Fig. 1. Crystal structure and emission spectra of perovskite^[21]. (a) Ideal crystal structure of perovskites; (b) steady-state photoluminescence spectra of CsPbX₃ nanocrystals (NCs)

Fig. 2. Device structure and working principle. (a) Device structure of PeLEDs; (b) working principle of PeLEDs; (c) device structure of FPeLEDs; (d) schematic of FPeLEDs in bending state

Fig. 3. Performance of flexible substrates. (a) Variation of thin layer resistance of flexible substrate and glass substrate after 30 min heat treatment at different temperatures^[40]; (b) transmittance spectra of neat PI film and PI/Ag NWs composite films with sheet resistance of 20, 10, and 8 Ω·sq^-1[44]; (c) functional relationship between sheet resistance and bending stress of PI/Ag NWs electrodes, inset shows the photograph of electrode bent at a specific radius of curvature^[33]; (d) transmission spectra of chitin nanofiber paper compared to other polymers^[46]

Fig. 4. Thin films and devices based on various flexible electrode materials. (a) SEM images of surface morphologies of the 7 nm ultrathin Au films deposited on bare glass, SU-8 modified glass, and MoO₃/SU-8 modified glass substrates^[53]; (b) picture of a large-area Ni-doped Ag-FTC being utilized in a circuit that lights up a green LEDs^[54]; (c) schematic of exciton quenching resulting from In and Sn atoms in ITO-based PeLEDs and circumvention of this process in graphene-based PeLEDs^[12]; (d) schematic of difunctional Ano-HTL and hole injection process from Ano-HTL into an overlaying semiconducting layer^［66］; (e) device structure of simplified PeLEDs^[66]; (f) schematic of device structure based on composite electrodes^[32]

Fig. 5. Preparation methods of flexible perovskite thin films. (a) Schematic of FLA method ^[34]; (b) schematic of CsPbBr₃ layer thermal vacuum co-evaporation deposition process in conjunction with in situ dynamic thermal crystallization^[81]; (c) schematic of fabrication processes of all-inkjet-printed FPeLEDs^[82]; (d) schematic of preparation process of perovskite thin film by scraping^[83]; (e) schematic of screen-printing sedimentary perovskite membrane^[84]

Fig. 6. Optimization of perovskite films in FPeLEDs. (a) Fracture energy and PLQY of perovskite films with or without various additives^[13]; (b) normalized EQE versus bending cycles at bending radii of 1 mm and 2 mm for the FPeLEDs with FPMAI additives^[13]; (c) principle diagram and SEM diagram of PEABr and PEG synergistic regulation of perovskite morphology^[85]; (d) SEM images of dense and dendritic CsSnI₃ films after 2000 bending cycles^［36］; (e) schematic of charge carriers transport and recombination process in dendritic CsSnI₃ film^[36]; (f) schematic of the interaction between EC and CsPbI₃^[35]

Fig. 7. Self-healing of perovskite thin films in FPeLEDs. (a) Schematic of formed silica network (left, TEOS; right, TFPTMS) and the interaction with perovskite^[88]; (b) degradation diagram of EQE under different cycles with a bending radius of 5 mm^[88]; (c) schematic of chemical structure of MDI-PU and interaction between perovskite crystals and MDI-PU^[33]; (d) J-V-L characteristics with insets showing the photographs of bending test and twisting test setups^[33]; (e) EQE-J diagram before and after 2000 bending and twisting experiments^[33]

Fig. 8. Interface engineering and energy level regulation in FPeLEDs. (a) FPeLEDs device structure, energy level structure, and device brightness under different bending radii and bending times using PFN as interface layer^[92]; (b) energy level structure and J-V curve of FPeLEDs with different concentrations of Zonyl added to PEDOT∶PSS^[67]; (c) preparation process of flexible transparent electrodes based on patterned ZnO^［14］; (d) EQE distribution statistics of FPeLEDs with planar and patterned flexible transparent electrodes^[14]