The bulk photovoltaic effect (BPVE), a photoelectric effect that generates photocurrent without a p–n junction, can persist even when a material’s average crystal structure remains centrosymmetric, a study from Institute of Science Tokyo has found. Researchers demonstrated this in CuCrP 2 S 6 , a van der Waals material that transitions from a non-centrosymmetric to a centrosymmetric average structure. The finding challenges the conventional view of BPVE and suggests a new strategy for enhancing photoelectric conversion.
Conventional solar cells generate photocurrent by separating and transporting light-generated charge carriers, typically through structures such as p–n junctions. This approach has a fundamental theoretical efficiency limit, known as the Shockley–Queisser limit. For an ideal single-junction silicon solar cell, this limit is about 33%.
The bulk photovoltaic effect (BPVE) offers an alternative way of generating photocurrent that is not subject to the same Shockley–Queisser limit. In BPVE, light interacts with a material to generate photocurrent without requiring a p–n junction. BPVE has traditionally been associated with non-centrosymmetric crystal structures.
A team of researchers led by former graduate student Ryoga Murata and Associate Professor Takao Sasagawa from Institute of Science Tokyo (Science Tokyo), Japan, have now demonstrated BPVE in a material whose time-averaged crystal structure becomes centrosymmetric at room temperature. They found that dynamic changes in atomic positions can sustain BPVE even when the material appears symmetrical over longer timescales. The study was made available online on 09 August 2026 and published in Volume36, Issue73 of the journal Advanced Functional Materials on 10 September 2026.
“According to conventional theory, the BPVE should vanish in the room-temperature phase because inversion symmetry is restored. Surprisingly, our experiments revealed that a clear zero-bias photocurrent persists at room temperature and is even enhanced to approximately 1.5 times the magnitude observed in the low-temperature phase,” says Sasagawa.
The effect was observed in copper chromium thiophosphate (CuCrP 2 S 6 ) (CCPS) crystals, a layered van der Waals material. CCPS transitions from a non-centrosymmetric structure at low temperatures to a centrosymmetric structure at higher temperatures. At low temperatures, the copper ions (Cu + ) occupy slightly displaced positions within sulfur octahedra. As the temperature increases, the Cu + ions become increasingly mobile and move between different positions in the crystal. Between approximately 145 and 190 K, the material enters a dynamically disordered state. At room temperature, its average crystal structure becomes centrosymmetric.
The researchers measured photocurrents in CCPS crystals using different electrode materials (silver and platinum) under various light conditions (polarization, power, and wavelength). Under laser illumination, they observed a photocurrent exclusively along the polar a-axis that persisted across the entire temperature range from 15 to 300 K. The observed photocurrent pattern matched the behavior typically associated with shift current, a BPVE mechanism usually seen in materials with asymmetric structures.
The researchers attributed this unexpected behavior to the dynamic motion of Cu + ions within the crystal lattice. At room temperature, these ions do not remain in fixed positions but fluctuate among several possible sites. Over a longer period, the positions of the Cu + ions average out, making the crystal appear symmetrical. However, the ions move much more slowly, over timescales of picoseconds (10 -12 s) or longer, than the electronic response to light, which occurs within femtoseconds (10 -15 s). Consequently, electrons respond to the instantaneous positions of the Cu + ions rather than their time-averaged positions, allowing these momentary asymmetric regions to contribute to the BPVE.
“Consequently, dynamically generated polar clusters can contribute to the shift-current response even though the macroscopic symmetry appears centrosymmetric in diffraction measurements,” explains Sasagawa.
The findings challenge the conventional view that strong BPVE requires a permanently non-centrosymmetric structure. Instead, they show that temporary, local symmetry breaking caused by atomic motion can contribute to and even enhance the effect.
These findings suggest that dynamic atomic motion could be harnessed to enhance photoelectric conversion. “The work suggests a new materials-design philosophy: rather than suppressing atomic motion, dynamic fluctuations can be deliberately utilized to strengthen photoelectric conversion. This concept may open new avenues for the development of next-generation photovoltaic materials, self-powered optoelectronic devices, and energy-harvesting technologies based on van der Waals materials,” says Sasagawa.
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About Institute of Science Tokyo (Science Tokyo)
Institute of Science Tokyo (Science Tokyo) was established on October 1, 2024, following the merger between Tokyo Medical and Dental University (TMDU) and Tokyo Institute of Technology (Tokyo Tech), with the mission of “Advancing science and human wellbeing to create value for and with society.”
Advanced Functional Materials
Experimental study
Not applicable
Unusual Temperature-Enhanced Bulk Photovoltaic Effect Beyond the Centrosymmetric Phase in CuCrP2S6 van der Waals Crystals
10-Sep-2026