Towards better solar cells: Exploring an anomalous phenomenon of electricity generation

The bulk photovoltaic (BPV) effect is an uncommon phenomenon that may enable certain materials to outperform the conventional p–n junctions used in solar cells. In a recent study, researchers from Japan have experimentally demonstrated the BPV effect in alpha-phase indium selenide (α-In₂Se₃) for the first time along the out-of-plane direction, validating previous theoretical predictions. The remarkable conversion efficiency recorded in their α-In₂Se₃ device signals a promising advancement for future solar cell technologies and photosensors.

A firm understanding of the photovoltaic effect, by which light can be converted into useful electrical energy, lies at the core of solar cell design and development. Today, most solar cells employ p–n junctions, leveraging the photovoltaic effect that occurs at the interface of different materials. However, such designs are constrained by the Shockley–Queisser limit, which puts a hard cap on their theoretical maximum solar conversion efficiency and imposes a tradeoff between the voltage and current that can be produced via the photovoltaic effect.

However, certain crystalline materials exhibit an intriguing phenomenon known as the bulk photovoltaic (BPV) effect. In materials lacking internal symmetry, electrons excited by light can move coherently in a specific direction instead of returning to their original positions. This results in what is known as “shift currents,” leading to the generation of the BPV effect. Although experts have predicted alpha-phase indium selenide (α-In₂Se₃) to be a possible candidate to demonstrate this phenomenon, it hasn’t yet been experimentally investigated.

To fill this knowledge gap, a research team from Japan led by Associate Professor Noriyuki Urakami from Shinshu University set out to explore the BPV effect in α-In₂Se_3. Their findings were published in Volume 125, Issue 7 of Applied Physics Letters on August 12, 2024 and made available online on August 14, 2024.

“This material has recently become a hot topic in the field of condensed matter physics, as it might be able to generate a shift current. Our study is the first to experimentally demonstrate this prediction,” shares Prof. Urakami.

First, the researchers produced a layered device composed of a thin α-In₂Se₃ layer sandwiched between two transparent graphite layers. These graphite layers served as electrodes and were connected to a voltage source and an ammeter to measure any generated currents upon light irradiation. Notably, the team employed this specific arrangement of layers because they focused on the shift currents occurring in the out-of-plane direction in the α-In₂Se₃ layer.

After testing with different external voltages and incident light of various frequencies, the researchers verified the existence of shift currents in the out-of-plane direction, confirming the abovementioned predictions. The BPV effect occurred throughout a wide range of light frequencies.

Most importantly, the researchers gauged the potential of the BPV effect in α-In₂Se₃ and compared it to that in other materials. “Our α-In₂Se₃ device demonstrated a quantum efficiency several orders of magnitude higher than other ferroelectric materials, and a comparable one to that of low-dimensional materials with enhanced electric polarization,” remarks Prof. Urakami. He further adds, “This discovery will guide material selection for the development of functional photovoltaic devices in the near future.”

The research team is hopeful that their efforts will eventually have a positive environmental impact by contributing to the field of renewable energy generation. “Our findings have the potential to further accelerate the spread of solar cells, one of the key technologies for environmental energy harvesting and a promising avenue towards a carbon neutral society,” concludes a hopeful Prof. Urakami.

We hope that this study paves the way for further studies to harness the BVP effect and vastly improve the performance of solar cells, as well as enhance the design of sensitive photodetectors.

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About Shinshu University

Shinshu University is a national university founded in 1949 and located nestling under the Japanese Alps in Nagano known for its stunning natural landscapes. Our motto, "Powered by Nature - strengthening our network with society and applying nature to create innovative solutions for a better tomorrow" reflects the mission of fostering promising creative professionals and deepening the collaborative relationship with local communities, which leads to our contribution to regional development by innovation in various fields. We’re working on providing solutions for building a sustainable society through interdisciplinary research fields: material science (carbon, fiber and composites), biomedical science (for intractable diseases and preventive medicine) and mountain science, and aiming to boost research and innovation capability through collaborative projects with distinguished researchers from the world. For more information visit https://www.shinshu-u.ac.jp/english/ or follow us on X (Twitter) @ShinshuUni for our latest news.