We report two investigations conducted by using photoconductivity decay lifetime measurement. The... more We report two investigations conducted by using photoconductivity decay lifetime measurement. The first is crystalline silicon (c-Si) surface passivation using quinhydrone/methanol (QM) for bulk minority-carrier lifetime measurement. QM shows great promise as a substitute for iodine-based solutions because of its superior stability and minimized surface recombination velocity in silicon. The second is interface passivation in an amorphous silicon (a-Si)/c-Si heterojunction structure as a parallel effort to develop and optimize heterojunction c-Si solar cells by hot-wire chemical vapor deposition (HWCVD). A thin buffer layer inserted between the a-Si and the c-Si substrate has been found to be much more effective than a directly deposited a-Si/c-Si interface in reducing the interface recombination velocity.
We report two investigations conducted by using photoconductivity decay lifetime measurement. The... more We report two investigations conducted by using photoconductivity decay lifetime measurement. The first is crystalline silicon (c-Si) surface passivation using quinhydrone/methanol (QM) for bulk minority-carrier lifetime measurement. QM shows great promise as a substitute for iodine-based solutions because of its superior stability and minimized surface-recombination velocity in silicon. The second is interface passivation in an amorphous silicon (a-Si)/c-Si heterojunction structure as a parallel effort to develop and optimize heterojunction c-Si solar cells by hot-wire chemical vapor deposition (HWCVD). A thin buffer layer inserted between the a-Si and the c-Si substrate has been found to be much more effective than a directly deposited a-Si/c-Si interface in reducing the interface recombination velocity.
The effects of oxygen and nitrogen on the mechanical properties of Czochralski (CZ) and float zon... more The effects of oxygen and nitrogen on the mechanical properties of Czochralski (CZ) and float zone silicon have been studied using nano-indentation. Nitrogen free FZ Si exhibited low hardness of 6.49 GPa and elastic modulus of 104 GPa. When doped with 2×1015cm−3 nitrogen, FZ Si hardness and elastic modulus increased to 8.2 and 182 GPa, respectively. In the near-surface denuded zone of N-doped CZ Si (N-CZ) the hardness correlates well with the O and N profiles. Distinct high hardness points, found in the O- and N- rich subsurface region, were attributed to precipitates. Nano-scratch tests of N-CZ Si confirmed the existence of hard phases, mostly small precipitates, whose density, estimated to be 2×1013cm−3, is in the range of previously suggested nuclei density in as-grown N-CZ silicon.
WO3 is an important material to study, not only due to its interesting electronic properties, but... more WO3 is an important material to study, not only due to its interesting electronic properties, but also because it has other applications in both electrochromics and energy storage. The mechanism behind the electrochromic effect has been debated for several decades [1]. We have studied two WO3 single crystals, a transparent and doped WO3-x, in an attempt to understand this effect. A photoluminescence center around 865 nm is observed after sub-band gap excitation at 405 nm with relatively higher intensity in the crystal containing oxygen vacancies. The center appears as a broad transition of 35 nm FWHM and does not appear to be correlated with temperature. However, polarization studies reveal at least two polarization dependent components of the center. [4pt] [1] Satyen K. Deb, Solar energy materials and solar cells 92, 245 (2008), and the references therein
In this study, synchrotron-based x-ray absorption microspectroscopy (µ-XAS) is applied to identif... more In this study, synchrotron-based x-ray absorption microspectroscopy (µ-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 (Cu 3 Si) 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 (µ-XRF), and a segregation coefficient is determined from experimental data to be at least (1-2)×10 3 . Additionally, µ-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.
Abstract: Nitrogen doped Czochralski (N-CZ) and Float Zone (N-FZ) silicon were measured by high r... more Abstract: Nitrogen doped Czochralski (N-CZ) and Float Zone (N-FZ) silicon were measured by high resolution synchrotron Fourier Transform IR spectroscopy (HR-FTIR). The chemical complexes were analyzed in specific regions with known extended defects, i.e., denuded or precipitated regions of annealed N-CZ Si wafers, in N-FZ Si with ring defects and on “N-Skin ” region. The absorption lines were assigned to chemical complexes previously studied by first principles calculations. In annealed N-CZ Si wafers, a strong correlation was observed between the absorption line intensity depth variations and the defect distributions revealed by an Oxygen Precipitate Profiler (OPP), and oxygen and nitrogen SIMS profiles. Transformation of chemical complexes from one type to another was observed. A defect band, visible as an OPP peak at the denuded zone-bulk interface was found to be related to vacancy defect enhancement of oxygen precipitation via production of mobile N2. For the as grown N-FZ, the...
