Abstract
In this paper, we propose a chemically grown titanium oxide (TiO2) on Si to form a heterojunction for photovoltaic devices. The chemically grown TiO2 does not block hole transport. Ultraviolet photoemission spectroscopy was used to study the band alignment. A substantial band offset at the TiO2/Si interface was observed. X-ray photoemission spectroscopy (XPS) revealed that the chemically grown TiO2 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 105 orders of magnitude higher for holes passing through TiO2 than for flow through SiO2. With 24-nm-thick TiO2, a Si solar cell achieves a 33.2 mA/cm2 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 TiO2 deposition. This structure enables light harvesting for a high photocurrent. The high photocurrent and ease of production of chemically grown TiO2 imply that it is a suitable candidate for future low-cost, high-efficiency solar cell applications.
Highlights
Heterojunction solar cells have recently become leading candidates for high-efficiency cells with a considerably low production cost
The high photocurrent and ease of production of chemically grown TiO2 imply that it is a suitable candidate for future low-cost, high-efficiency solar cell applications
Heterojunctions are widely used in optoelectronic devices such as light-emitting diodes (LEDs) [1,2,3,4], laser diodes [5,6], and photodiodes (PDs) [7,8,9] because they enable manipulating either hole or electron transport selectively by applying an appropriate band alignment
Summary
Heterojunction solar cells have recently become leading candidates for high-efficiency cells with a considerably low production cost. Several material systems have been proposed and demonstrated to achieve appropriate heterojunctions in Si solar cells. Amorphous Si is a well-known material system deposited on crystalline Si to construct a heterojunction for high-efficiency solar cells by setting front and back surface fields [10]. The a-Si:H-based heterojunction with intrinsic thin-film technology has demonstrated high efficiency, an alternative material that inherently exhibits excellent surface passivation and proper band alignment is required. Avasthi et al [16] used low temperature chemical vapor deposition (CVD)-grown TiO2 (3 nm) on a p-type wafer to block holes, thereby achieving a 7.02% photoelectric conversion efficiency. Chemically depositing TiO2 at a low temperature is proposed for attaining a high hole tunneling rate on an n-type Si substrate for heterojunction solar cells
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