Phosphorus Diffusion Gettering Research Articles

Production of upgraded metallurgical-grade silicon for a low-cost, high-efficiency, and reliable PV technology

Upgraded metallurgical-grade silicon (UMG-Si) has the potential to reduce the cost of photovoltaic (PV) technology and improve its environmental profile. In this contribution, we summarize the extensive work made in the research and development of UMG technology for PV, which has led to the demonstration of UMG-Si as a competitive alternative to polysilicon for the production of high-efficiency multicrystalline solar cells and modules. The tailoring of the processing steps along the complete Ferrosolar’s UMG-Si manufacturing value chain is addressed, commencing with the purification stage that results in a moderately compensated material due to the presence of phosphorous and boron. Gallium is added as a dopant at the crystallization stage to obtain a uniform resistivity profile of ∼1 Ω cm along the ingot height. Defect engineering techniques based on phosphorus diffusion gettering are optimized to improve the bulk electronic quality of UMG-Si wafers. Black silicon texturing, compatible with subsequent gettering and surface passivation, is successfully implemented. Industrial-type aluminum back surface field (Al-BSF) and passivated emitter and rear cell (PERC) solar cells are fabricated, achieving cell efficiencies in the range of those obtained with conventional polysilicon substrates. TOPCon solar cell processing key steps are also tested to further evaluate the potential of the material in advanced device architectures beyond the PERC. Degradation mechanisms related to light exposure and operation temperature are shown to be insignificant in UMG PERC solar cells when a regeneration step is implemented, and PV modules with several years of outdoor operation demonstrated similar performance to reference ones based on poly-Si. Life cycle analysis (LCA) is carried out to evaluate the environmental impact of UMG-based PV technology when compared to poly-Si-based technology, considering different scenarios for both the manufacturing sites and the PV installations.

Investigating Wafer Quality in Industrial Czochralski‐Grown Gallium‐Doped p‐Type Silicon Ingots with Melt Recharging

Herein, a systematic study of the electronic quality of gallium‐doped p‐type silicon wafers from Czochralski‐grown ingots with melt recharging is presented. It is found that in the as‐grown state, the ingots contain interstitial iron concentrations in the range of 3 × 109–2 × 1010 cm−3, with a trend of slightly higher concentrations toward the tail end of each ingot, and in subsequently grown ingots. However, analysis of the effective lifetimes indicates that iron–gallium pairs are not the dominant recombination centers in the as‐grown state. Moreover, when these wafers are subjected to a tabula rasa step, an increase in the iron concentration is observed in the range of 1 × 1010–6 × 1010 cm−3, with iron–gallium pairs becoming the dominant recombination centers. This is possibly caused by the dissolution of pre‐existing precipitated iron in the wafers. Nevertheless, the negative impact of iron contamination can be dramatically reduced by subjecting the wafers to a phosphorus diffusion gettering step, as is commonly incorporated in the fabrication of p‐type passivated emitter and rear cells. Therefore, it is concluded that the quality of the ingots is not limited by iron contamination, even after multiple ingots are pulled from the recharged melt.

High carrier lifetimes in UMG multicrystalline wafers after P- diffusion compatible with high-efficiency cell structures

High-quality multicrystalline Upgraded Metallurgical Grade Silicon (UMG-Si) offers significant advantages over conventional polysilicon-based PV technology, associated to lower energy budget. The aim of this study is twofold: on the one hand, to ascertain the efficiency potential of solar cells based on this material in terms of carrier lifetime; and on the other hand, to explore the adoption of high-efficiency cell architectures by establishing an effective rear-side passivation scheme for the implementation of passivated emitter rear contact (PERC) devices. The carrier lifetime and the surface passivation efficacy are investigated for different passivating layer configurations after single and double P-diffusion gettering processes. Layer stacks consisting of Al 2 O 3 , SiO x N y and a-SiN x :H capping overlayers have been optimized, on industrial size, saw-damage-etched UMG wafers and results compared to those obtained using reference iodine-ethanol (IE) passivation. Diagnosis based on minority carrier lifetime and implied V oc (iV oc ) measurements helped monitor the impact of parameter optimization on wafer quality, particularly after firing processes. Carrier lifetimes over 600 μs at Δn = 10 15 cm −3 injection level as well as up to 790 μs locally have been measured in UMG-Si wafers passivated with IE after a Phosphorus Diffusion Gettering (PDG), demonstrating the suitability of the material for high-efficiency cell architectures. Values higher than 300 μs have been obtained with Al 2 O 3 -based passivation layers for gettered UMG wafers, with implied V oc values up to 710 mV. These record-breaking lifetimes and iV oc figures obtained with p-type multicrystalline UMG-Si material demonstrate a significant upgrading of its electronic quality by means of industry-scalable technical processes. • UMG-Si as an environmentally friendly and low-cost material. • Successful PDG treatments on UMG-Si. • Study of different passivation structures. • Lifetime values up to 790 us locally and iVoc values up to 710 mV. • Figures compatible with high-efficiency cell structures.