Crystalline silicon continues to be the dominant semiconductor material used for terrestrial phot... more Crystalline silicon continues to be the dominant semiconductor material used for terrestrial photovoltaics. This paper discusses the scientific issues associated with silicon photovoltaics processing, and cell design that may yield cell and module performance improvements that are both evolutionary and revolutionary in nature. We first survey critical issues in ''thick'' crystalline silicon photovoltaics, including novel separations processes for impurity removal,
Crystalline silicon continues to be the dominant semiconductor material used for terrestrial phot... more Crystalline silicon continues to be the dominant semiconductor material used for terrestrial photovoltaics. This paper discusses the scientific issues associated with silicon photovoltaics processing and cell design that may yield cell and module performance improvements, both evolutionary and revolutionary in nature. We first survey critical issues in thick crystalline silicon photovoltaics, including novel separations processes for impurity removal, impurity and defect fundamentals, interface passivation, the role of hydrogen, and high-throughput, kinetically-limited materials processing. Second, we outline emerging opportunities for creation of a very different thin-layer silicon cell structure, including the scientific issues and engineering challenges associated with thin-layer silicon processing and cell design. INTRODUCTION Todays basic research advances in materials physics and materials synthesis and processing will provide the foundation for a large-scale industrial p...
This work centers on the use of small (2 mm X 2 mm) photovoltaic devices to determine the electri... more This work centers on the use of small (2 mm X 2 mm) photovoltaic devices to determine the electrical characteristics of selective grain boundaries. The ESP bicrystal sheet growth method allows crystal orientation to be determined prior to growth and as such is an excellent tool for selective examination. Photolithographic techniques of a special pattern and mesa etching were used to isolate areas on and around the grain boundaries for device characterization. Spreading resistance measurements over and next to the grain boundaries, as well as characterization, were used to study electrical effects of these grain boundaries.
The planning of a photovoltaic laboratory in a developing country is discussed. With a budget of ... more The planning of a photovoltaic laboratory in a developing country is discussed. With a budget of $500,000, equipment and materials are purchased for a four-year period. In the fourth year 1000W (peak power) of modules will be produced from blank silicon wafers.
The effects of various kinds of micro-defects on minority carrier lifetime in high purity disloca... more The effects of various kinds of micro-defects on minority carrier lifetime in high purity dislocation-free silicon single crystals were studied by float-zoning, photoconductivity attenuation and copper decoration combined with selective etching, X-ray topography and electron-beam-inducedcurrent analysis. We found that swirl defects (A or B type) and unidentified frozen-in defects are responsible for limiting minority carrier lifetimes in high purity silicon crystals. The A-type defect was found to be a recombination center with an effective diameter of approximately 30-40 pm. By altering crystal growth conditions such as growth rate and diameter, various thermal gradients and cooling rates were attained. These affected the micro<lefect characteristics and hence the lifetimes of the crystals grown.
Crystal growth parameter effects on minority carrier lifetime and solar cell efficiencies were in... more Crystal growth parameter effects on minority carrier lifetime and solar cell efficiencies were investigated using high purity techniques such as float zoning. Study objectives include the following: (1) optimize dopants and minority carrier lifetime in FZ material for high efficiency silicon solar cell applications; (2) improve the understanding of lifetime degradation mechanisms (point defects, impurities, thermal history, surface effects, etc.), and (3) crystallographic defect characterization of float zone and ribbon crystals via X-ray topography.
Presented at the 2001 NCPV Program Review Meeting: Preliminary work on a novel process for extern... more Presented at the 2001 NCPV Program Review Meeting: Preliminary work on a novel process for external gettering by creating a band of silicon dioxide precipitation on top of a denuded zone via nitrogen doping is presented.
