Titanium Hyperdoping of Silicon Solar Cells via Nanosecond-Plused Laser Melting and the Key Factors Affecting Their Performance

Hyperdoped Si has an impurity band, which lifts the limitation imposed by the intrinsic bandgap of Si regarding the absorption wavelength of the solar spectrum and can improve the power conversion efficiency (PCE) of monocrystalline Si (c-Si) solar cells. Nanosecond-pulsed laser melting (NPLM) is a suitable, inexpensive method to prepare hyperdoped Si solar cells with a high efficiency. However, the key factors affecting the generation and collection of carriers in these solar cells are unclear. Here, Ti hyperdoping of Si films on a c-Si solar cell substrate with a passivated emitter and rear cell (PERC) structure was achieved via the combination of electron-beam deposition and NPLM in air or low vacuum. The performance of the Ti-hyperdoped Si (Si:Ti) solar cells was studied to evaluate several factors such as Si:Ti film crystallinity, thickness, and O impurities introduced by NPLM. The results show that the crystallinity degree and thickness of the Si:Ti films determine the electrical activation rate of Ti impurities (or sub-bandgap optical absorption) and carrier transport in the Si:Ti films, respectively. The rough surface of the pyramidal microstructures due to the Si:Ti films enhances the intrinsic optical absorption of the Si:Ti solar cells. However, the O impurities preferably bind to Si and do not affect the concentration of the electrically activated Ti impurities. Thus, the Si:Ti solar cell prepared via NPLM in air achieved a high PCE of 19.4%, a 2.1% increase over the bare c-Si solar cell substrate (17.3%). The short-circuit current density and external quantum efficiency measurements indicate that such a PCE increase is caused by an increase in the intrinsic and sub-bandgap photocurrents. Since hyperdoped films are not restricted to solar cell substrates, selecting higher efficiency substrates can achieve further efficiency improvements as well as reduce the cost of existing commercial c-Si solar cells.