Carlos del Cañizo

A Comprehensive Model for the Gettering of Lifetime‐Killing Impurities in Silicon

A quantitative model for gettering processes is presented. First, lifetime-killing impurities that are fixed in the silicon lattice, either as precipitates, trapped atoms, or substitutionals, must be extracted. Then they travel as interstitials to a sink region where they cease to be harmful to the electron device behavior. The model is applied to several cases where aluminum or phosphorus gettering is performed. Agreement with experimental results commonly observed is found, and several interesting conclusions are reached. Application to a more complex situation provides a way of identifying the phenomena involved and their importance. This quantitative model is thought to be a useful tool for the interpretation of gettering experiments.

High-Performance and Traditional Multicrystalline Silicon: Comparing Gettering Responses and Lifetime-Limiting Defects

In recent years, high-performance multicrystalline silicon (HPMC-Si) has emerged as an attractive alternative to traditional ingot-based multicrystalline silicon (mc-Si), with a similar cost structure but improved cell performance. Herein, we evaluate the gettering response of traditional mc-Si and HPMC-Si. Microanalytical techniques demonstrate that HPMC-Si and mc-Si share similar lifetime-limiting defect types but have different relative concentrations and distributions. HPMC-Si shows a substantial lifetime improvement after P-gettering compared with mc-Si, chiefly because of lower area fraction of dislocation-rich clusters. In both materials, the dislocation clusters and grain boundaries were associated with relatively higher interstitial iron point-defect concentrations after diffusion, which is suggestive of dissolving metal-impurity precipitates. The relatively fewer dislocation clusters in HPMC-Si are shown to exhibit similar characteristics to those found in mc-Si. Given similar governing principles, a proxy to determine relative recombination activity of dislocation clusters developed for mc-Si is successfully transferred to HPMC-Si. The lifetime in the remainder of HPMC-Si material is found to be limited by grain-boundary recombination. To reduce the recombination activity of grain boundaries in HPMC-Si, coordinated impurity control during growth, gettering, and passivation must be developed.

Study of Internal versus External Gettering of Iron during Slow Cooling Processes for Silicon Solar Cell Fabrication

The eect of slow cooling after dierent high temperature treatments on the in- terstitial iron concentration and on the electron lifetime of p-type mc-Si wafers has been in- vestigated. The respective impacts of internal relaxation gettering and external segregation gettering of metal impurities during an extended phosphorous diusion gettering are studied. It is shown that the enhanced reduction of interstitial Fe during extended P-gettering is due to an enhanced segregation gettering while faster impurities like Cu and Ni are possibly reduced due to an internal gettering eect.

Optimizing phosphorus diffusion for photovoltaic applications: Peak doping, inactive phosphorus, gettering, and contact formation

The phosphosilicate glass (PSG), fabricated by tube furnace diffusion using a POCl3 source, is widely used as a dopant source in the manufacturing of crystalline silicon solar cells. Although it has been a widely addressed research topic for a long time, there is still lack of a comprehensive understanding of aspects such as the growth, the chemical composition, possible phosphorus depletion, the resulting in-diffused phosphorus profiles, the gettering behavior in silicon, and finally the metal-contact formation. This paper addresses these different aspects simultaneously to further optimize process conditions for photovoltaic applications. To do so, a wide range of experimental data is used and combined with device and process simulations, leading to a more comprehensive interpretation. The results show that slight changes in the PSG process conditions can produce high-quality emitters. It is predicted that PSG processes at 860 °C for 60 min in combination with an etch-back and laser doping from PSG layer results in high-quality emitters with a peak dopant density Npeak = 8.0 × 1018 cm−3 and a junction depth dj = 0.4 μm, resulting in a sheet resistivityρsh = 380 Ω/sq and a saturation current-density J0 below 10 fA/cm2. With these properties, the POCl3 process can compete with ion implantation or doped oxide approaches.

Towards the Tailoring of P Diffusion Gettering to As-Grown Silicon Material Properties

The evolution of Fe-related defects is simulated for di erent P di usion gettering (PDG) processes which are applied during silicon solar cell processing. It is shown that the introduction of an extended PDG is bene cial for some as-grown Si materials but not essential for all of them. For mc-Si wafers with an as-grown Fe concentration 14 cm3, a good reduction of the Fei concentration and increase of the electron lifetime is achieved during standard PDG. For mc-Si wafers with a higher as-grown Fe concentration the introduction of defect engineering tools into the solar cell process seems to be advantageous. From comparison of standard PDG with extended PDG it is concluded that the latter leads to a stronger reduction of highly recombination active Fei atoms due to an enhanced segregation gettering e ect. For an as-grown Fe concentration between 1014 cm3 and 1015 cm3, this enhanced Fei reduction results in an appreciable increase in the electron lifetime. However, for an as-grown Fe concentration >1015 cm3, the PDG process needs to be optimized in order to reduce the total Fe concentration within the wafer as the electron lifetime after extended PDG keeps being limited by recombination at precipitated Fe.

