Control of grain size and -orientation in multi-crystalline silicon ingots

Abstract

Two different cooling rates have been imposed during the early solidification of two multi-crystalline silicon ingots with 250mm diameter and 100mm height in a pilot scale directional solidification furnace. This has been done by opening a variable heat leak system below the crucible in order to achieve a high initial cooling rate in one of the ingots. The grain-structure and -orientation of these two ingots have been investigated by light microscopy (LM) and electron backscattered diffraction (EBSD), and their electrical properties by quasi-steady state photo-conductance (QSSPC) and surface photovoltage (SPV) method. The ingot with the high initial cooling rate shows predominantly grains which are significantly larger than what is usually found in mc-Si. The minority carrier diffusion lengths measured on the large grains in the ingot with high cooling rate show higher values than those measured on the ingot with smaller grains. These results indicate that principles of grain size and -orientation control in mc-Si ingots can be applied to a pilot scale furnace, and the potential for up-scaling to industrial ingots with improved electrical properties and, thus, higher solar cell conversion efficiency.

Grain control in directional solidification of photovoltaic silicon

“…However, the low supercooling in the nucleation stage and the low growth rate during grain growth may not be always positive. The recent results showed that the high initial cooling induced the dendritic growth that could lead to larger grains with better quality [4,15]. With an enhanced cooling design using argon flow for nucleation, we also observed similar results [16].…”

“…With an enhanced cooling design using argon flow for nucleation, we also observed similar results [16]. Therefore, this concept using dendrite casting for getting a high percentage of high symmetry grain boundaries, such as coincidence site lattice (CSL) boundaries and twins at the nucleation stage is getting recognized [4,6,15,16]. Because silicon is a highly anisotropic material, its growth habit is strongly affected by undercooling [17,18].…”

Engineering silicon crystals for photovoltaics

“…Recent results showed that the high initial cooling could induce dendritic growth that could lead to larger grains of better quality. [45][46][47][48][49][50]65 This approach is the so-called dendrite casting method, proposed by Nakajima's group at Tohoku University, as illustrated in Fig. 4(a).…”

Multicrystalline Silicon Crystal Growth for Photovoltaic Applications

“…A careful control of initial nucleation by slow cooling and grain competition through growth rate and the growth front shape are believed to be crucial [111]. Recent results showed that higher initial cooling induced dendritic growth that could lead to larger grains with better quality [61][62][63]112]. For example, a flat or slightly convex growth front is still believed to be beneficial to reduce the parasitic nucleation from the crucible wall, as well as the thermal stress as we discussed.…”