Self‐Powered Perovskite Photodetector Arrays with Asymmetric Contacts for Imaging Applications

Designing photodetectors (PDs) with fast response and low power consumption is important for the realization of photoelectric conversion in photoelectric integrated systems. The emerging metal halide perovskite is proven to be a promising material for PD array devices due to its excellent photoelectric performance and large‐area production. However, the array integration of self‐powered perovskite PDs still faces challenges due to the incompatibility between perovskite and micro‐nano processing, hindering practical applications. Here, a perovskite‐compatible device fabrication process is reported for the construction of a perovskite PD array (10 × 10 pixels) with asymmetric contacts, enabling self‐powered photodetection and high‐performance imaging applications. Au and ITO/ZnO electrodes are deposited on the substrate in sequence as the asymmetric contacts. Patterned CH3NH3PbI3 films with uniform and pinhole‐free morphology are then synthesized by a photolithography‐assisted vapor–solution fabrication method as the photosensitive material. The introduction of asymmetric electrodes enables the easy collection of holes at the Au electrode side and electrons at the ITO/ZnO electrode side with light irradiation, resulting in self‐powered photodetection performance. Moreover, the PD array exhibits a uniform distribution of light response and is successfully demonstrated for light imaging. This work opens up a new avenue to develop large‐scale high‐performance perovskite optoelectronic devices with low power consumption.

For traditional PDs, an external power source is needed to prevent the recombination of the electron-hole pairs generated by the photoelectricity in order to achieve the photoelectric detection behavior, which hinders the miniaturization of the device.At the same time, the involvement of external power sources will inevitably lead to high dark current, small optical on/off ratio and high power consumption. [28,29]32][33][34][35][36][37][38][39] It can work without an external power supply, which can meet the requirement of a small size and low power consumption.There have been many attempts in this area to achieve highperformance perovskite PDs.For example, the detectivity of the single self-powered PD based on CH 3 NH 3 PbI 3 thin film fabricated with solvent engineering has exceeded 1.22 × 10 13 Jones. [40]esides, the self-powered photodetector arrays (4 × 5 pixels) based on all-inorganic perovskite quantum dots are reported by Kai Shen and their colleagues demonstrate a high open-circuit voltage of 1.3 V, responsivity of 10.1 A W −1 , specific detectivity of 9.35 × 10 13 Jones, and on/off ratio up to 10 4 . [41]However, limited by the incompatibility between perovskite and micro-nano processing (photolithography), the design of large-scale, highsensitivity, low-power PD arrays is still in urgent need.
In this work, patterned PbI 2 array is induced as the intermediate to achieve CH 3 NH 3 PbI 3 (MAPbI 3 ) perovskite array, which shows nice compatibility with micro-nano processing, and thus enables the successful fabrication of high-performance self-powered PD arrays.A simple lithographic-assisted vaporsolution (LAVS) fabrication method is used to achieve the largescale MAPbI 3 array, where patterned PbI 2 is first achieved with a vapor evaporation method and then converted into MAPbI 3 in desired shape.Morphology and structure characterization indicate that the synthesized MAPbI 3 is of high quality with large crystal size.Based on the MAPbI 3 film arrays, self-powered PDs are fabricated, which show large on/off current ratio (up to 3 × 10 4 ) and rapid response time (850/750 μs) under light irradiation.Moreover, the PD arrays also exhibit excellent electrical stability and performance uniformity and are successfully used for imaging applications.The self-powered PD arrays may have potential in photosensitive imaging, electronic eyes, and other biomimetic fields.

