Sputtered Tungsten Oxide as Hole Contact for Silicon Heterojunction Solar Cells

Reactively sputtered tungsten oxide (WO
_x
) was investigated as hole contact on n-type crystalline silicon. Varying the oxygen gas flow during sputtering enables variation of the WO
_x
conductivity from 0.01 to 1000 Ω/cm, while the band bending at the interface and the implied fill factor (FF) change by 70 meV and 1.5%. Sputtered WO
_x
shows higher resistivity and higher absorption in the visible range compared with indium-tin-oxide (ITO). Therefore, stacks of WO
_x
and ITO are used in solar cells. It was found that at least 20 nm thick WO
_x
is needed to prevent detrimental effects of the ITO work function on the band bending at the junction, the implied FF, and the real FF of solar cells. WO
_x
hole contacts of different thicknesses and conductivity were applied in solar cells and it was found that the highest FF is achieved using about 20 nm thick interlayers of WO
_x
with the highest possible conductivity. It was found that sputtering enables a drastic improvement of WO
_x
/silicon solar cells compared with thermal evaporation, due to the precise control of the WO
_x
conductivity. Unfortunately, the resistivity of the sputtered WO
_x
is still limiting the FF of these devices.