Polarization-modulated programmable photovoltaic performance of a designed ferroelectric heterojunction

Domain: neuroscience
Published: 2026-01-28
Editorial outcome: major_revision

Abstract
Conventional photovoltaic devices based on PN or Schottky junctions are inherently constrained by the material bandgap, resulting in limited photoelectric conversion efficiency. The van der Waals ferroelectric material CuInP2S6 offers strong potential for efficient photocurrent generation via bulk photovoltaic effect, while heterostructure engineering provides additional degrees of freedom for performance modulation. Yet, the specificity in asymmetric CuInP2S6 heterojunctions remains poorly understood. Here, we design and fabricate an asymmetric photovoltaic device, consisting of Pt/CuInP2S6/Graphene heterostructure. The ferroelectric photovoltaic effect couples with the Cu+ migration dynamics, which could be manipulated by asymmetric interface barriers. Remarkably, the photovoltaic current is increased by 10 times and exhibits a positive-negative switching through polarization modulation. The heterojunction device achieves on-demand photovoltaic programming for in-sensor computing tasks, including edge detection with recognition rate F-score~1 and binary pattern classification with 100% accuracy, which provides new insights for next-generation visual technology.

Introduction
The development of a neuromorphic visual system demands photo- electric sensors that are ultrafast, high-responsivity, and reconfigurable for dynamic information processing1–3. In recent years, diverse reconfigurable optoelectronic devices have been developed to detect optical stimuli and process image information in parallel4–8, offering significant opportunities for enhanced image processing capabilities. However, conventional photovoltaic devices based on PN and Schottky junctions remain fundamentally limited by the material bandgap-manifested in the Shockley–Queisser limit and by carrier recombination losses9, which together constrain their photoelectric conversion efficiency. The limitations underscore the need for alternative material platforms capable of breaking bandgap restrictions and achieving superior photoelectric conversion performance.

The bulk photovoltaic effect (BPVE) of two-dimensional (2D) van der Waals (vdW) ferroelectric materials enables the separation of photogenerated carriers without a built-in electric field required by conventional PN or type II heterojunctions. This intrinsic capability can surpass the bandgap-limited photovoltage of traditional semiconductor devices, generating outputs well beyond the theoretical threshold10–15. Moreover, the spontaneous polarization and ultrafast switching dynamics of ferroelectric materials can emulate synaptic plasticity in biological vision systems, facilitating highly efficient and intelligent neuromorphic visual perception and processing systems16.

Over the past decade, various 2D ferroelectric materials have exhibited outstanding photovoltaic performances17–19. Among them, CuInP2S6 (CIPS) stands out as a promising candidate due to its room-temperature ferroelectricity and unique long-range Cu+ ions migration20–24, which significantly enhances the BPVE16,25. Beyond its intrinsic photovoltaic properties of CIPS, constructing asymmetric heterojunction interfaces offers a powerful route to regulate the interface potential barrier, leading to distinct photovoltaic behaviors and regulatory mechanisms. Such tunability not only deepens the mechanistic understanding of photovoltaic technologies, but also paves a new way for the design and optimization of multi-state photoelectronic applications. In particular, CuInP2S6/Graphene (CIPS/Gr) heterostructure exhibit intriguing properties, such as large interface barrier tunability and high carrier-regulation capability26,27, which indicates great application prospects in photovoltaic applications. Recently, theoretical calculations show that this heterostructure exhibit exceptional ferroelectric polarization-adjustable photovoltaic properties28. However, experimental evidence directly linking ferroelectric polarization states to photovoltaic performance in asymmetric CIPS heterostructures is still lacking, and the microscopic mechanisms governing polarization–carrier transport coupling remain to be elucidated.

In this study, the ferroelectric photovoltaic (FE-PV) characteristics of an asymmetric Pt/CuInP2S6/Graphene (Pt/CIPS/Gr) heterostructure are systematically investigated. The polarization modulation enhances the photovoltaic current by approximately ten times with a low light intensity. The device exhibits a remarkable voltage polarity-dependent bipolar switching behavior driven by residual polarization. Combining the experimental results with density functional theory (DFT) calculations, we have proposed a physical mechanism based on the synergistic interaction among Cu+ ions, interface barriers, and photogenerated charge carriers. Cu+ interlayer migration within vdW gaps could regulate photovoltaic performance, thereby inducing the programmable and non-volatile optoelectronic response. Leveraging this capability, the device performs in-sensor computing tasks-including edge detection with recognition rate F-score~1 and binary pattern classification with 100% accuracy—by directly mapping polarization states to synaptic weights. These findings not only elucidate the FE-PV physics of CIPS-based heterojunctions but also establish a versatile platform for next-generation functional neuromorphic-vision systems.

Results
Schematic representation and characterization of FE-PV heterojunction
The designed asymmetric FE-PV heterojunction device features a simple two-terminal structure. A 25nm Pt bottom electrode was deposited on a SiO2 (300nm)/Si substrate via magnetron sputtering, followed by sequential dry transfer of graphene and CIPS to form a vertical Pt/CIPS/Gr structure, as illustrated in Fig. 1a and Supplementary Fig. 1. Structural and optical characterization of CIPS and graphene confirms their suitability for subsequent photoresponse experiments (Supplementary Figs. 2–4). The photovoltaic effect of the device is modulated synergistically by laser illumination and a Vp applied to the bottom Pt electrode. This device is fabricated and applied to emulate the functionality of the human visual system, which could enable bio-inspired operation analogous to retinal photoreceptors. It could convert optical stimuli into neural impulses and realize preprocessing and temporary storage before relaying visual information to the cerebral cortex (Fig. 1b). As the key factor of a FE-PV device, the robust out-of-plane ferroelectricity of CIPS is confirmed by Piezoelectric force microscopy (PFM) measurements (Supplementary Figs. 5 and 6). Figure 1c indicates that the asymmetric interfacial barrier could be modulated by ferroelectric polarization, resulting in a nonlinear dependence of photogenerated carriers. This results in a nonlinear relationship between the scanning voltage and the photocurrent, thereby providing a more comprehensive dataset for image edge detection. As shown in Fig. 1d, the anomalous switching effect in the Vp pulse dependence of short-circuit current (Isc) is observed. It indicates the positive and negative inversion as increasing unidirectional positive polarization, forming the basis for in-device functions such as the binary pattern classification in brain-like neural computing.

Regulated FE-PV performance by ferroelectric polarization in heterojunction
To characterize the photovoltaic performance of the device, low-power-density laser irradiation was employed. The intrinsic photoelectric and photovoltaic properties of heterojunction devices were systematically investigated. As shown in Fig. 2a, the photocurrent increases steadily with optical power density from 0 to 2.516 mW/cm2. Under pulsed illumination at 0.505 mW/cm2, the device demonstrates stable photovoltaic sensitivity with a prominent photocurrent response Ilight/Idark ratio of ~900 (Fig. 2b). As shown in Supplementary Fig. 7, the device response remains stable over a testing period of 1000 seconds, which corresponds to 2500 on/off cycles. To systematically characterize neuromorphic-vision characteristics, we have performed measurements of the write kinetics, transient response, read non-destructiveness and −3dB bandwidth, and noise spectral density, as detailed in Supplementary Figs. 8–10.

We further investigate ferroelectric modulation of the photoresponsive behavior. As shown in Fig. 2c, the switchable photocurrent varies systematically with the applied Vp, indicating that the photovoltaic effect can be either enhanced or suppressed depending on the electric polarization state of CIPS. A Vp = +2V pulse reverses the photocurrent to a negative value, whereas a Vp = −2V pulse significantly enhanced the positive Isc. To elucidate the influence of remanent polarization on photovoltaic effect in Pt/CIPS/Gr heterojunction, distinct polarization states were induced by applying voltage pulses of opposite polarity. The 60-s polarizing time is intentionally adopted to ensure that Cu+ migration reaches a fully stabilized state (Supplementary Fig. 11). Current-voltage (I-V) curves and photovoltaic signals are then measured under 0.505 mW/cm2 illumination (Fig. 2d, e). The I-V curve exhibits opposite rectifying behaviors after polarization with negative and positive voltage. As Vp = +2V is applied, the current in the negative voltage scanning region is relatively higher than that in the positive voltage scanning region, whereas Vp = −2V produces the reverse trend (Fig. 2d). Notably, the absolute value of short-circuit current (|Isc|) after −2.5V polarizing is approximately ten times larger than that without polarization (Fig. 2e), and the enhancement under negative Vp is significantly greater than that under positive Vp. These results confirm the generation of an asymmetric polarity-related photocurrent, the origin of which will be discussed later. To further verify the ferroelectric manipulation of the photovoltaic behavior, temperature-dependent photocurrent measurements were performed (Fig. 2f). Below the Curie temperature (TC), Isc increases gradually with temperature, whereas above TC it drops sharply due to the ferroelectric-paraelectric phase transition29. These observations provide additional evidence that the photovoltaic effect in Pt/CIPS/Gr heterojunction is governed by ferroelectricity and help to distinguish the mechanism from photothermoelectric, pyroelectric, Schottky/Dember, trap-mediated photogating or interfacial charging effects at the graphene/CIPS interface.

Proposed mechanism of polarization-modulated FE-PV effect in heterojunction
The nonzero Isc observed experimentally confirm the existence of the FE-PV effect in heterojunction, which is significantly affected by a forward or reverse polarizing voltage Vp applied to the device. For polyvinylidene fluoride (PVDF) based devices, the external bias towards the polarization direction enhances the current shift caused by the FE-PV effect30, thereby increasing the photoelectric conversion efficiency. However, for CIPS, the external electric field causes the interlayer migration of Cu+ ions, which leads to a reversal of the local polarization direction of CIPS, making the correlation of external electric field and photoelectric conversion efficiency more complex.

From the traditional perspective, as we apply a Vp pointing towards Gr from CIPS, Cu+ ions accumulate at the interface, with the polarization direction pointing towards Gr (P↑) (Fig. 3a). The DFT calculation results show that a classical type II heterojunction is formed for this case, and there is an offset of CIPS conduction band points towards Gr (Voc > 0) (Fig. 3c and Supplementary Fig. 12). It enhances the transfer of photogenerated hot electrons from CIPS to Gr (Fig. 3b). When a Vp is applied pointing towards CIPS from Gr, the polarization direction points away Gr (P↓) (Fig. 3d) and electrons in Gr transfer to CPIS (Voc < 0), pushing the Dirac point of Gr to a higher energy level and causing a type II heterojunction with reduced offset (Fig. 3f and Supplementary Fig. 12). Additionally, the conduction band offset of CIPS deviates from Gr, which hinders the transfer of photogenerated hot electrons from CIPS to Gr (Fig. 3e). The polarization reversal caused by the interlayer migration of Cu+ ions under an electric field has overturned the traditional FE-PV effect. Therefore, we proposed the model that explains the observed polarization-modulated FE-PV effect in heterojunction by changing the polarization in Fig. 2.

Anomalous switchable FE-PV effect and kinetic mechanism of heterojunction
Furthermore, we investigate the kinetic process of polarization switching via the migration of Cu+ ions systematically. Figure 4a shows the I-V curves obtained under gradually increased positive polarization, revealing the sign reversals of Isc and the open-circuit voltage (Voc). The photovoltaic current changes from positive to negative, and further to positive (Supplementary Fig. 13), while the Voc varies from negative to positive and further to negative. As no Vp is applied, the curve I in Fig. 4a indicates Voc < 0 and Isc > 0. When the positive Vp increases, the Voc shifts toward positive, gradually reaches a maximum value as Vp = +2.0V. As positive Vp is continuously increased, the Voc shifts towards negative, and the Voc crosses zero point again and becomes negative for Vp = +3.0V. The Vp dependence of photovoltaic current is plotted in Fig. 4b.

The physical model of kinetic process to explain the anomalous switchable FE-PV effect is setup in Fig. 4c. At zero bias shown in Fig. 4c-Ⅰ, the ordered occupation is downward polarization (P↓) of Cu+ ions in CIPS. It leads to the existence of the FE-PV effect and current shift, which results in a nonzero dark current while Voc ≠ 0. Under a low positive Vp, Cu+ ions undergo intralayer migration, causing an upward local polarization (P↑) in CIPS. When the increased polarization cancels out the initial polarization, the overall polarization is zero (P=0), while both the FE-PV effect and current shift disappear, thus Voc = 0. As the Vp continues to increase, the overall polarization reverses to the upward polarization (P↑), so Voc becomes positive. After applying a relatively large Vp, Cu+ ions can achieve interlayer migration, and the polarization reverses back to the initial downward polarization (P↓) while Voc return to the original state.

Unlike the polarity reversal induced by positive Vp, no sign reversal of the Voc is observed as increasing negative Vp. This discrepancy originates from the initial ferroelectric state of the heterojunction is downward polarization (P↓). Figure 4d displays the (I-V) curves measured under linearly increasing negative Vp, revealing a distinct drift of the curve apex. The dependence of photovoltaic current on negative Vp is presented in Fig. 4e, with the Isc states similarly categorized into I, II, III, and IV. State I corresponds to the initial ferroelectric state with downward polarization (P↓), where Voc < 0, Isc > 0. As increasing the negative Vp, the majority of Cu+ ions migrate downward into the vdW gaps, while a fraction occupies the upper sites in the neighboring layer (indicated by the blue arrow). This generates localized upward polarization (P↑), attenuating the overall downward polarization and consequently reducing Voc in state II compared to state I. But it cannot completely offset the downward polarization of Cu+ ions that initially exist at the intralayer VDW layers. As the negative Vp further increases, more Cu+ ions undergo interlayer and intralayer migration to the neighboring layer, locating the downward polarization state (P↓). The resultant increase in the interface potential barrier elevates Voc (State III). Subsequently, under higher negative Vp, Cu+ ions repeat this migration behavior, driving Voc undergoing a cyclic evolution of weakening—strengthening—weakening. The positive and negative offsets determined by the initial polarization state can be switched through an applied external voltage bias. The reproducibility of this behavior across different devices, measurement cycles, and independent experimental runs is shown in Supplementary Figs. 14–16, certifying that the anomalous switchable FE-PV effect of heterojunction is robust, stable and highly repetitive.

