Next-generation solar power: Japanese researchers achieve BPVE breakthrough

In a conventional solar cell, two differently doped semiconductor layers form a p–n junction. At their interface, an internal electric field develops. When light strikes the cell, it generates electrons and their positively charged counterparts. The electric field rapidly drives them in opposite directions, creating a flow of current. However, this fundamental design comes with a built-in physical limit to voltage and efficiency – known as the Shockley–Queisser limit. In practical terms, even under ideal sunlight conditions, only about one-third of the light’s energy can be converted into electricity.

This is where the bulk photovoltaic effect (BPVE) comes into play. Unlike traditional solar cells, it doesn't rely on a p-n junction or an internal electric field. Instead, it harnesses the unique atomic structure of certain crystals that lack mirror symmetry. The effect arises when two symmetries are broken simultaneously: First, spatial mirror symmetry must be absent, allowing the asymmetric atomic arrangement to push electrons preferentially in one direction when exposed to light. Second, time-reversal symmetry must be broken by a magnetic material, so that forward and backward electron movements are no longer equivalent. When both conditions are met, light alone can generate a current – without a junction and beyond the Shockley–Queisser limit.

Kyoto researchers achieve BPVE breakthrough – solar cells controllable via magnetism

A research team at Kyoto University, led by physicist Kazunari Matsuda, has for the first time developed a solar cell without a conventional p–n junction, where both critical conditions are simultaneously met:

1. A single, atomically thin semiconductor layer ensures that the material lacks mirror symmetry.
2. An underlying magnetic crystal further breaks time-reversal symmetry.

Kyoto University announced the breakthrough on June 24. This allows the bulk photovoltaic effect (BPVE) to fully take hold: light directly drives electrons in one direction, generating current without the need for an internal electric field. The magnetic crystal functions like a finely adjustable control knob – applying an external magnetic field can switch the current on or off, or modulate its strength. In theory, solar cells based on the BPVE could harness more energy from sunlight while being ultra-thin, flexible and even tunable via magnetic fields.

The eight-page study, published in Nature Communications, is freely available online. While Kyoto University has not provided a timeline for commercialization, the technology remains in the early stages of development. Still, there are potential applications that could emerge in the near future – not just in energy generation, but also in sensor technology. For instance, ultra-thin BPVE films could serve as self-powered “mini power plants” on labels, wearables or environmental monitoring devices. These films wouldn’t just power temperature, humidity or motion sensors; their magnetic tunability could also enable the detection of light intensity, magnetic fields and even light polarization – all within a single, nearly invisible layer.