Depolarization Field Driven Photovoltaic Effect in 2D Ferroelectric α-In₂Se₃

The ultimate limit of bulk photovoltaic energy conversion efficiency in ferroelectrics at the nanoscale depends on the mechanism of electron and hole separation. Understanding the underlying mechanism will enable new design principles for utilizing the bulk photovoltaic effect (BPVE) in next generation optoelectronic devices, such as self-powered photodetectors or solar cells. In contrast to conventional ferroelectrics, 2D ferroelectric α-In₂Se₃ has two advantages for BPVE based solar cells. First, the band gap is 1.3 eV, close to the ideal value for utilizing the solar spectrum, and much smaller than most wide bandgap ferroelectric oxides. Second, they naturally exist at nanoscale dimensions with stable ferroelectricity down to a monolayer. The key goals are to characterize the thickness dependent BPVE in α-In₂Se₃ and understand the dominant mechanism.<br/>Here, we fabricate graphene-α-In₂Se₃-graphene heterostructures, where the thickness of α-In₂Se₃ varies between 18-50 nm, and characterize the transport under illumination. We use scanning photocurrent and photovoltage microscopy to map the short-circuit current density (<i>J_sc</i>) and open-circuit voltage (<i>V_oc</i>). We also measure the transport under varying intensities.<br/>The photocurrent and photovoltage maps show that both the <i>J_sc</i> and <i>V_oc</i> prevails only in the region overlapped by the top and bottom graphene. The<i> J_sc</i> is antiparallel to polarization. All these observations confirm the BPVE in the heterostructures. The transport measurements under illumination show that the <i>J_sc</i> and efficiency decay exponentially with thickness. The direction of the photocurrent and its exponential decay with thickness confirm the depolarization field as the origin of the photovoltaic effect in α-In₂Se₃. We also find that the depolarization field is inversely proportional to thickness and reaches 158 KV/cm for 18 nm α-In₂Se₃. Moreover, the depolarization field model allows us to predict a nonmonotonic trend of photoconductivity with thickness where it is expected to peak for 6 nm thick α-In₂Se₃. The transport measurements depict that <i>J_sc</i> ∝ Intensity<i>^β</i> where <i>β </i>varies from 0.6 to 1.2 with increasing thickness<i>. </i>The photoresponsivity reaches 1.8 mA/W under 1 W/cm² intensity for 18 nm α-In₂Se₃. This is comparable to other 2D ferroelectrics and almost 4 to 2200 times higher than conventional bulk ones.<br/>Our study demonstrates the relative importance of depolarization field in 2D ferroelectrics over other mechanisms, such as shift current or Schottky barrier effect, and provides design principles for optimizing the efficiency of BPVE based energy harvesting.