To confirm the existence of the Cu+ ion migration process within CIPS, consecutive switching spectroscopy PFM (SS-PFM) hysteresis loops were measured at a fixed location by tuning the tip bias. As shown in Supplementary Fig. 17, by applying a DC bias, the phase exhibits three switching loops with 180° difference. When the positive bias voltage increases, the polarization sequentially switches from P↑ to P↓ and then reverts to P↑, completing a polarization switching cycle. In contrast, a negative bias induces a P↑ → P↓ polarization switching. Notably, increasing the bias magnitude can generate a polarization direction antiparallel to the electric field, i.e., a positive bias can generate an abnormal P↓ state, or a negative bias can generate an abnormal P↑ state. It primarily stems from Cu+ ions migration within the vdW gap, consistent with the proposed mechanism to reveal the switchable FE-PV effect under unidirectional voltage. In order to prove the Cu+ ions migration directly, we have performed cross-sectional scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM-EDS) to visually confirm Cu+ ions migration (Supplementary Fig. 18). Specifically, we prepared a bulk CIPS sample with ~15-μm thick, sandwiched between two Au electrodes for in-situ bias tests. The cross-sectional structure is shown in Supplementary Fig. 18a. After applying a 3V bias pulse for 4 min, we immediately conducted EDS elemental mapping on the cross-section. As shown in Supplementary Fig. 18b, In, P, and S remain uniformly distributed, while Cu exhibits pronounced accumulation toward the cathode side, forming a clear concentration gradient. These results provide direct evidence that Cu+ ions migrate and redistribute within CIPS under an applied electric field.

We also fabricated a symmetric Gr/CIPS/Gr device and characterized its photovoltaic properties (Supplementary Fig. 19) under the same measurement conditions as used in Fig. 4. The results reveal a clear contrast in photocurrent behavior between the symmetric structure and the asymmetric Pt/CIPS/Gr device. As the positive or negative polarizing voltage increases gradually from 0 to 3V, both the short-circuit current (Isc) and open-circuit voltage (Voc) of the symmetric Gr/CIPS/Gr device exhibit polarity reversal. In contrast, there is an asymmetric switching behavior in the asymmetric Pt/CIPS/Gr device, manifesting as that Isc reverses as increasing positive polarizing voltage, but does not reverse with changes in negative bias voltage. To explain the difference in photovoltaic behavior between symmetric and asymmetric structures from an energy-band perspective, we have performed DFT calculations for the Gr/CIPS/Gr configuration (Supplementary Fig. 20). The results clearly show that when both contacts are graphene, their Fermi levels are aligned, and the band bending induced by Cu+ ions migration under an external electric field exhibits symmetric characteristics. This comparison strongly indicates that the previously reported anomalous current reversal originates from the structural asymmetry of the device interface contact, rather than the initial ferroelectric state.

Applications in edge detection
Edge detection is a fundamental step in image processing and computer vision. To demonstrate the potential of Pt/CIPS/Gr-based photovoltaic devices for machine vision, we simulated image edge detection using a convolutional neural network (CNN). Figure 5a indicates the schematic illustration of the edge detection process, while a 100×100 image showing a petal shape is used as the input image. The pixel values in this image are preprocessed by converting to grayscale, then normalized and binarized to 0 and 1, whose pixel values of black and white are defined as 1 and 0, respectively. When mapping the input image to the FE-PV heterojunction devices, the pixel value of 1 and 0 are represented by applying and removing illumination stimuli of an optical power of 48.3 μW/cm2 to the FE-PV heterojunction devices. To implement the convolution process, two 3×3 convolution kernels are applied with a stride of 1, where the kernel weights correspond to the photoresponsivity of the FE-PV heterojunction devices (see calculation method in Supplementary Note 1). This responsivity is directly modulated via ferroelectric polarization by adjusting pulse duration, pulse amplitude or light intensity that is applied to the FE-PV heterojunction devices. Figure 5b shows two Prewitt kernels, where the kernel 1 extracts the gradient in x direction (Gx) and the kernel 2 extracts the gradient in y direction (Gy), respectively. The output current Ix and Iy related to kernel 1 and 2 fully captures the Gx and Gy features, respectively, clearly extracting the edges between the petal and the background along the x and y direction, as shown in Fig. 5c, d. These two sets of output currents Ix and Iy can then be merged to find the absolute magnitude of the gradient at each pixel point (Fig. 5e). After these processes, the current values are further normalized and binarized to obtain the final output image (Supplementary Note 2). The edges of the input image are successfully detected in Fig. 5c–e. According to Fig. 5f, the F-score, which is a figure-of-merit in terms of true and false positives and negatives, is calculated to be 1, showing good performance of edge detection by using our heterostructure devices (Supplementary Fig. 23). Moreover, the symmetry of photoresponsivity weight can be improved by a programming algorithm to increase the photoresponsivity accuracy31. As the identical values of positive and negative photoresponsivity for +1 and −1 are programmed along with near-zero photoresponsivity for 0, the nonzero value of output currents beyond the detected edges could be eliminated (Supplementary Figs. 24 and 25). Details regarding reproducibility, preprocessing, and post‑processing associated with the image edge detection are summarized in Supplementary Figs. 26–28.

Applications in pattern classification
Pattern classification is another fundamental task in computer vision and machine learning. In Fig. 6a, two pattern sets representing the letters “X” and “T”-along with their noisy variants are used, each consisting of a 3×3 pixel array32. The training and test sets are identical, containing 50 patterns with possible repetitions. For each pattern, the pixel values of the body pattern pixels are defined as logical +1, while the pixel values of the background pixels are defined as logical −1. The sets of “X” and “T” correspond to the binary outputs of y = +1 and y = −1, respectively. A single-layer perceptron can be hardware-implemented with the FE-PV heterojunction devices (see Supplementary Note 3).

The classification weights w that yield correct classification can be obtained through the training process in the framework of an artificial neural network. Herein, we adopt the so-called ex situ method, the training is performed in software. As shown in Fig. 6b, the theoretical dimensionless weights +1 and −1 are programmed to the positive and negative photoresponsivity values of 9.5 and −10.5 mA/W, while the calculated weights +2 and −2 are programmed to the positive and negative photoresponsivity values of 19.3 and −21.5 mA/W, respectively. Using the MAC operation in a way like that used for edge detection, Fig. 6c presents the output current I during the inference of different input patterns, by using an illumination source with an optical power of 2.156 mW/cm2 to the FE-PV heterojunction devices. A pattern from the “X” class results in a positive output current, while a pattern from the “T” class results in a negative output current. Like that in the edge detection, the asymmetry for positive and negative photoresponsivity weights still exists for both ±1 and ±2. Moreover, the ratio of photoresponsivities corresponding to dimensionless 1 and 2 are slightly derived from 2. Despite the existence of non ideal factors, all the patterns are correctly separated, and thus the accuracy for this binary classification task is 100%.

In conclusion, we have achieved a switchable photovoltaic effect in a Pt/CuInP2S6/Graphene asymmetric heterojunction through ferroelectric polarization modulation. The device exhibits polarity-dependent rectifying behavior and a distinct positive-to-negative reversal of the photovoltaic current. DFT calculations reveal that this anomalous FE-PV effect originates from the coupling of Cu+ ion migration and interface barrier regulation. Furthermore, we demonstrate that the asymmetric heterojunction could be used in image processing, such as image edge detection and pattern classification. This work provides deep mechanistic insights and design strategies for exploiting advantageous heterojunction devices in next-generation neuromorphic and visual technologies.

Methods
Crystal growth
CIPS single crystals were synthesized by the chemical vapor transport method33. Stoichiometric amounts of high-purity Cu, In, P, and S powder were sealed into a vacuumed quartz tube and loaded into a dual-zone horizontal tube furnace. Initially, the hot and cold zones were maintained at 665 and 635 °C, respectively, for 34 h. The temperature gradient was then inverted, with the cold zone heated to 665 °C and the hot zone lowered to 635 °C for an additional 196 h. Following this thermal treatment, bulk CIPS single crystals were successfully obtained. Raman spectroscopy is utilized to confirm the high-quality crystal structure of the CIPS layer. Furthermore, a UV-vis absorption spectrum is obtained to characterize the optical absorption of CIPS.

Sample characterization
The thickness of CIPS and Graphene, measured via atomic force microscope (AFM) measurements (Bruker, Dimension Icon), is ~26 and 42 nm, respectively (Supplementary Fig. 2). Furthermore, Raman (Horiba, XploRA PLUS) spectroscopy of the CIPS (Supplementary Fig. 3) reveals characteristic peaks at 101, 162, 262, 315, 375, and 445 cm−1, which is assigned to P2S6 librations, the δ(S-P-P) mode, the δ(S-P-S) mode, cation oscillations, the δ(P-P) mode, and the δ(P-S) mode, respectively34. This demonstrates that the materials possess a well-defined crystal structure. To characterize the optical absorption of CIPS, UV-vis absorption spectra (Shimadzu, UV-3600) were measured. As shown in Supplementary Fig. 4, the absorption edge (λg) is determined to be 477 nm. Therefore, a 450 nm laser was selected for all subsequent experiments to stimulate the photoresponse of the heterojunction device.

Device fabrication
Following the patterns printed onto a SiO2 (300 nm)/Si substrate by photolithography, a 25 nm Pt electrode was deposited via the ultra-vacuum magnetron sputtering system, with a deposition rate of 0.1 nm/s. To minimize contamination, substrates were ultrasonically cleaned with ethanol and acetone before lithography. Photoresist was prebaked for 3 min to avoid charring, and electrodes were cleaned with fresh acetone to eliminate the residual photoresist. Subsequently, graphene and CuInP2S6 (CIPS) thin layers are obtained through mechanical exfoliation using scotch tape and attached to PDMS films, respectively. Then, under an optical microscope, we use a precise micro-manipulation system to precisely stack the target material in thin layers on the predefined electrode positions. Only flakes with smooth surfaces, clean edges, and uniform thickness were used. Graphene is used as the upper electrode by connecting a parallel Pt electrode on the periphery using a sputteringsystem. A vertical and asymmetrical Pt/CuInP2S6/Graphene heterojunction device was fabricated. The thickness of CIPS and Graphene is measured via an atomic force microscope (AFM).

Each heterojunction was optically inspected post-transfer. Only devices with uniform, bubble-free interfaces and full substrate contact were selected, ensuring the high quality of interfaces. Through statistics of approximately more than 100 instances of device fabrication experiments, the final transfer success rate (the proportion of complete heterojunction devices successfully fabricated and available for electrical testing) is more than 85%, providing a basis for the reproducibility of the research.

Ferroelectric photovoltaic (FE-PV) characterization
The FE-PV characterizations of the heterojunction device were obtained with a semiconductor characterization system (Keithley B1500A). Photovoltaic response measurements were conducted under a 450 nm wavelength optical excitation, with an optical power density ranging from 0.505 mW/cm2 to 2.516 mW/cm2. The incident laser power was precisely calibrated using a commercial optical power meter. Additionally, the time-dependent short-circuit current (Isc) under zero bias voltage was obtained. Under 405 nm laser illumination, a high density of photocarriers was generated, causing Isc to rapidly increase from the noise level (8×10−14 A). The specific measurement procedure for the FE-PV effect was as follows: In the dark, a polarizing voltage (Vp) was applied to the bottom Pt electrode to establish the polarization state, while the top graphene electrode was grounded. Subsequently, under laser illumination, the FE-PV characteristics were obtained by performing I-V scans within a ±1 V range. Furthermore, the I-V characteristics could be modulated by varying the magnitude, polarity, and duration of the applied Vp.

Density functional theory simulation
All calculations in this work are performed based on the density functional theory (DFT) within the Vienna ab initio simulation package (VASP)35–37, with the projector augmented wave (PAW) potentials38 to describe the electron-ionic core interaction. Based on the previous parameter test39, we chose the Perdew–Burke–Ernzerhof formulation (PBE)40 of the generalized gradient approximation (GGA) to describe the exchange-correlation interaction of electrons, and the DFT-D3 (Becke–Johnson) formulation41 was used to describe vdW interactions between layers. For structural relaxation of individual two-dimensional material, the wave functions are expanded in a plane-wave basis set with an energy cutoff of 650 eV for structural relaxation. The force on each ion is converged to be less than 0.01 eV/Å, and a precision of 10−6 eV is adopted to minimize the total energy of the system. The k spacing of 0.02 Å−1 grid in reciprocal space is used to ensure the convergence for the total energy self-consistent calculations. The Cu 3p63d104s1, In 4d105s25p1, P 3s23p3, S 3s23p4, and C 2s22p2 electrons are treated as valence electrons. VESTA software is used for visualization of crystal structures42. The experimental lattice parameters are used as the initial structure, and after fully relaxation, including both the external (lattice constants and including angles) and internal (atomic coordinates) parameters, the lattice parameters are a=4.303 Å, b=3.189 Å, γ=90° for orthorhombic Graphene, and a=6.091 Å, b=10.546 Å, c=13.788 Å, β=107.27° for CIPS, agreement with the experimental lattice parameters (a=6.096 Å, b=10.565 Å, c=13.623 Å, β=107.10°)20. Detailed information is available at 4.

Applications in image processing
In mapping the input image to the FE-PV heterojunction devices, pixel values of 1 and 0 correspond to the application and removal of 48.3 μW/cm2 illumination. Convolution is performed by sliding 3×3 kernels (e.g., Prewitt kernels for edge detection) over the image with a stride of 1. Kernel weights are mapped to the photoresponsivity (Rkl) of each device, adjustable through ferroelectric polarization and varying light intensity. The fundamental multiply-accumulate (MAC) operation of convolution is physically realized by summing, according to Kirchhoff’s law, the photocurrents generated from the illuminated 3×3 device group (Supplementary Note 5). For edge detection, convolution outputs are merged, normalized to the 0–1 range, and converted to binary using a threshold of 0.5. Performance is evaluated using the F-score. Pattern classification employs a single-layer perceptron: A flattened 3×3 pixel array with bias input feeds into an output neuron that applies a sign function to weighted inputs. Weights are trained externally using the perceptron learning rule on datasets, achieving perfect test accuracy. Detailed information is available at Supplementary Notes 2 and 3.

