FUTURE PV Innovation

Kōriyama, Japan

FUTURE PV Innovation

Kōriyama, Japan

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Kivambe M.,Massachusetts Institute of Technology | Powell D.M.,Massachusetts Institute of Technology | Castellanos S.,Massachusetts Institute of Technology | Ann Jensen M.,Massachusetts Institute of Technology | And 5 more authors.
Journal of Crystal Growth | Year: 2014

We evaluate minority-carrier lifetime and defect content of n-type photovoltaic silicon grown by the noncontact crucible method (NOC-Si). Although bulk impurity concentrations are measured by inductively coupled plasma mass spectroscopy to be less than one part per million, homogeneously throughout the as-grown material we observe lifetimes in the ~150 μs range, well below the theoretical entitlement of single-crystalline silicon. These observations suggest the presence of homogeneously distributed recombination-active point defects. We compare an industry-standard gettering profile to an extended gettering profile tailored for chromium extraction, to elucidate potential gains and limitations of impurity gettering. Near the ingot top, gettering improves lifetimes to 750 and >1800 μs for standard and extended profiles, respectively. Relatively lower gettered lifetimes are observed in wafers extracted from the ingot middle and bottom. In these regions, concentric-swirl patterns of low lifetime are revealed after gettering. We hypothesize that gettering removes a large fraction of fast-diffusing recombination-active impurities, while swirl microdefect regions reminiscent of Czochralski silicon can locally limit gettering efficiency and lifetime. Apart from these swirl microdefects, a low dislocation density of <103 cm-2 is observed. The millisecond lifetimes and low dislocation density suggest that, by applying appropriate bulk microdefect and impurity control during growth and/or gettering, n-type NOC-Si can readily support solar cells with efficiencies >23%. © 2014 Elsevier B.V.


Nakajima K.,FUTURE PV Innovation | Murai R.,FUTURE PV Innovation | Morishita K.,Kyoto University | Powell D.M.,Massachusetts Institute of Technology | And 2 more authors.
2014 IEEE 40th Photovoltaic Specialist Conference, PVSC 2014 | Year: 2014

A noncontact crucible method was proposed to obtain a crystal-diameter as large as a crucible-diameter. In this method, a Si melt used has a large low-temperature region in its central upper part to ensure Si crystal growth inside it. Therefore, the present method has several merits such as the convex shape of the growing interface in the growth direction, the possibility of growing large ingots even using a small crucible, and the growth of square-like single bulk crystals. In these ingots, dislocations in the ingot moved to the periphery of the ingot from its center during crystal growth, and the dislocation density was on the order of 102-103/cm2. The effective minority carrier lifetime was measured to be as high as 750 μs by the Quasi-Steady-State Photoconductance (QSSPC) method after phosphorus diffusion gettering and Al2O3 thin-film passivation. Especially, this method has a possibility to attain a high growth rate using a high cooling rate because the growth rate was determined by the expansion rate of the low-temperature region in Si melts. The growth rate increases as the cooling rate increases. At the cooling rate of 0.4 K/min, the horizontal growth rate became higher to 1.5 mm/min in the <110> direction. The vertical growth rate was determined as 0.3-0.6 mm/min, and it had a tendency to increase as the depth of Si melts increased. The diameter of ingots can be kept constant during crystal growth using a high cooling rate because the horizontal growth rate increases as the cooling rate increases. An ingot with a diagonal length of 24.5 cm was obtained using the high cooling rate of 0.4 K/min. The diagonal length was as large as 82% of the crucible diameter. © 2014 IEEE.


Nakajima K.,FUTURE PV Innovation | Murai R.,FUTURE PV Innovation | Ono S.,FUTURE PV Innovation | Morishita K.,Kyoto University | And 3 more authors.
Japanese Journal of Applied Physics | Year: 2015

The noncontact crucible method enables production of Si bulk single crystals without crucible contact by intentionally establishing a distinct low-temperature region in the Si melt. In this contribution, we correlate crystal growth conditions to crystal material properties. The shape of the growing interface was generally convex in the growth direction. The quality of the Si ingots was determined by the spatial distributions of dislocations, resistivity, oxygen concentration, and minority-carrier lifetime. In an ingot with a convex bottom, swirl patterns with higher resistivity are present in the top, middle, and bottom of the ingot. The dislocation density decreased from the top (first to solidify) to the bottom of the ingot because dislocations in the ingot moved to the periphery from the center of the ingot during crystal growth owing to the convex growing interface. The oxygen concentration was concentrically distributed on the seed axis owing to the convex growing interface. The lifetime was as high as 1.8ms after phosphorus diffusion gettering (PDG) and 205 μs before PDG at an injection level of 1 × 1015cm-3. The lifetime was not strongly affected by the dislocation density, which was as low as 102-103cm-2. © 2015 The Japan Society of Applied Physics


Nakajima K.,FUTURE PV Innovation | Morishita K.,Kyoto University | Murai R.,FUTURE PV Innovation
Journal of Crystal Growth | Year: 2014

We propose a high-speed growth based on noncontact crucible method for obtaining large ingots with a constant diameter. In this method, the Si melt used has a large low-temperature region in its central upper part to ensure Si crystal growth inside it. Therefore, this method has the possibility of attaining a high growth rate using a high cooling rate because the growth rate is determined by the rate of expansion of the low-temperature region in the Si melt. The horizontal and vertical growth rates in Si melts were experimentally determined. At a cooling rate of 0.4 K/min, the horizontal growth rate reached 1.5 mm/min in the <110> direction and 1.9 mm/min in the <100> direction. These growth rates are higher than that of the cast method. The growth rate increased with the cooling rate. The vertical growth rate was determined to be 0.3-0.6 mm/min, and it tended to increase with increasing depth of the Si melt. The diameter of an ingot remained constant during pulling due to a high cooling rate of 0.4 K/min because the horizontal growth rate increased as the cooling rate increased and the melt temperature markedly decreased. An ingot with a constant diameter of 21 cm and a height of 7 cm was obtained inside a Si melt by the high speed growth using a crucible with 33 cm diameter. © 2014 Elsevier B.V.

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