Evidence of TiO_x reduction at the SiO_x/TiO_x interface of passivating electron-selective contacts

摘要

A TiO_x layer is well known as an electron-selective contact material because of its asymmetric band offsets with respect to c-Si. When applying TiO_x layers as passivating electron-selective contacts, forming sub-stoichiometric TiO_x is important to obtain a low contact resistivity because oxygen vacancies increase the conductivity of TiO_x and provide n-type doping effects. In this work, oxygen vacancies at SiO_x/TiO_x interfaces are investigated by atomic depth profiling of XPS measurements. Three kinds of TiO_x layers are studied grown by either e-beam evaporation, atomic layer deposition or sputtering on c-Si. In all three TiO_x samples, a resulting stack of c-Si/SiO_x/TiO_x could be noticed XPS measurements that show SiO_x peaks near the c-Si/TiO_x interface. Moreover, clear TiO_2 peaks, which can be measured at the surface of all three TiO_x layer types, gradually change to Ti or TiSi_2 peaks near the SiO_x/TiO_x interface. This indicates that many oxygen vacancies seem to exist at the SiO_x/TiO_x interface. This TiO_x reduction may contribute to the formation of a dipole and increased downward band bending resulting in a lower contact resistivity in the electron-selective contacts. As a result, hetero-junction solar cells with i-a-Si:H/TiO_x/Ca/Al contacts exhibit a significant series resistance reduction of about 40 % compared to solar cells with i-a-Si:H/Ca/Al contacts.