High Mobility IWO for Improved Current in Heterojunction Technology Solar Cells

Silicon heterojunction technology (HJT) enables record efficiencies for silicon-based solar cells [1]. The main advantage is the excellent surface passivation offered by hydrogenated amorphous silicon (a-Si:H), enabling record open circuit voltages. On the other hand, the need to use transparent conducting oxides for contacting and current spreading combined with a-Si:H leads to parasitic absorption and thus sub optimal short circuit current (JSC) for both side contacted solar cells. As a result, the advantage of this technology versus competing ones is particularly evident for back contacted solar cells [1]. However, thanks to the reduced complexity, and thus lower cost per watt, both side contacted solar cells have a much higher market penetration. It is therefore highly desirable to reduce as much as possible this parasitic absorption. In this context, high mobility TCOs like hydrogen doped indium oxide (IO:H) offers mobilities in excess of 100 cm2/(V.s) and enables higher JSC than use of the industrial standard indium tin oxide (ITO) [2]. Process control, in particular the incorporation of hydrogen, is however challenging with this material. We propose here the use of indium tungsten oxide (IWO) deposited by RF sputtering as a high mobility TCO for use in HJT solar cells. Preparing this TCO in standard conditions, i.e. with a base pressure below 5 E-7 mbar in our Oerlikon Clusterline tool already leads to mobilities higher than our best ITO process (>50 vs ~30 cm2/(V.s)) for conditions compatible with HJT solar cell processing. Furthermore, by controlling the chamber base pressure and using only argon and oxygen as reactive gases we are able to tailor the material properties to even higher mobilities (>80 cm2/(V.s)) without needing a water source or hydrogen line for doping. These layers were successfully included in HJT solar cells as evidenced by the current and efficiency gains associated with the use of this TCO.