Researchers from the University of Tokyo, RIKEN Center for Emergent Matter Science and Tohoku University have demonstrated a gigantic bulk photovoltaic response in high-quality epitaxial thin films of the lead-free ferroelectric halide perovskite CsGeI₃.
Ferroelectric halide perovskites are attractive because they combine a built‑in electric polarization with strong visible‑light absorption. In such materials, light can generate current through the bulk photovoltaic effect, which does not require a conventional p–n junction. A key part of this effect is the “shift current”, a special kind of photocurrent that comes directly from the quantum‑mechanical motion of electrons when they absorb light. In this work, the team grew high‑quality epitaxial CsGeI₃ thin films using molecular beam epitaxy. The films show clear ferroelectric behavior and allowed the researchers to probe the intrinsic bulk photovoltaic response with minimal influence from defects. Under visible‑light illumination, they found that CsGeI₃ produces an exceptionally large shift current.
The measurements show several clear fingerprints of shift current. The photocurrent changes sign at different photon energies, and it strongly depends on the polarization of the incoming light. In addition, the current can be reversibly controlled by an external electric field, which switches the ferroelectric polarization and flips the direction of the photocurrent.
Remarkably, the normalized shift current magnitude in CsGeI₃ is more than an order of magnitude larger than previously reported values in other materials. This sets a new benchmark for bulk photovoltaic performance in a lead‑free perovskite. The results highlight ferroelectric halide perovskites as promising candidates for environmentally friendly solar cells, photodetectors and other optoelectronic devices that make direct use of quantum‑mechanical effects rather than relying only on conventional junction-based architectures.
