Anomalous photovoltaic effect originates from shift current under interband excitation in ferroelectrics

Shift current plays a critical role in the anomalous photovoltaic effect (APE) in ferroelectric semiconductors. This is apparent during studies of ultrafast photocurrent dynamics using terahertz (THz) emission spectroscopy, but it is unclear exactly how shift-current dynamics correlate to APE in ferroelectrics.

New results by Sotome et al. describe the relationship between shift currents and APE in tin hypothiodiphosphate (Sn₂P₂S₆) and provide new insights into advancing photovoltaic technology.

By zapping the ferroelectric material with sub-picosecond lasers and analyzing the resulting waveform and spectrum using THz spectroscopy, the researchers discovered that the ultrafast photocurrent originates from the shift current under intraband excitation.

Shift currents are steady-state photocurrents created in certain materials when electrons shift during photon absorption. Their results indicate that these electron shifts are a driving force in charge transfer and carrier separation that enables APE, which occurs when the photovoltage is much larger than the band gap of the corresponding material.

Since the transition temperature of Sn₂P₂S₆ is close to room temperature with a band gap in the visible spectrum, the researchers expect the material to become a benchmark model for studying other ferroelectric semiconductors. Furthermore, THz emission spectroscopy can be applied to study ferroelectric-based photovoltaic materials or solar cells.

“Sn₂P₂S₆ can be used as a testbed material to unveil the universality of shift excitation in ferroelectric semiconductors,” said Sotome. “Our study also shows the potency of THz emission spectroscopy in investigating ferroelectric dynamics that have not been studied before, such as the waveform and spectrum.”

Source: “Ultrafast spectroscopy of shift-current in ferroelectric semiconductor Sn₂P₂S₆,” by M. Sotome, M. Nakamura, J. Fujioka, M. Ogino, Y. Kaneko, T. Morimoto, Y. Zhang, M. Kawasaki, N. Nagaosa, Y. Tokura, and N. Ogawa, Applied Physics Letters (2019). The article can be accessed at https://doi.org/10.1063/1.5087960 .