Surface modifications by wet oxidation method removing getter layer in crystalline silicon cells.

Reducing the impurity atom content in crystalline silicon (c-Si) can effectively reduce the recombination current density (J 0) and improve the photoelectric conversion efficiency (PCE) of solar cells. Phosphorus diffusion gettering (PDG) has been proven to be an effective method to remove impurity atoms from c-Si. However, the research studies show that the traditional tube thermal diffusion method will cause a large number of dislocations on the silicon surface during the oxidation process, reducing the effectiveness of gettering. In this paper, the wet oxidation method is systematically used to remove phosphorus-rich layers (PRL) and modify the surface. The gettering effectiveness is measured by the minority carrier lifetime (τ eff) and bulk carrier lifetime (τ bulk) of silicon wafers. The results show that wet oxidation can reduce J 0 by 27.0% and increase τ eff by 26.3%. For the bulk region, the average τ bulk can be increased by more than 6-14%. In addition, with the final PCE comparison, the efficiency of the wet oxidation cell will be improved by 0.12%. These works indicate that the wet oxidation method can significantly improve the gettering effectiveness and the PCE of c-Si solar cell fabrication.

Impact of pre-fabrication treatments on n-type UMG wafers for 21% efficient silicon heterojunction solar cells

Silicon heterojunction solar cells achieve high conversion efficiency due to the excellent surface passivation provided by the hydrogenated intrinsic amorphous silicon films. However, they require a high-quality wafer as a starting material because their low-temperature processing does not allow for gettering. Czochralski-grown upgraded metallurgical-grade (UMG-Cz) silicon is a low-cost alternative to electronic-grade silicon for silicon solar cells, but is often limited in lifetime by grown-in defects. We have previously shown that pre-fabrication treatments, namely tabula rasa, phosphorus diffusion gettering, and hydrogenation, can significantly improve the bulk quality of UMG-Cz wafers. These help to mitigate the impact of grown-in oxygen precipitate nuclei and metallic impurities. In this work, we fabricate rear-junction silicon heterojunction solar cells on both as-grown and pre-treated UMG-Cz and electronic-grade wafers. We show that pre-fabrication treatments have a marked impact on solar cell efficiencies. With pre-fabrication treatment, the efficiency improves from 18.0% to 21.2% for the UMG-Cz cells and 21.2%–22.7% for the electronic-grade cells. Comparison of the open-circuit voltages of the as-grown and pre-treated UMG-Cz and electronic-grade cells using Quokka simulations reveals that the bulk lifetime remains the primary limiting factor for the UMG-Cz wafers.

Gettering of transition metals in high-performance multicrystalline silicon by silicon nitride films and phosphorus diffusion

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.

Lifetime Degradation by Oxygen Precipitation Combined with Metal Contamination in Czochralski Silicon for Solar Cells

Metal impurities which cause degradation of the carrier lifetime are incorporated into the device from the feedstock, during the slicing process or the thermal process. Oxygen precipitates in Czochralski silicon (Cz-Si) have been used as internal gettering sources for metals in the integrated circuit device fabrication. In this study, lifetime degradation caused by oxygen precipitates combined with metal contamination in Cz-Si for solar cells was evaluated for the samples with different interstitial oxygen concentrations using photoluminescence imaging and infrared light scattering tomography. Samples with and without oxygen precipitation thermal treatment were intentionally contaminated with FeCl3 solution, and then phosphorus diffusion gettering (PDG) was carried out. It was confirmed that PDG is effective for the samples with low oxygen concentration, however the lifetime was not recovered for the samples with high oxygen concentration when subjected to intentional metal contamination even after PDG. The oxygen precipitation promotion process made it more difficult to recover from the metallic contamination. We have shown that the high density and large size oxygen precipitates have negative effect on the lifetime in combination with metal impurities.

