Andrei A. Istratov

Metal content of multicrystalline silicon for solar cells and its impact on minority carrier diffusion length

Instrumental neutron activation analysis was performed to determine the transition metal content in three types of silicon material for cost-efficient solar cells: Astropower silicon-film sheet material, Baysix cast material, and edge-defined film-fed growth (EFG) multicrystalline silicon ribbon. The dominant metal impurities were found to be Fe (6×1014u2009cm−3 to 1.5×1016u2009cm−3, depending on the material), Ni (up to 1.8×1015u2009cm−3), Co (1.7×1012u2009cm−3 to 9.7×1013u2009cm−3), Mo (6.4×1012u2009cm−3 to 4.6×1013u2009cm−3), and Cr (1.7×1012u2009cm−3 to 1.8×1015u2009cm−3). Copper was also detected (less than 2.4×1014u2009cm−3), but its concentration could not be accurately determined because of a very short decay time of the corresponding radioactive isotope. In all samples, the metal contamination level would be sufficient to degrade the minority carrier diffusion length to less than a micron, if all metals were in an interstitial or substitutional state. This is a much lower value than the actual measured diffusion length of these samples...

Metal precipitation at grain boundaries in silicon : Dependence on grain boundary character and dislocation decoration

Synchrotron-based analytical microprobe techniques, electron backscatter diffraction, and defect etching are combined to determine the dependence of metal silicide precipitate formation on grain boundary character and microstructure in multicrystalline silicon (mc-Si). Metal silicide precipitate decoration is observed to increase with decreasing atomic coincidence within the grain boundary plane (increasing Σ values). A few low-Σ boundaries contain anomalously high metal precipitate concentrations, concomitant with heavy dislocation decoration. These results provide direct experimental evidence that the degree of interaction between metals and structural defects in mc-Si can vary as a function of microstructure, with implications for mc-Si device performance and processing.

Synchrotron-based investigations of the nature and impact of iron contamination in multicrystalline silicon solar cells

Synchrotron-based microprobe techniques were used to obtain systematic information about the size distribution, spatial distribution, shape, electrical activity, chemical states, and origins of iron-rich impurity clusters in multicrystalline silicon (mc-Si) materials used for cost-effective solar cells. Two distinct groups of iron-rich cluster have been identified in both materials: (a) the occasional large (diameter ⩾1μm) particles, either oxidized and/or present with multiple other metal species reminiscent of stainless steels or ceramics, which are believed to originate from a foreign source such as the growth surfaces, production equipment, or feedstock, and (b) the more numerous, homogeneously distributed, and smaller iron silicide precipitates (diameter ⩽800nm, often ⩽100nm), originating from a variety of possible formation mechanisms involving atomically dissolved iron in the melt or in the crystal. It was found that iron silicide nanoprecipitates account for bulk Fe concentrations as high as 1014–...

Impact of metal silicide precipitate dissolution during rapid thermal processing of multicrystalline silicon solar cells

Synchrotron-based analytical x-ray microprobe techniques were employed to study the dissolution of iron, copper, and nickel silicide precipitates at structural defects in cast multicrystalline silicon in response to rapid thermal processing (RTP). A direct correlation was observed between iron silicide precipitate dissolution, increased minority carrier recombination, and decreased device performance after high-temperature (1000°C) RTP. In contrast, iron precipitates comparable in size to as-grown material remained after lower-temperature RTP (860°C); in this case the material exhibited higher minority carrier diffusion length and better solar cell performance. RTP at both temperatures effectively dissolved nickel and copper silicide precipitates. It is concluded that iron dissolved from structural defect reservoirs detrimentally affects the cell performance, likely by forming distributed point defects and smaller precipitates. For cast multicrystalline silicon, higher performance can be expected by inhib...

X-ray beam induced current—a synchrotron radiation based technique for the in situ analysis of recombination properties and chemical nature of metal clusters in silicon

A synchrotron radiation based x-ray microprobe analytical technique, x-ray beam induced current (XBIC), is suggested and demonstrated at the Advanced Light Source at the Lawrence Berkeley National Laboratory. The principle of XBIC is similar to that of electron/laser beam induced current with the difference that minority carriers are generated by a focused x-ray beam. XBIC can be combined with any other x-ray microprobe tool, such as the x-ray fluorescence microprobe (μ-XRF), to complement chemical information with data on the recombination activity of impurities and defects. Since the XBIC signal, which carries information about the recombination activity of defects in the sample, and the μ-XRF signal, which contains data on their chemical nature, can be collected simultaneously, this combination offers a unique analytical capability of in situ analysis of the recombination activity of defects and their chemical origin with a high sensitivity and a micron-scale spatial resolution. Examples of an applicat...

