Giant photocurrent response observed in lead-free perovskite within the visible light spectrum

A joint research group including Trainee Koma Miki (at the time of the research, currently a graduate student at the Graduate School of Engineering, the University of Tokyo), Senior Visiting Scientist Masao Nakamura (at the time of the research, currently a professor at the Graduate School of Science, Tohoku University), and Group Director Masashi Kawasaki (Professor at the Graduate School of Engineering, the University of Tokyo) of the Strong Correlation Interface Research Group at the RIKEN Center for Emergent Matter Science; along with Group Director Naoki Ogawa of the Emergent Photodynamics Research Group; Group Director Yoshinori Tokura (Distinguished Professor at the University of Tokyo) of the Strong Correlation Physics Research Group; and Team Director Satoshi Okamoto (Research Planning Manager, Sumitomo Chemical Corporate Research Laboratories) of the Correlated-materials-based Ecological-device Laboratory at the Transformative Research Innovation Platform of RIKEN (TRIP) Headquarters, has successfully observed a giant photocurrent response within the visible light spectrum using ferroelectric, lead-free halide perovskite thin films. This breakthrough is expected to accelerate the development of highly environmentally friendly, next-generation photoelectric conversion materials. The findings were published in the online edition of PNAS.

Perovskite solar cells are garnering significant anticipation as a next-generation photovoltaic technology originating from Japan. Although materials containing lead (Pb) are currently the primary focus of research, their environmental impact and toxicity to the human body remain major concerns for practical applications. Consequently, research into lead-free alternatives has been advancing, with germanium (Ge)-based halide perovskites showing particularly excellent ferroelectric properties.

In materials where spatial inversion symmetry is broken, such as ferroelectrics, a "shift current" is generated due to the quantum geometric effects of electronic wave functions. The shift current is a photovoltaic force that generates without requiring the conventional p-n junctions used in standard solar cells. Because it is less susceptible to scattering caused by defects and impurities and exhibits an ultra-fast response, it is highly anticipated as a novel photoelectric conversion principle that could enhance the performance of solar cells and photodetectors.

Among these materials, cesium germanium iodide (CsGeI₃) possesses large ferroelectric polarization and a bandgap well-suited for absorbing sunlight, leading to expectations for the generation of substantial shift currents. However, fabricating high-quality thin films of CsGeI₃ with excellent crystallinity and uniformity has been difficult using conventional solution-based methods, leaving its photoelectric properties largely unclarified until now.

The joint research group had previously developed a proprietary molecular beam epitaxy system optimized for the growth of halide thin films. Utilizing this apparatus, they succeeded for the first time in fabricating high-quality epitaxial thin films of CsGeI₃ with precisely aligned crystal orientations.

The researchers illuminated the fabricated thin-film samples and measured the photocurrent generated under zero external voltage (unbiased photocurrent). The photocurrent began to rise from approximately 1.6 eV, corresponding to the bandgap of CsGeI₃, inverted its sign from positive to negative around 2.9 eV, and subsequently exhibited a negative peak near 3.0 eV. This type of photocurrent sign inversion cannot be explained by conventional photocurrents induced by electric fields near electrodes, making it a characteristic behavior unique to shift currents.

Furthermore, when comparing these experimental results with the shift current spectrum obtained through first-principles calculations in prior studies, the observed characteristics, including the sign inversion and the negative peaks, aligned well. These findings conclusively demonstrate that the observed photocurrent is indeed a shift current.

The team also confirmed that when an electric field was applied to the thin film to control the direction of the ferroelectric polarization, the magnitude of the unbiased photocurrent changed reversibly in accordance with the direction of the electric field. This further substantiates that the observed photocurrent is a shift current tightly coupled with the ferroelectric polarization of CsGeI₃.

Additionally, when comparing the shift current response observed in the CsGeI₃ thin film with representative materials reported in the past, its figure of merit within the visible light spectrum was found to outperform existing published values by more than an order of magnitude. This indicates that CsGeI₃ possesses an exceptionally high capacity for shift current generation, establishing ferroelectric halide perovskites as a highly potent candidate for next-generation photoelectric conversion materials.

Moving forward, precise control over the thin film's crystallinity, strain, and ferroelectric domain structures is expected to yield even greater enhancements in shift currents and finer control of photocurrents via external electric fields. Furthermore, as a lead-free, environmentally friendly material, it holds great promise for applications in next-generation solar cells, high-speed photodetectors for next-generation communications, terahertz-band high-speed photoelectric conversion devices, and nonlinear optical elements.

Journal Information
Publication: PNAS
Title: Record-high Glass coefficient in the shift current response of a ferroelectric halide perovskite
DOI: 10.1073/pnas.2602252123