Surface and interface states investigated in Si based thin films for photovoltaic applications

Recently, hydrogenated nanocrystalline silicon oxynitride (nc-SiOxNy:H) has been proposed as a suitable substitute for a-Si:H as heteroemitter in SHJ cells. The latter presents strong parasitic absorption of light, which causes losses in short circuit current, and prevents to achieve higher power conversion efficiencies. Previous studies on nc-SiOxNy:H thin layers have shown for these films optical bandgap values as high as 2.5 eV and very high dark conductivities, up to 44 S/cm, which could lead to significant efficiency improvements. This thesis reports the investigation of nc-SiOxNy thin films, deposited by Plasma-Enhanced Chemical Vapour Deposition (PECVD) on c-Si and glass substrates, conducted by means of surface photovoltage (SPV) measurements and electrical characterization. The study concerned in particular the evaluation of the influence of the oxygen content and the thermal treatment, on the optoelectronic and electrical properties of the SiOxNy layers. SPV measurements have revealed the excellent passivation quality of the as-deposited sample with low oxygen content. In addition, it allowed for the identification of possible quantum confinement effects at the silicon nanocrystals in the sample with low oxygen, 3 h annealed. Furthermore, the thermal treatment have demonstrated to yield a significant improvement of the electrical properties of nc-SiOxNy thin films. The results this study show that the optimization of nc-SiOxNy for its application in SHJ solar cells still requires further study. In order to achieve the required high conductivity, annealing treatments are required; however, such treatments strongly degrades the passivating quality of the layer. Similar high conductivities can be achieved using higher H2 dilution during the deposition stage, that boosts the layer crystallinity.