Data availability
The authors declare that all data supporting the findings of this study are contained within the article and its supplementary information. Additional raw data are available from the corresponding author upon reasonable request.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Meijiao Men et al. present a manuscript entitled “Polarization-modulated Programmable Photovoltaic Performance of a Designed Ferroelectric Heterojunction.” The work addresses an important issue in ferroelectric photovoltaics by demonstrating programmable photovoltaic responses in Pt/CIPS/graphene heterojunctions and showcasing potential applications in in-sensor computing. The experiments are carefully designed, and the results are of interest to the optoelectronic and neuromorphic device community. Overall, this is a good piece of work. However, in order to ensure clarity, reproducibility, and stronger positioning of the results, I suggest the authors address the following points before the manuscript can be considered for publication.\n1. In Fig. 1a, supplementary Fig. 1, and in device fabrication methods, the dry transfer process of graphene and CIPS is mentioned briefly. Could the authors provide more fabrication details of transfer yield, possible contamination, or interface quality that may influence device reproducibility?\n2. In Supplementary Fig. 2, the PFM data confirm ferroelectricity of CIPS. Could the authors clarify whether both out-of-plane and in-plane polarization switching were characterized, and how stable the domains are under repeated cycling?\n3. In Fig. 2b, the device exhibits a large Ilight/Idark ratio (~900). While impressive, the stability and reproducibility of this value remain unclear. Over how many cycles and what duration was this stability verified? Such details are important to assess the robustness of the photovoltaic response.\n4. The claim of this work is that Cu⁺ ion migration governs the anomalous switchable FE-PV effect, overturning the traditional FE-PV. While the I–V characteristics and SS-PFM loops are suggestive, they do not provide direct proof of Cu⁺ redistribution. Have the authors considered more direct probes of STEM-EDX/EELS mapping or operando XPS to conclusively verify ionic migration? Without this, the mechanism remains somewhat speculative.\n5. The manuscript describes multiple reversals of Voc and Isc as a function of Vp (Fig. 4a–b). However, the reproducibility of this behavior across different devices, cycles, or measurement runs is not shown. Given that ionic processes can be highly device-to-device dependent; the authors should clarify whether this anomalous switching is robust or sample-specific.\n6. The proposed mechanism suggests that the initial ferroelectric state (P↓) dictates the asymmetric switching behavior under positive versus negative bias. However, this assumption seems critical: how is the initial P↓ state determined experimentally in the as-fabricated heterojunction? Could other factors, such as electrode work function mismatch or interfacial defects, bias the system towards this orientation? Addressing this would help solidify the foundation of the model.\n7. The F-score is reported as 1 in Fig. 5f, whether the score was calculated over multiple test images or only one input. This clarification is important for reproducibility.\n8. The device is claimed to perform edge detection and binary pattern classification for brain-like neural computing. Could the authors benchmark their device performance of accuracy and speed against existing neuromorphic optoelectronic systems to highlight the advantages and limitations of their FE-PV heterojunction approach?", "reviewer_2": "In this manuscript, the authors investigate how the ferroelectric photovoltaic effect couples to Cu⁺ migration dynamics, yielding up to a 10x increase in photocurrent and polarization-dependent polarity switching. By using simple structure of Pt/CIPS/Gr, they demonstrate in-sensor computing application, including edge detection and pattern classification. The results are promising, but several fundamental questions remain before the full potential and limitations of this approach can be assessed:\n1- Figure 2 analyzes the effects of illumination power and polarization separately, but their combined influence is not addressed. Under an applied VP, does illumination further modulate the device current? Please provide comparative data (e.g., I-V or I-t curves) under dark vs. illuminated conditions at fixed VP for both polarization states, and quantify any interaction.\n2- Figure 2e shows negligible contrast between +P and −P at 1 V, whereas a clear polarization-dependent change emerges above ~1.5 V. Yet Figure 4 centers on VP = ±1V. Please justify this choice (e.g., device reliability, leakage, or comparability constraints).\n3- The introduction claims “ultrafast switching dynamics” in ferroelectrics, yet Figure 2 reports a 60 s poling time to set polarization. Please reconcile this discrepancy. Clarify whether 60 s is a stabilization/poling dwell rather than the intrinsic FE switching time, and quantify the minimum pulse width needed to achieve saturated (or ≥80%) Pr. Including switching-current transient’s data would help separate true ferroelectric switching from slower Cu⁺ migration effects.\n4- In Figure 4, the authors propose a voltage-driven polarization mechanism in CIPS attributed to Cu⁺ migration, but the evidence provided is largely inferential. Please substantiate this claim with direct, mechanism-specific measurements (e.g., in-situ biasing TEM to track Cu site occupancy or switching-spectroscopy PFM and PUND versus pulse width/temperature to disentangle ionic migration from intrinsic ferroelectric switching). Absent such data, the mechanism should be presented as a hypothesis rather than a confirmed explanation.\n5- Please clarify how intralayer vs interlayer Cu⁺ migration in CIPS maps onto the in-plane (Px, Py) and out-of-plane (Pz) polarization components. Is there a direct correspondence (e.g., interlayer displacement → Pz ; intralayer displacement → Px, Py)? A brief explanation, ideally with a schematic and expected measurement signatures (vertical vs. lateral PFM, SHG) would improve completeness and help interpret the Figure 4 evidence.\n6- The devices use ~25 nm-thick CIPS, yet prior reports indicate a pronounced thickness dependence, often with larger remanent polarization and improved domain stability in thicker flakes (10.1002/smll.201904529). Please justify the choice of 25 nm.\n7- It appears the Prewitt operator is applied directly to raw inputs without denoising or contrast normalization. These can cause the risk of noise-induced spurious edges. If pre-processing was performed, please detail the methods and parameters; if not, please justify its omission. In addition, specify any post-processing (e.g., non-maximum suppression, thresholding method/parameters, kernel size) and assess the sensitivity of the results to these choices (an ablation would be helpful).\n8- Minor editorial issues:\n• In the Introduction, the following sentence appears twice in succession; please remove the duplicate:\n“Moreover, the spontaneous polarization and ultrafast switching dynamics of ferroelectric materials can emulate synaptic plasticity in biological vision systems, facilitating highly efficient and intelligent neuromorphic visual perception and processing systems.”\n• Reference 18 does not appear to be related to ferroelectric materials; please verify its relevance or replace it with an appropriate citation.\n• Line 101: “Pt/Gr/CIPS” should be corrected and the stack notation standardized (Pt/CIPS/Gr).", "reviewer_3": "This manuscript reports a ferroelectric–photovoltaic (FE-PV) device based on an asymmetric Pt/CuInP₂S₆/graphene (Pt/CIPS/Gr) heterojunction whose photoresponse can be programmed via ferroelectric polarization and associated Cu⁺ ion migration. The device shows polarity-dependent rectification, sign-reversible short-circuit photocurrent under poling, and ~10× enhancement in the magnitude of the short-circuit photocurrent with appropriate polarization. The physical picture is supported by density-functional theory (DFT) band-alignment analysis and by temperature-dependent measurements near the CIPS Curie temperature. The authors further connect the programmable photoresponsivity to in-sensor computing tasks (edge detection and 3×3 pattern classification), outlining a path toward neuromorphic vision hardware. In my view, this paper is a promising contribution that can become publication-ready after substantial revisions focused on quantitative benchmarking, direct mechanism validation, and clearer presentation. I therefore recommend Minor Revision\n1. The manuscript attributes the sign-programmable photoresponse to Cu⁺-ion migration coupled with ferroelectric barrier tuning, supported by SS-PFM and DFT. However, PFM/SS-PFM and DFT are not species-specific and therefore cannot uniquely exclude look-alike mechanisms such as (i) pure ferroelectric barrier modulation without ions, (ii) trap-mediated photogating or persistent interfacial charging at the graphene/CIPS interface, (iii) photothermoelectric or pyroelectric effects, (iv) Schottky/Dember photovoltages, or (v) electrochemical/adsorbate-induced work-function shifts in graphene. To elevate the claim from “consistent with” to “caused by” Cu⁺ motion, please add direct, chemistry-resolved evidence (e.g., bias-treated ToF-SIMS or gentle-sputter XPS depth profiles; pre/post-poling STEM-EELS), quantify post-poling drift versus temperature to extract an ionic activation energy, and include discriminating controls (ultrathin ion-blocking interlayers, symmetric-contact devices, and thickness/environment/capping sweeps).\n2. To complete the neuromorphic-vision story end-to-end, please address: write (poling) kinetics—report the minimum pulse width/amplitude and corresponding energy per write required to reach each non-volatile state, and include a 2D pulse-duration × voltage state map; read speed and bandwidth—quantify short-circuit photocurrent rise/fall times under modulated illumination and the −3 dB optical/electrical bandwidth at the stated read condition, with full measurement setup and load; transients—show representative time-domain traces for write→read and light on/off to extract rise, fall, latency, and jitter, and clarify whether reads are non-destructive; and noise/SNR—report noise spectral density, NEP, and specific detectivity at the read condition to bound achievable frame rate and MAC accuracy.\n3. The paper has included fabrication and measurements of an asymmetrical Pt/CuInP₂S₆/graphene heterojunction and reports FE-PV tests under 450 nm illumination with DFT/SS-PFM support, which establishes the basic operation and mechanism narrative, but it does not include a symmetric-contact control to isolate the role of contact asymmetry in the polarity-switching sequence (the device is explicitly “vertical and asymmetrical,” and no Pt/CIPS/Pt or Gr/CIPS/Gr control appears in the SI contents). I suggest the author fabricate and report a symmetric (or quasi-symmetric) control and/or a contact-swapped variant (Gr/CIPS/Pt vs. Pt/CIPS/Gr), and tabulate polarity, open-circuit voltage, and short-circuit photocurrent across these variants; additionally, include KPFM/CPD and scanning-photocurrent maps to localize interfacial barrier changes and decisively link the switching sequence to contact asymmetry."}, "author_response": "Dear reviewers,\nThank you very much for your comments and suggestions that have helped us to\nsignificantly improve the quality of our manuscript “Polarization-modulated\nProgrammable Photovoltaic Performance of a Designed Ferroelectric Heterojunction”.\nWe have replied to these comments and suggestions in a point-by-point way and made\na major revision of the manuscript (highlighted in a yellow background). Our responses\nto each of the reviewers’ comments are as follows.\nWe believe that the revised manuscript could bring general interest of the\nreadership and deserve the publication in Nature Communications.\nThank you for all your time and effort. We look forward to receiving your favorable\ndecision!\nBest Regards!\nXiaolei Wang\nSchool of Physics and Optoelectronic Engineering\nBeijing University of Technology\nBeijing 100124, China\nEmail: xiaoleiwang@bjut.edu.cn\nTel: 86-13581688590\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nMeijiao Men et al. present a manuscript entitled “Polarization-modulated\nProgrammable Photovoltaic Performance of a Designed Ferroelectric\nHeterojunction.” The work addresses an important issue in ferroelectric\nphotovoltaics by demonstrating programmable photovoltaic responses in\nPt/CIPS/graphene heterojunctions and showcasing potential applications in in-\nsensor computing. The experiments are carefully designed, and the results are of\ninterest to the optoelectronic and neuromorphic device community. Overall, this\nis a good piece of work. However, in order to ensure clarity, reproducibility, and\nstronger positioning of the results, I suggest the authors address the following\npoints before the manuscript can be considered for publication.\nReply: We thank the reviewer for the positive assessment of our work as a carefully\ndesigned study with results of genuine interest to the optoelectronic and neuromorphic\ncommunities. We have supplemented cross-sectional SEM-EDS mapping, systematic\ntests across multiple devices, cycles, and measurement runs to improve the clarity and\nreproducibility of the results. Specifically, the point-by-point response to all thoughtful\ncomments are addressed in the following.\n1. In Fig. 1a, supplementary Fig. 1, and in device fabrication methods, the dry\ntransfer process of graphene and CIPS is mentioned briefly. Could the authors\nprovide more fabrication details of transfer yield, possible contamination, or\ninterface quality that may influence device reproducibility?\nReply: We are very grateful to the reviewer for your attention to this important technical\ndetail. We fully agree that the quality of the dry transfer process is crucial to the\nperformance and reproducibility of the devices. Below is a detailed description of the\nspecific measures and evaluation methods adopted to enhance the transfer success rate,\nmitigate contamination, and optimize interface quality.\n(1) Our detailed dry transfer is carried out as follows. First, graphene and CuInP₂S₆\n(CIPS) thin layers are obtained through mechanical exfoliation by the “Scotch Tape”\nand attached to PDMS films, respectively. Then, under an optical microscope, we use\na precise micro-manipulation system to precisely stack the target material thin layers\non the predefined electrode positions—CIPS first, followed by graphene. When the\nsample on the PDMS makes full contact with the silicon substrate, heating to 80 °C and\nthen cooling back to room temperature can facilitate the transfer of the two‑dimensional\nmaterials from the PDMS onto the substrate.