therefore, the solar cell efficiency of LTA was up to 19.01%, which is 0.28%abs higher than LDE due to the higher open circuit voltage (Voc) and short-circuit current (Isc). The Internal Quantum Efficiency (IQE) measurement with better long wavelength response showed the wafer quality was much improved after LTA diffusion. To further investigate diffusion gettering performance, light-induced-degradation (LID) was analyzed both for LDE and LTA diffusion. The LID of LDE diffusion with 1.15%rel. is obviously higher than LTA diffusion of 0.46%rel., attributing to the higher iron concentration of LDE diffusion emitter to result in higher iron-boron pair dissociation. [ABSTRACT FROM AUTHOR]

This paper presents direct experimental evidence for the strong impurity gettering effects associated with the formation of both phosphorus and boron doped polysilicon/oxide passivating contacts for silicon solar cells, doped via thermal diffusion from POCl 3 or BBr 3 sources. Ion-implanted iron is used as a marker to quantify the gettering effectiveness via carrier lifetime measurements. The process conditions for fabricating optimum polysilicon passivating contacts are found to remove more than 99.9% of the iron from the silicon wafer bulk. The gettering effects of POCl 3 and BBr 3 diffused polysilicon/oxide contacts mainly arise from the dopant diffusions, as opposed to gettering by structural defects in the polysilicon films. The thin oxide interlayer hinders the gettering effectiveness at low diffusion temperatures, although its blocking effect becomes small at the moderate temperatures used to fabricate optimum polysilicon contacts. The gettering effectiveness increases with increasing diffusion temperature. The gettering of iron from the silicon wafer bulk to the surface layers is found to have a negligible impact on their ability to suppress recombination at the interface with the silicon wafer. Therefore, the formation of polysilicon/oxide passivating contacts, via thermal diffusion from POCl 3 and BBr 3 sources, not only achieves high quality surface and contact passivation but also has the net additional benefit of achieving very effective gettering of unwanted impurities in the silicon wafer bulk. [ABSTRACT FROM AUTHOR]

In this work, we report a simple and efficient purification technique of solar grade silicon wafers suitable for high efficiency and low-cost silicon solar cells. This gettering method requires the elaboration of silicon nanowire thin layer followed by an annealing treatment and removing impurity loaded silicon nanowires. ICP-AES analysis shows a significant decrease in the metallic impurity concentration resulting considerable enhancement in the silicon purity grade. Electrical characterizations reveal a global decrease of the majority carrier concentration and a substantial improvement of the minority carrier lifetime. Furthermore, the application of this novel gettering process induces an enhancement in the silicon solar cell electrical parameters. These results give evidence of the effectiveness of the proposed purification method to produce efficient solar cells starting from low-quality silicon wafers. [ABSTRACT FROM AUTHOR]

Carrier-selective contact schemes, like polysilicon on oxide (POLO), provide low contact resistivities while preserving an excellent passivation quality. These junctions offer an important additional feature compared to a-Si/c-Si heterojunctions. We find that the formation of n -type POLO junctions lead to a huge increase of the Shockley-Read-Hall (SRH) lifetime of the substrate, a prerequisite for highly efficient solar cells. The SRH lifetime improvement can be observed for both bulk polarities and for a variety of bulk resistivities. Thus we suggest that the highly doped POLO junction getters impurities that have more or less symmetric SRH capture cross sections. We are able to achieve SRH lifetimes of > 50 ms. By applying POLO junctions to interdigitated back contact cells, we achieve cells with an efficiency of 25%. [ABSTRACT FROM AUTHOR]

