Isabelle Périchaud

Direct correlation of transition metal impurities and minority carrier recombination in multicrystalline silicon

Impurity and minority carrier lifetime distributions were studied in as-grown multicrystalline silicon used for terrestrial-based solar cells. Synchrotron-based x-ray fluorescence and the light beam induced current technique were used to measure impurity and lifetime distributions, respectively. The purpose of this work was to determine the spatial relation between transition metal impurities and minority carrier recombination in multicrystalline silicon solar cells. Our results reveal a direct correlation between chromium, iron, and nickel impurity precipitates with regions of high minority carrier recombination. The impurity concentration was typically 5×1016 atoms/cm2, indicating the impurity-rich regions possess nanometer-scale precipitates. These results provide the first direct evidence that transition metal agglomerates play a significant role in solar cell performance.

Nanometer-scale metal precipitates in multicrystalline silicon solar cells

In this study, we have utilized characterization methods to identify the nature of metal impurityprecipitates in low performance regions of multicrystalline silicon solar cells. Specifically, we ha ...

Multicrystalline silicon prepared by electromagnetic continuous pulling: recent results and comparison to directional solidification material

The electromagnetic continuous pulling (EMCP) is a new growth process competing with directional solidification for the production of massive silicon ingots. The advantages of the EMCP are higher production rates, no crucible consumption and a more uniform crystalline structure of the wafers. The present work is devoted to the characterization of the EMCP material by means of minority carrier diffusion length measurements (L) which allow the comparison between raw, annealed, phosphorus-diffused, annealed and gettered or hydrogenated wafers. The EMCP wafers are compared with conventionally cast wafers. It is shown that L is relatively high in the raw material, but L is degraded by annealings at temperatures higher than 600°C, while hydrogenation treatments drastically improve the material. In the present state, the EMCP silicon appears as a particular form of multicrystalline silicon materials with promising abilities. However, it requires low temperature processing steps or additional hydrogenation treatments to make low-cost and efficient solar cells.

n-p Junction formation in p-type silicon by hydrogen ion implantation

Hydrogen ion implantations at an energy of 250 keV and a dose of 3 × 10 16 cm -2 were applied to float zone, Czochralski grown silicon wafers and to multicrystalline samples. It was found that after annealing at 350°C <T<550°C for 1 h a n-p junction is formed and a photovoltaic behaviour is observed. Spectral responses show that the photocurrent in the near infrared part of the spectrum is comparable to that given by a standard silicon solar cell. The depth of the junction is about 2μm and C-V measurements show that the junction is graduated. Hydrogen plasma immersion leads to similar results. The conversion of p- to n-type silicon is explained by the formation of shallow donor levels associated to a high concentration of hydrogen.

Trapping of gold by nanocavities induced by H+ or He++ implantation in float zone and Czochralski grown silicon wafers

In silicon, implantation of He++ or H+ ions and subsequent annealing can lead to the formation of nanocavities below the implanted surface of the wafers. These nanocavities, which behave as trapping sites for metallic impurities, can be located near the devices in integrated circuits in order to induce a proximity gettering. In this article, we investigate, in float zone (FZ) and Czochralski (Cz) wafers, the trapping of gold by nanocavities formed by implantation of He++ or H+ ions at 250 keV and at a dose of 3×1016 cm−2 followed by subsequent annealing(s) at 750  °C for 1 h. Deep level transient spectroscopy profiles show that substitutional gold concentration decreases near the cavity band in FZ and Cz samples. Gold profiles obtained by secondary ion mass spectroscopy show that there is a strong trapping of gold in the cavity band in all samples. In the case of He++ implanted wafers, this trapping also occurs in the region between the implanted surface and the cavities, and the higher the oxygen concent...

INFLUENCE OF PHOSPHORUS DIFFUSION ON THE RECOMBINATION STRENGTH OF DISLOCATIONS IN FLOAT ZONE SILICON WAFERS

Dislocation arrays are investigated in float zone (FZ) grown silicon wafers by the light beam induced current (LBIC) mapping technique at various wavelengths and by deep level transient spectroscopy (DLTS). The LBIC technique allows us to recognize and detect these arrays and to evaluate their recombination strength. Dislocations are found to be less recombining in (100)‐oriented FZ samples than in (111) oriented ones. In FZ dislocated wafers, a phosphorus diffusion strongly attenuates the LBIC contrast of dislocations, depending on the duration and temperature of the treatment. Electrical activity of the defects, which are still physically present, as verified by x‐ray topography, seems to disappear. Simultaneously, the peak intensities of DLTS spectra related to dislocations are reduced and this reduction depends on the phosphorus diffusion temperature and duration.

Oxygen-Related Thermal Donor Formation in Dopant-Rich Compensated Czochralski Silicon

The thermal donor (TD) generation in dopant-rich compensated Czochralski silicon was studied by pulling an ingot from a feedstock containing large amounts of donors and acceptors. In a wafer located in the vicinity of the change of conductivity type, thermal donors were formed and the evolution of their concentration was similar to that in noncompensated lowly boron-doped silicon. Thus, simultaneous high densities of both boron and phosphorus do not have a significant impact on the TD formation. This brings the experimental evidence that for a given oxygen concentration and annealing temperature, the TD formation is controlled by the electron density.

Behaviour of Light Induced Defect Generation and Carrier Lifetime Degradation in Solar Grade Silicon

Light-induced defect generation seriously reduces the minority-carrier lifetime of crystalline silicon (c-Si) wafers which causes a decrease in solar cell efficiency. In this paper we investigate the impact of boron-oxygen complexes and iron impurities on the light induced minority-carrier lifetime degradation in c-Si, comparing electronic grade and upgraded metallurgical grade materials. For the later, the characteristic of the decay process is shown to be composed of a fast initial decay and a subsequent slow asymptotic decay. We conclude that the dissociation of iron-boron pairs must be taken into account to explain the light-induced lifetime reduction.

Electrical and photovoltaic properties through a large multicrystalline Si ingot

Large multicrystalline cast silicon ingots (>310 kg) are cost effective in the photovoltaic industry and attenuate the feedstock shortage. The bulk lifetime τn and diffusion length Ln of minority carriers vary through the height due to the segregation of metallic impurities during the directional solidification. The native impurity concentrations increase from the bottom to the top of the ingot, which is solidified last, while the ingot bottom, which is solidified first, is contaminated by the contact with the crucible. It was found that τn and Ln are the smallest in the top and in the bottom of the ingot. In solar cells, the evolution is similar, however in the central part of the ingot Ln is strongly increased due to the in-diffusion of hydrogen from the SiN-H antireflection coating layer. The variations along the ingot height of the conversion efficiency η and of τn in raw wafers are well correlated, that can predict the values of η, allowing an in-line sorting of the wafers, before solar cells are made. If τn is smaller than 1 μs, as observed at the extremities of the ingot, η will be limited to 10% only; if τn is higher than 2.5 μs η achieve 15 % at least. In addition, impurity segregation phenomena around grain boundaries are observed at the extremities of the ingots, linked to the long duration of the solidification process. Reducing the height of the ingots could suppress these phenomena and not much material must be discarded. Another problem can come from the use of upgraded metallurgical silicon feedstock in which the densities of boron and phosphorus are very close. Due to the difference in the segregation coefficients, ingots may be entirely or partly p or n type, suggesting that a purification step tawards the dopants is required.