Results and Discussion
The photolithographic-assisted vapor-solution (LAVS) fabrication method is schematically displayed in Figure 1a.First, a pat-terned photoresist template was covered on the substrate after photolithographic process.Subsequently, PbI 2 thin film was vaporized in a vacuum chamber at 10 −4 Pa using thermally physical vapor deposition (PVD).Since PbI 2 thin films are compatible with acetone, we use acetone after this step to remove the photoresist film.Then, a solution of MAI dissolved in IPA (20 mg mL −1 ) was spin-coated on the prepared structurally homogeneous patterned PbI 2 film at 3000 rpm for 30 s in the glove box, followed immediately by annealing on a heating table at 100 °C for 10 min.As can be seen, the key to obtain high quality patterned MAPbI 3 film is to obtain a dense homogeneous PbI 2 layer.Figure S1 (Supporting Information) shows the obtained PbI 2 with different deposition rates.With a high deposition rate of 0.15 nm s −1 , a PbI 2 film with leaf-like structures and many irregularly shaped microcrystals were obtained.When the deposition rate was reduced to 0.04 nm s −1 , a dense and homogeneous patterned PbI 2 film was achieved, as observed in the top-view scanning electron microscopy (SEM) image, which further enables the formation of a dense and homogeneous patterned MAPbI 3 film on the substrate.Figure S2 (Supporting Information) compares PbI 2 films prepared by traditional liquid-phase method and the vapor-phase deposition method.Under the same conditions, the vapor-phase deposition method can obtain more dense, homogeneous and high-quality PbI 2 films with high repeatability than the liquid-phase method.
Figure 1b shows a MAPbI 3 film with a school emblem shape fabricated through LAVS fabrication method.It can be seen from the magnified SEM images that this method can realize the preparation of patterned MAPbI 3 with the shape from macroscopic to microscopic, and the surface of MAPbI 3 film is compact and the quality is high, which is much better than that obtained by the traditional liquid-phase method (Figure S3, Supporting Information).As shown in Figure 1c and Figures S4 and  S5 (Supporting Information), MAPbI 3 arrays with different patterns can be realized on any substrates including rigid substrate and flexible substrate with different photomasks, demonstrating the feasibility and universality of the synthesis method.At the same time, this process also provides the basis for us to fabricate high-pixel material to meet the processing requirements of PD. Figure 1d,e shows the rectangular pixel array as well as the linear array with high accuracy, where the minimum line width is 5 μm.[44][45][46] At the same time, the analysis shows that the half-width of the spectrum is 39.5 nm, slightly smaller than previous reports, indicating that the perovskite is of good quality. [42,47]The XRD characterization of the as-prepared PbI 2 film and the transformed MAPbI 3 film shown in Figure 1g indicates that PbI 2 has been completely con-verted to PVK after the second step.Choosing a suitable PbI 2 film thickness is of great importance, when the thickness of the PbI 2 film is too thick, the XRD spectrum of the transformed MAPbI 3 film will contain the peak position of PbI 2 , as shown in Figure S7 (Supporting Information), indicating that the transformation is not completely carried out.
Based on the LAVS fabrication method, a large-scale PD array is assembled.Considering the incapability of perovskite and polar solution, we first constructed bottom interdigital electrodes array, and then grow MAPbI 3 arrays at the desired position on each pair of electrodes (Figure S8, Supporting Information).In order to well separate the photogenerated electron-hole pairs, an asymmetric Au-ITO contact structure is introduced, where a ZnO array (≈8 nm thick) is deposited on the ITO electrodes as transport layer.Figure 2a shows the schematic illustration of the device fabrication process.Bottom electrodes array is first achieved, followed by the patterned growth of MAPbI 3 array through LAVS two-step method.Schematic structure of the integrated device and the circuit configuration can be seen in Figure 2b,c, respectively.Figure 2d presents the SEM images and partially magnified images of the PD arrays with regular shape and compact surface.Figure 2e is the Optical image of 10 × 10 pixelated PD arrays on PET substrate.The energy levels and carrier transport process schematic of the device are shown in Figure 2f.[50][51] So the structure of ITO-ZnO-MAPbI 3 can provide a good electron transport for perovskite.The work function of Au is 5.1 eV, which is close to the valence band of MAPbI 3 and can provide good hole transport. [32]This structural design enables the entire device to have an asymmetric electrical transport under illumination, thereby obtaining the photovoltaic performance.As shown in Figure 2g, when a bias voltage from −1 to 1 V is applied to the device, a clear photovoltaic phenomenon can be observed, accompanied by obvious short-circuit voltage and open-circuit current.In contrast, the devices with Au-Au and ITO-ITO symmetric electrodes show symmetric current transport under light illumination, where an external electrical filed is essential for the photodetection.
