To appreciate why the timing of additive delivery matters, it helps to understand what makes mixed-halide perovskites both brilliant and brittle. Perovskites are crystalline materials built from a framework of lead and halide ions, typically iodide and bromide in red-emitting compositions, interspersed with organic or inorganic cations. By blending halides, engineers can tune the bandgap and therefore the emission color with remarkable precision. But that same blend is the source of the material’s Achilles heel. Under the electric field inside a working LED, halide ions migrate through the crystal lattice, segregating into iodide-rich and bromide-rich domains. The emission color drifts during operation, a phenomenon that has plagued spectrally stable mixed-halide devices for years. At the same time, defects, uncoordinated lead atoms, missing halides, and disordered grain boundaries, act as traps where injected electrons and holes recombine without emitting a single photon, draining efficiency.

The conventional response has been additive engineering: sprinkle passivating molecules into the precursor solution and hope they find their targets. The problem, as the new work makes clear, is that a perovskite film does not form all at once. It crystallizes in stages, from nucleation through grain growth to final film consolidation, and an additive introduced into the precursor is present from the very beginning whether it is needed there or not. A molecule that is ideal for patching grain boundaries late in the process may instead interfere with early nucleation if it is present from the start. Shao and Yang’s insight was to treat film formation as a choreography rather than a single event, matching each additive to the stage where it can do the most good.

The first additive, 4-vinylbenzylammonium iodide, or VBAI, goes into the precursor solution itself. As an ammonium salt, it belongs to a family of molecules well known in the perovskite community for defect passivation. The ammonium group can hydrogen-bond with the perovskite framework, while the iodide counterion helps replenish halide vacancies, one of the most damaging defect types in these materials. The vinylbenzyl portion adds a bulky aromatic character that can help anchor the molecule at the perovskite surface and, in principle, stiffen the lattice against ion migration. By placing VBAI in the precursor, the researchers ensure it is incorporated during crystallization, where it can influence grain growth and passivate defects as they form rather than after the fact.

The second additive, octylphosphonic acid, or OPA, takes a different route entirely: it is dissolved into the antisolvent, the liquid dripped onto the spinning precursor film to trigger rapid crystallization. Phosphonic acids carry a distinctive dual personality. The phosphonic acid head is a strong Lewis base that coordinates to undercoordinated lead atoms at the perovskite surface, neutralizing deep traps that would otherwise swallow charge carriers. The long octyl chain, meanwhile, is hydrophobic and insulating, and previous work has shown that octylphosphonate species can self-assemble into robust hydrogen-bonded networks that wrap around perovskite grains, shielding them from moisture and thermal stress. Delivering OPA through the antisolvent means it arrives precisely at the moment the film is solidifying, positioning it at surfaces and grain boundaries where its protective chemistry is most valuable, without contaminating the early nucleation chemistry.

The complementary division of labor between the two additives is the conceptual heart of the paper. VBAI, present from the start, shapes the crystal growth itself; OPA, arriving with the antisolvent, seals and protects the finished grains. Combined experimental characterization and density functional theory calculations, performed with the widely used plane-wave codes developed by Kresse and Furthmüller, support this picture. The dual-additive films show improved morphology and crystallinity, meaning larger, better-ordered grains with fewer boundaries where ions could hop and traps could lurk. The measured trap density drops, and with it the rate of non-radiative recombination, the silent efficiency killer in every LED. The films also hold up better against heat and resist the spectral drift that normally afflicts mixed-halide emitters under sustained operation.

The device numbers translate the materials science into something display engineers and optoelectronics researchers can actually use. The optimized deep-red PeLEDs emit at an electroluminescence peak of 734 nanometers, squarely in the deep-red region where perovskite performance has historically lagged behind green and red-emitting cadmium-based and organic alternatives. The maximum external quantum efficiency of 21.70 percent means that more than one in five electrons injected into the device emerges as a useful photon, a figure that places these diodes among the more efficient deep-red perovskite emitters reported to date. Just as important for any practical application, the operational lifetime, quantified as T50, the time for brightness to fall to half its initial value, reaches 26.39 hours. For a device class where spectral instability and rapid degradation have often limited lifetimes to minutes or a few hours, that is a meaningful step forward, even if it remains far from the tens of thousands of hours demanded of commercial display panels.

The strategy also fits into a broader and rapidly maturing research landscape. The paper’s reference list reads like a map of the field’s recent trajectory: Nature Photonics work on interfacial reactions for bright and stable deep-red devices, Advanced Materials reports on space-confined fabrication of cesium lead iodide nanocrystal films and lateral zero-dimensional/three-dimensional heterostructures, and a growing literature on dual-additive and dual-site strategies borrowed from perovskite solar cell research. Ammonium salts have been shown in multiple studies to passivate different defect types selectively, and phosphonic acid modifiers have already proven their worth in blue perovskite LEDs and high-efficiency solar cells. What Shao and Yang add to this conversation is the stage-dependent logic, the recognition that where and when an additive enters the process can matter as much as which molecule you choose. That principle is transferable: the same reasoning could guide additive selection for blue and green perovskite emitters, for wide-bandgap compositions in tandem solar cells, and for any application where crystallization kinetics and defect chemistry must be tamed simultaneously.

