A spherical Si solar cell with a semi-light-concentration system was successfully fabricated using a spherical Si crystal produced by a dropping method. The dropping method has great potential for low-waste and low-cost fabrication because the Si spheres can be made directly from dropping melted Si without the cutting and polishing processes of Si ingots. The fabricated Si spheres were generally multicrystalline due to the crystal growth through homogeneous crystal nucleation in containerless states. Spherical Si solar cells were fabricated using a Si sphere with a diameter of 1 mm as a solar cell substrate and then mounted on a reflector cup with a hexagonal aperture to complete the semi-light-concentration system. The current–voltage (J–V) measurement of the cell demonstrated an energy conversion efficiency of 10.4%. The key parameters for achieving higher efficiency are discussed with J–V data analysis, and quantum efficiency and laser-beam-induced current measurements.

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Spherical silicon can be produced directly from melted silicon allowing them to solidify into spherical shape by surface tension; several organizations have carried out trial investigation for fabricate solar cells. Spherical silicon produced by this method are generally polycrystalline and solar cells fabricated from these product are strongly affected by the crystallinity. In this report, an X-ray pole plot analysis of crystal structure of spherical silicon is described. We use pole figure measurement in X-ray diffraction, because distribution and number of the small crystals are directly observable. From (111) pole figure of single crystal as well as polycrystal silicon wafers, we found four poles for the (100) single crystal test sample wafer. In case of the polycrystal, the number of poles is proportional to the number of crystal grains. We have also successfully analyzed the crystallinity of spherical silicon by the pole figure measurement. The (111) pole figure has only four poles in the case of the single crystal spherical silicon. The limitation of sampling position is also discussed.

We have investigated the crystal growth mechanism of spherical Si fabricated by a dropping method. The Si spheres were classified into two categories by surface morphology. The grain size of a Si sphere with a smooth surface is larger than that with a rough surface. To investigate the crystal growth mechanism of the spheres, Si samples with various crystal sizes were observed. Si sample size is related to Si droplet size of, which influences the ease of solidification. Large Si droplets take longer to solidify than small Si droplets. Si samples 4, 2 and 1 mm in diameter correspond to the initial, intermediate and final stages of crystal growth, respectively. In the Si sample 4 mm in diameter, a disk of (111) plane crystals is observed. This result suggests that the initial crystal growth of the Si spheres consisting of large grains involves the formation of a disk of ( 111) plane Si crystals. The crystal growth mechanism for a Si sphere 1 mm in diameter is proposed.

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