Effects of high temperature incubation on solid phase crystallisation of silicon films and properties of polycrystalline silicon thin-film solar cells on glass

High temperature, 640°C or 680°C, and 8 to 60 min long treatment was applied to amorphous Si films on glass prior to solid-phase crystallisation (SPC) at 600°C to selectively enhance and shorten the “incubation-only” period in the SPC process. Effects of such enhanced-incubation anneal and its conditions on SPC kinetics and resulting properties of polycrystalline silicon thin-film solar cells are studied. The incubation period at 600°C can be shortened by 50-80% (by 200-400 min) and the crystal growth rate can be doubled after only a few minute long enhanced-incubation anneal at 640 or 680°C. However, cell performance parameters degrade for all such pre-treatments in a direct correlation with treatment intensity: the higher enhanced-incubation temperature and the longer its time, the worse the cell performance. Both open circuit voltage and short circuit current gradually decrease with longer and hotter enhanced-incubation annealing. It is the cell blue response that is mostly affected by the treatment, while the red response stays similar. Accordingly, the modeled minority carrier diffusion length is shorter in the cell emitter and it does not change in the absorber after the enhanced-incubation anneal. The increasing diode ideality factor means more depletion region recombination, which also indicates that the applied treatment predominantly affects the front, glass-side layers of the cell, where the junction and the emitter are located. The observed results are unexpected as the cell performance worsens for treatments conducted within the incubation period only, i.e. before nucleation and crystal growth start to take place and the Si film is still fully amorphous. The results draw attention to the importance of the incubation period to the final electronic quality of the polycrystalline Si.