We discuss the importance of gettering and hydrogenation for next-generation silicon solar cells in the context of industrial cell fabrication. Gettering and hydrogenation play a vital role for p-type cell technologies in improving the silicon material's minority charge carrier lifetime. These mechanisms are naturally incorporated during screen-printed cell fabrication through the phosphorus emitter diffusion, silicon nitride deposition and subsequent metallisation firing processes. While the transition towards emitters with lower dopant concentrations and/or thermal oxide passivation can reduce surface recombination, it can negatively impact the ability to getter common impurities such as iron. For cell technologies with alternative low-temperature metallisation approaches, the ability to hydrogenate bulk defects is greatly reduced. Ultra-high efficiency n-type technologies tend to use heterojunction structures rather than diffused layers, but in doing so, do not benefit from phosphorus gettering. Also, particularly for amorphous silicon-based heterojunction structures, the imposed temperature constraints strongly limit the ability to passivate bulk defects. As a result, high-efficiency n-type technologies rely on the use of 'high-quality' wafers or would require the deliberate addition of gettering and hydrogenation processes before cell fabrication. A potential high-efficiency hybrid homojunction/heterojunction structure is then discussed that could naturally enable gettering and bulk hydrogenation throughout cell fabrication. Calibrated implied open circuit voltage ( Voc) map of a p-type mono-crystalline wafer highlighting the impact of pre-hydrogenating the top half of the wafer. [ABSTRACT FROM AUTHOR]

High-efficiency solar cell designs such as the passivated emitter and rear cell (PERC) raise the quality requirements for silicon substrates, favoring monocrystalline materials. Seed-cast quasi-monocrystalline silicon (qm-Si) is a promising alternative for Czochralski (Cz) Si with potential benefits of lower cost and reduced energy footprint. However, the purity and crystalline quality of qm-Si is not on par with Cz-Si, which can cause efficiency losses for example in the form of light-induced degradation (LID). In this contribution, we study the LID phenomena that can be present in qm-Si PERC solar cells, and compare them to the Cz-Si PERC. Degradation and regeneration are analyzed especially from the viewpoint of Cu impurity, which has until very recently been omitted as a source of LID for this device type. Subsequently, differences in LID behavior between qm-Si and Cz-Si are investigated considering the density of dislocations in the bulk. The results imply that slurry-based wafer slicing may introduce contamination that is capable of causing considerable LID in PERC devices fabricated of Si with inherently high defect density. [ABSTRACT FROM AUTHOR]

Impurities and defects are of significant interest in multicrystalline silicon, due to the detrimental effect they can have on carrier lifetimes and electrical properties. In view of that, it is important to incorporate certain processing steps to decrease the recombination activities. In this study, a novel experiment was applied as a beneficial approach to improve the electronic quality of low-resistivity mc-Si substrates via a two-step process. Initially, the first step involves gettering multicrystalline substrates using sacrificial porous silicon layer on both sides, which was introduced as a simple sequence for efficient extrinsic gettering schemes. The gettering experiment was performed at 600–900 °C, and optimum results were obtained at 900 °C. Then, the second step involves coating the front surface of gettered mc-Si at 900 °C with vanadium oxide that serves as an excellent antireflection layer and leads to improve furthermore the electrical properties. Significant improvements were obtained after the deposition of vanadium oxide antireflection coating, in view of the fact that gettered mc-Si substrate at 900 °C provides the highest minority carrier lifetime and the lowest effective surface recombination velocity. An overall increase of the electrical properties was obtained after the described two-step process. The conversion efficiency increases from 6% (reference) and reached 13.7%. [ABSTRACT FROM AUTHOR]

The γ-ray irradiation hardness of arrayed silicon microhole-based radial p–n junction (ASMRJ) solar cells (SCs) has been experimentally studied. It was found that the sidewall morphology of the microhole arrays had an important effect on the radiation hardness, so the 4 µm-pitch ASMRJ SCs with hole arrays' sidewalls both unpassivated and passivated were made and referred to as 4 µm-U-ASMRJ and -P-ASMRJ SCs, respectively. On increasing the radiation doses, in contrast with the monotonous and rapid degradation of short circuit current density and open circuit voltage for the planar SCs, these parameters for the 4 µm-U-ASMRJ SCs show a small increase in the initial stage of γ-ray irradiation and then a slow decline. Average conversion efficiency shows an initial slight ascent by 4.5%. Additionally, the average conversion efficiency for the 2 µm-U-ASMRJ SCs shows an initial slight ascent by 5.7%. When the radiation doses grow to 8 × 106 rad, the average conversion efficiency degradation rates for the 2 µm- and 4 µm-U-ASMRJ SCs are 14% and 15%, respectively, whereas it is 39% for the planar SCs. The radiation-gettering mechanism is suggested to explain the radiation-hardened properties of the U-ASMRJ SCs. [ABSTRACT FROM AUTHOR]

