Sputtered Tungsten Oxide as Hole Contact for Silicon Heterojunction Solar Cells

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