Studies On Passivation Of P-type Si Surface By The Sol-gel Al₂O₃ Films

At present, the industrial crystalline silicon solar cells are dominant in the photovoltaic industry. Passivation is very important for high efficiency crystalline silicon solar cells. Traditional screen printed Aluminum Back-Surface Field (BSF) is used, posing problems with wafer bowing when applied to wafer thinner than 200μm. Moreover, with the research and development of n-type solar cells, it needs to find new passivation techniques for p-type crystalline silicon. Experimental studies on test structures have shown that Al2O3 films would be ideally suited for this purpose, with high density of negative fixed charges in the order of 1012-1013 cm-2.Now Al2O3 synthesized by atomic layer deposition (ALD) is studied as surface passivation layer, showing that Al2O3 can provide an excellent level of surface passivation on p-type c-Si. ALD, however, is not suitable for industrial, large-scale production because of its low throughout. In comparison with ALD, sol-gel method has more cost advantages with simple equipment, process easy to control, high product purity and uniformity.In this paper, the passivation characteristics of the aluminum oxide films by sol-gel method were studied in detail, focusing on the effect of annealing temperature, annealing time and annealing atmosphere. The passivation effect of aluminum oxide films on p-type c-Si with different doping concentration was further studied. Some meaningful results were achieved:1. Sol-agent, solution ph value, temperature and water added in the process had great influence on the characteristic of Al2O3 colloid. In this experiment, HNO3 as the sol-agent, reaction temperature was between 85-90℃the initial value of H2O:A1 (OC3H7) 3 was 70-80 ml/g, the colloid showed very steady and transparent when the solution ph value was controlled between 4.4 and 4.6.2. The aluminum oxide films by sol-gel method can provide an excellent level of field-induced surface passivation on lightly doped p-type silicon wafer after proper annealing treatment. The reason for the negative charge at the Al2O3/Si interface is that the Al ions rearrange in tetrahedrally coordinated Al sites with a net negative charge bonded directly to the O atoms of the SiO2 interfacial. The passivaion effect leapt dramatically between 450℃and 500℃. Moreover, annealing in air is better than annealing in nitrogen.3. After annealing at high temperature, the bonding structure rearranged was not stable. The net amount of negative fixed charge was decreased with time. With the increase of annealing time and film thickness, the passivation effect will be more stable and better.4. A high density of negative fixed charges in the order of 1012 cm-2 was detected in the Al2O3 films. The surface recombination velocity was about 2 cm/s.5. The aluminum oxide films by sol-gel method provided a good level of field-induced surface passivation on p+emitter of n-type solar cells.