\nTo ensure the reproducibility of the process, we only select thin layers with smooth\nsurfaces, neat edges, and uniform thickness under the optical microscope for device\nfabrication. Through statistics of approximately more than 100 instances of device\nfabrication experiments, the final transfer success rate (the proportion of complete\nheterojunction devices successfully fabricated and available for electrical testing) is\nmore than 85%, providing a basis for the reproducibility of the research.\n(2) To reduce the potential sources of contamination, the following attempts were\nmade i) To reduce the contamination from substrate cleaning and residues, we\nalternately ultrasonically clean the SiO₂/Si substrate with high-purity anhydrous ethanol\nand acetone, and only proceed with the subsequent process after confirming that the\nsurface is clean and flawless under an optical microscope. ii) For photolithography\nprocess, we strictly control the photoresist curing or pre-baking time at 3 minutes avoid\nover-baking, which could cause \"charring\" and the formation of residues that are\ndifficult to remove. iii) During the lift-off process, we use fresh high-purity acetone for\nthorough soaking and gentle ultrasonication to ensure clear electrode patterns and\ncomplete removal of residual photoresist.\n(3) The core challenge for the heterojunction interface quality lies in the microbubbles\nformed by gas trapped during the transfer process. These bubbles can cause local\nsuspension of the material, seriously affecting the interface coupling and electrical\nperformance. To address this issue, we immediately examine the morphology of the\nentire heterojunction region using an optical microscope after each dry transfer step.\nWe have established rigorous device selection criteria: only devices exhibiting no\nobvious bubbles, wrinkles, or other defects at the heterojunction interface—with the 2D\nmaterial making uniform contact with the substrate—are selected for subsequent\nelectrical measurement and performance characterization. This strict selection criterion\nensures that the data reported in this paper are all from devices with excellent interface\nquality, thereby guaranteeing the reliability and consistency of the electrical results.\nIn the method section of the main text, descriptions about the transfer details\nhave been added: “To minimize contamination, substrates were ultrasonically cleaned\nwith ethanol and acetone before lithography. Photoresist was pre-baked for 3 mins to\navoid charring, and electrodes were cleaned with fresh acetone to eliminate the residual\nphotoresist. Subsequently, graphene and CuInP₂S₆ (CIPS) thin layers are obtained\nthrough mechanical exfoliation by the “Scotch Tape” and attached to PDMS films,\nrespectively. Then, under an optical microscope, we use a precise micro-manipulation\nsystem to precisely stack the target material thin layers on the predefined electrode\npositions. Only flakes with smooth surfaces, clean edges, and uniform thickness were\nused.” (line 408-414 in the revised text)\n“Each heterojunction was optically inspected post-transfer. Only devices with\nuniform, bubble-free interfaces and full substrate contact were selected, ensuring the\nhigh quality of interfaces. Through statistics of approximately more than 100 instances\nof device fabrication experiments, the final transfer success rate (the proportion of\ncomplete heterojunction devices successfully fabricated and available for electrical\ntesting) is more than 85%, providing a basis for the reproducibility of the research.”\n(line 418-423 in the revised text)\n2. In Supplementary Fig. 2, the PFM data confirm ferroelectricity of CIPS. Could\nthe authors clarify whether both out-of-plane and in-plane polarization switching\nwere characterized, and how stable the domains are under repeated cycling?\nReply: We thank the reviewer for raising this important point. As suggested, we have\ncharacterized the out-of-plane polarization switching behavior, and observed the\nstability of the out-of-plane ferroelectric domains under repeated cycling by using\npiezoresponse force microscopy (PFM).\nWe have supplemented additional PFM tests for repeated P-E loop cycles (Fig. R1)\nas suggested. The results show that after 50 P-E loop cycles, the overall polarization\nstate remains almost unchanged, with negligible local changes in the domain structure.\nIt indicates great stability of the material under repeated cycling conditions.\nWe have added Fig. R1 as Supplementary Fig. 6 in the Supplementary\ninformation and revised the description: “the robust out-of-plane ferroelectricity of\nCIPS was confirmed by Piezoelectric force microscopy (PFM) measurements\n(Supplementary Fig. 5 and Supplementary Fig. 6).” (line 111-112 in the revised\nmanuscript)\nFig. R1 (Supplementary Fig. 6) | Robust out-of-plane ferroelectric domains in CIPS. PFM\nphase images of the same region after the domain is written for various P-E loop cycles. Scale\nbars, 1 μm.\nFurthermore, this study focuses on vertical heterostructures and our analysis\nprimarily emphasizes the out-of-plane ferroelectric polarization of the 25 nm-thick\nCIPS film. Although recent studies suggest the coexistence of both out-of-plane and in-\nplane polarization components in CIPS [Appl. Phys. Rev. 11, 041423 (2024)], it is\ngenerally accepted that the in-plane component is relatively weak and often negligible\nin most contexts [Nat. Commun. 16, 4462 (2025)]. Notably, previous research has\ndemonstrated that in CIPS films thinner than 90 nm, the in-plane polarization\ncomponent is virtually absent [Small 16, 1904529 (2020)]. Therefore, the influence of\nin-plane polarization was not considered in this work.\nWe have improved the description of the mentioned original Supplementary\nFig. 2 (Supplementary Fig. 5 in the revised version) and clearly labeled it as an\n“out-of-plane” PFM measurement (line 111 in the main text).\n3. In Fig. 2b, the device exhibits a large I light/I dark ratio (~900). While impressive,\nthe stability and reproducibility of this value remain unclear. Over how many\ncycles and what duration was this stability verified? Such details are important to\nassess the robustness of the photovoltaic response.\nReply: We are grateful to the reviewers for their thorough review and constructive\nfeedback. In response, we have incorporated additional data and clarifications. As\nshown in Fig. R2 (Supplementary Fig. 7), the device response remained stable over\na testing period of 1000 seconds, which corresponds to 2500 on/off cycles. The high\nI /I ratio of ~900 was consistently maintained throughout this entire cycling test,\nlight dark\ndemonstrating excellent operational stability. The stability of the photocurrent was\nevaluated under 405 nm pulsed illumination (0.505 mW/cm²) with a pulse duration and\ninterval of 200 ms.\nWe have added Fig. R2 as Supplementary Fig. 7 in the Supplementary\ninformation and revised the description: “As shown in Supplementary Fig. 7, the\ndevice response remains stable over a testing period of 1000 seconds, which\ncorresponds to 2500 on/off cycles.” (line 140-141 in the revised manuscript)\nFig. R2 (Supplementary Fig. 7) | The stability of photovoltaic response. a, Under a pulsed\nlight of 450 nm with an intensity of 0.505 mW/cm², the photovoltaic current remained stable\nwithin 2500 pulse cycles. b, The enlarged Current-Time image within 584.5 s to 590.5 s.\n4. The claim of this work is that Cu⁺ ion migration governs the anomalous\nswitchable FE-PV effect, overturning the traditional FE-PV. While the I–V\ncharacteristics and SS-PFM loops are suggestive, they do not provide direct proof\nof Cu⁺ redistribution. Have the authors considered more direct probes of STEM-\nEDX/EELS mapping or operando XPS to conclusively verify ionic migration?\nWithout this, the mechanism remains somewhat speculative.\nReply: We sincerely thank the reviewer for the careful reading and insightful\nsuggestions. Accordingly, we have performed scanning electron microscopy–energy\ndispersive X-ray spectroscopy (SEM-EDS) measurements to verify the migration of\nions.\nTo provide more direct evidence of Cu⁺ redistribution, we supplemented cross-\nsectional SEM-EDS measurements to visually confirm copper ion migration.\nSpecifically, we prepared a bulk CIPS sample approximately 15 μm thick, sandwiched\nbetween two Au electrodes for in situ biasing tests. The cross-sectional structure is\nshown in Fig. R3a. After applying a 3 V bias pulse for 4 minutes, we immediately\nconducted EDS elemental mapping on the cross-section sample. As shown in Fig. R3b,\nIn, P, and S remain uniformly distributed, while Cu exhibits pronounced accumulation\ntoward the cathode side, forming a clear concentration gradient. These results provide\ndirect evidence that Cu⁺ ions migrate and redistribute within CIPS under an applied\nelectric field, ruling out interference from other ionic migration or elemental\nsegregation phenomena. Combined with I-V characteristics and SS-PFM loops, it\njointly indicates that the polarization switching phenomenon of the heterojunction\ndevice is caused by the migration of Cu⁺.\nFig. R3 (Supplementary Fig. 18) | The cross-sectional scanning electron microscopy-\nenergy dispersive spectroscopy (SEM-EDS) spectra of CIPS. a, The cross-sectional view of\nbulk CIPS sandwiched between two gold electrodes (pink color) for the EDS measurement. b,\nThe redistribution of Cu, In, P and S element in the whole cross-sectional area.The dotted lines\nrepresent the interfaces between material and electrodes.\nFurthermore, the migration of copper ions in CIPS under an electrical field has also\nbeen well documented in previous work. The intrinsic ferroelectricity of two-\ndimensional CIPS originates from the breaking of spatial inversion symmetry. When an\nupward electric field is applied, copper ions can switch to align the polarization\ndirection with the field [Nano Lett. 21, 995–1002 (2021)]. For instance, aberration-\ncorrected scanning transmission electron microscopy (AC-STEM) has been used to\ndirectly visualize the migration pathways of Cu+ within CIPS [Nat. Commun. 15, 10152\n(2024)], further corroborating the mechanism we propose.\nWe have inserted Fig. R3 as Supplementary Fig. 18 into the Supplementary\nInformation, and added the following description in the text from line 277 to 287:\n“In order to prove the Cu⁺ ions migration directly, we have performed cross-sectional\nscanning electron microscopy–energy dispersive X-ray spectroscopy (SEM-EDS) to\nvisually confirm Cu⁺ ions migration (Supplementary Fig. 18). Specifically, we prepared\na bulk CIPS sample with approximately 15 μm thick, sandwiched between two Au\nelectrodes for in-situ bias tests. The cross-sectional structure is shown in\nSupplementary Fig. 18a. After applying a 3 V bias pulse for 4 minutes, we immediately\nconducted EDS elemental mapping on the cross-section. As shown in Supplementary\nFig. 18b, In, P, and S remain uniformly distributed, while Cu exhibits pronounced\naccumulation toward the cathode side, forming a clear concentration gradient. These\nresults provide direct evidence that Cu⁺ ions migrate and redistribute within CIPS under\nan applied electric field.”\n5. The manuscript describes multiple reversals of Voc and Isc as a function of Vp\n(Fig. 4a–b). However, the reproducibility of this behavior across different devices,\ncycles, or measurement runs is not shown. Given that ionic processes can be highly\ndevice-to-device dependent; the authors should clarify whether this anomalous\nswitching is robust or sample-specific.\nReply: We are grateful for the reviewer’s valuable suggestion. To clarify the anomalous\nswitching is robust, we have presented the data in Fig. R4-6. The results which are\nobtained across different devices, measurement cycles, and independent experimental\nruns, provide robust evidence for its reproducibility.\n(1) Following the reviewer’s advice, we have supplemented different devices to\ndemonstrate the robustness of this anomalous switching behavior. Using the same\nfabrication process, we prepared two additional devices and performed repeated\nexperiments, as summarized in Fig. R4 (Supplementary Fig. 14). Similar to the device\nin Fig. 4 of the main text (Device A), the short-circuit current (I ) and open-circuit\nsc\nvoltage (V ) of Devices B and C are measured under the same experimental conditions\noc\nafter poling at various voltages. The results confirm that under positive polarizing\nvoltages, the short-circuit current in both devices undergoes a sign reversal as the\nvoltage amplitude increases. In contrast, no such reversal is observed under\nprogressively increasing negative polarizing voltages. The switching behavior is\nconsistent across devices, though the specific switching voltage magnitude varies from\ndevice to device. Therefore, all measured devices show the same anomalous switching\nphenomenon, which is reproducibly observed across different devices and is not\nsample-specific.\n(2) In addition, we conducted multiple cycling tests on the same device, as\nshown in Fig. R5 (Supplementary Fig. 15). It shows the evolution of the short-circuit\ncurrent over six consecutive cycles under negative (0 to –3 V) and positive (0 to 3 V)\npolarizing voltages, respectively. For all cycles, no sign reversal of I occurred under\nsc\nnegative voltage poling, whereas positive poling consistently induced current direction\nswitching. These observations align with the anomalous switching effect described in\nthe manuscript and confirm that the phenomenon remains stable across multiple\noperational cycles of the same device.\n(3) Furthermore, we performed repeated independent measurement runs of\nthe same device to further assess reproducibility. As illustrated in Fig. R6, the\nmultiple reversal behaviors of V and I with respect to the polarizing voltage (V )—\noc sc p\nsimilar to those shown in Figs. 4a and 4b—are independent of the measurement setup\nor specific test run, and are solely governed by the initial polarization state of CIPS.\nIn summary, all supplementary experimental results are highly consistent and in\nfull agreement with theoretical predictions, strongly supporting that the observed\nanomalous switching is not an isolated or incidental occurrence, but a robust\ncharacteristic governed by an intrinsic physical mechanism. To maintain the clarity and\nfocus of the main text, detailed repeatability test data, statistical results, and the related\nexplanations have been supplemented in the Supplementary Information (see\nSupplementary Fig. 14-16).\nWe have added the following description in the text from line 262 to 265: “The\nreproducibility of this behavior across different devices, measurement cycles, and\nindependent experimental runs is shown in Supplementary Figs. 14-16, certifying that\nthe anomalous switchable FE-PV effect of heterojunction is robust, stable and highly\nrepetitive.”