Source Material Design for Realizing >50% Indium-Saving Transparent Electrode toward Sustainable Development of Silicon Heterojunction Solar Cells

Abstract

Indium (In) reduction is a hot topic in transparent conductive oxide (TCO) research. So far, most strategies have been focused on reducing the layer thickness of In-based TCO films and exploring In-free TCOs. However, no promising industrial solution has been obtained yet. In our work, we adopt the emerging reactive plasma deposition (RPD) approach and provide our In-reduced solution by directly reducing the In content from the source material. We designed the indium zinc oxide (IZO) target with a composition of Zn₃In₂O₆ (i.e., (ZnO)₃·In₂O₃). Density functional theory (DFT) calculation shows that the introduction of a large amount of ZnO significantly perturbs the conduction band of the In₂O₃ host, resulting in a limitation of exploring high-mobility IZO films. For TCOs used in solar cell application, low resistivity with high carrier mobility is required. Via RPD process optimization, we obtained the minimal resistivity value of 6.08 × 10^-4 Ω·cm, which is comparable to our lab-standard tin-doped indium oxide (ITO) film. The corresponding electron mobility and carrier concentration are 31 cm² V^-1 s^-1 and 3.37 × 10²⁰ cm^-3, respectively. Our IZO film is in an amorphous state. The optical band gap is ∼3.6 eV. X-ray photoelectron spectroscopy (XPS) data show that the film composition is In:Zn:O = 21.60:28.75:49.65 (at. %). Damp heat tests show strong stability of our IZO film, and no aging effects have been observed. Furthermore, we demonstrated wafer-scale silicon heterojunction (SHJ) solar cells with IZO films. As compared with our reference hydrogenated cerium-doped indium oxide (ICO)-based solar cells, the IZO-based devices show even higher fill factor parameters. Our amorphous state stable In-reduced IZO film could find versatile application in the sustainable development of temperature-sensitive devices such as SHJ and perovskite/silicon tandem solar cells, as well as flexible OLEDs.

Keywords: indium zinc oxide (IZO); indium-reduced target; reactive plasma deposition (RPD); silicon heterojunction (SHJ) solar cell; transparent conductive oxide (TCO).