Metal halide perovskite materials have rapidly developed in optoelectronic devices such as light-emitting diodes, solar cells, lasers, photodetectors, and image sensors due to their advantages of solution processability, high absorption coefficient, tunable bandgap, and long carrier diffusion distance. As a promising electroluminescent material, perovskite materials combining organic and inorganic semiconductor advantages have attracted significant attention in light-emitting diodes (LEDs). Since the first room-temperature perovskite light-emitting diode (PeLED) was introduced in 2014, the external quantum efficiencies (EQE) in the near-infrared, red, and green light regions have exceeded 20%. However, traditional rigid substrate PeLEDs cannot meet the growing demand for flexible display and wearable electronic devices, with emphasis on the need for flexible perovskite LEDs (FPeLEDs).

For practical applications of flexible devices, each layer of FPeLEDs requires sound flexibility and stability, including substrates, electrodes, emitting layers, and interface layers. In 2014, Kim et al. achieved the first flexible device with a flexible plastic substrate instead of a rigid glass substrate to realize an EQE of 0.125% with a maximum bending radius of 1.05 cm. Over the years, significant research progress has been made for FPeLEDs. However, their EQE still lags behind rigid glass-based devices, limiting their application in high-performance wearable devices. Therefore, summarizing existing research is necessary to identify challenges and future directions for FPeLEDs' development.

Suitable substrates for FPeLEDs must exhibit excellent flexibility, high transmittance, and sound stability. Various transparent polymer substrates are reported (Table 1). Among them, polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) are commonly employed. However, they suffer from deformation and increased resistance at high temperatures. Flexible polyimide (PI) substrates with high-temperature resistance have been explored with cost limitations. Enhancements in mechanical flexibility and strain release have been achieved by incorporating silver nanowires (Ag NWs) into PI substrates. Additionally, biodegradable substrates and mica with high transparency and flexibility are being developed as alternatives.

Traditional indium tin oxide (ITO) electrodes adopted in rigid devices are not compatible with flexible substrates due to high-temperature deposition requirements. As alternatives, metal electrodes, carbon electrodes, and conductive polymers have been explored (Fig. 4). Metals (such as metal films and metal nanowires) are widely utilized as flexible electrodes in flexible optoelectronic devices due to their high conductivity and good mechanical flexibility. Carbon electrodes, including graphene and carbon nanotubes (CNTs), provide high transparency, carrier mobility, and flexibility. Strategies like passivation layers, chemical post-treatment, and doping have been employed to enhance the conductivity and surface morphology of carbon electrodes. Conductive polymers like PEDOT: PSS are ideal electrode materials due to their conductivity and flexibility. Incorporating solvents or additives can further enhance their conductivity. Composite electrodes that combine different materials have also been developed to realize improved performance compared with single-component electrodes.

Perovskite emissive layers play a critical role in device performance, and their film quality is of utmost importance. Achieving high-performance devices requires well-formed films with uniform grain size. Various deposition methods have been developed to prepare flexible perovskite films, including spin coating, dual-source thermal evaporation, inkjet printing, blade coating, and screen printing, each with its advantages and challenges (Fig. 5). Meanwhile, perovskite thin films typically have dense polycrystalline structure, which limits their flexibility and application in FPeLEDs. To this end, researchers have focused on improving the flexibility of perovskite films through grain size control, micro/nanostructure construction, and physical dispersion/chemical cross-linking (Fig. 6). Additionally, the quantum dot strategies and incorporating self-healing properties in perovskite layers are also discussed.

To address charge injection and transport imbalances in FPeLEDs, methods such as introducing buffer layers, doping, and post-processing of charge transport layers and electrodes are commonly adopted. These approaches reduce non-radiative recombination losses and improve energy level alignment between layers to enhance FPeLEDs' efficiency and stability. Lee et al. employed the conjugated polymer electrolyte PFN as an interface layer between the electron transport layer (SPW-111) and Ag NWs electrode, lowering the electron injection barrier. Their flexible devices maintained 80% initial brightness after 400 bending cycles with a 2 mm radius. Lee et al. modified the hole transport layer by Zonyl FS-300 to enhance hole injection and reduce emission quenching at the PEDOT∶PSS/perovskite interface. These modifications increased the device's efficiency and maintained it even after 1000 bending cycles with a 2.5 mm radius. In FPeLEDs, not all the generated photons are emitted into free space but captured by emission layers, electrodes, and substrates. Therefore, improving the outcoupling efficiency is a key factor for further improving device performance. Shen et al. achieved high-efficiency photon generation and improved light output coupling efficiency by utilizing rational interface engineering and patterned ZnO in a flexible thin-film structure, resulting in devices with an EQE approximately 1.4 times higher than planar devices (Fig. 8).

We discuss the influence of flexible substrates, electrodes, perovskite emissive layers, and interface energy level alignment on the flexibility, stability, and efficiency of FPeLEDs. We summarize strategies for optimizing the performance of each functional layer. FPeLEDs show significant potential in wearable and display lighting applications, overcoming the limitations of rigid PeLEDs. However, challenges remain, such as studying performance degradation during bending, optimizing thin film design and fabrication, and improving interlayer adhesion. Addressing these challenges will enhance the performance and reliability of FPeLEDs, and realize their practical applications in various fields.