Methods to Improve Bulk Lifetime in n-Type Czochralski-Grown Upgraded Metallurgical-Grade Silicon Wafers

This paper investigates the potential of three different methods— tabula rasa (TR), phosphorus diffusion gettering (PDG), and hydrogenation, for improving the carrier lifetime in n-type Czochralski-grown upgraded metallurgical-grade (UMG) silicon samples. Our results show that the lifetimes in the UMG wafers used in this study were affected by both mobile metallic impurities and as-grown oxygen precipitate nuclei. Thus, the dissolution of grown-in oxygen precipitate nuclei via TR and the removal of mobile impurities via PDG step were found to significantly improve the electronic quality of the UMG wafers. Finally, we report bulk lifetimes and 1-sun implied open-circuit voltages of the UMG wafers after boron and phosphorus diffusions, as typically applied in n-type cell fabrication.

Mitigating Light and Elevated Temperature Induced Degradation in Multicrystalline Silicon Wafers and PERC Solar Cells Using Phosphorus Diffusion Gettering

Reducing light and elevated temperature induced degradation (LeTID) is important for the industrial success of PERC (passivated emitter and rear cell) solar cells fabricated on high‐performance multicrystalline silicon (multi‐Si) wafers. Although there has been a lot of progress in understanding the degradation kinetics, the defect(s) responsible for LeTID in multi‐Si wafers at elevated temperatures (≈80 °C, 1 Sun) has yet to be identified. In this study, the authors look at the possibility of using phosphorus diffusion gettering (PDG) for reducing LeTID in multi‐Si wafers and solar cells. By measuring light induced defect concentrations in multi‐Si wafers before and after LeTID, the authors observe that PDG can substantially reduce the average defect concentration. Trace element analysis using inductively coupled plasma mass spectrometry reveals that multi‐Si wafers from the edge of the ingot contain a high concentration of Cu, Ni and Ti in grains that degrade more than neighboring grains. To explore PDG for reducing LeTID in multi‐Si PERC solar cells, the authors fabricate cells with two different emitter profiles. Etching back a heavily diffused emitter to obtain a high sheet resistance is observed to improve the LeTID performance of the solar cells, an effect that is very likely related to a reduced impurity concentration within the wafer.

Phosphorus Diffusion Gettering Efficacy in Upgraded Metallurgical-Grade Solar Silicon

In the context of the continuous price reduction in photovoltaics (PV) in recent years, Si feedstock continues to be a relevant component in the cost breakdown of a PV module, highlighting the need for low-cost, low-capital expenditure (CAPEX) silicon technologies to further reduce this cost component. Upgraded metallurgical-grade silicon (UMG Si) has recently received much attention, improving its quality and even attaining, in some cases, solar cell efficiencies similar to those of conventional material. However, some technical challenges still have to be addressed when processing this material to compensate efficiently for the high content of impurities and contaminants. Adaptation of a conventional solar cell process to monocrystalline UMG Si wafers has been studied in this work. In particular, a tailored phosphorus diffusion gettering step followed by a low-temperature anneal at 700°C was implemented, resulting in enhanced bulk lifetime and emitter recombination properties. In spite of the need for further research and material optimization, UMG Si wafers were successfully processed, achieving efficiencies in the range of 15% for a standard laboratory solar cell process with aluminum back surface field.

Deep-level transient spectroscopy characterization of electrical traps in p-type multicrystalline silicon with gettering and hydrogenation process

In p-type multicrystalline silicon solar cells, defects in silicon bulk are well known to act as carrier recombination centers and considerably affect the conversion efficiency of cells. In this study, effective minority carrier lifetime (τeff) mapping and deep-level transient spectroscopy (DLTS) data were utilized to evaluate gettering efficiency and hydrogenation passivation in bulk defects. Microwave photoconductivity decay method was used to illustrate the effect of phosphorus gettering behavior and hydrogenation process on the bulk carrier lifetime of multicrystalline silicon. Results showed that τeff drastically improved after gettering and further increased after hydrogenation. DLTS results further revealed six traps corresponding to interstitial iron (Fei), Fe–O complex, FeB complex, vanadium, Fei complex and divacancy oxygen. The main trap in a nongettered sample was related to Fei, with two deep levels at about EV + 0.373 and EV + 0.470 eV. However, the Fei complex was the main trap in the gettered sample, with two deep traps at about EV+0.390 and EV+0.442 eV. Finally, we concluded that Fei defect and FeB complex can be significantly removed by phosphorus diffusion gettering. Moreover, divacancy-oxygen complexes, vanadium and Fe–O defects were effectively passivated by hydrogenation process. Fei complex, however, was hardly removed by these two process.

