Thermal Stability of Hole-Selective Tungsten Oxide: In Situ Transmission Electron Microscopy Study

In this study, the thermal stability of a contact structure featuring hole-selective tungsten oxide (WO_x) and aluminum deposited onto p-type crystalline silicon (c-Si/WO_x/Al) was investigated using a combination of transmission line measurements (TLM) and in situ transmission electron microscopy (TEM) studies. The TEM images provide insight into why the charge carrier transport and recombination characteristics change as a function of temperature, particularly as the samples are annealed at temperatures above 500 °C. In the as-deposited state, a ≈ 2 nm silicon oxide (SiO_x) interlayer forms at the c-Si/WO_x interface and a ≈ 2-3 nm aluminum oxide (AlO_x) interlayer at the WO_x/Al interface. When annealing above 500 °C, Al diffusion begins, and above 600 °C complete intermixing of the SiO_x, WO_x, AlO_x and Al layers occurs. This results in a large drop in the contact resistivity, but is the likely reason surface recombination increases at these high temperatures, since a c-Si/Al contact is basically being formed. This work provides some fundamental insight that can help in the development of WO_x films as hole-selective rear contacts for p-type solar cells. Furthermore, this study demonstrates that in situ TEM can provide valuable information about thermal stability of transition metal oxides functioning as carrier-selective contacts in silicon solar cells.