We obtained single-crystal YBa 2Cu 307 and ErBa 2Cu 307_5 platelets by eutectic growth from CuO-r... more We obtained single-crystal YBa 2Cu 307 and ErBa 2Cu 307_5 platelets by eutectic growth from CuO-rich mixtures of Y203 (or Er203), BaCO3, and CuO. Ground and mixed powders were loaded in an alumina crucible and heated to 1150-1180°C in air, held for 0.5-2 h, then cooled under programmed temperature and time conditions. Individual crystals of both YBa 2Cu 307.5 and ErBa 2Cu 307 up to 4 mm wide have been grown in voids that form in the charge. Similarly, compositions and temperature programs that allow single-crystal growth of Bi-Sr-Ca-Cu-~0 platelet crystals up to 4 mm wide that resembled mica in their delamination tendency were found. Low field AC magnetic susceptometry was used to characterize the temperature dependence of the superconducting transition in small single crystals of the three superconductors. Both the real x' and imaginary x" components of magnetic susceptibility were measured.
An oscillating temperature method was used for iodine chemical vapor transport growth of CuInSe2 ... more An oscillating temperature method was used for iodine chemical vapor transport growth of CuInSe2 in the solid phase regime between 665 and 810°C.Crystals grown at 730°Cand cooled at 10°C/mm exhibited a chalcopyrite phase structure. The resistivity of the crystals ranged from 1 to 650 ohm cm, and all were p-type. The mobility of the majority carriers in the high resistivity crystals was 15 cm 2/V 5
This paper describes a simple, internally-heated high pressure furnace for liquid-encapsulated sy... more This paper describes a simple, internally-heated high pressure furnace for liquid-encapsulated synthesis and melt growth at temperatures up to 1200 C and pressures up to 100 bar. The hot zone is contained in a 102-mm-ID x 127-mm-OD x 508-mm-long tubular vessel made of 304-series stainless steel. The vessel is externally water cooled and has self-sealing O-ring end closures. A Kanthal
We report two investigations conducted by using photoconductivity decay lifetime measurement. The... more We report two investigations conducted by using photoconductivity decay lifetime measurement. The first is crystalline silicon (c-Si) surface passivation using quinhydrone/methanol (QM) for bulk minority-carrier lifetime measurement. QM shows great promise as a substitute for iodine-based solutions because of its superior stability and minimized surface recombination velocity in silicon. The second is interface passivation in an amorphous silicon (a-Si)/c-Si heterojunction structure as a parallel effort to develop and optimize heterojunction c-Si solar cells by hot-wire chemical vapor deposition (HWCVD). A thin buffer layer inserted between the a-Si and the c-Si substrate has been found to be much more effective than a directly deposited a-Si/c-Si interface in reducing the interface recombination velocity.
We report two investigations conducted by using photoconductivity decay lifetime measurement. The... more We report two investigations conducted by using photoconductivity decay lifetime measurement. The first is crystalline silicon (c-Si) surface passivation using quinhydrone/methanol (QM) for bulk minority-carrier lifetime measurement. QM shows great promise as a substitute for iodine-based solutions because of its superior stability and minimized surface-recombination velocity in silicon. The second is interface passivation in an amorphous silicon (a-Si)/c-Si heterojunction structure as a parallel effort to develop and optimize heterojunction c-Si solar cells by hot-wire chemical vapor deposition (HWCVD). A thin buffer layer inserted between the a-Si and the c-Si substrate has been found to be much more effective than a directly deposited a-Si/c-Si interface in reducing the interface recombination velocity.
The effects of oxygen and nitrogen on the mechanical properties of Czochralski (CZ) and float zon... more The effects of oxygen and nitrogen on the mechanical properties of Czochralski (CZ) and float zone silicon have been studied using nano-indentation. Nitrogen free FZ Si exhibited low hardness of 6.49 GPa and elastic modulus of 104 GPa. When doped with 2×1015cm−3 nitrogen, FZ Si hardness and elastic modulus increased to 8.2 and 182 GPa, respectively. In the near-surface denuded zone of N-doped CZ Si (N-CZ) the hardness correlates well with the O and N profiles. Distinct high hardness points, found in the O- and N- rich subsurface region, were attributed to precipitates. Nano-scratch tests of N-CZ Si confirmed the existence of hard phases, mostly small precipitates, whose density, estimated to be 2×1013cm−3, is in the range of previously suggested nuclei density in as-grown N-CZ silicon.
