Integration of atomic layer deposited aluminum oxide as surface passivation layer into silicon solar cell

The efficiency of the industrial back surface field (BSF) solar cells is limited by the strong recombination at the rear surface of the solar cell. The enormous number of dangling bonds on the silicon surface create recombination centers, trap the charge carriers separated by the sunlight, and thus lower significantly the solar cell efficiency. This detrimental effect is stressed in this dissertation by passivating the silicon surface with atomic layer deposition (ALD) aluminum oxide (Al2O3). It is shown that the passivation quality of ALD Al2O3 is highly sensitive to the surface condition before the ALD process. Compared with an HF cleaned silicon surface, an oxidized surface resulting from HCl solution results in a passivation layer with better thermal stability. Ozone is found to be a better oxidant than water for the industrialization of ALD Al2O3 passivation in terms of passivation quality and throughput. High negative charge density up to 7×1012 cm-2 can be reached with ozone, providing stronger field-effect passivation for the passivated surface. Increasing the concentration of ozone can improve the chemical passivation, resulting in a better final passivation quality. However, adding water dose in the ozone-based process can further reduce the destructive interface hydrogen content, thus leading to better passivation than the process with pure ozone or pure water. In addition to pure ALD Al2O3 film, various layers of materials can be deposited within one ALD batch with precise control. This thesis shows that titanium doping in the ALD Al2O3 enhances the surface passivation of silicon. In addition to surface passivation, ALD can provide good optical properties for the solar cells as it is shown that high refractive index material, such as titanium dioxide and zinc oxide, can be deposited within the same ALD batch as capping layers or anti-reflection coatings. Furthermore, a lower-cost precursor, dimethylaluminium chloride, is analyzed in terms of ALD growth and passivation quality. Better thermal stability is revealed than that with trimethylaluminum. This thesis pays attention also to the blistering phenomenon of ALD films during the post-annealing. Several suggestions are made to eliminate the blisters in the post-annealed films. Finally, PERC solar cells are fabricated in an industrial-scaled production line, which integrates a batch ALD reactor with a loading capacity of 600 solar cell wafers. The solar cell with an ALD Al2O3 passivation layer exhibits improved efficiency at 20.8%, compared to the traditional BSF solar cell with 19.7% efficiency.