Noncentrosymmetric bulk crystals generate photocurrent without any bias voltage.One of the dominant mechanisms, shift current, comes from a quantum interference of electron wave functions being distinct from classical current caused by electrons' drift or diffusion. The dissipation-less nature of shift current, however, has not been fully verified presumably due to the premature understanding on the role of electrodes. Here we show that the photocurrent dramatically enhances by choosing electrodes with large work function for a p-type ferroelectric semiconductor SbSI. An optimized device shows a nearly constant zero-bias photocurrent despite significant variation in photocarrier mobility dependent on temperature, which could be a clear hallmark for the dissipation-less nature of shift current. Distinct from conventional photovoltaic devices, the shift current generator operates as a majority carrier device.The present study provides fundamental design principles for energy-harvesting and photo-detecting devices with novel architectures optimal for the shift current photovoltaic effect.

Lattice defect is a major cause of energy dissipation in conventional electric current due to the drift and diffusion motions of electrons. Different nature of current emerges when noncentrosymmetric materials are excited by light. This current, called the shift current, originates from the change in the Berry connection of electrons’ wave functions during the interband optical transition. Here, we demonstrate the defect tolerance of shift current using single crystals of ferroelectric semiconductor antimony sulfoiodide (SbSI). Although the dark conductance spreads over several orders of magnitude in each crystal due to the difference in the density of defect levels, the observed shift current converges to an identical value. We also reveal that the shift current is scarcely disturbed by the surface defects while they drastically suppress the conventional photocurrent. The defect tolerance is a manifestation of the topological nature of shift current, which will be a crucial advantage in optoelectronic applications.

We have investigated the photovoltaic action in a ferroelectric semiconductor SbSI with a potentiometry for a multiterminal device under local laser irradiation. The DC-voltage characteristics indicate that the device characteristics are understood to be equivalent to a series circuit composed of dark-resistances for the unirradiated parts and the parallel circuit of a current source and photo-resistance for the irradiated part. The results clearly guide us toward designing photovoltaic and photodetective devices based on ferroelectric materials.

Photoresponsive ferroelectrics are recently under intense study due to their potential application to photovoltaic devices. Antimony sulfoiodide (SbSI) is a prototypical compound that possesses both ferroelectricity and a strong visible-light-response. However, most of the SbSI films reported so far have a polycrystalline structure with a randomly oriented polarization axis. In this study, we have fabricated c-axis textured SbSI thin films through annealing of amorphous films deposited in a molecular beam epitaxy system, employing Sb2S3 and SbI3 sources. The fabricated films are highly uniform and have the polarization axis ordered vertical to the film plane. We have confirmed that the films show a strong visible-light-response and ferroelectricity in accord with bulk samples. These results will stimulate the development of photovoltaics employing narrow bandgap ferroelectric compounds.

The measuring technique of Raman gain efficiency along the transmission line composed of optical fibers and connectors based on the OTDR is proposed. It is a simple method of estimating Raman gain efficiency from the backscattered powers with and without pump power based on the OTDR. Experiments of Raman gain efficiency are carried out for the optical transmission line composed of four 3-km long single-mode fibers. By our present method, the Raman gain efficiency distribution is successfully estimated.