Atomic layer deposited alumina on black silicon: passivation, electrical properties and application to high-efficiency solar cells

The term ’black silicon’ (bSi) refers to a particular silicon texture with surface features smaller than the wavelength of light in the ultraviolet to near-infrared range, which results in a reflectance close to zero. This material is consequently highly considered for solar cells applications. In those structures, carrier recombination can be critical due to the large surface area, and eventually affects the solar cell performance. This thesis proposes ways to improve control and understanding of recombination mechanisms in bSi. This work first introduces atomic layer deposition (ALD) processes for Al2O3 growth that rely on ozone to improve the standard water-based process. Significantly higher passivation quality is obtained with ozone-based processes and is shown to depend on ozone concentration. This can be explained by definite improvements in terms of electrical interface characteristics, which are studied in detail and discussed in relation with the chemical composition of the silicon/dielectric interface. While the quality of the dielectric film is essential for bSi passivation, bSi geometry also plays a decisive role. A relationship between bSi surface recombination velocity and surface charge is determined empirically; it indicates that the electric field in bSi is significantly stronger than in planar substrates. As a result, one type of charge carriers is efficiently repelled from the bSi surface, and surface recombination velocities below 7 cm/s are obtained in lowly-doped n-type bSi passivated by Al2O3. Such performance is typically relevant for interdigitated back-contact (IBC) solar cells, which rely on a lowly-doped and well-passivated front surface. IBC cells of nand p polarities are fabricated, and an external quantum efficiency close to 95 % is obtained in most of the sunlight spectrum due to low reflectance and effective surface passivation. This results in an excellent bSi solar cell efficiency of 22.1 %. Finally, recombination mechanisms are studied in highly-doped bSi for applications in front contact solar cells. This work shows that the excessive recombination often encountered in diffused emitters can be avoided by ion implantation, which enables strict control of dopant dose while preserving the low bSi reflectance, and by Al2O3 passivation. High-efficiency solar cells with front side emitter can thus also be envisaged.