Low-Temperature, Chemically Grown Titanium Oxide Thin Films with a High Hole Tunneling Rate for Si Solar Cells

In this paper, we propose a chemically grown titanium oxide (TiO 2 ) on Si to form a heterojunction for photovoltaic devices. The chemically grown TiO 2 does not block hole transport. Ultraviolet photoemission spectroscopy was used to study the band alignment. A substantial band offset at the TiO 2 /Si interface was observed. X-ray photoemission spectroscopy (XPS) revealed that the chemically grown TiO 2 is oxygen-deficient and contains numerous gap states. A multiple-trap-assisted tunneling (TAT) model was used to explain the high hole injection rate. According to this model, the tunneling rate can be 10 5 orders of magnitude higher for holes passing through TiO 2 than for flow through SiO 2 . With 24-nm-thick TiO 2 , a Si solar cell achieves a 33.2 mA/cm 2 photocurrent on a planar substrate, with a 9.4% power conversion efficiency. Plan-view scanning electron microscopy images indicate that a moth-eye-like structure formed during TiO 2 deposition. This structure enables light harvesting for a high photocurrent. The high photocurrent and ease of production of chemically grown TiO 2 imply that it is a suitable candidate for future low-cost, high-efficiency solar cell applications.