Abstract: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.

In this work, we demonstrated a heterojunction Si solar cell utilizing chemically grown titanium oxide (TiOx) as an electron-selective contact layer at its rear surface. With TiOx, the rear surface was passivated to reduce carrier recombination. The reverse saturation current, which is an indicator of carrier recombination, exhibited a 4.4-fold reduction after placing a TiOx layer on the rear surface. With reduced recombination, the open-circuit voltage increased from 433 mV to 600 mV and consequently, the power conversion efficiency (PCE) increased from 9.57 to 14.70%. By x-ray photoemission spectroscopy, the surface passivation was attributed to a silicon oxide interfacial layer formed during the chemical growth process. This passivation results in a 625 cm/s surface recombination velocity for the TiOx-passivated Si surface, which is 2.4 times lower than the sample without TiOx, ensuring the carriers pass through the rear contact without extensive recombination. According to these results, the band alignment for the heterojunction solar cell with and without a TiOx rear contact layer was plotted, the reduced interfacial recombination and the electron and hole blocking structure are the main reasons for the observed efficiency enhancement.

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