Photoresponse characteristics of the self-powered PD arrays were systematically investigated by the current-voltage (I-V) measurement of each pixel.Figure 3a shows the I-V curves of a typical designed PD under dark, 450, 520, and 633 nm laser irradiation, respectively (The optical power density is 12.01 mW cm -2 ).As can be seen, the PD not only exhibited low dark current (1 pA) but also a prominent open-circuit voltage (V oc ) and short-circuit current (I sc ).Moreover, the device shows the strongest response with light wavelength of 633 nm, which can be attributed to the direct interband absorption of perovskite at 1.59 eV. Figure 3b shows the I-V curves of the device with different light intensity at 633 nm.The current increases gradually and the photovoltaic effect becomes more obvious with the increase of the number of generated electron-hole pairs.The similar tendency is also observed under 450 and 520 nm laser illumination (Figure S6, Supporting Information).Then the V oc and I sc were calculated in Figure 3c, where maximum I sc and V oc are measured to be 213 nA and 0.58 V, respectively.
Figure 3d shows the time-resolved light response at different incident power density, illustrating good optical switching characteristics.When the laser is turned on and off periodically, the I sc rises and falls accordingly.It can be also observed that the saturation level increases gradually with the increase of electronhole pairs by increasing optical power density, but the dark current still maintains at a relatively low level, indicating nice cut off performance, a similar trend was observed under 520 nm and 420 nm laser illumination (Figure S9, Supporting Information).As shown in Figure S10 (Supporting Information), compared with the symmetrical Au-Au electrode structure, the device with asymmetric ITO/ZnO-Au electrode structure has not only low power consumption, but also smaller dark current.Figure S11 (Supporting Information) shows the optical switching characteristics by changing the timing and period of the pulsed laser.It can be seen that the self-powered PD has stable light response under different illumination intensities and different pulsed laser.Meanwhile, the self-powered photodetector also possesses a rapid rise time of 850 μs and a fall time of 750 μs (Figure 3e), which are faster than those of the previously reported typical perovskite PDs. [3,38]The on/off current ratio (I on /I off ) of the device under different light power density was further characterized respectively with bias voltage of 0 V and 1 V (Figure 3f; Figure S8, Supporting Information), it is obvious that under the self-powered mode, the on/off ratio is larger than that under bias voltage of 1 V.When the light intensity reaches 10 mW cm −2 , the on/off current ratio of the device under the self-powered mode can reach four orders of magnitude, which is much better than the PD arrays based on other materials and structures (Table S1, Supporting Information).
In large-scale photoelectric imaging system, self-powered PD arrays have great practical application value in imaging applications.When the array device is applied to the imaging process, each individual PD will act as a pixel, the stability and uniformity of photoelectric characteristics of all pixels determine the imaging functions.As shown in Figure 4a, the self-powered PD runs 300 s (≈150 cycles), and the performance of the PD has little change.As can be seen in Figure S14 (Supporting Information), the device still possesses nice switch characteristics, and very weak attenuation in the photocurrent is observed after continuous operation for ≈1 h, indicating good stability.An ideal imaging sensor requires the performance of all pixels to be basically the same, which is convenient to coordinate the work among pixels.One of the ways to evaluate the performance differences of each pixel is to measure the dark state current and photocurrent of each single pixel.The dark current and photocurrent are closely related to the crystallinity of the material and directly represent the photoresponse performance of pixels.Figure 4b shows the dark state current and photocurrent of 100 pixels under the incident power of 12.01 mW cm −2 at self-powered mode.It can be found that the dark state current of most pixels is very small, distributed in the interval of 10 −14 to 10 −13 A. For the on-state current, most of the devices are larger than 10 −9 A. The above results show that the photoelectric properties of pixels of the device have satisfactory stability and uniformity.
Finally, in order to verify the imaging capability of the selfpowered PD arrays, we conducted a prototype experiment on the principle of imaging.A pre-designed shadow mask was placed between the light source and the device.When light passed through the pre-designed shadow mask, speck spots were formed on the pixels of the self-powered PD array, as shown in Figure 4c.The pixels under illumination have higher current values, while the others blocked by the shadow mask show no light response.We used masks of "H", "N", and "U" respectively to obtain the 2D planar graph as shown in Figure 4d-f.Clear letters of "H", "N", and "U" could be identified from the mapping results, and the resulting pattern was consistent with the pattern of light spot, indicating that each pixel of self-powered PD array could work independently and had basic light intensity reflection ability.that means the self-powered PD array has reliable imaging capability, showing potential application prospects in visual sensing.