Abstract: A multidiode simulation determines the absolute solar cell efficiency gain due to gettering. The simulation predicts current/voltage characteristics of lateral inhomogeneous industrial solar cells from luminescence images measured before and after gettering or any other processing step. Improved lifetime distributions lead to a predicted increase in solar cell efficiency Δηabs = +0.46% for a POCl3 diffusion on multicrystalline silicon wafers. The simulative approach determines the gettering efficiency of any process step on any wafer material without preselection. [Copyright &y& Elsevier]

The extraction of metal impurities during phosphorus diffusion gettering (PDG) is one of the crucial process steps when fabricating high-efficiency solar cells using low-cost, lower-purity silicon wafers. In this work, we show that for a given metal concentration, the size and density of metal silicide precipitates strongly influences the gettering efficacy. Different precipitate size distributions can be already found in silicon wafers grown by different techniques. In our experiment, however, the as-grown distribution of precipitated metals in multicrystalline Si sister wafers is engineered through different annealing treatments in order to control for the concentration and distribution of other defects. A high density of small precipitates is formed during a homogenization step, and a lower density of larger precipitates is formed during extended annealing at 740 °C. After PDG, homogenized samples show a decreased interstitial iron concentration compared to as-grown and ripened samples, in agreement with simulations. [ABSTRACT FROM AUTHOR]

Abstract: To improve PV module''s lifetime and reliability, it is essential to understand the mechanisms and causes that degrade module performance over time under different climatic conditions. In this study, our main aim is to correlate the degradation of module power with physical changes that are generated in the cells over time. The poly-crystalline silicon 200W modules are subjected to long term accelerated tests (testing to be continued until failure occurs) and cell level defects are characterized using electroluminescence (EL) technique. The characterization is done periodically to analyze the trend of physical changes occurring in the module under aggravated stresses and degrading module''s performance. Also, change in series resistance under thermal stress is analyzed as it has a direct impact on module''s output power. Iterative simulation based software and EL images have been used to estimate series resistance of the cells. [Copyright &y& Elsevier]

Abstract: This study is focussed on the growth of multicrystalline silicon ingots with large grains by controlling the silicon melt cooling rate to initiate dendritic nucleation in the initial stage of the solidification. Two ingots were grown with different undercooling rates and compared with a reference ingot grown by standard cooling conditions. All ingots were grown in a lab scale directional solidification system. The wafers cut from all three ingots have been characterized for resistivity, minority carrier lifetime and dislocation density measurements by four point probe, quasi steady state photo conductance and PV Scan, respectively. The wafers were converted into solar cells and their electrical parameters have been measured. The cells fabricated from ingot 2 show slightly higher efficiencies in comparison with ingot 1 and the reference one. The present cooling rate was not enough to initiate the dendrite nucleation in the beginning of the solidification. Hence, there is no significant difference was observed in the crystal quality of the grown ingots 1 and 2. [Copyright &y& Elsevier]

