The bulk photovoltaic effect (BPVE) offers a promising pathway for light energy harvesting, potentially exceeding the Shockley-Queisser efficiency limit. Unlike conventional photovoltaic cells relying on pn junctions, BPVE generates photocurrent and photovoltage within a single material without a built-in electric field. However, BPVE traditionally requires materials lacking inversion symmetry, excluding centrosymmetric materials. This research explores the possibility of inducing BPVE in centrosymmetric materials by breaking inversion symmetry through structural disorder. Previous attempts to induce BPVE in centrosymmetric van der Waals (vdW) materials involved strain gradients, strain-induced polarization, or reduced dimensionality, but these methods have limitations in practical applications due to challenges in device fabrication and the need for large external fields. Introducing structural disorder, specifically point defects, provides a potentially simpler and more practical approach. While defects can locally break inversion symmetry and enable second-harmonic generation (SHG) in centrosymmetric materials, the possibility of inducing BPVE through defects remained experimentally unproven. This study uses PtSe₂, a centrosymmetric semiconducting vdW material, to investigate this possibility, aiming to demonstrate defect-induced BPVE and expand the range of materials suitable for photovoltaic applications.

The literature extensively discusses the BPVE in non-centrosymmetric materials and its potential advantages over conventional photovoltaic cells. Several studies have explored ways to induce BPVE in centrosymmetric materials, primarily focusing on strain engineering or external electric fields. The impact of defects on the optical properties of 2D materials, particularly the induction of SHG in centrosymmetric materials, has also been reported. However, a clear experimental demonstration of defect-induced BPVE in centrosymmetric materials was lacking before this study. The unique properties of PtSe₂, including its centrosymmetry and semiconducting nature in few-layer forms, make it an ideal candidate for investigating this phenomenon. Previous research has highlighted the role of point defects like Se and Pt vacancies in modifying the electronic structure of PtSe₂, leading to phenomena like spin-orbit splitting and Rashba interaction, which are relevant to the BPVE. This prior work lays the groundwork for understanding the potential connection between defects and BPVE in PtSe₂.

Ultrathin PtSe₂ samples were prepared using two methods: regular tape exfoliation (RE) and Au-assisted exfoliation (AE). AE, known for producing large-area ultrathin crystals, was hypothesized to introduce more defects. Raman spectroscopy and aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) were employed to characterize the structural quality and defect density of the samples. Raman spectroscopy revealed a significant reduction in the intensity and redshift of the E_g⁰ peak in AE samples, indicating a higher defect concentration, possibly due to Se vacancies (V_Se). HAADF-STEM directly confirmed the presence of Se vacancies and cluster-like defects. Polarization-controlled scanning photovoltage microscopy was used to investigate the photoresponse of both pristine (RE) and defective (AE) bilayer PtSe₂ samples. Multi-terminal devices were fabricated for measuring photovoltage under homogenous illumination. The light helicity was controlled using a quarter-wave plate, allowing for the investigation of both LPGE and CPGE. A phenomenological expression was used to fit the photovoltage dependence on polarization angle. First-principles calculations, combining density functional theory (DFT), maximally localized Wannier functions (MLWF), and the non-equilibrium Green's function (NEGF) formalism, were performed to simulate light-matter interactions in bilayer PtSe₂ with Se vacancies. The simulations investigated the spatial distribution of photocurrent, its electron and hole components, and the impact of Se vacancies on the band structure. A wedge-shaped potential model was used to understand the scattering of carriers by Se vacancies. Finally, a device with an array of circularly oriented electrodes was fabricated to determine the crystallographic orientation of PtSe₂ using LPGE and compare it with AFM measurements.

The AE method produced PtSe₂ samples with significantly higher defect concentrations, as evidenced by Raman spectroscopy and HAADF-STEM. Pristine PtSe₂ exhibited a weak photovoltaic response primarily originating from the electrode/PtSe₂ interface. In contrast, defective PtSe₂ showed a strong spontaneous photoresponse under homogenous illumination, generating both LPGE and CPGE. The photovoltage in defective PtSe₂ was maximal when the laser spot was centered between the electrodes, indicating that the photovoltage originated from the bulk material itself and not from the interfaces. The amplitudes of both LPGE and CPGE were substantially larger in defective PtSe₂ compared to pristine PtSe₂. First-principles calculations showed that the presence of Se vacancies breaks the electron-hole symmetry, resulting in a net photocurrent. Se vacancies act as asymmetric trigonal scattering centers, reflecting holes more effectively than electrons, leading to an anisotropic net current flow. A model based on wedge-shaped potentials at Se vacancy sites accurately describes the LPGE. By analyzing the polarization dependence of LPGE from a device with an array of electrodes, the crystallographic orientation of PtSe₂ was determined, providing strong experimental evidence that Se vacancies are the source of LPGE. This is supported by the close match between optically and electrically determined edge directions (around 5° difference).

The findings demonstrate for the first time that structural disorder, specifically Se vacancies, can induce BPVE in a centrosymmetric material. The observation of both LPGE and CPGE in defective PtSe₂ highlights the complexity of defect-induced BPVE. The strong LPGE, explained by the asymmetric scattering of holes at Se vacancies, opens exciting possibilities for designing photovoltaic devices based on centrosymmetric materials. The ability to determine crystallographic orientation using LPGE provides a novel technique for characterizing vdW materials. The close agreement between experimental results and first-principles calculations strongly supports the proposed mechanism of defect-induced BPVE. The absence of a substantial interfacial photovoltaic response in defective PtSe₂, likely due to Fermi level pinning and defect states, further supports the conclusion that the observed photoresponse originates from the bulk BPVE.

This study successfully demonstrates the induction of BPVE in the centrosymmetric material PtSe₂ through the introduction of structural disorder. Se vacancies act as asymmetric scattering centers, resulting in a significant LPGE, while cluster-like defects contribute to CPGE. First-principles calculations support these experimental findings, and a wedge-shaped potential model accurately describes the scattering at Se vacancies. Furthermore, the LPGE provides a novel method for determining the crystallographic orientation of vdW materials. Future research could explore other centrosymmetric materials with similar defect engineering strategies to further broaden the range of materials applicable for BPVE-based devices and investigate the impact of different types and concentrations of defects on the BPVE.

The study primarily focuses on bilayer PtSe₂. The extent to which these findings generalize to other thicknesses or materials needs further investigation. The specific types and distribution of defects induced by the Au-assisted exfoliation method may influence the results, and a more controlled introduction of defects would enhance the understanding of the relationship between defect characteristics and BPVE. The phenomenological model for LPGE simplifies the complex interactions, and a more detailed theoretical framework may be required for a complete description of the effect. The first principles calculations were performed on a simplified model of the defective material, neglecting the complex interactions among various defects present in the experimental samples.