Symmetric Dopant-Free Si Solar Cells Enabled by TiO_x Nanolayers: An In-Depth Study on Bipolar Carrier Selectivity

Area of Science:

• Materials Science
• Photovoltaics
• Semiconductor Physics

Background:

• Efficient solar cells depend on effective charge extraction using specialized contact structures.
• Current technologies often rely on extrinsic doping for selective contacts, adding complexity.

Purpose of the Study:

• To demonstrate a novel hole- and electron-selective passivating contact for crystalline silicon solar cells.
• To utilize a single material, amorphous titanium oxide (TiO_x), without extrinsic doping.
• To elucidate the charge generation mechanism at the silicon/metal oxide interface.

Main Methods:

• Atomic layer deposition (ALD) of amorphous titanium oxide (TiO_x) layers (≈5 nm).
• Tailoring TiO_x selectivity through oxidation processes and Ti precursor selection.
• Ex situ and in situ X-ray photoelectron spectroscopy (XPS) for interface analysis.
• Postdeposition oxidation to form a SiO_y nanolayer.

Main Results:

• Demonstrated bipolar carrier selectivity using only amorphous TiO_x.
• Hole-selective TiO_x induced significant band bending (Φ≈0.7 eV) in n-Si, creating a p-type inversion layer.
• Electron-selective TiO_x induced smaller band bending (Φ<0.35 eV).
• Identified negative fixed charges in TiO_x as the origin of selectivity, dependent on ALD parameters.
• Confirmed the critical role of a hydrogen-containing SiO_y nanolayer for chemical passivation.

Conclusions:

• Amorphous TiO_x offers a versatile, single-material solution for selective contacts in silicon solar cells.
• The selectivity mechanism is linked to controllable fixed charges within TiO_x.
• Interface engineering, including SiO_y formation, is key for high-performance passivation and charge extraction.