Conclusion
In summary, we have demonstrated the large-scale self-powered photodetector arrays based on the MAPbI 3 film for photosensing and imaging.The large-scale patterned MAPbI 3 film array with precise position, uniform, and pinhole-free morphology was synthesized by the facile photolithograpy-assisted vapor-solution (LAVS) fabrication process as photosensitive material.The LAVS process enables high-density integration and low-cost preparation, which can be applied to other vapor-solution-processed perovskites.Meanwhile, asymmetric Au-ITO electrode pairs are designed to achieve self-powered PDs.Large on/off current ratio (up to 3 × 10 4 ), and fast rise/decay times of 850/750 μs are successfully demonstrated, which can be attributed to the efficient carrier generation, separation, and transport resulting from favorable array structure and energy band arrangement.We believe that the device has potential applications in optical communication, digital display, and optical sensing and imaging applications.

Experimental Section
Fabrication of Self-Powered PD Arrays: Asymmetric electrodes were fabricated through UV lithography technique (MIDAS MDA-400M), metal deposition, and lift-off process.Cr/Au (10/60 nm) electrode patterns and ITO/ZnO (60/10 nm) electrode arrays were obtained separately by UV lithography technique on the pre-treated substrate with a typical engraving process.Cr and Au were deposited by vacuum thermal evaporation, while ITO and ZnO were achieved with RF magnetron sputtering (PVD75, Kurt J. Lesker).At this point, the bottom electrode arrays were complete, there were 100 pairs of interdigital electrodes where both electrode width and interelectrode space length were 10 μm.The active area of each PD was 9 × 10 −3 mm 2 .MAPbI 3 films were synthesized by the previously reported LAVS fabrication method.Alignment lithography was performed to pattern 100 rectangle patterns on the prefabricated interdigital electrodes, where the size of each rectangle pattern was 100 μm × 100 μm.PbI 2 thin film (The thickness was ≈90 nm) was vaporized in a vacuum chamber at 10 −4 Pa using thermally physical vapor deposition.Then the photoresist was removed with acetone.Next, CH 3 NH 3 I solution (in isopropyl alcohol, 20 mg mL −1 ) was spin-coated on the as-prepared PbI 2 films at 3000 rpm for 30 s in the vacuum glove box, followed by immediately annealing treatment at 100 °C for 10 min to enhance the crystallization.Finally, the device was packaged by PMMA to cut off air and water.
Characterizations and Measurements: The morphological and phase analyses of the as-synthesized MAPbI 3 perovskite arrays were carried out with field-emission scanning electron microscopy (UK) and X-ray diffraction (X' Pert3 Powder) -2 scan with Cu K radiation.Optical properties of perovskite film were measured by a UV-vis-NIR spectrophotometer (UV-3600, Shimadzu) and a full-function fluorescence spectrometer (FLS980-S2S2-stm).UV exposure machine (MIDAS MDA-400M) and RF magnetron sputtering (PVD75, Kurt J. Lesker) were adopted for the fabrication of designed electrodes.The electrical and optoelectronic properties of the self-powered photodetector arrays were measured in vacuum (10 −4 mbar) using a Lake Shore Probe Station and an Agilent-B1500 semiconductor analyzer at room temperature.The time response of the device was measured by switching the 450, 520, and 633 nm laser on and off with an internal square-wave trigger source (Thorlabs ITC 4001).

Figure 1 .
Figure 1.Synthesis process and characterizations of perovskite arrays.a) Schematic illustration of synthesis process of MAPbI 3 arrays.b) Optical, SEM images of large-scale MAPbI 3 arrays and magnified SEM images of the film surface.c) SEM image of patterned MAPbI 3 arrays.d,e) SEM of large-scale patterned MAPbI 3 films with different patterns and accuracy.f) PL spectrum of MAPbI 3 thin film (under excitation wavelength of 488 nm).g) XRD diffraction measurements of as-synthesized PbI 2 and MAPbI 3 arrays.

Figure 2 .
Figure 2. Fabrication process and structure of self-powered PD arrays.a) Schematic illustration of the device fabrication process.b) Schematic structure of the self-powered PD arrays.c) Structural representation of the circuit with asymmetric electrodes and enlarged image of a single pair of electrodes.d) SEM image of the PD arrays and enlarged SEM image of a single PD in device.e) Optical image of 10 × 10 pixelated self-powered PD arrays.f) The energy levels and carrier transport process schematic of the device.g) Typical logarithm I-V curves of three different electrode structures' single device under 633 nm laser with the light intensity of 0.71 mW cm −2 .

Figure 3 .
Figure 3. Photoresponse characteristics of the self-powered PD arrays.Logarithm I-V characteristics of individual PD measured with a) light source with different wavelength (450, 520, and 650 nm) and b) with incident light power density from 0 to 39.96 mW cm −2 (wavelength: 633 nm).c) The I sc and V oc statistics of single device under 633 nm laser illumination.d) Photo-switching property of single device at the photovoltaic mode under 633 nm laser illumination with the power densities vary from 0.014 to 39.96 mW cm −2 , respectively.e) Time-resolved photoresponse measured with V ds = 0 V (0.71 mW cm −2 ).f) The on/off current ratio of individual pixel under different illumination intensities.

Figure 4 .
Figure 4. Demonstration of light imaging application.a) Repeatable photoresponse of the individual pixel, with no bias voltage, under an illumination intensity of 12.01 mW cm −2 b) Dark current and photocurrent statistics of all device pixels with illumination intensity of 12.01 mW cm −2 .c) Schematic illustration of the self-powered PD arrays to detect multipoint light distribution.d-f) Imaging results of the device with different shape of masks.