Crystalline Silicon Surface Passivation Using Aluminum Oxide Fundamental Understanding And Application To Solar Cells PDF Download

High-efficiency silicon solar cells have been attracting an increased interest in recent years. Surface passivation is essential for various types of high-performance solar cells, particularly when thinner silicon wafers are used to reduce the material cost. Passivating dielectric thin films have been widely studied and used in solar cells designs, these include atomic-layer deposited (ALD) aluminium oxide, plasma-enhanced chemical vapour deposited (PECVD) silicon nitride and PECVD intrinsic amorphous silicon. The aim of this thesis is to develop and optimise an alternative deposition method for surface passivating films: sputtering. Sputtering is especially attractive for industrial production, due to its high throughput, easy and safe operation and global cost-effectiveness. This thesis has focussed on optimising the reactive sputtering of aluminium oxide, using an aluminium target, and the non-reactive sputtering of amorphous silicon, using a silicon target. A key innovation has been the addition of hydrogen to the mix of gasses that form the plasma, which permits to incorporate hydrogen into the films, leading to a significantly improved surface passivation quality compared to non-hydrogenated films. We have achieved the best surface passivation results by sputtered aluminium oxide to date, with an effective surface recombination velocity of 1 cm/s on 1.5 ohm-cm n-type silicon. This result is similar to the SRV of 0.9 cm/s measured on aluminium oxide films deposited by PA-ALD on the same substrates. Good passivation was also achieved on p-type silicon. The investigations into the reactive sputtering process have shown that the film properties are closely related to the oxidation level of the aluminium target, which can be controlled by adjusting process parameters. It has also been found that the presence of hydrogen in the plasma is beneficial for establishing the optimum conditions of the deposition; not only does the surface passivation quality improve, but the reactive sputtering process becomes easier to control as well.We have also shown - for the first time - that intrinsic amorphous silicon (a-Si:H) films by sputtering deposition are capable of providing an excellent passivation of crystalline silicon surfaces. A SRV of 1.5 cm/s on 1.5 ohm-cm n-type silicon and SRV of 9 cm/s on 1 ohm-cm p-type silicon have been achieved, which are comparable to the commonly used PECVD deposited a-Si:H films. After investigating the film properties using Fourier Transform Infrared Spectroscopy (FTIR), we observe that our sputtered a-Si:H films have a characteristic signature in terms of chemical bonding configurations, where several types of silicon-hydrogen bonds exist. From those measurements we have estimated that there is approximately a 4% hydrogen concentration in the films, sufficient to achieve excellent surface passivation. Finally, the thesis also presents initial attempts at developing doped amorphous silicon films, which could enable the development of an all-sputtered silicon heterojunction solar cell technology. Lightly doped a-Si:H films were deposited using a 1% boron doped silicon target and a 0.01% phosphorus doped silicon target. We have found an appropriate way to avoid surface passivation degradation caused by the doped layer deposition onto an intrinsic a-Si:H layer.