Spherical Silicon Crystal Formed by Semisolid Process in Drop Tube

Abstract

Thus far, spherical Si crystals with $1 mm for solar cells have been grown by ejecting the melt above the melting point in a drop tube. The large undercooling prior to nucleation, however, results in severe polycrystallinity, which retrogrades the energy conversion efficiency. In the present study, we applied the semisolid process to the crystallization of Si droplets, where the melt was ejected with small solid particles formed by iterative cooling and heating of the melt with magnetic stirring at the melting point. The small solid particles that act as a nucleus efficiently suppressed the melt to be undercooled, resulting in an increase in the percentage of the spheres with few grains from 9.6 to 30%.

Spherical crystallization of Si during free fall in drop-tubes

“…We used this implication to modify the process so that sample ejection occurs under the semi-solid state, allowing droplets to crystallize at the low undercooling temperature during free fall, where the small solid particles can play the role of the preferential nucleation site. By using this modified process, the yields of samples crystallized at low undercoolings were improved from 9.6% to an average of 30% [11]. In spite of this change, the experimental difficulty in sustaining the semi-solid state means that the reproducibility of the process is poor.…”

Microstructures and Optical Properties of Silicon Spheres for Solar Cells

“…A high undercooling temperature (ΔT > 200 K) induced many grains and poor crystallinity. 8) In terms of solar cell performance, grain boundaries in spherical Si solar cells act as carrier recombination sites. Thus, care should be given to avoiding grain boundaries.…”

“…3,4,[6][7][8][9][10][11][12] Flexible and light weight spherical Si solar cells are also anticipated, and will be valuable in situations such as responding to emergency disasters. The conversion ef ciencies of spherical Si solar cells can be increased by producing high-quality Si spheres, 3,4,[6][7][8][9][10][11][12][13][14][15] forming textured Si surface structures, 16) deactivating defects by passivation, 17) optimizing the concentrator module and re ector cup 18,19) and preparing anti-re ection (AR) lms on the spherical Si solar cells. 12,[20][21][22] Si spheres have typically been grown by a dropping method.…”

High Temperature In-situ Observation of Si on the Porous Ceramic Substrate Repelling Si Melt for Growth of Spherical Si Crystal

“…Some of these studies focus on spherical Si due to its conservation of Si material and reduction of Si consumption per unit electric power generation to less than 1/5 that of wafers because cutting process is not required to fabricate spherical Si crystals [4,5]. Spherical Si is currently produced by droptube method [6]. In this method, Si melt was solidified during freefall in a drop tower, but it is often composed of fine crystal grains due to the very fast cooling process.…”