Observation of transition metals at shunt locations in multicrystalline silicon solar cells

By employing a combination of analytical tools including lock-in thermography and synchrotron-based x-ray fluorescencemicroscopy,transition metals have been identified at shunting locations in two types of low-cost multicrystalline silicon (mc-Si) solar cellmaterials: cast multicrystalline and ribbon growth on substrate (RGS). At a shunting location in the cast mc-Si cell, silver and titanium, both contact strip materials, have been identified at the shunting location, suggesting a process-induced error related to contact metallization. At a shunting location in the RGS cell, a material-specific shunting mechanism is described, involving channels of inverse conductivity type, where copper and iron are found. The possible roles of these metals in this shunting mechanism are discussed. These results illustrate the wide range of physical mechanisms involved with shunting in solar cells.

Analysis of copper-rich precipitates in silicon: chemical state, gettering, and impact on multicrystalline silicon solar cell material

In this study, synchrotron-based x-ray absorption microspectroscopy (mu-XAS) is applied to identifying the chemical states of copper-rich clusters within a variety of silicon materials, including as-grown cast multicrystalline silicon solar cell material with high oxygen concentration and other silicon materials with varying degrees of oxygen concentration and copper contamination pathways. In all samples, copper silicide (Cu3Si) is the only phase of copper identified. It is noted from thermodynamic considerations that unlike certain metal species, copper tends to form a silicide and not an oxidized compound because of the strong silicon-oxygen bonding energy; consequently the likelihood of encountering an oxidized copper particle in silicon is small, in agreement with experimental data. In light of these results, the effectiveness of aluminum gettering for the removal of copper from bulk silicon is quantified via x-ray fluorescence microscopy (mu-XRF),and a segregation coefficient is determined from experimental data to beat least (1-2)103. Additionally, mu-XAS data directly demonstrates that the segregation mechanism of Cu in Al is the higher solubility of Cu in the liquid phase. In light of these results, possible limitations for the complete removal of Cu from bulk mc-Si are discussed.

Nickel solubility in intrinsic and doped silicon

Solubility of nickel in intrinsic, moderately, and heavily doped n-type and p-type silicon was determined using instrumental neutron activation analysis. The solubility data for intrinsic silicon were found to be in good agreement with the literature data. In heavily doped p-type silicon the enhancement of nickel solubility, if present, was close to the error margins of the experiment, indicating that interstitial nickel is predominantly neutral in silicon and suggesting that its donor level lies close to the valence band edge, if not within the valence band itself. No dependence of nickel solubility on doping level of the samples was observed in n-type silicon. This is consistent with the model reported in the literature of two acceptor levels of substitutional nickel located in the upper half of the band gap, one of the levels close to the conduction band edge. Consequently, unlike copper or iron, nickel does not segregate in heavily p-type or n-type doped areas of silicon wafers.

Transition metal interaction and Ni-Fe-Cu-Si phases in silicon

In the present article we characterize several intermetallic phases of the Cu-Ni-Fe-Si system found as precipitates in the misfit dislocation layer of intentionally contaminated and slowly cooled Si1−xGex∕Si-heterostructures. The clusters showed a characteristic phase speciation into a Cu-rich part similar to Cu3Si and an Fe-Ni-Cu-Si phase similar to NiSi2. It is suggested that the precipitate formation of the investigated intermetallic silicides involves a homogeneous precursor phase at higher temperatures that later decomposes into the observed phases. Our results indicate that chemical reactions between metals and silicon during precipitation may reduce the lattice mismatch compared to single-metal precipitates, rendering mixed-metal-silicide precipitates more stable and energetically favorable.

Local measurements of diffusion length and chemical character of metal clusters in multicrystalline silicon

We present a comprehensive description of synchrotron-based analytical microprobe techniques used to locally measure the diffusion length and chemical character of metal clusters in multicrystalline silicon (mc-Si) solar cell material. The techniques discussed are (a) X-ray fluorescence microscopy, capable of determining the spatial distribution, elemental nature, size, morphology, and depth of metal-rich particles as small as 30 nm in diameter; (b) X-ray absorption microspectroscopy, capable of determining the chemical states of these metal-rich precipitates, (c) X-ray beam induced current (XBIC), which maps the minority carrier recombination activity, and (d) Spectrally-resolved XBIC, which maps the minority carrier diffusion length. Sensitivity limits, optimal synchrotron characteristics, and experimental flowcharts are discussed. These techniques have elucidated the nature and effects of metal-rich particles in mc-Si and the physical mechanisms limiting metal gettering from mc-Si, and have opened several promising new research directions.