Impact of Extended Contact Cofiring on Multicrystalline Silicon Solar Cell Parameters

During the temperature spike of the contact cofiring step in a solar cell process, it has been shown that the concentration of lifetime-killer dissolved metallic impurities increases, while adding an annealing after the spike getters most of the dissolved impurities toward the phosphorus emitter, where they are less detrimental. The contact cofiring temperature profile, including the after-spike annealing, has been called extended contact cofiring, and it has also been proposed as a means to decrease the emitter saturation current density of highly doped emitters, thus benefiting a wide range of materials in terms of detrimental impurity content. The aim of the present work is to determine the effect of performing this additional annealing on contact quality and solar cell performance, looking for an optimal temperature profile for reduction of bulk and emitter recombination without affecting contact quality. It presents the effect of the extended cofiring step on fill factor, series resistance, and contact resistance of solar cells manufactured with different extended cofiring temperature profiles. Fill factor decreases when extended cofiring is performed. Series resistance and contact resistance increase during annealing, and this happens more dramatically when the temperature peak is decreased. Scanning electron microscopic images show silver crystallites in contact with silver bulk before the annealing that allow a direct current path, and silver crystallites totally surrounded by glass layer (>100 nm thick) after annealing. Glass layer redistribution and thickening at low temperatures at the semiconductor-metal interface can be related to the series resistance increase. Degradation of series resistance during the temperature spike, when it is below the optimum one, can also be attributed to an incomplete silicon nitride etching and silver crystallite formation. To make full use of the beneficial effects of annealing, screen-printing metallic paste development supporting lower temperatures without a thick glass layer growth is needed.

Electrically-inactive phosphorus re-distribution during low temperature annealing

An increased total dose of phosphorus (P dose) in the first 40 nm of a phosphorus diffused emitter has been measured after Low Temperature Annealing (LTA) at 700 °C using the Glow Discharge Optical Emission Spectrometry technique. This evidence has been observed in three versions of the same emitter containing different amounts of initial phosphorus. A stepwise chemical etching of a diffused phosphorus emitter has been carried out to prepare the three types of samples. The total P dose in the first 40 nm increases during annealing by 1.4 × 1015 cm–2 for the sample with the highly doped emitter, by 0.8 × 1015 cm–2 in the middle-doped emitter, and by 0.5 × 1015 cm–2 in the lowest-doped emitter. The presence of surface dislocations in the first few nanometers of the phosphorus emitter might play a role as preferential sites of local phosphorus gettering in phosphorus re-distribution, because the phosphorus gettering to the first 40 nm is lower when this region is etched stepwise. This total increase in phosp...

Ultra-dense energy storage utilizing high melting point metallic alloys and photovoltaic cells

A novel concept for energy storage utilizing high melting point metallic alloys and photovoltaic cells is presented. In the proposed system, the energy is stored in the form of latent heat of metallic alloys and converted to electricity upon demand by infrared sensitive photovoltaic cells. Silicon is considered in this paper due to its extremely high latent heat (1800 J/g), melting point (1410°C), low cost (~

Thin absorbers for defect-tolerant solar cell design

1.7/kg) and abundance on earth. The proposed solution enables an enormous energy storage density of ~ 500 Wh/kg and ~ 1 MWh/m3, which is 12 times higher than that of lead-acid batteries, 4 times than that of Li-ion batteries and 10 to 20 times than that of the molten salts utilized in CSP systems.

Upgrading the silicon IBC to the 40% efficiency

Thin silicon wafers provide a pathway to lower cost and lower capital intensity PV module manufacturing. They can also produce higher-efficiency devices with less expensive feedstock and crystallization processes because they require shorter diffusion lengths and operate at higher carrier injection. Through simulation, we show that thin Si wafers can be incorporated into high-efficiency cells with greater defect tolerance than thick wafers. Experimentally, we demonstrate the importance of excellent surface passivation to realizing the efficiency potential of thin silicon solar cells and show that such passivation can be achieved in silicon/amorphous silicon heterojunction devices.