Recombination activity of grain boundaries in high-performance multicrystalline Si during solar cell processing

In this work, we applied internal quantum efficiency mapping to study the recombination activity of grain boundaries in High Performance Multicrystalline Silicon under different processing conditions. Wafers were divided into groups and underwent different thermal processing, consisting of phosphorus diffusion gettering and surface passivation with hydrogen rich layers. After these thermal treatments, wafers were processed into heterojunction with intrinsic thin layer solar cells. Light Beam Induced Current and Electron Backscatter Diffraction were applied to analyse the influence of thermal treatment during standard solar cell processing on different types of grain boundaries. The results show that after cell processing, most random-angle grain boundaries in the material are well passivated, but small-angle grain boundaries are not well passivated. Special cases of coincidence site lattice grain boundaries with high recombination activity are also found. Based on micro-X-ray fluorescence measurements, a change in the contamination level is suggested as the reason behind their increased activity.

Black silicon significantly enhances phosphorus diffusion gettering

Black silicon (b-Si) is currently being adopted by several fields of technology, and its potential has already been demonstrated in various applications. We show here that the increased surface area of b-Si, which has generally been considered as a drawback e.g. in applications that require efficient surface passivation, can be used as an advantage: it enhances gettering of deleterious metal impurities. We demonstrate experimentally that interstitial iron concentration in intentionally contaminated silicon wafers reduces from 1.7 × 1013 cm−3 to less than 1010 cm−3 via b-Si gettering coupled with phosphorus diffusion from a POCl3 source. Simultaneously, the minority carrier lifetime increases from less than 2 μs of a contaminated wafer to more than 1.5 ms. A series of different low temperature anneals suggests segregation into the phosphorus-doped layer to be the main gettering mechanism, a notion which paves the way of adopting these results into predictive process simulators. This conclusion is supported by simulations which show that the b-Si needles are entirely heavily-doped with phosphorus after a typical POCl3 diffusion process, promoting iron segregation. Potential benefits of enhanced gettering by b-Si include the possibility to use lower quality silicon in high-efficiency photovoltaic devices.

Combining Low-Temperature Gettering With Phosphorus Diffusion Gettering for Improved Multicrystalline Silicon

We have investigated low-temperature (≤500 °C) gettering in combination with phosphorus diffusion gettering with a view to improving poor quality multicrystalline silicon. Low-temperature gettering applied after standard phosphorus diffusion gettering is found to provide a >40% improvement in minority carrier lifetime in samples from the top and bottom of an ingot. The best results are achieved at 300 °C with very long annealing times (>24 h). Improvements in the lifetime do not correlate with changes in interstitial iron concentration. Experiments are performed to assess whether the presence of a phosphorus-diffused emitter affects low-temperature gettering, and results from sister samples show the low-temperature gettering behavior is not affected by the existence of an emitter. Further experiments show that low-temperature gettering prior to phosphorus diffusion results in a 20% higher lifetime after phosphorus diffusion. Low-temperature gettering can, therefore, enhance lifetime even when used in conjunction with standard phosphorus diffusion gettering.

Modeling of recombination strength at grain boundaries after phosphorus diffusion gettering and the effect of hydrogen passivation

We present a method for extracting local recombination rates from photoluminescence images of double side passivated wafers, using simulations of lateral charge carrier diffusion in two dimensions. By fitting the simulated carrier lifetime map with a calibrated photoluminescence image, the recombination profiles in bulk and at grain boundaries (GBs) can be extracted. This method can be used to quantify the GB recombination velocity even if the carrier concentration in bulk is simultaneously affected by recombination at multiple GBs. High performance multicrystalline wafers from a commercially cast ingot with low impurity concentrations were studied. The GBs were completely passivated applying a hydrogenation step simulated contact firing of a hydrogen rich silicon nitride (SiN) film. The bulk carrier recombination profile from simulations of carrier diffusion after phosphorus diffusion gettering is similar to the measured lifetime after phosphorus diffusion gettering and firing. This indicates that the main role of hydrogen passivation is deactivation of the GB recombination. No additional enhancement of the bulk lifetime is observed after hydrogen passivation.

Effect of Phosphorus and Boron Diffusion Gettering on the Light Induced Degradation in Multicrystalline Silicon Wafers

Multicrystalline silicon (multi-Si) is currently the most widely used material for crystalline silicon solar cells. Although there has been substantial improvement in cell efficiency with the use of high-performance (HP) multi-Si, the PERC (passivated emitter and rear cell) devices fabricated from this material suffer from strong light-induced degradation (LID). This study aims to investigate the possibility of reducing LID in HP multi-Si wafers using diffusion gettering. By using inductively coupled plasma mass spectrometry, we measured the impurity concentration in sister samples subjected to different diffusion conditions. The results showed that a 50-Ω/sq POCl3 diffusion and 60-Ω/sq BBr3 diffusion at 800°C can getter impurities effectively. The interstitial Fe (Fei) and light-induced defect concentration was measured before and after gettering, using surface photovoltage measurements. After degrading the samples at 80°C for 24 hours, we observed that the light-induced defect concentration is lowest in HP multi-Si wafers that underwent phosphorus diffusion gettering. Samples subjected to boron diffusion gettering also show less LID compared to non-diffused sister samples.

