S. Martinuzzi

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 ...

Influence of dislocations on electrical properties of large grained polycrystalline silicon cells. I. Model

The influence of the density (Ndis) and of the recombination activity (Sd) of dislocations on the photocurrent (Jsc), the spectral dependence of Jsc and the effective electron diffusion length (Leff) of a P+‐N junction solar cell are computed by means of a model which makes use of the Green’s function method. In this model Sd is the surface recombination velocity of the space‐charge cylinder surrounding the dislocation line and the dislocations are assumed to be perpendicular to the illuminated surface of the cells and homogeneously recombining. The results obtained indicate that the values of Jsc and their spectral variations, as well as values of Leff, are dependent on both Ndis and Sd, especially when Ndis and Sd are larger than 103 cm−2 and 104 cm s−1, respectively. The base thickness (d) of the cells and the value of electron diffusion length (Ln) in the undislocated regions of the wafers are also introduced in the model as independent parameters.

Influence of iron contamination on the performances of single-crystalline silicon solar cells: Computed and experimental results

In this paper, the impact of iron contamination on the conversion efficiency of single-crystalline p-type silicon solar cells is investigated by means of the combination of numerical simulations and experimental data, taking into account the more recent results about the properties of iron in single-crystalline silicon. Numerical simulations highlight the fill factor losses due to the injection-level dependence of the bulk lifetime, which attenuates the decrease of the open circuit voltage and thus that of the solar cell conversion efficiency with iron concentration. Gettering and hydrogenation effects are quantified by means of experimental results obtained from voluntarily contaminated solar cells and integrated in the simulations. The results show that iron appears to be a metallic impurity rather well tolerated in p-type single-crystalline silicon solar cells, because its injection-level dependent bulk lifetime, like its abilities to be gettered and to be passivated by hydrogenation, limits its influe...

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.

Effect of intentional bulk contamination with iron on multicrystalline silicon solar cell properties

The influence of intentional iron bulk contamination on the performances of boron doped p-type multicrystalline silicon solar cells was investigated. Solar cells were made from iron contaminated wafers, with an initial dissolved iron concentration 100 times higher than that of standard wafers. Nevertheless, the conversion efficiency of these cells was not impacted by this intentional contamination. We showed that this tolerance toward iron was due to the efficiency of the gettering and hydrogenation effects, complementary in this material. While phosphorus diffusion (extracting more than 99% of the iron from the bulk) is slightly limited in regions of high dislocation density, hydrogen diffuses through the whole thickness of the wafer and passivates defects and remaining impurities, with its diffusion being faster along extended defects

Influence of dislocations on electrical properties of large grained polycrystalline silicon cells. II. Experimental results

The predictions of the model given in part I are tested by means of a set of diode arrays which allow us to measure the electron effective diffusion length (Leff) and the short circuit photocurrent (Jsc) in large grained polycrystalline silicon cells. The experimental dependencies of Leff and Jsc on the average of the dislocation density (Ndis) fit well with the computed curves, and then the mean value of the recombination strength of dislocations (Sd) may be evaluated. As predicted by the model, the reduction of the value of Sd by a passivation technique like hydrogenation, significantly increases the values of Leff and Jsc. The good agreement between computed and experimental results indicate that dislocations are the most harmful defects.

Influence of substitutional metallic impurities on the performances of p-type crystalline silicon solar cells: The case of gold

The influence of a gold bulk contamination on the performances of boron doped p-type crystalline silicon solar cells is investigated for different base doping levels and different kinds of materials, such as float zone Si, Czochralski Si, and multicrystalline Si. Solar cells are made from intentionally contaminated silicon wafers. By monitoring the evolution of the electrically active substitutional gold concentration by means of bulk lifetime and minority carrier diffusion length measurements, this paper highlights the eventual gettering or hydrogenation effects occurring throughout the whole process but also of the danger of such an impurity in materials containing large densities of extended defects generating recombination centers by means of the impurity-defect interaction.

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...