\nFig. R4 (Supplementary Fig. 14) | Anomalous switchable FE-PV effect in the different\nheterojunction devices. a and b, Individual I-V curves of the heterojunction device B with the\nV tuned from 0 V to +3 V (a) and 0 V to -3 V (b) in log scale. c and d, Various I-V curves of\np\nthe heterojunction device C with V tuned from 0 V to +3 V (c) and 0 V to -3 V (d) in log scale.\np\nFig. R5 (Supplementary Fig. 15) | The reproducibility of the anomalous switchable FE-\nPV effect across different cycles. a, Plot of I dependent on negative V . b, Plot of I\nsc p sc\ndependent on positive V .\np\nFig. R6 (Supplementary Fig. 16) | The reproducibility of the anomalous switchable FE-\nPV effect across different measurement runs. a, Initially, under an negative V sweep (from\np\nsc p\nI undergoes a full polarity reversal. c, Finally, upon reapplying the negative sweep, I\nsc sc\nconsistently remains positive without reversal.\n6. The proposed mechanism suggests that the initial ferroelectric state (P↓) dictates\nthe asymmetric switching behavior under positive versus negative bias. However,\nthis assumption seems critical: how is the initial P↓ state determined\nexperimentally in the as-fabricated heterojunction? Could other factors, such as\nelectrode work function mismatch or interfacial defects, bias the system towards\nthis orientation? Addressing this would help solidify the foundation of the model.\nReply: We appreciate the reviewer’s comment and sorry for the confusing description.\nWe have improved the statement in the manuscript and emphasized that it is the\nasymmetric structure of device which controls the asymmetric switching.\nThe asymmetric switching behavior is dictated by asymmetric device\nstructure rather than the initial ferroelectric state. Even if there exists randomness\nin the initial polarization state, the core of our model—i.e., the non-volatile modulation\nof photovoltaic properties by the migration of Cu⁺ ions under a polarization electric\nfield—remains valid. Consequently, the initial migration state of Cu+ does not affect\nthe functional performance of our device in the field of artificial intelligence neuro-\nvision. To substantiate this conclusion, we have performed additional experiments and\ncalculations on symmetric-contact devices (Gr/CIPS/Gr) to further clarify the role of\ncontact asymmetry in the polarity-switching sequence of the photovoltaic response.\n(1) We fabricated a symmetric Gr/CIPS/Gr device and characterized its\nphotovoltaic properties under the same measurement conditions as those used for Fig.\n4 in the main text (see Fig. R7). The results reveal a clear contrast in photocurrent\nbehavior between the symmetric structure and the asymmetric Pt/CIPS/Gr device: as\nthe positive or negative polarizing voltage increases gradually from 0 to 3 V, both the\nshort-circuit current (I ) and open-circuit voltage (V ) of the symmetric device\nsc oc\nexhibit polarity reversal. Notably, the anomalous asymmetry polarity-switching\nbehavior of I observed in the asymmetric device is absent in the symmetric device.\nsc\nThis comparison further indicates that the previously reported anomalous current\nreversal originates from the structural contact asymmetry of the device.\n(2) To explain the difference in photovoltaic behavior between symmetric and\nasymmetric structures from an energy-band perspective, we supplemented the study\nwith DFT calculations for the Gr/CIPS/Gr configuration (Fig. R8). The results\nclearly show that when both contacts are graphene, their Fermi levels align, and the\nband bending induced by Cu⁺ migration under an external electric field exhibits\nsymmetric characteristics. This finding is consistent with previous reports on the bulk\nphotovoltaic effect in CIPS [Nat. Commun. 12, 5896 (2021)]. This result further\nconfirms, from an electronic structure standpoint, that in a symmetric contact\nconfiguration, changes in polarization direction only leads to reversal of the\nphotocurrent direction, without inducing the complex response patterns (asymmetry\npolarity-switching) unique to asymmetric interfacial structures.\nWe have respectively added Figures R7 and R8 to Supplementary Information\n(Supplementary Fig. 19 and Supplementary Fig. 20). The corresponding textual\ndescription has been added from line 288 to line 304 in the text: “We also fabricated\na symmetric Gr/CIPS/Gr device and characterized its photovoltaic properties\n(Supplementary Fig. 19) under the same measurement conditions as used in Fig. 4. The\nresults reveal a clear contrast in photocurrent behavior between the symmetric structure\nand the asymmetric Pt/CIPS/Gr device. As the positive or negative polarizing voltage\nincreases gradually from 0 to 3 V, both the short-circuit current (Isc) and open-\ncircuit voltage (Voc) of the symmetric Gr/CIPS/Gr device exhibit polarity reversal. In\ncontrast, there is an asymmetric switching behavior in the asymmetric Pt/CIPS/Gr\ndevice, manifesting as that Isc reverses as increasing positive polarizing voltage, but\ndoes not reverse with changes in negative bias voltage. in the symmetric structure. To\nexplain the difference in photovoltaic behavior between symmetric and asymmetric\nstructures from an energy-band perspective, we have performed DFT calculations for\nthe Gr/CIPS/Gr configuration (Supplementary Fig. 20). The results clearly show that\nwhen both contacts are graphene, their Fermi levels are aligned, and the band bending\ninduced by Cu⁺ ions migration under an external electric field exhibits symmetric\ncharacteristics. This comparison strongly indicates that the previously reported\nanomalous current reversal originates from the structural asymmetry of the device\ninterface contact, rather than the initial ferroelectric state.”\nFig. R7 (Supplementary Fig. 19) | Nomal switchable FE-PV effect in the Gr/CIPS/Gr\nheterojunction. a, Individual I-V curves of the heterojunction device as V is set from 0 V to\np\n+3 V in log scale. b, Various I-V curves of the heterojunction device as V is set from 0 V to -3\np\nV in log scale. c, Plot of I dependent on positive V . d, Plot of I dependent on negative V .\nsc p sc p\nFig. R8 (Supplementary Fig. 20) | Crystal structure (upper) and LDOS (lower) of\nGr/CIPS/Gr heterostructure with polarization in CIPS perpendicular to the graphene.\n7. The F-score is reported as 1 in Fig. 5f, whether the score was calculated over\nmultiple test images or only one input. This clarification is important for\nreproducibility.\nReply: Thanks for reviewer’s valuable suggestion. We have done multiple test images\nfor edge detection using the same CNN and Prewitt kernels as done in Fig. 5.\nHerein, we have shown another three examples in below Fig. R9 (a)-(c). Note the F-scores for\nthree examples are all calculated to be 1 with FP=0 and FN=0 using the formulas in\nSupplementary Note 1, confirming the reproducibility of the Fe-PV device- based edge\ndetection application. We have included Fig. R9 as Supplementary Fig. 26 in the\nSupplementary Information and added the description in the text from line 237 to\n238: “Details regarding reproducibility, pre‑processing, and post‑processing\nassociated with the image edge detection are summarized in Supplementary Fig. 26-\n28.”\nFig. R9 (Supplementary Fig. 26) | Edge detection application with multiple test\nimage.\n8. The device is claimed to perform edge detection and binary pattern\nclassification for brain-like neural computing. Could the authors benchmark their\ndevice performance of accuracy and speed against existing neuromorphic\noptoelectronic systems to highlight the advantages and limitations of their FE-PV\nheterojunction approach?\nReply: The reviewer's suggestion is highly valuable. In accordance with the reviewer's\nsuggestion, we have conducted a comprehensive comparative analysis of the\nperformance of our device, focusing on its accuracy (image recognition accuracy) and\nspeed (photoelectric conversion—photocurrent response time), against existing\nneuromorphic photoelectric systems.\nWe supplemented the photoelectric transient response data of the device as\nshown in Fig. R10. Representative time-domain traces of the photocurrent under\npulsed illumination are presented in Fig. R10. The rise and fall times (measured\nbetween 10% and 90% of the peak current) are ~40 and ~45 μs, respectively. The\nresponse latency (measured between 10% and 50% of the peak current) from the optical\ntrigger is about 15 μs.\nFig. R10 | Transient photocurrent response.\nThe comparison results are systematically compiled in the Table R1. The core\nadvantage of our work lies in the realization of authentic in-sensor computing, which\nintegrates optical sensing, information preprocessing, and even partial neural network\ncomputation within a single device. Furthermore, by modulating the ferroelectric\npolarization, we have achieved dynamic and programmable visual perception. This\nenables the device to adapt online to varying lighting conditions or perform more\ncomplex tasks, much like a biological visual system, without requiring changes to the\nhardware. Moreover, the device exhibits a fast photoresponse speed of 40 μs, which\nenables rapid response under illumination and enhances the speed of in-sensor\ncomputing.\nHowever, the approach also presents limitations, the most significant of which is\nendurance. The fatigue effect in ferroelectric materials after repeated polarization\nswitching can lead to performance degradation. Addressing this issue is a critical\ndirection for future research aimed at practical applications.\nWe have included Table R1 (Supplementary Table 1) and the corresponding\ncomparative analysis in the Supplementary Information.\nTable R1. The performance comparison of existing neuromorphic optoelectronic devices\nReviewer #2 (Remarks to the Author)\nIn this manuscript, the authors investigate how the ferroelectric photovoltaic\neffect couples to Cu⁺ migration dynamics, yielding up to a 10x increase in\nphotocurrent and polarization-dependent polarity switching. By using simple\nstructure of Pt/CIPS/Gr, they demonstrate in-sensor computing application,\nincluding edge detection and pattern classification. The results are promising, but\nseveral fundamental questions remain before the full potential and limitations of\nthis approach can be assessed:\nReply: Thank you for your insightful comments and valuable suggestions, which have\nsignificantly contributed to enhancing our manuscript. According to your suggestions,\nthe point-by-point response to all thoughtful comments are addressed below.\n1- Figure 2 analyzes the effects of illumination power and polarization separately,\nbut their combined influence is not addressed. Under an applied VP, does\nillumination further modulate the device current? Please provide comparative\ndata (e.g., I-V or I-t curves) under dark vs. illuminated conditions at fixed VP for\nboth polarization states, and quantify any interaction.\nReply: We thank the reviewer for this suggestion. Accordingly, we have conducted the\nproposed experiment, which confirms the combined influence of illumination power\nand polarization on the device photoresponse.\nAfter poling at both positive and negative voltages, I-V characteristics under dark\nand illumination conditions have been supplemented, as shown in Figure R9(a-c) with\nthe applying polarizing voltages of -2 V, 0 V, and +2 V, respectively. In all fixed\npolarization states, the photocurrent increases with enhancing light intensity. In addition,\nunder a constant light intensity but different polarizing voltages (V ), the device exhibits\np\na distinct change in the rectifying characteristics compared to the initial state.\nSpecifically, after poling at V = -2 V, the current in the positive bias region is\np\nsignificantly higher than that in the negative bias region. In contrast, after poling at V\np\n= +2 V, the difference in current between the positive and negative bias regions becomes\nnegligible.\nFig. R11 | The photovoltaic effect in the Pt/CIPS/Gr heterojunction as a function of light\nintensity for various polarization states. a-c, I–V curves under V = -2 V, 0 V and +2 V,\np\nrespectively.\n2- Figure 2e shows negligible contrast between +P and −P at 1 V, whereas a clear\npolarization-dependent change emerges above ~1.5 V. Yet Figure 4 centers on VP\n= ±1 V. Please justify this choice (e.g., device reliability, leakage, or comparability\nconstraints).\nReply: We sincerely appreciate the reviewer’s meticulous review. Fig. 2e and Fig. 4\ndemonstrate consistent trends. At low poling voltages (±1 V), the short-circuit currents\nremain similar because the electric field is insufficient to induce strong polarization.\nA significant disparity between positive and negative poling only emerges at higher\nvoltages (>1.5 V). In Fig. 4, the ±1 V range on the horizontal axis represents the\nmeasurement scanning voltage, where the actual poling effect is negligible. In Fig. 4,\nthe ±1 V range on the horizontal axis represents the scanning voltage for measurement,\nat which the actual poling effect on the device is negligible. Therefore, this is consistent\nwith Fig. 2e, where the short-circuit currents under ±1 V polarizing are nearly identical.\nThis figure is intended to systematically illustrate the evolution of the device’s\nphotovoltaic current after applying different polarizing voltages. The experimental\nresults indeed show that at ±1 V, the difference in photovoltaic signals between the +P\nand –P states is negligible, whereas a clear and stable polarization-dependent\nphotovoltaic contrast emerges when the polarizing voltage reaches or exceeds ±1.5 V.\nBased on Figure 2e, polarization voltages need to be higher (e.g., ±1.5 V, ±2 V, ±2.5 V,\n±3 V), and are applied to set the polarization state (+P or –P) of device in a non-volatile\nmanner in the experiments presented in Figure 4. In contrast, scanning voltage need to\nbe small, and are applied after removing the polarizing voltage to perform current–\nvoltage (I–V) scans to extract the corresponding open-circuit voltage (V ) and short-\noc\ncircuit current (I ).\nsc\nThe scanning range between -1 and +1 V was chosen for measurement\nprimarily to minimize perturbation to the pre-set polarization state, ensuring a\nnon-destructive readout process while concurrently safeguarding high accuracy.\nAs demonstrated in Figure 2e, the polarization state remains stable under a 1 V bias and\nis not inadvertently switched. Therefore, using a ±1 V scanning voltage allows us to\nreliably probe the non-volatile modulation of the photovoltaic effect by previous poling\nhistory, without altering the preset polarization state. Moreover, this voltage range is\nsufficient to accurately capture variations in key photovoltaic parameters such as V\noc\nand I , while minimizing high-field leakage currents that could introduce noise or cause\nsc\ndevice degradation during repeated measurements. This approach significantly\nenhances data reliability and device longevity throughout testing.