Fig. 1. (Color online) (a) Schematic illustration of the electromagnetic spectrum ranging from visible to infrared regions. (b) Spectral response ranges of commonly used materials with different dimensions for infrared photodetection.

Fig. 2. (Color online) (a) Schematic illustration of a photoconductor photodetector array. (b) Schematic illustration of a photodiode photodetector array. (c) Schematic illustration of a phototransistor photodetector array.

Fig. 3. (Color online) (a) Schematic illustration of the fabrication process for a TiN/GeSn heterojunction photodetector, and (b) corresponding optical photograph^[45]. (c) Schematic illustration of epitaxial AlAs and In_xAl_1−xAs arrays directly grown on a GaAs substrate, and (d) corresponding SEM cross-sectional image^[46].

Fig. 4. (Color online) (a) Schematic illustration of an infrared photodetector based on UCNPs and BHJ, with corresponding mechanism diagram. (b) Normalized absorbance spectra of DPPTT−Tin solution, resulting film, and UCNP upconversion fluorescence spectrum^[57]. (c) Chemical structures of two narrow bandgap semiconductors, PBTT and PBTB. (d) and (e) Absorption spectra of PBTT and PBTB in solution and as fabricated films, respectively^[58].

Fig. 5. (Color online) (a) Schematic illustration of various 2D TMDs and end-functionalized polymers. (b) Absorbance spectra for a range of two-dimensional materials. (c) Schematic and optical photographs of a photodetector array composed of MoSe₂ films exfoliated by PS−NH₂, scale bars: (ⅰ) 6 mm; (ⅱ) 500 mm. (d) Schematic illustration of a flexible infrared detector using MoSe₂−PS−NH₂ composite films and its film morphologies, scale bars: (ⅲ and ⅳ) 500 nm. (e) Schematic illustration of a flexible infrared detector using MoSe₂/MoS₂−PS−NH₂ composite films and its EDS mappings, the scale bar, 2 mm^[84].

Fig. 6. (Color online) (a) Schematic illustration of the PbS quantum dot photodiode structure accompanied by its cross-sectional SEM image, and (b) the corresponding energy level diagram. (c) Schematic of the integration of PbS quantum dots with polyimide^[88]. (d) Absorption spectra of quantum dot solutions with and without polyimide after 24-h storage. The inset is the optical photograph of quantum dot solutions; the left without PI, and the right with PI^[89]. (e) Schematic illustration of a device utilizing CQDs for the infrared-sensitive layer. (f) Schematic of mask imaging under infrared illumination^[90].

Fig. 7. (Color online) (a) Schematic illustration of one-dimensional polymer nanowires with donor−acceptor (D−A) core-shell heterojunction structure^[92]. (b) Schematic illustration of a Ga−In₂O₃ nanowire phototransistor. (c) Performance comparison of I_light/I_dark ratios with similar devices^[93]. (d) Morphologies of Te nanomeshes directly grown on various substrates^[94].

Fig. 8. (Color online) (a) Schematic illustration of the fabrication process for a flexible InAs photodetector, employing molecular beam epitaxy and epitaxial lift−off techniques. (b) Schematic diagram of the device with vertical stacking structure^[47].

Fig. 9. (Color online) (a) Schematic illustration of the physical vapor deposition setup for depositing Sb₂Te₃. (b) Film morphology and composition after varying deposition times^[103]. (c) SEM image of directly epitaxial Sb₂Se₃ films grown on mica substrates and (d) corresponding XRD spectra. (e) I−V curve comparisons for photodetectors fabricated from epitaxial and non-epitaxial Sb₂Se₃ films^[44].

Fig. 10. (Color online) (a) Schematic illustration of the fabrication process for flexible electronic devices using metal−organic chemical vapor deposition. (b) Optical photograph and structural diagram of large-area MoS₂ prepared on a flexible parylene-C substrate^[109].

Fig. 11. (Color online) (a) Process diagram for the fabrication of PbS and ZnO quantum dot heterostructure via spin-coating and (b) schematic illustration of the completed device. (c) Absorption spectra of PbS, ZnO, and PbS/ZnO films. (d) I−V curves of the PbS/ZnO quantum dot heterojunction photodetector under various light intensities^[112]. (e) Process diagram for fabricating flexible NIR photodetectors using all-template printing. (f) I−V curves at different radii of curvature^[114].

Fig. 12. (Color online) (a) Chemical structure diagrams of YZ and YZ1 and (b) PCE-10. (c) Absorption spectra of YZ, YZ1, and PCE-10 films. (d) Energy level and structural diagrams of the corresponding photodiode^[118]. (e) Mu−tau product for spray-coated FAPbI₃ and PEA₂FA₃Pb4I₁₃ films. (f) Charge distribution across the PEA₂FA₃Pb₄I₁₃ film at different wavelengths, modeled from diffusion lengths and absorption spectra. (g) Normalized external quantum efficiency responses of perovskite photodetectors with varying halide compositions^[119].

Fig. 13. (Color online) (a) Schematic illustration of the device structure and working mechanism for a SWCNT/graphene and MoS₂ dual heterojunction and (b) corresponding optical photograph. (c) Schematic of the fabrication process for the SWCNT/graphene and MoS₂ dual heterojunction photodetector^[125].

Fig. 14. (Color online) Schematic illustrations of the device on a stretchable substrate in (a) bent and (b) stretched configurations^[130]. (c) Schematic of the electrode fabrication process and flexible photodetector using direct writing with a pencil and Chinese brush^[135].

Fig. 15. (Color online) Schematic illustration of a 3D integration strategy for (a) integrated sensor system^[156] and (b) multilayer two-dimensional material integration^[157]. (c) Schematic illustration of a multifunctional integration strategy for full-color recognition^[162].

Fig. 16. (Color online) (a) Schematic illustration of a flexible photodetector for detecting PPG signals on the wrist^[167]. (b) Schematic illustration of a NIR photodetector designed for remote health monitoring. (c) Schematic illustration of a photodetector array for NIR biomimetic curved imaging^[93].

Fig. 17. (Color online) Schematic illustrations of curved photodetectors fabricated using various strategies (a) ultrathin substrate design^[150], (b) origami/kirigami design, scale bar: 1 mm^[175], (c) island−bridge structure^[176], (d) fractal web structure^[177], and (e) in situ growth of nanowires^[178].