Metal Halide Perovskites for Violet and Ultraviolet Light Emission

Emissive metal halide perovskites (MHPs) have emerged as excellent candidates for next-generation optoelectronics due to their sharp color purity, inexpensive processing, and bandgap tunability. However, the development of violet and ultraviolet light-emitting MHPs has lagged behind due to challenges related to material and device stability, charge carrier transport, tunability into the ultraviolet spectrum, toxicity, and scalability. Here, we review the progress of both violet and ultraviolet MHP nanomaterials and light-emitting diodes, including materials synthesis and device fabrication across various crystal structures and dimensions (e.g., bulk thin films, 2D thin films, nanoplatelets, colloidal nanocrystals, and more) as well as lead-free platforms (e.g., rare-earth metal halide perovskites). By highlighting several pathways to continue the development of violet and ultraviolet light-emitting MHPs while also proposing tactics to overcome their outstanding challenges, we demonstrate the potential of state-of-the-art violet and ultraviolet MHP materials and devices for important applications in public health, 3D printing, nanofabrication, and more.

💡 Research Summary

This review comprehensively surveys the state‑of‑the‑art in metal‑halide perovskite (MHP) materials and devices that emit in the violet (400–435 nm) and ultraviolet (λ < 400 nm) spectral regions. The authors begin by contrasting MHP‑based light‑emitting diodes (PeLEDs) with conventional III‑V UV LEDs, emphasizing that the latter require costly metal‑organic chemical vapor deposition (MOCVD) processes, toxic precursors, and lattice‑matched substrates, whereas MHPs can be processed by low‑temperature solution or thermal‑evaporation techniques, offering a potentially cheaper and more scalable route.

The review then details the fundamental crystal chemistry of perovskites. By varying the A‑site cation (Cs⁺, MA⁺, FA⁺, bulky organic ligands), the B‑site metal (Pb²⁺, Sn²⁺, Bi³⁺, Sb³⁺, Cu⁺, etc.), and the halide X‑site (Cl⁻, Br⁻, I⁻), the lattice parameters and electronic band structure can be tuned across a wide range. In particular, moving from iodide to bromide to chloride raises the valence‑band energy and widens the bandgap, enabling emission down to ~410 nm in CsPbCl₃ nanocrystals. B‑site doping with smaller ions (e.g., Cd²⁺) or trivalent metals (Sb³⁺, Bi³⁺) further contracts the lattice and pushes the bandgap higher, while also influencing defect states and radiative recombination rates.

Dimensionality control is highlighted as a second lever for achieving V/UV emission. The authors discuss three‑dimensional bulk films, two‑dimensional layered perovskites (Ruddlesden‑Popper and Dion‑Jacobson phases) formed with large organic spacers such as phenethylammonium, one‑dimensional chain structures, and zero‑dimensional isolated octahedra or quantum dots. Reduced‑dimensional systems exhibit large exciton binding energies (often >300 meV) and quantum confinement, which blue‑shifts emission and improves photoluminescence quantum yields (PLQYs). For example, CsPbCl₃ quantum dots with side lengths below the exciton Bohr radius emit at ~380 nm with PLQY >70 %.

The review also covers lead‑free alternatives motivated by toxicity concerns. Rare‑earth‑based perovskites (e.g., Ce³⁺, Eu³⁺ doped) and Cu⁺‑containing halides can emit in the UV‑C range (200–280 nm) while eliminating lead. However, these systems currently suffer from lower carrier mobilities and PLQYs, requiring further compositional engineering and surface passivation.

Device architecture is examined in depth. Typical PeLED stacks (ITO/PEDOT:PSS/active layer/electron‑transport layer/metal electrode) are discussed, with emphasis on energy‑level alignment between transport layers and the perovskite emissive layer. Strategies such as using ZnO or TiO₂ electron‑transport layers, PTAA or TCTA hole‑transport layers, and interfacial modifiers (LiF, PFN) are shown to improve charge injection balance and reduce non‑radiative losses. The authors note that the best reported V/UV PeLEDs achieve electroluminescence at ~380 nm with external quantum efficiencies (EQEs) around 3 %, still far below the >30 % EQEs of green/red PeLEDs, indicating that charge imbalance, trap‑mediated recombination, and environmental degradation remain major hurdles.

Finally, the authors propose a roadmap for future research: (1) systematic halide‑site engineering to push bandgaps beyond 4 eV; (2) hybrid dimensional designs that combine 2D quantum‑well confinement with 0D quantum‑dot emission to maximize both exciton binding and charge transport; (3) development of high‑performance lead‑free compositions with optimized B‑site chemistry; (4) scalable deposition methods (slot‑die coating, ink‑jet printing) compatible with low‑temperature processing; and (5) robust encapsulation strategies to mitigate moisture and oxygen‑induced degradation. By addressing these challenges, violet and ultraviolet perovskite LEDs could become low‑cost, high‑performance alternatives to existing III‑V technologies, unlocking new applications in sterilization, high‑resolution lithography, optical communications, and advanced manufacturing.