Moving copper ions keep a junction-free photocurrent flowing in a crystal that looks symmetric

"Symmetric" here means symmetric on average. At room temperature the copper ions in CuCrP2S6 do not stay put. They move among several possible sites, and over longer periods their positions average out into a structure that looks symmetrical ^[12]. Diffraction measurements see that averaged structure as centrosymmetric ^[14]. The textbook view ties the bulk photovoltaic effect to crystals without that symmetry ^[3].

Electrons do not average. The copper ions move over picoseconds or longer, while the electronic response to light happens within femtoseconds ^[13]. Those orders of magnitude make the electrons at least about a thousand times quicker than the ions ^[16]. According to the team, electrons therefore respond to where the ions are at that instant, and these momentary asymmetric regions contribute to the current ^[13]. "Consequently, dynamically generated polar clusters can contribute to the shift-current response even though the macroscopic symmetry appears centrosymmetric in diffraction measurements," Sasagawa said ^[14].

The crystal supplies its own control. When cold, its copper ions sit slightly displaced inside sulfur octahedra and the structure lacks inversion symmetry ^[8]. As it warms, the ions grow mobile, pass through a dynamically disordered state between about 145 and 190 K, and leave a centrosymmetric average structure at room temperature ^[8]. A single sample measured from 15 to 300 K covers both regimes. Under laser light the current flowed only along the polar a-axis across that whole range ^[10], in a pattern that matched shift current, a bulk photovoltaic mechanism usually seen in asymmetric materials ^[11]. The team also tested silver and platinum electrodes and varied the light's polarization, power and wavelength ^[9].

The surprise was the direction of the change. "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," Sasagawa said ^[7]. The current is larger in the phase where the ions move most ^[7][8]. The researchers attribute the effect to that motion ^[12]. The evidence for the ion picture is a correlation across a phase transition plus a timing argument. It is a good argument. It is still an inference.

Conventional cells separate charge carriers at structures such as p-n junctions and face the Shockley-Queisser limit, about 33% for an ideal single-junction silicon cell ^[4]. The bulk photovoltaic effect is not subject to that same limit ^[5]. The findings challenge the idea that strong BPVE needs a permanently noncentrosymmetric structure, and show that temporary, local symmetry breaking can add to the effect ^[15]. I think that does widen the candidate list. Crystals that diffraction calls centrosymmetric but whose ions move are now worth testing, and the authors describe the result as a new strategy for improving photoelectric conversion ^[17].

The phys.org account does not report a power conversion efficiency or an absolute current density, and the measurements used laser illumination on one material ^[10]. The 1.5-fold figure compares CCPS with its own cold phase ^[7]. Comparing it with a silicon cell would take a different experiment.