\n3- The introduction claims “ultrafast switching dynamics” in ferroelectrics, yet\nFigure 2 reports a 60 s poling time to set polarization. Please reconcile this\ndiscrepancy. Clarify whether 60 s is a stabilization/poling dwell rather than the\nintrinsic FE switching time, and quantify the minimum pulse width needed to\nachieve saturated (or ≥80%) Pr. Including switching-current transient’s data\nwould help separate true ferroelectric switching from slower Cu⁺ migration effects.\nReply: We thank the reviewer for their careful attention to the experimental details and\nfor raising this valid point. In our supplementary experiments, the minimum pulse width\nrequired to reach polarization saturation at 2 V was determined to be 13.5 seconds.\nSince the saturation time varies with the applied voltage, a standardized pulse duration\nof 60 seconds was used for all poling voltages to ensure complete and uniform\npolarization across the sample.\nAs the reviewer suggested, we clarify that the 60-second poling time used in our\nexperiments does not reflect the intrinsic ferroelectric switching speed, but rather\nis intended to ensure that Cu⁺ migration reaches a fully stabilized state.\nFerroelectric switching speed corresponds to the time required for the spontaneous\npolarization of a ferroelectric domain to reverse by 180 degrees under an external\nelectric field. However, the ferroelectric domains may undergo reversible recovery\nduring this period. In our work, by applying a sustained polarizing voltage, the\nferroelectric material is driven to a fully stabilized state, where its polarization and\ndomain structure have relaxed into a minimum-energy equilibrium. Although the initial\nresponse of Cu⁺ to an external electric field can be rapid, the final stable distribution of\nions among lattice sites requires a finite relaxation period [Nat. Commun. 15, 10822\n(2024)]. 60s poling time allows us to achieve a persistent and reproducible polarization\nconfiguration for reliable measurement.\nTo determine an appropriate poling duration, we systematically investigated the\nminimum time required for the device conductance to stabilize under different voltage\nbiases. As shown in Fig. R12 (Supplementary Fig. 11), the results confirm that the\ntime for the device current to reach a steady state after applying polarization\nvoltages of various amplitudes is consistently less than 60 seconds. Therefore, the\nuniform use of a 60-second poling period in the measurements related to Figure 2\nrobustly ensures the stability of the polarization state and prevents potential interference\nfrom rapid depolarization effects during subsequent photocurrent characterization.\nWe fully agree with the reviewer that analyzing switching current transients would\nbe highly beneficial to decouple the contributions of intrinsic ferroelectric reversal and\nionic migration. Regrettably, our current experimental setup lacks the necessary signal\nacquisition capabilities (e.g., sufficient temporal resolution and bandwidth) to reliably\ncapture these fast transients, preventing their inclusion in this work. As a\ncompensatory approach, our conventional I-t characterization shows a clear\ncurrent saturation trend under continuous bias (Fig. R12a), strongly suggesting\nthat the ferroelectric domains have evolved into a fully stabilized configuration.\nThis stabilization process is our focus. We plan to specifically design experiments to\nprobe these dynamics in follow-up work, which will allow us to more precisely resolve\nthe polarization response of CIPS across different time scales.\nWe have inserted Fig. R12 as Supplementary Fig. 11 into the Supplementary\nInformation, and added the following description in the text from line 152 to 154:\n“The 60-second polarizing time is intentionally adopted to ensure that Cu⁺ migration\nreaches a fully stabilized state (Supplementary Fig. 11).”\nFig. R12 (Supplementarry Fig. 11) | The ferroelectric device reached a stabilized\npolarization state under a constant polarizing voltage. a, When V = 2 V，the measured\np\ncurrent gradually saturated within 13.5 s. b, The stabilization time for the device conductance\ndid not exceed 60 seconds across the range of applied polarization voltages.\n4- In Figure 4, the authors propose a voltage-driven polarization mechanism in\nCIPS attributed to Cu⁺ migration, but the evidence provided is largely inferential.\nPlease substantiate this claim with direct, mechanism-specific measurements (e.g.,\nin-situ biasing TEM to track Cu site occupancy or switching-spectroscopy PFM\nand PUND versus pulse width/temperature to disentangle ionic migration from\nintrinsic ferroelectric switching). Absent such data, the mechanism should be\npresented as a hypothesis rather than a confirmed explanation.\nReply: We sincerely thank the reviewer for their insightful comments and constructive\nsuggestions. In response to the points raised, we have performed SEM-EDS and SS-\nPFM experiments and provided further discussion regarding the proposed mechanism,\nas detailed below:\n(1) We have performed cross-sectional scanning electron microscopy coupled\nwith energy-dispersive X-ray spectroscopy (SEM-EDS) to in-situ track Cu site\noccupancy (Fig.R13). Au electrodes are deposited on both surfaces of a CIPS bulk\nsample (~15 μm thick). After applying a 3 V voltage pulse to the bottom electrode for\n4 minutes to induce ion migration, we conducted elemental mapping of Cu, In, P, and\nS. The results clearly reveal a pronounced accumulation of Cu⁺ near the cathode, while\nthe distributions of In, P, and S remained uniform. This provides direct evidence of bias-\ninduced Cu⁺ migration in CIPS, which disentangle ionic migration from intrinsic\nferroelectric switching.\nFig. R13 (Supplementarry Fig. 18) | The cross-sectional scanning electron microscopy-\nenergy dispersive spectroscopy (SEM-EDS) spectra of CIPS. a, The cross-sectional view of\nbulk CIPS sandwiched between two gold electrodes (pink color) for the EDS measurement. b,\nThe redistribution of Cu, In, P and S element in the whole cross-sectional area.The dotted lines\nrepresent the interfaces between material and electrodes.\n(2) Switching spectroscopy piezoresponse force microscopy (SS-PFM)\nmeasurements were also carried out, as shown in Supplementary Fig. 17 and Fig.\nR14. As applying a DC bias, the phase exhibits three switching loops with 180°\ndifference. When the positive bias voltage increases, the polarization sequentially\nswitches from P↑ to P↓ and then reverts to P↑, completing a polarization switching\ncycle. In contrast, a negative bias induces a P↑ → P↓ polarization switching. Notably,\nincreasing the bias magnitude can generate a polarization direction antiparallel to the\nelectric field, i.e., a positive bias can generate an abnormal P↓ state or a negative bias\ncan generate an abnormal P↑ state. It primarily stems from Cu+ ions migration within\nthe van der Waals gap, consistent with the proposed mechanism to reveal the switchable\nFE-PV effect under unidirectional voltage.\nFig. R14 (Supplementarry Fig. 17) | Consecutive switching spectroscopy PFM (SS-PFM)\nhysteresis loop. As the DC bias voltage applied to the tip increases, an anomalous\npolarization reversal occurs in CIPS.\nFurthermore, the mechanism of electric-field-driven Cu⁺ migration as the origin of\npolarization switching in CIPS is also supported by literature [Nano Lett. 21, 995–1002\n(2021)]. As a two-dimensional van der Waals ferroelectric, the out-of-plane\nspontaneous polarization in CuInP₂S₆ arises from the off-center displacement of Cu⁺\nwithin sulfur octahedra. Under an external electric field, Cu⁺ ions can hop between\nadjacent crystallographic sites, leading to a transition from an ordered to a disordered\nconfiguration or a reorientation of polar domains, thereby modulating the macroscopic\npolarization state.\nTherefore, the above results collectively demonstrate that the voltage-driven\npolarization mechanism in CIPS is attributed to Cu⁺ migration.\nWe have inserted Fig. R13 as Supplementary Fig. 18 into the Supplementary\nInformation, and added the following description in the text from line 277 to 287:\n“In order to prove the Cu⁺ ions migration directly, we have performed cross-sectional\nscanning electron microscopy–energy dispersive X-ray spectroscopy (SEM-EDS) to\nvisually confirm Cu⁺ ions migration (Supplementary Fig. 18). Specifically, we prepared\na bulk CIPS sample with approximately 15 μm thick, sandwiched between two Au\nelectrodes for in-situ bias tests. The cross-sectional structure is shown in\nSupplementary Fig. 18a. After applying a 3 V bias pulse for 4 minutes, we immediately\nconducted EDS elemental mapping on the cross-section. As shown in Supplementary\nFig. 18b, In, P, and S remain uniformly distributed, while Cu exhibits pronounced\naccumulation toward the cathode side, forming a clear concentration gradient. These\nresults provide direct evidence that Cu⁺ ions migrate and redistribute within CIPS under\nan applied electric field.”\n5- Please clarify how intralayer vs interlayer Cu⁺ migration in CIPS maps onto the\nin-plane (Px, Py) and out-of-plane (Pz) polarization components. Is there a direct\ncorrespondence (e.g., interlayer displacement → Pz ; intralayer displacement →\nPx, Py)? A brief explanation, ideally with a schematic and expected measurement\nsignatures (vertical vs. lateral PFM, SHG) would improve completeness and help\ninterpret the Figure 4 evidence.\nReply: We are grateful to the reviewer for raising this important theoretical question,\nwhich provides us with a valuable opportunity for clarification. The ferroelectricity in\nour 25-nm-thick CIPS is exclusive to the out-of-plane direction, as confirmed by PFM\nmeasurements. We have also systematically differentiated between intralayer and\ninterlayer Cu⁺ migration as follows.\n(1) Intralayer migration is defined as the z-oriented shift of a Cu⁺ ion within\nits native sulfur octahedral cage. Interlayer migration, conversely, refers to the\nprocess where a Cu⁺ ion crosses the van der Waals gap along the z-direction and\npermanently incorporates into a sulfur octahedron in the adjoining layer, as show\nin Fig. R15. Specifically, Cu⁺ in CIPS can occupy three positions: the center position\nof the sulfur atom framework, the off-center position of the sulfur atom framework, and\nthe van der Waals gap [Phys. Rev. B 56(17), 10860 (1997); Nat. Commun. 15, 10152\n(2024); Nano Lett. 21, 995-1002 (2021)]. Therefore, when an electric field is applied\nalong the out-of-plane direction (Z direction), Cu⁺ can either shift within the same sulfur\natom layer (intralayer migration) or cross the van der Waals gap to enter the adjacent\nlayer (interlayer migration). Both of these migration modes modulate the average\ndisplacement of Cu⁺ in the out-of-plane direction, thereby altering the out-of-plane\npolarization component.\nFig. R15 | A schematic diagram for characterizing the intralayer and interlayer Cu⁺\nmigration in CIPS.\n(2) The in-plane polarization (Px, Py) of CIPS can be ignored in the thin-layer\nsamples used in this study. Our previous research has proved that, when the thickness\nis less than approximately 90 nm, the in-plane polarization component basically\ndisappears [Small 16, 1904529 (2020)]. The CIPS used in this study has a thickness of\n25 nm, thus effectively avoiding the interference of in-plane polarization. PFM\nmeasurements revealed a clear vertical ferroelectric response (Fig. R16), but no\nlateral ferroelectricity was detected. The out-of-plane ferroelectric domain structures\nwith obvious contrasts are observed in the phase and amplitude images, demonstrating\nrobust ferroelectricity of CIPS. Saturated PFM amplitude exhibits well-defined\nbutterfly loops accompanied by distinct 180° phase switching.\nFig. R16 | The out-of-plane PFM characterization of CIPS nanoflakes. a, The phase image\nof CIPS. b, The amplitude image of CIPS. c, Ferroelectric polarization switching characteristics\nfor CIPS flakes.\n(3) This work is based on Pt/CIPS/Gr vertical heterojunction devices, whose\nstructural design itself focuses on detecting and regulating out-of-plane\npolarization (Pz). The PFM data provided in the supplementary information is based\non the out-of-plane phase and amplitude image, which further confirm that the selected\n25 nm CIPS film has significant and stable out-of-plane ferroelectric switching\nproperties.\nBased on the above analysis, we believe that in the vertical device structure studied\nin this paper, the observed electrical behavior mainly results from the regulation of out-\nof-plane polarization (Pz), and the microscopic mechanism corresponds to the\nintralayer and interlayer migration of Cu⁺ under the action of an electric field, without\ninvolving the contribution of the in-plane polarization component.\nIn response, we have revised the description accordingly and clearly labeled it as\nan “out-of-plane” measurement (line 111 in the main text)to make the description\nmore clearly.\n6- The devices use ~25 nm-thick CIPS, yet prior reports indicate a pronounced\nthickness dependence, often with larger remanent polarization and improved\ndomain stability in thicker flakes (10.1002/smll.201904529). Please justify the\nchoice of 25 nm.\nReply: We thank the reviewer for your insightful comment regarding the thickness\nselection. The reasons for choosing ~25 nm-thick CIPS are as follows: On one side, it\nis sufficient to eliminate any in-plane polarization component, thus enabling a clear\nstudy of out-of-plane ferroelectric effects. On the other side, it provides an optimally\npronounced photoresponse for our investigation.\n(1) The use of 25 nm-thick CIPS effectively eliminates any contribution\nfrom in-plane polarization, allowing us to focus exclusively on out-of-plane\n∼\nferroelectric effects. The reference cited by the reviewer reports that in-plane\npolarization vanishes below a critical thickness of 90 nm in CuInP₂S₆ (CIPS) crystals,\nas characterized by piezoresponse force microscopy and contact resonance imaging. In\nour two-terminal vertical device architecture, which is designed to probe and\nmanipulate out-of-plane polarization states, the use of 25 nm-thick CIPS allowing us\nto focus exclusively on out-of-plane ferroelectric effects.\n∼\n(2) The choice of 25 nm thickness also enables a pronounced photocurrent\nresponse. The bulk photovoltaic effect (BPVE) in CIPS exhibits a strong\ndimensionality dependence. As reported in Nature Communications 12, 5896 (2021),\nthe photovoltaic effect becomes significant when the thickness of CIPS is below 80\nnm, while it drops to nearly noise level beyond this threshold. Therefore, the 25 nm\n∼\nthickness not only ensures purely out-of-plane ferroelectric operation but also lies well\nwithin the optimal range for strong bulk photovoltaic performance.