Low-Temperature High-Mobility Amorphous IZO for Silicon Heterojunction Solar Cells

Parasitic absorption in the transparent conductive oxide (TCO) front electrode is one of the limitations of silicon heterojunction (SHJ) solar cells efficiency. To avoid such absorption while retaining high conductivity, TCOs with high electron mobility are preferred over those with high carrier density. Here, we demonstrate improved SHJ solar cell efficiencies by applying high-mobility amorphous indium zinc oxide (a-IZO) as the front TCO. We sputtered a-IZO at low substrate temperature and low power density and investigated the optical and electrical properties, as well as subband tail formation-quantified by the Urbach energy (E_U)-as a function of the sputtering oxygen partial pressure. We obtain an E_U as low as 128 meV for films with the highest Hall mobility of 60 cm²/V ·s. When comparing the performance of a-IZO films with indium tin oxide (ITO) and hydrogenated indium oxide (IO:H), we find that IO:H (115 cm²/V · s) exhibits a similar E_U of 130 meV, while ITO (25 cm²/V · s) presents a much larger E_U of up to 270 meV. The high film quality, indicated by the low E_U, the high mobility, and low free carrier absorption of the developed a-IZO electrodes, result in a significant current improvement, achieving conversion efficiencies over 21.5%, outperforming those with standard ITO.