Fig. 1. (Color online) The schematic diagram of the topics in this review, including the preparation methods, performance optimization, and device applications of perovskite PDs. Preparation methods. Reprinted with permission from Ref. [20]. Copyright 2022 Springer Nature. Reprinted with permission from Ref. [21]. Copyright 2022 Wiley-VCH. Reprinted with permission from Ref. [22]. Copyright 2022 Springer Nature. Reprinted with permission from Ref. [23]. Copyright 2022 Wiley-VCH. Performance optimization. Reprinted with permission from Ref. [24]. Copyright 2023 Wiley-VCH. Device applications. Reprinted with permission from Ref. [25]. Copyright 2023 Springer Nature. Reprinted with permission from Ref. [3]. Copyright 2021 American Chemical Society. Reprinted with permission from Ref. [26]. Copyright 2020 Springer Nature. Perovskite crystal structure. Reprinted with permission from Ref. [27]. Copyright 2014 Springer Nature.

Fig. 2. (Color online) Primary structures of PDs and their working principles. (a) Photodiodes. (b) Photoconductors. (c) Phototransistors. Reprinted with permission from Ref. [28]. Copyright 2024 American Chemical Society.

Fig. 3. (Color online) (a) Schematic illustration of the fabrication processes of FAPbI₃ film. Reprinted with permission from Ref. [23]. Copyright 2022 Wiley-VCH. (b) Schematic diagram of the CVD setup for the synthesis of Cs₃Cu₂I₅ nanosheets. Reprinted with permission from Ref. [21]. Copyright 2022 Wiley-VCH. (c) Schematic diagram of 2T-CVD processes. Reprinted with permission from Ref. [33]. Copyright 2021 Wiley-VCH. (d) Illustration of the formation process for the composition-graded films. Reprinted with permission from Ref. [45]. Copyright 2023 Springer Nature. (e) Preparation of spin-coating the Cs₃Bi₂Br₉ films. Reprinted with permission from Ref. [46]. Copyright 2021 Springer Nature. (f) Illustration of the home-made spray-coating setup. Reprinted with permission from Ref. [40]. Copyright 2022 American Chemical Society. (g) Schematic illustration of the EHD printing fabrication process and perovskite transformation from MAPbX₃ ink to polycrystalline film. Reprinted with permission from Ref. [47]. Copyright 2021 Wiley-VCH.

Fig. 4. (Color online) (a) and (b) Top-view SEM image of the Cs₃Cu₂I₅ perovskite films without/with MA (Inset: size distribution of the Cs₃Cu₂I₅ perovskite films without/with MA). Reprinted with permission from Ref. [67]. Copyright 2020 Elsevier B.V. (c) Photograph for growing: (ⅰ) MSCs and (ⅱ) finished products under room light. Reprinted with permission from Ref. [68]. Copyright 2023 Wiley-VCH. (d) Representative TEM images of Cs₂AgBiBr₆ nanosheets with different magnification. Reprinted with permission from Ref. [69]. Copyright 2021 Springer Nature. (e) Characterizations of MAPbBr₃ SCMWAs. Fluorescence images of straight (ⅰ) and curved (ⅱ) MAPbBr₃ SCMWAs excited by a 405 nm laser. SEM images of straight (ⅲ) and curved (ⅳ) MAPbBr₃ SCMWAs. Reprinted with permission from Ref. [70]. Copyright 2020 Wiley-VCH. (f) SEM images of the heterojunction structure. (g) Pb, I, Br elements distribution pattern of the heterojunction. Reprinted with permission from Ref. [71]. Copyright 2022 Wiley-VCH. (h) High-resolution TEM images of the Cs₃Bi₂Br₉ QDs encapsulated in a BiOBr matrix. Reprinted with permission from Ref. [72]. Copyright 2020 The Royal Society of Chemistry.

Fig. 5. (Color online) (a) Energy band alignments of CuI, CsCu₂I₃, and GaN. (b) Schematic energy band diagram of CuI/CsCu₂I₃/GaN heterojunction under light illumination. Reprinted with permission from Ref. [78]. Copyright 2022 Elsevier. (c) Device structure of the hybrid perovskite PD. (d) Energy diagram of the perovskite PD under a slight reverse bias. (e) Current density−voltage curves of PDs with and without the hole-blocking layer. PD1, without hole-blocking layers; PD2, with BCP as the hole-blocking layer; and PD3, with PFN as the hole-blocking layer. Reprinted with permission from Ref. [18]. Copyright 2014 Springer Nature. (f) Schematic illustration of the fabrication process of the gradient-O CdS/perovskite PDs. (g) The schematic representation of gradient energy levels, and carrier transport at the gradient-O CdS/perovskite interface. Reprinted with permission from Ref. [79]. Copyright 2019 Wiley-VCH. (h) The structure diagram of BDASnI₄ BGTC FET and the chemical structure of ASIs. Reprinted with permission from Ref. [80]. Copyright 2023 Wiley-VCH. (i) Energy band diagram of Au/p-CsCu₂I₃/n-Ca₂Nb_3−xTa_xO₁₀/MXenes device. WF: work function. Reprinted with permission from Ref. [81]. Copyright 2022 Wiley-VCH.

