Sputtered Tungsten Oxide as Hole Contact for Silicon Heterojunction Solar Cells

Abstract

Reactively sputtered tungsten oxide ( $\text{WO}_x$ ) was investigated as hole contact on n-type crystalline silicon. Varying the oxygen gas flow during sputtering enables variation of the $\text{WO}_x$ 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 $\text{WO}_x$ shows higher resistivity and higher absorption in the visible range compared with indium–tin–oxide (ITO). Therefore, stacks of $\text{WO}_x$ and ITO are used in solar cells. It was found that at least 20 nm thick $\text{WO}_x$ 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. $\text{WO}_x$ 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 $\text{WO}_x$ with the highest possible conductivity. It was found that sputtering enables a drastic improvement of $\text{WO}_x$ /silicon solar cells compared with thermal evaporation, due to the precise control of the $\text{WO}_x$ conductivity. Unfortunately, the resistivity of the sputtered $\text{WO}_x$ is still limiting the FF of these devices.