Abstract: The effects of boron diffusion were investigated on electronic grade n-type single- and multicrystalline silicon wafers for solar cells application. Doped p+-layers were formed using specific spin-on dopant (SOD) source annealed in a tube furnace. Homogeneous diffused layers with sheet resistance values of 70Ω/sq were achieved on large area silicon wafers. Bulk electrical properties were investigated measuring the effective carrier lifetime after the diffusion step. A significant degradation of the bulk electrical properties was observed using the SOD technique. Unlike for the single-crystalline silicon, it was determined that this effect was not due to an interstitial iron contamination originating from the SOD, but rather by the thermal degradation of the material. Boron diffusion was followed by a phosphorous diffusion sequence, necessary in the fabrication process for the formation of the back surface field. This additional step leads to an improvement of effective lifetime values due to an efficient gettering of the impurities. Finally, bulk carrier lifetimes from 100μs to over 300μs were deduced on n-type multicrystalline silicon after boron and phosphorus diffusion. This underlines the potential of this material for photovoltaic application and its stability after high temperature treatments provided that a gettering step is included after the boron diffusion step. [Copyright &y& Elsevier]

Abstract: Ultimately, alternative ways of silicon purification for photovoltaic applications are developed and applied. There is an ongoing debate about what are the acceptable contamination levels within the purified silicon feedstock to specify the material as solar grade silicon. Applying a simple model and making some additional assumptions, we calculate the acceptable contamination levels of different characteristic impurities for each fabrication step of a typical industrial mc-Si solar cell. The acceptable impurity concentrations within the finished solar cell are calculated for SRH recombination exclusively and under low injection conditions. It is assumed that during solar cell fabrication impurity concentrations are only altered by a gettering step. During the crystallization process, impurity segregation at the solid-liquid interface and at extended defects are taken into account. Finally, the initial contamination levels allowed within the feedstock are deduced. The acceptable concentration of iron in the finished solar cell is determined to be ppma whereas the concentration in the silicon feedstock can be as high as 12.5ppma. In comparison, the titanium concentration admitted in the solar cell is calculated to be ppma and the allowed concentration of ppma in the feedstock is only two orders of magnitude higher. Finally, it is shown theoretically and experimentally that slow cooling rates can lead to a decrease of the interstitial Fe concentration and thus relax the purity requirements in the feedstock. [Copyright &y& Elsevier]

Abstract: Nowadays the photovoltaic (PV) market suffers the severe shortage of silicon. One possible solution is to produce SoG-Si via a direct metallurgical route, followed by a final casting step. The use of such lower quality materials in solar cell production depends on the possibility of improving the electrical quality during the cell processing and requires a deep understanding of the interaction between defects. The aim of this work is to study the electrical properties and the minority charge carrier recombination behaviour of extended defects in a mc-Si ingot grown from metallurgical Si produced directly by carbothermic reduction of very pure quartz and carbon. The combined application of photoluminescence, infrared spectroscopy, electron beam induced current technique and transmission electron microscopy succeeded in identifying oxygen precipitates, decorated grain boundaries and dislocations as the defects which limit the quality of the metallurgical mc-Si and, therefore, the efficiency of the related solar cells. [Copyright &y& Elsevier]

Abstract: Multicrystalline silicon materials for photovoltaic applications inherently contain extended defects like grain boundaries, dislocations, microdefects and in some cases also second phase precipitates due to high concentrations of light elements (carbon, nitrogen or oxygen) and transition metal impurities. The latter are known to reduce the minority carrier lifetime and hence should be removed by gettering during solar cell processing. This paper discusses the influence of extended defects on the spatial distribution of copper- and nickel-related silicide precipitates for a model system containing a small angle grain boundary and in one part silicon oxide pecipitates partly associated with punched-out dislocations. Phosphorus-diffusion gettering under conditions of mostly precipitated metal impurities is discussed in terms of quantitative simulations. It is shown that two regimes can be distinguished where gettering kinetics are either limited by precipitate dissolution or phosphorus in-diffusion. Finally, binding of metal impurities to dislocations is considered and its effect on gettering kinetics is illustrated in terms of gettering simulations. [Copyright &y& Elsevier]