WO3 is an important material to study, not only due to its interesting electronic properties, but... more WO3 is an important material to study, not only due to its interesting electronic properties, but also because it has other applications in both electrochromics and energy storage. The mechanism behind the electrochromic effect has been debated for several decades [1]. We have studied two WO3 single crystals, a transparent and doped WO3-x, in an attempt to understand this effect. A photoluminescence center around 865 nm is observed after sub-band gap excitation at 405 nm with relatively higher intensity in the crystal containing oxygen vacancies. The center appears as a broad transition of 35 nm FWHM and does not appear to be correlated with temperature. However, polarization studies reveal at least two polarization dependent components of the center. [4pt] [1] Satyen K. Deb, Solar energy materials and solar cells 92, 245 (2008), and the references therein
In this study, synchrotron-based x-ray absorption microspectroscopy (µ-XAS) is applied to identif... more In this study, synchrotron-based x-ray absorption microspectroscopy (µ-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 (Cu 3 Si) 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 (µ-XRF), and a segregation coefficient is determined from experimental data to be at least (1-2)×10 3 . Additionally, µ-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.
Abstract: Nitrogen doped Czochralski (N-CZ) and Float Zone (N-FZ) silicon were measured by high r... more Abstract: Nitrogen doped Czochralski (N-CZ) and Float Zone (N-FZ) silicon were measured by high resolution synchrotron Fourier Transform IR spectroscopy (HR-FTIR). The chemical complexes were analyzed in specific regions with known extended defects, i.e., denuded or precipitated regions of annealed N-CZ Si wafers, in N-FZ Si with ring defects and on “N-Skin ” region. The absorption lines were assigned to chemical complexes previously studied by first principles calculations. In annealed N-CZ Si wafers, a strong correlation was observed between the absorption line intensity depth variations and the defect distributions revealed by an Oxygen Precipitate Profiler (OPP), and oxygen and nitrogen SIMS profiles. Transformation of chemical complexes from one type to another was observed. A defect band, visible as an OPP peak at the denuded zone-bulk interface was found to be related to vacancy defect enhancement of oxygen precipitation via production of mobile N2. For the as grown N-FZ, the...
Crystalline silicon continues to be the dominant semiconductor material used for terrestrial phot... more Crystalline silicon continues to be the dominant semiconductor material used for terrestrial photovoltaics. This paper discusses the scientific issues associated with silicon photovoltaics processing, and cell design that may yield cell and module performance improvements that are both evolutionary and revolutionary in nature. We first survey critical issues in &#39;&#39;thick&#39;&#39; crystalline silicon photovoltaics, including novel separations processes for impurity removal,
Crystalline silicon continues to be the dominant semiconductor material used for terrestrial phot... more Crystalline silicon continues to be the dominant semiconductor material used for terrestrial photovoltaics. This paper discusses the scientific issues associated with silicon photovoltaics processing and cell design that may yield cell and module performance improvements, both evolutionary and revolutionary in nature. We first survey critical issues in thick crystalline silicon photovoltaics, including novel separations processes for impurity removal, impurity and defect fundamentals, interface passivation, the role of hydrogen, and high-throughput, kinetically-limited materials processing. Second, we outline emerging opportunities for creation of a very different thin-layer silicon cell structure, including the scientific issues and engineering challenges associated with thin-layer silicon processing and cell design. INTRODUCTION Todays basic research advances in materials physics and materials synthesis and processing will provide the foundation for a large-scale industrial p...
This work centers on the use of small (2 mm X 2 mm) photovoltaic devices to determine the electri... more This work centers on the use of small (2 mm X 2 mm) photovoltaic devices to determine the electrical characteristics of selective grain boundaries. The ESP bicrystal sheet growth method allows crystal orientation to be determined prior to growth and as such is an excellent tool for selective examination. Photolithographic techniques of a special pattern and mesa etching were used to isolate areas on and around the grain boundaries for device characterization. Spreading resistance measurements over and next to the grain boundaries, as well as characterization, were used to study electrical effects of these grain boundaries.
