Researchers from the School of Electronics and Information Engineering at Hebei University of Technology and the State Key Laboratory of Luminescence Science and Technology at the Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, report how to balance defect passivation with the quantum confinement effect (QCE) in ligand-free blue perovskite emitters. The study appears inLaser & Photonics Reviews and shows that selecting weakly coordinating additives enables efficient, spectrally stable, sky-blue CsPbBr₃ light-emitting devices while avoiding the redshift that often accompanies strong passivation chemistry.

Blue light-emitting diodes based on metal halide perovskites have lagged behind their red and green counterparts because techniques that widen the bandgap can introduce instability or deep defects. Mixed-halide strategies face halide migration and phase separation, and quasi-2D routes relying on long-chain organic ligands often suffer from multiphase distributions and poor charge transport. A ligand-free, pure-bromide approach—using Cs₄PbBr₆ to confine CsPbBr₃—offers narrow spectra and better charge conduction, but intrinsic defects at grain boundaries still suppress the photoluminescence quantum yield (PLQY). The question the team addressed was how to passivate those defects without dismantling the nanoconfinement that makes the emission blue.

The researchers systematically compared four Lewis-base additives with distinct electron-donating motifs—urea (C=O), guanidine sulfamate (S=O), 18-crown-6 (C–O–C), and 2PACz (P=O)—in ligand-free Cs-Pb-Br films that rely on Cs₄PbBr₆-confined CsPbBr₃ domains for blue emission. Coordination analyses, combined with spectroscopy and modeling, indicated a clear mechanistic divide. Strongly coordinating additives (18-crown-6 and 2PACz) bound Pb²⁺ centers tightly and hindered the transformation of CsPbBr₃ into Cs₄PbBr₆, weakening the QCE and producing a pronounced redshift toward green. In contrast, weakly coordinating additives (urea and guanidine sulfamate) passivated defects while minimally disturbing the confined architecture, retaining sky-blue emission with small spectral drift.

The research process followed a standard thin-film path in the past tense: the team prepared ligand-free Cs-Pb-Br perovskite films with a high CsBr/PbBr₂ ratio, introduced controlled amounts of each additive, and then evaluated optical and structural responses. They measured PLQY and time-resolved photoluminescence to track radiative and non-radiative channels, used X-ray and photoelectron spectroscopies to probe coordination and lattice signatures, and performed density-functional calculations to quantify additive–surface binding energies and the thermodynamics of Cs₄PbBr₆ formation. Together, these tools built a consistent picture: passivation strength needed to be “just enough”—too strong, and the nanoconfined scaffold that ensures blue emission collapsed; too weak, and traps dominated.

Guanidine sulfamate (GuaS) emerged as the best-balanced choice. Films with an optimized GuaS content showed a higher PLQY than control samples and longer exciton lifetimes, indicating fewer non-radiative pathways. Devices based on these films—configured as ITO/PEDOT:PSS (hole transport)/perovskite/TPBi/LiF/Al—emitted at ~489 nm with a peak external quantum efficiency (EQE) of 10.5%. Operational stability also improved: the T₅₀ (time to half the initial luminance) reached approximately 56 minutes at 100 cd m⁻², outlasting many sky-blue devices that rely on quasi-2D structures or mixed halides. Notably, when the additive dose exceeded the optimal range, aggregation and roughness increased, mobility fell, and efficiency dropped—reinforcing the central claim that passivation must be balanced against preserved confinement.

The study’s results are best understood as a materials-chemistry “tuning curve” rather than a single recipe. Strong additive–perovskite bonds effectively neutralized defects but interfered with the formation or stability of Cs₄PbBr₆ spacers, shifting emission to longer wavelengths. Moderate bonds (as with GuaS or urea) still coordinated unsaturated Pb and formed hydrogen bonds with halides, yet they allowed the Cs₄PbBr₆/CsPbBr₃ composite to persist, keeping emission in the sky-blue region with narrow linewidths and low bias sensitivity. This balance—maintaining QCE while suppressing traps—explains the simultaneous gains in spectral stability, efficiency, and device lifetime.

From an application standpoint, the work advances a pathway toward reproducible, ligand-free blue PeLEDs with fewer failure modes. The approach reduced reliance on long-chain organic spacers, which are known to impede charge transport and degrade under stress, and it avoided chloride incorporation that can invite ion migration. The authors argue that additive selection can be treated as a design handle: choose chemistries that passivate deep states and mitigate halide motion, but that do not lock the lattice so tightly that the confining Cs₄PbBr₆ framework disassembles. Because the method stayed within mainstream solution processing and standard device stacks, it should be compatible with interface engineering and optical outcoupling strategies that are already common in perovskite displays and lighting.

The broader significance is twofold. Scientifically, the paper clarifies why “stronger passivation” does not always translate to “better devices” in quantum-confined perovskites; coordination strength alters not only defect chemistry but also the very nanostructure that sets the emission color. Technologically, the results suggest a pragmatic route to sky-blue pixels with competitive EQE and longer run times under constant drive—two practical bottlenecks for next-generation microdisplays and solid-state lighting. Further gains, the authors note, could come from interface modifications or new weak-binding additives that preserve confinement while further suppressing traps.