Flexible perovskite light-emitting diodes: recent progress, applications and challenges

Flexible light-emitting diodes (FLEDs), recognized for their excellent mechanical properties and lightweight features, have garnered significant attention for a range of emerging optoelectronic technologies, such as unconventional lighting, flexible and foldable displays, consumer electronics, and healthcare monitoring. Currently, the predominant technology in FLEDs is based on organic LEDs (OLEDs), which utilize the deposition of organic light-emitting and charge transport materials onto flexible substrates. 

Despite the advantages of OLEDs, including high EQE, color purity, and bending capabilities, the organic molecules or polymers used often require complex synthesis processes. Coupled with the high cost of these raw materials, this poses substantial barriers to device production, especially for large-sized flexible display panels. Moreover, the luminescent color of OLED materials depends on their chemical structure, and achieving color adjustments typically necessitates the synthesis of varied materials, which limits color tunability.

Recently, metal halide perovskite materials have emerged as promising candidates for high-performance LEDs, demonstrating substantial potential for next-generation displays and lighting solutions due to their excellent optical properties. Generally, metal halide perovskite can be expressed as the formula ABX₃, where the “A” site is commonly occupied by monovalent cations such as methylamine (MA⁺), formamidine (FA⁺), and cesium (Cs⁺). The “B” site is a divalent metal cation like lead (Pb²⁺) or tin (Sn²⁺), and the “X” represents halide anions such as chloride (Cl⁻), iodide (I⁻), and bromide (Br⁻). The distinctive feature of perovskites is that ions at all sites can be substituted either completely or partially, allowing for bandgap tuning and thus influencing the emission wavelength. This compositional tunability endows perovskite materials with a broad color gamut. The unique bandgap structure of perovskites allows for high tolerance to shallow and intra-band defects, which is beneficial for realizing high photoluminescence quantum yield (PLQY).

Additionally, compared to conventional commercialized OLEDs and quantum dot LEDs (QLEDs), PeLEDs offer higher color purity. The full width at half maximum of the electroluminescent emission peak of PeLEDs is typically much narrower than that of OLEDs and QLEDs. This narrow emission bandwidth provides the potential for more vivid and accurate color reproduction in displays. Given these advantages, remarkable progress has been achieved on PeLEDs. For instance, the EQE and luminance of green PeLEDs have reached levels comparable to state-of-the-art OLEDs and QLEDs. Furthermore, unique low-temperature synthesis methods for perovskites, such as solution spin-coating and thermal evaporation, facilitate the fabrication of high-performance flexible PeLEDs. By employing comprehensive optimization techniques—including active layer morphology manipulation, charge transport layer refinement, and light outcoupling efficiency enhancement—the EQE of flexible PeLEDs has significantly improved. Unlike organic semiconductor materials, the lower cost of raw materials and simpler synthesis processes for flexible PeLEDs greatly reduce overall fabrication costs, essential for large-scale FLED production and commercialization.