Differential Ion Motion in Perovskite Light Emitting Electrochemical Cells




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Perovskite light emitting diodes (PeLED) have shown promising progress as next-generation efficient electroluminescent devices. However, PeLEDs suffer from low lifetimes and color instability during operation that limits its insertion into most practical applications. To address this concern, I investigated a form of perovskite light-emitting device termed perovskite light-emitting electrochemical cells (PeLECs) that utilize a phenomenon of selective differential ion motion in perovskite devices. I have been exploring an interesting phenomenon of “differential ion motion” in PeLECs, where under applied bias, additive ions (LiPF6) selectively move while restricting the motion of intrinsic perovskite ions. This interplay of intrinsic and additive ions enhances the efficiency and operational stability of PeLECs. In differential ion motion, the perovskite structure remains stable while sacrificial additive ions move in response to the applied electric field. These additive ions accumulate at respective electrodes (anions at anode and cations at the cathode) and improve electronic charge injection (electron and holes) by the formation of electrical double layers (EDLs) at the electrode interfaces. Specifically, I fabricated and characterized PeLECs, directly measuring their luminance-currentvoltage characteristics, quantum efficiency, power efficiency, electroluminescence (EL) spectra, operational stability. To understand the fundamental materials science behind device performance, I have performed numerous materials and device characterizations such as electron microscopy (SEM, TEM), spectroscopy (XPS, UV-Vis), crystallography (XRD), force microscopy (AFM), reliability testing, electrochemical circuit design, and photoluminescence (PL) spectra, lifetime, and quantum yield. We demonstrated that optimized Li salt additive improves thin film morphology, increases PL stability and quantum yield, reduces charge traps, and strengthens the perovskite chemical bonding. Then, we hypothesized differential ion motion phenomenon and showed long lifetimes at constant current, calculated EDLs thickness by using electrochemical impedance circuit model. We also demonstrated voltage-controlled color-tunable perovskite host-ionic guest (Ir-ionic transition metal complex) LECs. We observed the benefits of differential ion motion in pure blue light-emitting mixed-halide perovskite, where we effectively suppressed detrimental halide segregation under intense photoexcitation and electrical bias that facilitated us to obtain longawaited stable blue PeLECs satisfying technological emission standards. Additionally, we integrated highly emissive zero-dimensional perovskite into a 3D perovskite matrix through a novel solvent engineering method that demonstrated high quantum efficiency and operational stability facilitated by differential ion motion. PeLECs have shown superior operational stability (initial luminance level of 3200 cd/m2, 120 h— extrapolates to 30,000 h half-life at 100 cd/m2, the common industrial benchmark for a lifetime) and high color-purity (Full-width half maximum of EL spectra ≤ 18 nm). Pure Blue emission from PeLECs meets all the National Television System Committee (NTSC) requirements. These PeLECs are simple single-layer devices that offer ease of processing (low-temperature and costeffective) for facile fabrication of large-area display and lighting applications. Leveraging differential ion motion in PeLECs demonstrates a new pathway of utilizing simple and smart, wearable devices for the internet of things (IoT) for digital communication and fashion.



Engineering, Materials Science, Physics, Optics, Chemistry, General