A key efficiency-limiting factor in silicon-based photovoltaic (PV) devices is the quality of the silicon material itself. With evolving cell architectures that better address other efficiency-loss channels in the device, the final device efficiency becomes increasingly sensitive to the contaminants in the silicon wafer bulk. However, due to cost constraints, silicon materials for PV are inherently less pure and further contamination during device fabrication is commonly found, especially in mass production environments. Metallic impurities are ubiquitous and abundant, and they are strong efficiency-loss channels in the device if not removed. Gettering is the process of removing metallic impurities to a less harmful region of the device, and is therefore an essential aspect of the cell fabrication process. This article presents an up-to-date review of the gettering techniques and processes in silicon solar cells, providing a complete picture of the possible gettering sinks and routes in various cell architectures. The article starts by explaining the common nomenclatures in gettering and summarising recent updates to the solubility and diffusivity data of the common 3d transition metals in silicon. Then the three-step gettering process (release, diffusion, capture) is explained, and its implications for solar-grade cast-grown silicon (in terms of release) and various cell architectures (in terms of diffusion) are discussed. The main focus of the article is to summarise and review the various capture approaches in the context of silicon PV. These include phosphorus diffusion, boron diffusion, selective doping via ion implantation, state-of-the-art polycrystalline-silicon/oxide passivating contact structures, dielectric films (silicon nitride and aluminium oxide), aluminium alloying, surface damaged regions including black silicon, and internal gettering in cast-grown silicon by existing crystallographic defects. Their gettering effects, current understanding of the gettering mechanisms, modelling, improvement strategies, implementation in processing and potential impacts on cell performance are reviewed. [ABSTRACT FROM AUTHOR]

Upgraded metallurgical grade (UMG) silicon (Si) has raised interest as an alternative material for solar cells due to its low cost, low environmental impact and low CAPEX. Maximum cell efficiencies at the level of those obtained from high purity poly-Si have been reported. However, a higher defect density and the compensated doping character result in UMG-based cell efficiencies varying over wider ranges in frequency distribution charts. In this report we characterize mc-Si UMG samples with different defect densities, comparing them with monocrystalline silicon (mono-Si) UMG and commercial high-performance multicrystalline silicon (mc-Si) samples, analysing the impact of carrier trapping by means of photoconductance (PC) decay measurements, and its evolution after applying a phosphorous diffusion gettering (PDG) process. When analyzing the decay time constant of the PC measurements, slow (66.8 ± 14.3 ms) and fast (16.1 ± 3.5 ms) traps are found in mc-Si samples, while no evidence of trapping is found in mono-UMG samples. Slow traps are effectively removed after the PDG process, while fast traps do remain. The influence of dislocations clusters and the possible role of oxygen, as revealed by Fourier-transform infrared spectroscopy (FTIR) is discussed. Finally, the improvement in minority carrier lifetime due to the PDG treatment is reported for each sample type, reaching values up to 140 μs in mc-Si samples with neither slow traps nor interstitial oxygen FTIR-peaks. [Display omitted] • UMG-Si as an environmentally friendly and low cost material. • Slow and fast traps identified by means of PC decay. • Complementary studies on interstitial oxygen and dislocations altering lifetime. • Successful PDG treatments on UMG-Si. • Average 25-fold lifetime improvement after PDG. [ABSTRACT FROM AUTHOR]

Abstract: This work investigates the photo-thermal treatment of solar grade (SG) silicon to reduce impurities to a low level suitable for high efficiency low-cost solar cells application. It describes experiment carried out by using a tungsten lamps furnace (rapid thermal processing, RTP) to purify solar grade silicon wafers using a combination of porous silicon (PS) and silicon tetrachloride. This process enables to attract the impurities towards the porous layer where they react with SiCl4 to form metallic chlorides. The gettering effect was studied using the Hall Effect and the Van Der Pauw methods to measure the resistivity, the majority carrier concentration and mobility. We have obtained a significant improvement of the majority carrier mobility after such thermo-chemical treatment. The gettering efficiency is also evaluated by the relative increase of the minority carrier diffusion length L, measured by the light beam induced current (LBIC) technique. [Copyright &y& Elsevier]