Fig. 6. (Color online) (a) SEM image of the 600 nm-perovskite film. (b) Magnified SEM image of the film surface. Reprinted with permission from Ref. [82]. Copyright 2022 The Royal Society of Chemistry. (c) Magnified SEM images of the moiré perovskite at different positions. Reprinted with permission from Ref. [83]. Copyright 2022 Wiley-VCH. (d) Designed geometrical parameters of BNA arrays with MIM configuration. (e) and (f) The E-field distribution (|E|²/|E₀|²) under 775 nm (LSPR mode) in x−y and x−z plane. Reprinted with permission from Ref. [84]. Copyright 2020 Wiley-VCH. (g) Schematic diagram of fabrication process of CN-patterned PDMS stamp. (h) Electric field of CN for x-(top), and y-(bottom) polarized light (670 nm). Reprinted with permission from Ref. [85]. Copyright 2024 Wiley-VCH.

Fig. 7. (Color online) (a) The transmission mode oximetry. (b) The reflection mode oximetry. Reprinted with permission from Ref. [88]. Copyright 2023 Wiley-VCH. (c) Schematic diagram of the working principle of the PPG test in transmission mode. Volumetric changes in the blood vessels modulate the transmitted light intensity. (d) Photograph of the FPD attached on finger pulp as PPG sensor for recording blood pulse signal. (e)−(g) Comparison of PPG signals detected by the FPD under different incident light intensities (72, 4.6, and 2 mW·cm⁻²) when the CE was applied with 0 and 0.1 V. The calculated blood pulse frequency was 67 beats per minute. a.u. arbitrary units. Reprinted with permission from Ref. [25]. Copyright 2023 Springer Nature. (h) Schematic illustration of the application of flexible mixed Sn−Pb (FMSP) PPD in wearable remote health monitoring (TX: transmitter RX: receiver). (i) and (j) The pulse signal measured from the FMSP PPD (transmitted pulse signal) and the received pulse signal by optical communication at rest and after-run conditions. Reprinted with permission from Ref. [89]. Copyright 2022 Wiley-VCH. (k) The detailed structure diagram and physical schematic of the flexible PPG signal sensor (inset: Left: flexible perovskite PD; right: red led used as light source). Scale bar: 0.5 cm. The photoplethysmography (PPG) signals of the fingers are under different swelling degrees. (l) Schematic diagram of varied working modes of PPG signal sensor. (m) The detailed waveforms of PPG signals corresponding to fingers with different swelling degrees. (n) The basic waveforms of PPG signals corresponding to fingers with different swelling degrees under 635 and 532 nm (the degree of swelling increases from left to right). (o) Calculated blood-oxygen saturation values according to the PPG signals under 635 and 532 nm. Reprinted with permission from Ref. [90]. Copyright 2023 Wiley-VCH. (p) Schematic illustration of the perovskite photodetector-based PPG sensor. The inset is the working principle diagram of the PPG sensor and the schematic diagram of signal components received by the photodetector. Photograph of the PPG sensor in the nonworking (top) and working (bottom) conditions. (q) PPG signals under different illumination intensities of 8.16, 2, 0.423, and 0.055 mW·cm⁻², respectively. Reprinted with permission from Ref. [91]. Copyright 2024 Wiley-VCH. (r) Schematic illustration showing the UV monitor works in daily life. The data can be transmitted to the user’s mobile phone and uploaded to the cloud. (s) Photograph of a wearable flexible UV monitor. Reprinted with permission from Ref. [92]. Copyright 2022 Elsevier. (t) Diagram of the brief working principle of UV monitoring. The data can be transmitted to the user’s mobile phone via Bluetooth terminal for real-time display. (u) Photograph of the designed flexible circuit board on arm (the inset shows the flexible device). Reprinted with permission from Ref. [93]. Copyright 2024 Royal Society of Chemistry.

Fig. 8. (Color online) (a) Schematic illustration of the folding steps to create a cubic PD from a 2D pattern. (b) A Cartesian coordinate system is built based on the cubic PD. (c) The signals (normalized current) of pixels on each face when the cubic PD was illuminated along different directions. Reprinted with permission from Ref. [35]. Copyright 2017 American Chemical Society. (d) The ΔV and PDCR of the perovskite PD under sunny, cloudy, and room lighting conditions. The sunny and cloudy days occurred at N 22°18′30″ and E 39°06′20″ at 11:00 on March 16th, 2017 and 15:00 on March 21st, 2017, respectively. Reprinted with permission from Ref. [95]. Copyright 2018 Wiley-VCH. (e) (ⅰ) Digital image of curved solar-blind PDs array attached on the hemisphere support. Inset is the corresponding 2D plane diagram. (ⅱ) Current variation of the pixels at different positions with the flame (220 µW·cm⁻²) close to the A5 pixel. Current distribution of the curved solar-blind PDs array under single flame (f) and multi-flame (g) irradiation. Reprinted with permission from Ref. [96]. Copyright 2023 Wiley-VCH.

Fig. 9. (Color online) (a) The schematic design of SnO₂/CABB PD based transmittance imaging system. (b) and (c) Imaging results under a light intensity of 0.5 and 5 μW·cm⁻², respectively (the scale bar is 0.4 cm). (d) SNRs of the imaging results extracted from (b) and (c). Reprinted with permission from Ref. [64]. Copyright 2021 Elsevier. (d) Schematic diagram of the diffuse reflection imaging system. The diffuse reflection images of "butterfly outline" detected by (e) commercial silicon photodiode S2386 and (f) our flexible PD. Reprinted with permission from Ref. [101]. Copyright 2022 Wiley-VCH. Reflective single-pixel imaging based on the MAPbBr₃ microwire arrays (MWAs) single-pixel detector (SPD). (g) Schematic of reflective single-pixel imaging (SPI) experimental setup. (h) The reconstructed images of objects "1" and "2" with different colors and backgrounds. Reprinted with permission from Ref. [102]. Copyright 2022 Wiley-VCH.