The sputter deposition of broadband transparent and highly conductive cerium and hydrogen co-doped indium oxide and its transfer to silicon heterojunction solar cells
Indium oxide doped with tin (ITO) is the most commonly used material for lateral transport window layers in silicon heterojunction (SHJ) solar cells, as it currently offers the best combination of physical properties, industrial deposition capability, and module reliability. However, typically applied ITO layers by far do not exploit the full electro-optical potential of the indium oxide material class, resulting in optical and electrical losses limiting the solar cell efficiency. In this work, cerium and hydrogen co-doped indium oxide thin films are developed for their application as high-performance transparent conductive oxide layers in SHJ devices. Amorphous In2O3:Ce,H layers are fabricated via radio frequency magnetron sputtering, before being crystallized during post-deposition thermal treatments compatible with the SHJ temperature stability. The resulting excellent electro-optical film properties are on par with values so far solely reported for reactive plasma deposited films. It is shown that the surface morphology of the substrate (planar or textured) has a strong impact on the film properties, and further, the critical role of the atmosphere present during post-deposition annealing is elucidated. Finally, the large potential of an optimally processed In2O3:Ce,H window layer in SHJ cells is demonstrated, quantified by a gain in short circuit current density of 0.6 mA/cm2 without impairing the resistive losses in comparison to the usage of a baseline ITO layer.