Pinhole Formation by Nucleation-Driven Phase Separation in TOPCon and POLO Solar Cells: Structural Dynamics and Optimization

SiOx/poly-Si passivating carrier-selective contacts are one of the most promising concepts for the next generation of high-efficiency silicon solar cells called TOPCon and POLO cells. Experiments found that their ultrathin SiOx layer may be susceptible to the formation of Si-rich pinholes. In TOPCon cells, these pinholes act as undesirable recombination centers, while POLO cells are designed with pinholes being crucial for their operation. Motivated by remarkable experimental results, in this paper, we report extensive molecular dynamics simulations of the structural dynamics of TOPCon stacks during the elevated temperature thermal treatments of the cell fabrication. In particular, we investigated the effects of hydrogen on the structural dynamics of the TOPCon stacks. Our main findings were the following. (1) Pinholes formed via nucleated phase separation already in the nonhydrogenated SiOx layer but only at higher, POLO-relevant temperatures. (2) The effect of hydrogen was distinctly different at low and at high concentrations. At low H concentration, hydrogen reduced the concentration of interface dangling bonds, which are well-known recombination centers. However, at high H concentration, hydrogen dramatically lowered the energy of the pinhole configurations and their nucleation barriers and thus induced the formation of pinholes. These proto-pinholes reversed the trend and increased the dangling bond concentration. (3) We found that pinhole formation was reversible: the extraction of hydrogen dissolved the pinholes.