Full recovery of red zone in p-type high-performance multicrystalline silicon

Between 10% and 30% of commercial cast silicon ingots is discarded due to contamination caused by the casting process. A significant contaminant in the scrap volume is metal precipitates, which are difficult to getter effectively and degrade minority charge carrier lifetime, hence limiting solar cell efficiency potential. We show here that the unusable red zone can be restored in high-performance multicrystalline silicon wafers. Adding a high temperature dissolution anneal prior to phosphorus diffusion dissolves the metal precipitates within the wafer bulk, and leaves the metal point defects in a mobile state to be readily gettered by phosphorus diffusion gettering during the solar cell process. The efficiency of the dissolution gettering treatment increases with increasing temperature, with a temperature of 1150°C eliminating the very low lifetime region of the wafers completely. Additionally, we find that the red zone does not re-emerge after a 60min oxidation anneal at 900°C, confirming that the achieved benefit is tolerant to any high temperature processing following the phosphorus diffusion gettering process.

Electronic Quality Improvement of Highly Defective Quasi‐Mono Silicon Material by Phosphorus Diffusion Gettering

Quasi‐mono silicon (QM‐Si) attracts interest as a substrate material for silicon device processing with the promise to yield single‐crystalline silicon quality with multicrystalline silicon cost. A significant barrier to widespread implementation of QM‐Si is ingot edge‐contamination caused by the seed material and crucible walls during crystal growth. This work aims to recover the scrap material in QM‐Si manufacturing with a process easily adaptable to semiconductor device manufacturing. A phosphorus diffusion process at 870 °C for 60 min significantly improves the electronic quality of a QM‐Si wafer cut from a contaminated edge brick. The harmonic minority carrier recombination lifetime of the wafer, a key predictor of ultimate device performance, experiences a tenfold increase from 17 to 178 µs, which makes the scrap QM‐Si material usable for device fabrication. Local areas with suboptimal (<50 µs) lifetimes remaining can be further improved by a high temperature anneal before the phosphorus diffusion process.

The Effect of Phosphorus Diffusion Gettering on Recombination at Grain Boundaries in HPMC-Silicon Wafers

The influence of phosphorus diffusion gettering on recombination at grain boundaries has been studied in a commercially cast high performance multicrystalline silicon block. Wafers from four different heights have been studied with high resolution photoluminescence-imaging. The recombination at grain boundaries was studied from linescans perpendicular to the grain bounadary of interest. The change in recombination activity at grain boundaries after gettering has been correlated with grain orientation measured by electron backscatter diffraction and classified according to Brandon criterion. The relative change in carrier lifetime after gettering depends on the height in the cast, and is very sensitive to the injection level. Iron concentrations were also found from photoluminescence-imaging of iron in Fei and FeB states, respectively. After gettering recombination grain boundaries in the middle section of the ingot has become strongly recombination active. Fei, has been efficiently removed and no longer constitute the main recombination path at any height.

Effect of grain boundary character of multicrystalline Si on external and internal (phosphorus) gettering of impurities

AbstractWe investigated the effect of the grain boundary (GB) character of multicrystalline Si (mc‐Si) on the efficiency of external and internal gettering of impurities during phosphorus diffusion gettering (PDG). We utilized seed crystals with an artificially designed GB configuration to grow mc‐Si ingots with different artificial GB characters. PDG combined with an originally developed multiple‐cycle gettering technique at low temperature was introduced on intentionally Fe‐contaminated mc‐Si samples to enhance external and internal gettering. A significant positive PDG effect was observed after PDG combined with the multiple‐cycle technique, as evidenced by the increase in lifetimes after PDG. A bright cloud‐like photoluminescence signal around contaminated GBs was observed for artificial Σ5‐GBs and tilt‐GBs after PDG, suggesting the enhancement of the internal gettering efficiency by leaving a cleaner area around the GBs. This result suggests the importance of the control of crystal defect character as well as impurities in mc‐Si ingots, which could strongly affect the PDG efficiency. Copyright © 2016 John Wiley & Sons, Ltd.