\n7- It appears the Prewitt operator is applied directly to raw inputs without\ndenoising or contrast normalization. These can cause the risk of noise-induced\nspurious edges. If pre-processing was performed, please detail the methods and\nparameters; if not, please justify its omission. In addition, specify any post-\nprocessing (e.g., non-maximum suppression, thresholding method/parameters,\nkernel size) and assess the sensitivity of the results to these choices (an ablation\nwould be helpful).\nReply: We thank the reviewer for raising this important point. The detailed descriptions\nof the pre-processing and post-processing steps, as well as an ablation study on the\nthreshold in the binarization operation, have been provided as follows.\n(1) Pre-Processing\nIn fact, the original image whether is colorful or not is converted into grayscale,\nthen normalized between 0 and 1, and finally binarized into 0 and 1, being transformed\nto black and white image, where 0 is represented by black color and 1 is represented by\nwhite color. Subsequently, the Prewitt operator is applied to these black and white\nimages as input. Strictly speaking, these image pre-processing bring about the effect of\ndenoising and contrast normalization. These techniques are widely used in image\nprocessing. One of the main reasons for converting images to grayscale is to reduce\ncomputational complexity while preserving critical information, as RGB images\ncontain information from three channels: red, green, and blue, while grayscale images\nonly contain intensity information. The normalization operation can adjust the pixel\nvalues of an image to a specified range, such as between 0 and 1. The binarization\noperation is achieved by setting a threshold to divide grayscale pixels into two\ncategories such as 0 and 1. After these pre-processing, the pixel value of 1 and 0 are\nrepresented by applying and removing illumination stimuli when mapping the input\nimage to the Fe-PV heterojunction devices,\nIn the original manuscript, we have stated “Fig. 5a shows the schematic illustration of\nthe edge detection process. A 100 × 100 image showing a petal shape was used as the\ninput image. The pixel values in this image were converted to grayscale and then were\nbinarized, whose pixel values of black and white were defined as 1 and 0, respectively.”\nand illustrated the “Grayscale & Binarization” in Fig. 5(a).\nConsidering reviewer’s suggestion, they are changed to be “Figure 5a shows the\nschematic illustration of the edge detection process. A 100 × 100 image showing a petal\nshape was used as the input image. The pixel values in this image were pre-processed\nby converting to grayscale, then were normalized and binarized into 0 and 1, whose\npixel values of black and white were defined as 1 and 0, respectively.” (line 308-312 in\nthe revised manuscript) and illustrated the “Grayscale, Normalization & Binarization”\nin Fig. 5(a) in the revised manuscript.\nAdditionally, these pre-processing are provided in detail as follows in\nSupplementary Fig. 27.\nStep 1: Original image (Fig. R17 (a)) is converted into grayscale image (Fig. R17\n(b)), whose pixel values range between 0 and 255;\nStep 2: The normalization operation is performed to adjust the pixel values of a\ngrayscale image to a specified range between 0 and 1 (Fig. R17 (c));\nStep 3: The binarization operation is carried out by setting a threshold to divide\ngrayscale pixels into two categories such as 0 and 1 following the equation below (Fig.\nR17 (d)):\n0, x < t\nBinarized pixel value = ,\n1, x ≥ t\n{\nwhere t is a threshold value, and t = 0.5 was used in this work.\n(a) (b) Grayscale image\n200\n150\n100\n50\n0\n(c) After normalization (d) After binarization\n1 1\n0.8 0.8\n0.6 0.6\n0.4 0.4\n0.2 0.2\n0 0\nFig. R17 (Supplementary Fig. 27) | Pre-processing of original images before feeding into\nthe edge detection application.\n(2) Post-Processing\nAfter the convolution operation using the Prewitt kernels G and G , the obtained\nx y\noutput current I and I are then merged, normalized and binarized to obtain the final\nx y\noutput detected edges. These post-processing are stated in our original manuscript as:\n“Figure 5b shows two Prewitt kernels, where the kernel 1 extracts the gradient in x\ndirection (Gx) and the kernel 2 extracts the gradient in y direction (Gy), respectively.\nThe output current Ix and Iy related to kernel 1 and 2 fully captures the Gx and Gy\nfeatures respectively, clearly extracting the edges between the petal and the background\nalong the x and y direction, as shown in Figs. 5c and 5d. These two sets of output\ncurrents Ix and Iy can then be merged to find the absolute magnitude of the gradient at\neach pixel point (Fig. 5e). After these processes, the current values are further\nnormalized and binarized to obtain the final output image (Supplementary Note 2).” In\nparticularly, the post-processing operation is provided in detail in Supplementary Note\n2, in which the normalization and binarization are similar to that for pre-processing\nwith the same threshold of t=0.5.\n(3) An Ablation of Threshold in Binarization Operation\nAs discussed, the thresholding method is adopted in both pre- and post-processing.\nFactually, the thresholding is a fundamental method in image processing that classifies\npixels into different categories, mainly used to separate targets and backgrounds in\nimages. Herein, the fixed threshold method is employed, which directly set a global\nthreshold to mark points with pixel values above or below this value as foreground or\nbackground, respectively. The results in the manuscript are obtained using the threshold\nof t=0.5.\nAs the reviewer suggested, an ablation is performed to assess the sensitivity of the\nresults to threshold value in the post-processing of binarization operation, as shown in\nFig. R18. Figure R18 (a) shows the output edge images with different threshold value\nof t (= 0.1, 0.3, 0.5, 0.7 and 0.9) during the post-processing of binarization operation.\nFigure R18 (b) shows the magnification of output image, where the pixels that represent\nTP (blue), FP (pink), and FN (green) are distinctly labeled. From Fig. R18 (c), as t\nincreases, TP increases and then decreases, FP decrease and FN increases, all of which\nwould affect precision p and the recall r and thereby the F -score (Supplementary Fig.\n1\n28). When t is smaller than 0.5, non-edge pixels are detected as edge pixels, i.e., FP≠0,\nso p<1, making F -score below 1 (Fig. R18 (d)). When t is larger than 0.5, edge pixels\n1\nare detected as non-edge pixels, i.e., FN≠0, so r<1, also making F -score below 1 (Fig.\nt=0.5 offers the best F -score.\n1\nt=0.1 t=0.3 t=0.5 t=0.7 t=0.9\n1 1 1 1 1\n0 0 0 0 0\nTP = 38 TP = 39 TP = 40 TP = 29 TP = 12\nFP = 69 FP = 21 FP = 0 FP = 0 FP = 0\nFN = 0 FN = 0 FN = 0 FN = 11 FN = 28\n85.5\n86.5\n87.5\n88.5\n89.5\n90.5\n91.5\n92.5\n93.5\n94.5\n95.5\nTP True positive FP Falsepositive TNTrue negative FN Falsenegative\n60\n40\n20\n0\n0.0 0.2 0.4 0.6 0.8 1.0\nFig. R18 (Supplementary Fig. 28) | Simulation results with different threshold value of t\nduring the post-processing of binarization operation. a, Output edge images, b, local\nmagnification of output image, c, number of TP, FP and FN, d, precision, e, recall, and f, F -\n1\nscore.\nrebmuN\n1.2\nT F P P 1.0\nFN 0.8\n0.6\n0.4\n0.2\n0.00.0 0.2 0.4 0.6 0.8 1.0\nt\nnoisicerP\n1.2\np 1.0\n0.8\n0.6\n0.4\n0.2\n0.00.0 0.2 0.4 0.6 0.8 1.0\nt\nllaceR\n1.2\n1.0\n0.8\nr 0.6\n0.4\n0.2\n0.00.0 0.2 0.4 0.6 0.8 1.0\nt\nerocs-1F\n(a)\n(b)\n(c) (d) (e) (f)\nF1-score\nt\nWe have included Fig. R17-18 as Supplementary Fig. 27-28 in the\nSupplementary Information and added the description in the text from line 237 to 238:\n“Details regarding reproducibility, pre‑processing, and post‑processing associated with\nthe image edge detection are summarized in Supplementary Fig. 26-28.”\n8- Minor editorial issues:\n• In the Introduction, the following sentence appears twice in succession; please\nremove the duplicate:\n“Moreover, the spontaneous polarization and ultrafast switching dynamics of\nferroelectric materials can emulate synaptic plasticity in biological vision systems,\nfacilitating highly efficient and intelligent neuromorphic visual perception and\nprocessing systems.”\n• Reference 18 does not appear to be related to ferroelectric materials; please\nverify its relevance or replace it with an appropriate citation.\n• Line 101: “Pt/Gr/CIPS” should be corrected and the stack notation standardized\n(Pt/CIPS/Gr).\nReply: We sincerely thank the reviewer for the thorough review and valuable feedback.\nIn response to the comments raised, we have carefully revised the manuscript as follows:\n(1) We have highlighted the sentence in the Introduction: “Moreover, the\nspontaneous polarization and ultrafast switching dynamics of ferroelectric materials\ncan emulate synaptic plasticity in biological vision systems, facilitating highly efficient\nand intelligent neuromorphic visual perception and processing systems.” The\nrepetitive descriptions have been deleted in the revised article: “which facilitates\nthe realization of highly efficient and intelligent neuromorphic visual perception and\nprocessing systems”.\n(2) We have updated Reference 18 to [Adv. Funct. Mater. 32, 2109625 (2022)],\nwhich is more appropriate in this context and more closely aligned with the\ndevelopment of ferroelectric photovoltaic effects discussed in our study.\n(3) On Line 102 of the main text, we have corrected the device structure from\n“Pt/Gr/CIPS” to “Pt/CIPS/Gr” to accurately reflect the actual heterojunction\nconfiguration.\nReviewer #3 (Remarks to the Author)\nThis manuscript reports a ferroelectric–photovoltaic (FE-PV) device based on an\nasymmetric Pt/CuInP₂S₆/graphene (Pt/CIPS/Gr) heterojunction whose\nphotoresponse can be programmed via ferroelectric polarization and associated\nCu⁺ ion migration. The device shows polarity-dependent rectification, sign-\nreversible short-circuit photocurrent under poling, and ~10× enhancement in the\nmagnitude of the short-circuit photocurrent with appropriate polarization. The\nphysical picture is supported by density-functional theory (DFT) band-alignment\nanalysis and by temperature-dependent measurements near the CIPS Curie\ntemperature. The authors further connect the programmable photoresponsivity to\nin-sensor computing tasks (edge detection and 3×3 pattern classification),\noutlining a path toward neuromorphic vision hardware. In my view, this paper is\na promising contribution that can become publication-ready after substantial\nrevisions focused on quantitative benchmarking, direct mechanism validation,\nand clearer presentation. I therefore recommend Minor Revision\nReply: We thank the reviewer for the positive assessment of our work as a promising\ncontribution with potential for neuromorphic vision hardware applications. We have\naddressed all the thoughtful comments as below.\n1. The manuscript attributes the sign-programmable photoresponse to Cu⁺-ion\nmigration coupled with ferroelectric barrier tuning, supported by SS-PFM and\nDFT. However, PFM/SS-PFM and DFT are not species-specific and therefore\ncannot uniquely exclude look-alike mechanisms such as (i) pure ferroelectric\nbarrier modulation without ions, (ii) trap-mediated photogating or persistent\ninterfacial charging at the graphene/CIPS interface, (iii) photothermoelectric or\npyroelectric effects, (iv) Schottky/Dember photovoltages, or (v)\nelectrochemical/adsorbate-induced work-function shifts in graphene. To elevate\nthe claim from “consistent with” to “caused by” Cu⁺ motion, please add direct,\nchemistry-resolved evidence (e.g., bias-treated ToF-SIMS or gentle-sputter XPS\ndepth profiles; pre/post-poling STEM-EELS), quantify post-poling drift versus\ntemperature to extract an ionic activation energy, and include discriminating\ncontrols (ultrathin ion-blocking interlayers, symmetric-contact devices, and\nthickness/environment/capping sweeps).\nReply: We thank the reviewer for raising these critical points regarding the uniqueness\nof our proposed mechanism. We have supplemented SEM-EDS, SS-PFM, the\ntemperature-dependent short-circuit current and the critical symmetric-device control\nexperiments to provide more direct evidence and to distinguish our Cu⁺ migration\nmodel from other possible mechanisms.\nThe migration of Cu⁺ ions within CIPS and its direct role in polarization switching\nhave been previously demonstrated via STEM [Nat. Commun. 15, 10152 (2024)].\nHowever, the high resistivity of CIPS—characteristic of its non‑conventional\nsemiconducting nature—poses a significant challenge for extracting the ionic activation\nenergy through conventional electrical transport measurements. So, to provide more\ndirect evidence of Cu⁺ redistribution, we supplemented cross-sectional scanning\nelectron microscopy–energy dispersive X-ray spectroscopy (SEM-EDS)\nmeasurements to visually confirm copper ion migration. Specifically, we prepared\na bulk CIPS sample approximately 15 μm thick, sandwiched between two Au electrodes\nfor in situ biasing tests. The cross-sectional structure is shown in Fig. R19a. After\napplying a 3 V bias pulse for 4 minutes, we immediately conducted EDS elemental\nmapping on the cross-section. As shown in Fig. R19b, In, P, and S remain uniformly\ndistributed, while Cu exhibits pronounced accumulation toward the cathode side,\nforming a clear concentration gradient. These results provide direct evidence that Cu⁺\nions migrate and redistribute within CIPS under an applied electric field, ruling out\ninterference from other ionic migration or elemental segregation phenomena.\nCombined with I-V characteristics and SS-PFM loops (Supplementary Fig. 17), it\njointly indicates that the polarization switching phenomenon of the heterojunction\ndevice is caused by the migration of Cu⁺.\nIn order to distinguishing the origination from pure ferroelectric barrier modulation\nand other alternative mechanisms. We have provided the following discussion:\ni) We exclude the mechanism of pure ferroelectric barrier modulation without\nions. The combination of our new EDS evidence with the SS-PFM phase hysteresis\nloops (Supplementary Fig. 17)-which exhibit characteristic multi-step switching-allows\nus to rule out a scenario of pure ferroelectric barrier modulation without ionic migration.\nThe multi-step switching is a signature of the coupled ion-ferroelectric system in\nCIPS rather than pure ferroelectric barrier modulation.\nii) We have performed additional control experiments to discriminate against\nother look-alike mechanisms. We measured the I -T (short-circuit current vs\nsc\ntemperature) characteristics (Fig. 2f). The short-circuit current exhibits a sharp drop at\nthe Curie temperature, demonstrating its close coupling to the ferroelectric order in\nCIPS. Such a rapid response at the phase transition is inconsistent with the gradual\ndynamics expected for a trap-mediated photogating process.