Wide energy band gap materials for next generation heterojunction solar cells

Due to the increase of energy demand and the growth of atmospheric pollution, in the last few years renewable energy sources had a large growth. Among these, photovoltaic solar cells played a central role, thanks to the reduction of their cost partially due to the wide research developed on them. Currently, silicon crystalline solar cells’ state of the art is based on heterojunction structures that combines crystalline silicon with a wider band gap semiconductor material like hydrogenate amorphous silicon (a-Si:H). This technology detains current efficiency record of 26.7 %. However, this structure has still certain aspects that can be improved such as the high light absorption of the amorphous silicon layers (a-Si:H), which limits the overall energy that can be converted into current, and their low thermal stability, which does not allow to overcome their deposition temperature (typically 200°C) in the following fabrication steps, limiting and making more difficult and costly their fabrication. Furthermore, during the amorphous silicon layers’ deposition, some highly hazardous and toxic gases are used and it would be desired to find an alternative. In this work are studied two new materials that can help to overcome the aforementioned drawbacks of amorphous silicon layers. The first material is hydrogenated amorphous Silicon sub-Oxide (a-SiOx:H) which during this work was demonstrated to be more transparent and more thermally stable than a-Si:H, preserving the passivation properties of amorphous silicon layers. Different heterojunction solar cells based both on a-SiOx:H and a-Si:H were made and characterized, confirming the higher transparency and thermal stability of the former set with respect to the latter group. Furthermore it was proved the compatibility between deposition process of a-SiOx:H and a-Si:H, confirming this material as a suitable candidate to replace amorphous silicon layers in industry. Moreover, thanks to the oxygen presence inside the film, the a-SiOx:H layers showed a high chemically compatibility with metal oxides which, may be suitable to replace a-Si:H layers as selective contacts in case they have a high work function. Among these metal oxides it was chosen non-stoichiometric Molybdenum Oxide (MoOx) as a second material to be investigated, since unlike doped a-Si:H layer does not need the use of hazardous and toxic gases during the deposition process. MoOx layer was investigated in terms of material analysis and its characteristics and was developed together with a-SiOx:H layer to obtain high transparency and stability at industrial level. Hence the MoOx layer was successfully experimented in combination with a-SiOx:H buffer inside a complete heterojunction solar cell which was described and characterized, exploiting their high overall transparency allowing the absorption of a wider portion of sunlight spectrum in comparison to heterojunction solar cells based on a-Si:H layers. The experimental work of my Ph.D. thesis has been mainly carried out at the “ENEA Casaccia” laboratories, some experiments have been performed at “ENEA Portici” laboratories.