ENGINEERING SiO_2/TiO_2 STACKS FOR IMPROVED ELECTRON SELECTIVE CONTACTS

摘要

To increase the conversion efficiency of crystalline silicon solar cells, reduction of the recombination losses associated with the contacts is a must. Therefore, a contact structure that simultaneously passivates the c-Si surface while selectively extracting only one type of charge carrier (i.e., either electrons or holes) is desired. Therefore, recently such passivating contacts in c-Si solar cells has become an important research objective. Oxide/poly-Si concepts have been used to successfully reduce contact recombination. However, the use of poly-Si, namely at the front contact, introduces unwanted absorption of the sunlight. So, in addition to the electrical properties, the optical losses have to be minimized both on the front and rear sides of the solar cell for any contact structure. Furthermore, we aim at developing high quality selective contacts that can be deposited easily and economically. To address the problem caused by absorption in the top contact we studied different stacks using SiO_2 and Ti and TiO_2 layers with variable combinations, along two routes: high temperature route, in which the silicon oxide is obtained by thermal oxidation and a low temperature route where the silicon oxide is obtained by chemical oxidation or by e-beam evaporation. The results show that the introduction of the additional Ti layer is not beneficial when compared to stacks without this layer. High temperature thermal SiO_2 (900C) gives lower lifetimes when compared to the low temperature thermal oxide (775C). Annealing is crucial to enhance the final lifetimes in all explored stacks.