\niii) The photocurrent behavior of a symmetric Gr/CIPS/Gr device is\nconducted experimentally. It shows fundamental difference and lacks the anomalous\nswitching observed in the asymmetric Pt/CIPS/Gr structure (Fig. R23)—which rules\nout electrochemical/adsorbate-induced work-function shifts as the operative\nmechanism. This strongly implicates the role of contact asymmetry and the associated\ninterface barriers, which are modulated by ferroelectric polarization, in the observed\nphenomenon. This control helps to distinguish the mechanism from\nphotothermoelectric, pyroelectric, or Schottky/Dember effects, which would likely\nexhibit different symmetry dependencies. The new DFT calculations for the symmetric\nstructure (Fig. R24) further support this interpretation by showing symmetric band\nbending.\nIn summary, the direct observation of Cu⁺ migration via EDS, the distinctive\nswitching behavior in SS-PFM, the temperature-dependent short-circuit current, and\nthe critical symmetric-device control experiments, collectively allows us to robustly\nattribute the sign-programmable photoresponse to Cu⁺-migration-coupled ferroelectric\npolarization switching, and to confidently exclude the other alternative mechanisms.\nc) We have added the following description in the revised manuscript (line 167\nto 171): “These observations provide additional evidence that the photovoltaic effect\nin Pt/CIPS/Gr heterojunction is governed by ferroelectricity and help to distinguish the\nmechanism from photothermoelectric, pyroelectric, Schottky/Dember, trap-mediated\nphotogating or interfacial charging effects at the graphene/CIPS interface.”\nWe have inserted image R19 as Supplementary Fig. 18 into the Supplementary\nInformation, and added the following description in the text from line 277 to 287:\n“In order to prove the Cu⁺ ions migration directly, we have performed cross-sectional\nscanning electron microscopy–energy dispersive X-ray spectroscopy (SEM-EDS) to\nvisually confirm Cu⁺ ions migration (Supplementary Fig. 18). Specifically, we prepared\na bulk CIPS sample with approximately 15 μm thick, sandwiched between two Au\nelectrodes for in-situ bias tests. The cross-sectional structure is shown in\nSupplementary Fig. 18a. After applying a 3 V bias pulse for 4 minutes, we immediately\nconducted EDS elemental mapping on the cross-section. As shown in Supplementary\nFig. 18b, In, P, and S remain uniformly distributed, while Cu exhibits pronounced\naccumulation toward the cathode side, forming a clear concentration gradient. These\nresults provide direct evidence that Cu⁺ ions migrate and redistribute within CIPS under\nan applied electric field.”\nFig. R19 (Supplementary Fig. 18) | The cross-sectional scanning electron microscopy-\nenergy dispersive spectroscopy (SEM-EDS) spectra of CIPS. a, The cross-sectional view of\nbulk CIPS sandwiched between two gold electrodes (pink color) for the EDS measurement. b,\nThe redistribution of Cu, In, P and S element in the whole cross-sectional area.The dotted lines\nrepresent the interfaces between material and electrodes.\n2. To complete the neuromorphic-vision story end-to-end, please address:\nwrite (poling) kinetics—report the minimum pulse width/amplitude and\ncorresponding energy per write required to reach each non-volatile state, and\ninclude a 2D pulse-duration × voltage state map; read speed and bandwidth—\nquantify short-circuit photocurrent rise/fall times under modulated illumination\nand the −3 dB optical/electrical bandwidth at the stated read condition, with full\nmeasurement setup and load; transients—show representative time-domain traces\nfor write→read and light on/off to extract rise, fall, latency, and jitter, and clarify\nwhether reads are non-destructive; and noise/SNR—report noise spectral density,\nNEP, and specific detectivity at the read condition to bound achievable frame rate\nand MAC accuracy.\nReply: We thank the reviewer for these insightful suggestions regarding the\ncomprehensive characterization of our neuromorphic vision device. In contrast to\ntraditional synaptic devices, our approach innovatively utilizes a polarizing voltage to\nmodulate the polarization state of device. The corresponding photovoltaic current is\nthen read out across different states, enabling image detection. We have performed the\nfollowing additional measurements to address the above points raised:\n(1) Write (Poling) Kinetics. To determine the minimum pulse width/amplitude\nand the corresponding write energy required to reach each non‑volatile state, we applied\na 2.5 V polarizing voltage pulse with a duration of 1 s to program the device. As shown\nin Fig. R20, the device successfully entered a non‑volatile state. The electrical energy\nconsumption per write operation can be calculated using the following formula:\nwhere I is the current read at 1 V, V is the amplitude of the polarizing voltage (2.5 V),\n𝐸 = ∫𝑉×𝐼𝑑𝑡\nand t is the pulse duration (1 s). Based on this formula, the energy required to write\neach non‑volatile state is calculated to be 1.4 μJ.\nWe systematically investigated the effect of electrical pulse parameters on the\nshort-circuit current (I ). A 2D state map was constructed with polarizing voltage\nsc\n(V) as the x-axis and pulse duration (s) as the y-axis, and color-coded by the\nnormalized change in I after poling (Fig. R21). The map reveals distinct non-\nsc\nvolatile states achievable through different voltage-duration combinations. Regions in\nred correspond to a significant positive enhancement of I , while blue regions indicate\nsc\nsuppression. Notably, under a +2 V poling bias, the I can even be switched to a\nsc\nnegative value. This map effectively visualizes the parameter space for programming\ndifferent polarization states in the device.\n(2) Transients and Read Non-Destructiveness. Representative time-domain traces\nof the photocurrent under pulsed illumination are presented in Fig. R22a. The rise and\nfall times (measured between 10% and 90% of the peak current) are ~40 and ~45\nμs, respectively. The response latency (measured between 10% and 50% of the\npeak current) from the optical trigger is about 15 μs. Jitter is negligible, and the\nphotocurrent reliably returns to the baseline (~0 nA) after each pulse, confirming that\nthe readout process is non-destructive and does not alter the programmed polarization\nstate.\n(3) Read Speed and Bandwidth. The optical-to-electrical bandwidth of the\nheterojunction device was characterized under modulated 450 nm illumination. As\nshown in Fig. R22b, the −3 dB bandwidth is approximately 825 Hz, determined from\nthe frequency at which the normalized photoresponse drops by 3 dB. It shows that the\nframe rate of the device can reach 825 Hz.\n(4) Noise/SNR. The noise spectral density of the device at the readout condition is\nplotted in Fig. R22c. The noise floor stabilizes at a level of ~10⁻¹⁴ A/Hz¹/² for\nfrequencies above 10 Hz. This noise level limits the achievable frame rate and the\nprecision for multiply-accumulate (MAC) operations. The corresponding noise-\nequivalent power (NEP) can be calculated using the following formula:\n𝑖𝑛 ×√∆𝑓\nwhere i\nn\nis the noise current de𝑁n𝐸si𝑃ty,= ∆f is t\n𝑅\nhe noise equivalent bandwidth (1Hz), R\nis the photoresponse (52.4 mA/W). Based on this formula, the noise-equivalent power\n(NEP) is calculated to be 1.9×10-13 W/Hz1/2. The specific detectivity (D*) can be\ncalculated using the following formula:\n∗ √𝐴×∆𝑓\nwhere A is the effective photos𝐷ens=itive area (77.65 μm2). Based on this formula,\n𝑁𝐸𝑃\nthe specific detectivity (D*) is calculated to be 4.638×107 Jones. The NEP and D* will\nbe key metrics for subsequent performance optimization.\nWe have respectively added Figures R20-22 to Supplementary Information\n(Supplementary Fig. 8-10). The corresponding textual description has been added\nfrom line 141 to line 144 in the text: “To systematically characterize neuromorphic-\nvision characteristics, we have performed measurements of the write kinetics, transient\nresponse, read non-destructiveness and -3 dB bandwidth, and noise spectral density, as\ndetailed in Supplementary Figs. 8-10.”\nFig. R20 (Supplementary Fig. 8) | Achieving a nonvolatile state via a polarizing voltage\npulse. a, A 2.5 V voltage pulse with a duration of 1 s drives the device into a non‑volatile state.\nb, Under this pulse, the electrical energy consumed per write operation is 1.4 μJ.\nFig. R21 (Supplementary Fig. 9) | Modulation of the short-circuit current in the 2D pulse-\nduration × voltage space.\nFig. R22 (Supplementary Fig. 10) | Detector performance of the heterojunction device. a,\nTransient photocurrent response. b, Frequency response showing the –3 dB bandwidth. c,\nNoise power spectral density.\n3. The paper has included fabrication and measurements of an asymmetrical\nPt/CuInP₂S₆/graphene heterojunction and reports FE-PV tests under 450 nm\nillumination with DFT/SS-PFM support, which establishes the basic operation\nand mechanism narrative, but it does not include a symmetric-contact control to\nisolate the role of contact asymmetry in the polarity-switching sequence (the device\nis explicitly “vertical and asymmetrical,” and no Pt/CIPS/Pt or Gr/CIPS/Gr\ncontrol appears in the SI contents). I suggest the author fabricate and report a\nsymmetric (or quasi-symmetric) control and/or a contact-swapped variant\n(Gr/CIPS/Pt vs. Pt/CIPS/Gr), and tabulate polarity, open-circuit voltage, and\nshort-circuit photocurrent across these variants; additionally, include\nKPFM/CPD and scanning-photocurrent maps to localize interfacial barrier\nchanges and decisively link the switching sequence to contact asymmetry.\nReply: We thank the reviewer for their valuable suggestion. In response, we have\nperformed additional experiments and calculations on symmetric-contact devices\n(Gr/CIPS/Gr) to further clarify the role of contact asymmetry in the polarity-switching\nsequence of the photovoltaic response as follows.\n(1) We fabricated a symmetric Gr/CIPS/Gr device and characterized its\nphotovoltaic properties under the same measurement conditions as those used for\nFigure 4 in the main text (see Fig. R23). The results reveal a clear contrast in\nphotocurrent behavior between the symmetric structure and the asymmetric Pt/CIPS/Gr\ndevice: as the positive or negative polarizing voltage increases gradually from 0 to 3 V,\nboth the short-circuit current (I ) and open-circuit voltage (V ) of the symmetric\nsc oc\ndevice exhibit polarity reversal. Notably, the anomalous asymmetry polarity-\nswitching behavior of I observed in the asymmetric device is absent in the symmetric\nsc\ndevice. This comparison further indicates that the previously reported anomalous\ncurrent reversal originates from the structural contact asymmetry of the device.\n(2) To explain the difference in photovoltaic behavior between symmetric and\nasymmetric structures from an energy-band perspective, we supplemented the study\nwith DFT calculations for the Gr/CIPS/Gr configuration (Fig. R24). The results\nclearly show that when both contacts are graphene, their Fermi levels align, and the\nband bending induced by Cu⁺ migration under an external electric field exhibits\nsymmetric characteristics. This finding is consistent with previous reports on the bulk\nphotovoltaic effect in CIPS [Nat. Commun. 12, 5896 (2021)]. This result further\nconfirms, from an electronic structure standpoint, that in a symmetric contact\nconfiguration, changes in polarization direction only leads to reversal of the\nphotocurrent direction, without inducing the complex response patterns (asymmetry\npolarity-switching) unique to asymmetric interfacial structures.\nFig. R23 (Supplementary Fig. 19) | Nomal switchable FE-PV effect in the Gr/CIPS/Gr\nheterojunction. a, Individual I-V curves of the heterojunction device as V is set from 0 V to\np\n+3 V in log scale. b, Various I-V curves of the heterojunction device as V is set from 0 V to -3\np\nV in log scale. c, Plot of I dependent on positive V . d, Plot of I dependent on negative V .\nsc p sc p\nFig. R24 (Supplementary Fig. 20) | Crystal structure (upper) and LDOS (lower) of\nGr/CIPS/Gr heterostructure with polarization in CIPS perpendicular to the graphene.\nAs direct KPFM measurement under poling voltage could bring high risk of\ninstrument damage from the necessary external circuitry, it is difficult to be conducted.\nInstead, our combined experimental and theoretical approach provides compelling\nevidence. The variation in the interface barrier is definitively proven by short-circuit\ncurrent measurements and DFT calculations as above. Subsequently, by contrasting\nasymmetric and symmetric device structures (see comparative experiments above),\nwe are able to attribute the polarity-switching sequence specifically to contact asymmetry.\nWe have respectively added Figures R23 and R24 to Supplementary\nInformation (Supplementary Fig. 19 and Supplementary Fig. 20). The\ncorresponding textual description has been added from line 288 to line 304 in the\ntext: “We also fabricated a symmetric Gr/CIPS/Gr device and characterized its\nphotovoltaic properties (Supplementary Fig. 19) under the same measurement\nconditions as used in Fig. 4. The results reveal a clear contrast in photocurrent\nbehavior between the symmetric structure and the asymmetric Pt/CIPS/Gr device. As\nthe positive or negative polarizing voltage increases gradually from 0 to 3 V, both the\nshort-circuit current (Isc) and open-circuit voltage (Voc) of the symmetric Gr/CIPS/Gr\ndevice exhibit polarity reversal. In contrast, there is an asymmetric switching behavior\nin the asymmetric Pt/CIPS/Gr device, manifesting as that Isc reverses as increasing\npositive polarizing voltage, but does not reverse with changes in negative bias voltage.\nin the symmetric structure. To explain the difference in photovoltaic behavior between\nsymmetric and asymmetric structures from an energy-band perspective, we have\nperformed DFT calculations for the Gr/CIPS/Gr configuration (Supplementary Fig.\n20). The results clearly show that when both contacts are graphene, their Fermi levels\nare aligned, and the band bending induced by Cu⁺ ions migration under an external\nelectric field exhibits symmetric characteristics. This comparison strongly indicates\nthat the previously reported anomalous current reversal originates from the structural\nasymmetry of the device interface contact, rather than the initial ferroelectric state.”"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have satisfactorily addressed all of my queries and concerns with clear and appropriate explanations. The revisions have significantly improved the clarity and quality of the manuscript. In its present form, the manuscript meets the required standards and can be considered for publication.", "reviewer_2": "The authors have addressed all questions and comments. In my opinion, the revised manuscript is suitable for publication in Nature Communications, and I recommend its acceptance."}, "author_response": ""}]

Decision letter