Bulk Photovoltaic Effect in Graphene-α-In₂Se₃-Graphene van der Waals Heterostructures

The photoelectric efficiency in p-n junction solar cells is fundamentally constrained by the Shockley-Queisser limit. Ferroelectrics surpass the limit because their lack of inversion symmetry leads to a bulk photovoltaic effect. Van der Waals ferroelectrics, such as α-In₂Se₃ have two features which make them compelling candidates for bulk photovoltaic solar cells. First, the band gap is 1.4 eV, close to the ideal bandgap for utilizing the solar spectrum, and much smaller than most ferroelectric oxides with bandgaps of 2.5-4 eV. Second, they naturally exist at nanoscale dimensions, with thicknesses 10-50 nm, well below the mean free path of hot carriers. The key goals are to characterize the domain structures of α-In₂Se₃ and understand how the ferroelectric domains and switching affects the performance of nanoscale solar cells utilizing the bulk photovoltaic effect in α-In₂Se₃.<br/>Here, we first use scanning transmission electron microscopy (STEM), and piezoelectric force microscopy (PFM) to characterize the domain configuration of α-In₂Se₃. We observe uniform domains of α-In₂Se₃ which possess a net dipole moment due to the crystal asymmetry leading to the out of plane ferroelectricity. In addition, we observe the existence of atomically sharp lateral charged domain walls, where the polarization of opposite direction meets to form head to head or tail to tail domain boundaries. Our PFM measurement indicates that the converse piezoelectric coefficient enhances by 800% in the charged domain wall region due to the electrostatic potential in the wall.<br/>Finally, we fabricate sandwiched graphene-α-In₂Se₃-graphene heterostructures, where the top and bottom graphene serve as the electrodes, to characterize and compare how the ferroelectric domains affect the device transport in dark, and under illumination with scanning photocurrent microscopy. In the absence of light, the current vs voltage curve shows an asymmetric resistance. The value of the resistance is 200 MΩ or 2GΩ as an electric field of 0.05 V/nm is applied along or against the polarization direction. Under the illumination of 532 nm laser, the heterostructures display an open circuit voltage of approximately 400 mV. The short circuit photocurrent density scales as intensity^α, where α = 0.73 and reaches a peak value of 0.8 mA/cm² under 1 W/cm² power. Moreover, we modulate the polarization of the material by sweeping from -0.1 V/nm to +0.1 V/nm and observe a modulation of zero-bias short circuit current from -0.5 nA to 1.5 nA. Both the short circuit current and open circuit voltage demonstrate considerable bulk photovoltaic effect of α-In₂Se₃. These values are similar to the ones observed in other 2D bulk photovoltaic material. Finally, we observe that the existence of charged domain walls enhances the photocurrent generation by more than an order of magnitude.<br/>Our study demonstrates the potential utilization of bulk photovoltaic effect in van der Waals α-In₂Se₃ to realize next generation solar cells.