Anomalous Photocurrent Reversal for the Same Polarization Direction in van der Waals Ferroelectric CuInP2S6

The bulk photovoltaic effect (BPVE) in ferroelectric materials has attracted much attention due to its above-band-gap photovoltage and switchable photoresponse. However, the exact correlation between the photocurrent and spontaneous polarization remains unclear. Here, we report an anomalous reversal of the photocurrent, even for the same polarization direction, in van der Waals (vdW) ferroelectric <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><a:msub><a:mrow><a:mi>Cu</a:mi><a:mi>In</a:mi><a:mi mathvariant="normal">P</a:mi></a:mrow><a:mn>2</a:mn></a:msub><a:msub><a:mrow><a:mrow><a:mi mathvariant="normal">S</a:mi></a:mrow></a:mrow><a:mn>6</a:mn></a:msub></a:math> (CIPS). Careful analysis reveals that the relocation of <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><f:mi>Cu</f:mi></f:math> ions into the vdW gap upon thickness reduction causes the current reversal, even though it does not change the polarization direction. By combining theoretical and experimental studies, we show clearly the decoupling of the BPVE photocurrent and electric polarization. This discovery significantly improves our understanding of the BPVE and demonstrates yet another example of the lattice dimensionality effect in vdW ferroelectrics.