Flexible perovskite light‐emitting diodes (LEDs) have been highly expected to realize advanced wearable optoelectronic applications due to the excellent optoelectronic properties of perovskites. However, the poor water and oxygen stability and limited flexibility of perovskites prevent their commercialization and applications in flexible LEDs. Herein, the low‐cost and green biomass materials‐ethyl cellulose (EC) is added in the CsPbI3 nanocrystals (NCs), acting as a cross‐linker between neighboring halide octahedra through hydrogen bonds and PbO coordination bonds. It reduces the defect densities of NCs, leading to improved photoluminescence quantum yield. Simultaneously, the synergistic effect of efficient defect passivation and hydrophobic ether groups of EC significantly improve the environment stability of NCs. Additionally, the favorable flexibility of EC and cross‐linking between EC and perovskite NCs improve the deformation resistance of the perovskite layer with stable photoluminescence and negligible cracks after repeated bending. Consequently, flexible LEDs based on the EC‐passivated CsPbI3 NCs achieved a record external quantum efficiency of 12.1% and significantly enhanced operational stability. Moreover, the flexible LEDs show small luminance degradation after bending for 1000 cycles at a radius of 3 mm, and still retain high performance even after repeatedly bending at an ultrasmall bending radius of 1 mm.

Oleylamine and oleic acid are common organic capping ligands used in the hot injection preparation of perovskite quantum dots (QDs). Their labile nature is responsible for the poor colloidal stability and conductivity that affect the performance of perovskite QD light-emitting diodes (LEDs). We introduced 4trifluoro phenethylammonium iodide (CF 3 PEAI) directly in the synthesis and found that CF 3 PEAI efficiently modified the I − vacancy defects on the QD surface and partially substituted the surface capping ligand oleylamine. The strong electron pulling ability of F in CF 3 PEAI results in a more positive −NH 3 + terminal compared to that of PEAI, which promotes tight bonding of CF 3 PEAI on the surface of CsPbI 3 QDs. As a result, we achieved bright QDs with a photoluminescence quantum yield of 92% and efficient red LEDs. The maximal luminance was improved to 4550 cd m −2 for 685 nm red light, which was nearly 4.6-fold of the LEDs with plain CsPbI 3 QDs. Additionally, the peak external quantum efficiency reached 12.5%.

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