The planning of a photovoltaic laboratory in a developing country is discussed. With a budget of ... more The planning of a photovoltaic laboratory in a developing country is discussed. With a budget of $500,000, equipment and materials are purchased for a four-year period. In the fourth year 1000W (peak power) of modules will be produced from blank silicon wafers.
The effects of various kinds of micro-defects on minority carrier lifetime in high purity disloca... more The effects of various kinds of micro-defects on minority carrier lifetime in high purity dislocation-free silicon single crystals were studied by float-zoning, photoconductivity attenuation and copper decoration combined with selective etching, X-ray topography and electron-beam-inducedcurrent analysis. We found that swirl defects (A or B type) and unidentified frozen-in defects are responsible for limiting minority carrier lifetimes in high purity silicon crystals. The A-type defect was found to be a recombination center with an effective diameter of approximately 30-40 pm. By altering crystal growth conditions such as growth rate and diameter, various thermal gradients and cooling rates were attained. These affected the micro<lefect characteristics and hence the lifetimes of the crystals grown.
Crystal growth parameter effects on minority carrier lifetime and solar cell efficiencies were in... more Crystal growth parameter effects on minority carrier lifetime and solar cell efficiencies were investigated using high purity techniques such as float zoning. Study objectives include the following: (1) optimize dopants and minority carrier lifetime in FZ material for high efficiency silicon solar cell applications; (2) improve the understanding of lifetime degradation mechanisms (point defects, impurities, thermal history, surface effects, etc.), and (3) crystallographic defect characterization of float zone and ribbon crystals via X-ray topography.
Presented at the 2001 NCPV Program Review Meeting: Preliminary work on a novel process for extern... more Presented at the 2001 NCPV Program Review Meeting: Preliminary work on a novel process for external gettering by creating a band of silicon dioxide precipitation on top of a denuded zone via nitrogen doping is presented.
We obtained single-crystal YBa 2Cu 307 and ErBa 2Cu 307_5 platelets by eutectic growth from CuO-r... more We obtained single-crystal YBa 2Cu 307 and ErBa 2Cu 307_5 platelets by eutectic growth from CuO-rich mixtures of Y203 (or Er203), BaCO3, and CuO. Ground and mixed powders were loaded in an alumina crucible and heated to 1150-1180°C in air, held for 0.5-2 h, then cooled under programmed temperature and time conditions. Individual crystals of both YBa 2Cu 307.5 and ErBa 2Cu 307 up to 4 mm wide have been grown in voids that form in the charge. Similarly, compositions and temperature programs that allow single-crystal growth of Bi-Sr-Ca-Cu-~0 platelet crystals up to 4 mm wide that resembled mica in their delamination tendency were found. Low field AC magnetic susceptometry was used to characterize the temperature dependence of the superconducting transition in small single crystals of the three superconductors. Both the real x' and imaginary x" components of magnetic susceptibility were measured.
An oscillating temperature method was used for iodine chemical vapor transport growth of CuInSe2 ... more An oscillating temperature method was used for iodine chemical vapor transport growth of CuInSe2 in the solid phase regime between 665 and 810°C.Crystals grown at 730°Cand cooled at 10°C/mm exhibited a chalcopyrite phase structure. The resistivity of the crystals ranged from 1 to 650 ohm cm, and all were p-type. The mobility of the majority carriers in the high resistivity crystals was 15 cm 2/V 5
This paper describes a simple, internally-heated high pressure furnace for liquid-encapsulated sy... more This paper describes a simple, internally-heated high pressure furnace for liquid-encapsulated synthesis and melt growth at temperatures up to 1200 C and pressures up to 100 bar. The hot zone is contained in a 102-mm-ID x 127-mm-OD x 508-mm-long tubular vessel made of 304-series stainless steel. The vessel is externally water cooled and has self-sealing O-ring end closures. A Kanthal
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