Caveats remain, as they always do on the road from laboratory record to living-room screen. The T50 lifetime of roughly 26 hours was measured under the specific operating conditions of the study, and translating such results to the higher brightness and harsher duty cycles of commercial displays will require further gains in encapsulation, thermal management, and ion-migration suppression. Lead toxicity continues to shadow the entire perovskite field, motivating parallel work on lead-free compositions and on recycling strategies. Still, the study demonstrates that some of the perovskite LED’s most stubborn problems, defect-assisted recombination, halide segregation, and thermal degradation, can be attacked with nothing more exotic than two well-chosen small molecules added at the right moments in a standard solution-processing workflow. No new deposition equipment, no high-temperature steps, no rare-earth chemistry. If the stage-dependent additive philosophy proves as general as the authors suggest, the humble antisolvent drip may turn out to be one of the most powerful levers in perovskite engineering, and deep-red perovskite LEDs may finally be on a credible path from the optics bench to the display stack.

Subject of Research: Stage-dependent dual-additive engineering of mixed-halide perovskite films for efficient and stable deep-red light-emitting diodes

Article Title: Efficient and stable deep-red mixed-halide perovskite LEDs via dual-additive engineering

Article References: Shao, Y., & Yang, J. (2026). Efficient and stable deep-red mixed-halide perovskite LEDs via dual-additive engineering. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13948-6

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13948-6

Keywords: perovskite LEDs, deep-red emission, mixed-halide perovskites, additive engineering, defect passivation, ion migration, VBAI, octylphosphonic acid, external quantum efficiency, spectral stability, crystallization kinetics, optoelectronics

The context for this advance is the remarkable maturation of p-i-n architecture perovskite solar cells built on self-assembled monolayer contacts. Such devices have now reached a record efficiency of 27 percent, and they promise low-cost, industrially scalable photovoltaics that could complement or even challenge silicon. But the organic-inorganic hybrid perovskite film at the heart of these cells is a soft lattice in which ions drift readily under constant illumination, heat and moisture. Ion migration degrades performance over time, and the defects that accumulate at the interface between the perovskite and its charge transport layers are widely recognized as the critical origin of instability and degradation. Any route to commercialization must confront this interface head-on.

The most popular strategy has been to cap the 3D perovskite with a thin layer of 2D perovskite, whose bulky organic spacers physically restrict ion migration. Researchers have used phenethylammonium to form Ruddlesden-Popper type 2D layers, 3-aminomethylpiperidine to build Dion-Jacobson quasi-2D structures lattice-matched to the underlying crystal, and guanidinium to generate ACI-type phases that passivate surface defects. Yet this approach carries a hidden cost. Introducing large spacer cations to replace the native A-site cations, formamidinium and methylammonium, drives uncontrolled ion exchange that leaves vacancies behind and produces a fluctuating n-value, the number of inorganic layers in the 2D structure on which its properties depend. Worse, previous studies have shown that spacer cations can undergo deprotonation under irradiation, with the decomposition products reacting with the perovskite to release volatile hydrogen iodide and ammonia, corroding the very heterostructure meant to protect it.

The new work takes a fundamentally different route. Instead of importing a foreign ammonium cation, the team used n-hexyl phosphonic acid, or HPA, a molecule whose phosphonic acid head group forms multiple hydrogen bonds with the perovskite’s own formamidinium cations. Spectroscopic evidence for this interaction was unambiguous: proton nuclear magnetic resonance showed peak splitting of the formamidinium CH and NH2 signals, Fourier transform infrared spectra shifted in the same direction, and solid-liquid mass spectrometry detected reaction products corresponding to formamidinium bound to HPA and to HPA dimers. When HPA diffuses into the perovskite lattice, it locks onto formamidinium through these hydrogen bonds, and the resulting HPA-FA cations act as spacer cations that induce 2D perovskite formation in place, without any ion exchange displacing the native A-site population.

The team synthesized this 2D phase, dubbed HF-PVK with the formula (HPA-FA)2PbI4, directly from HPA, formamidinium iodide and lead iodide. X-ray diffraction revealed characteristic reflections attributed to the (001), (121), (002) and (003) planes of the 2D structure, while transmission electron microscopy confirmed lattice spacings of 9.3, 5.7 and 3.8 angstroms matching those planes. Ultraviolet-visible absorption showed the characteristic peaks of a 2D perovskite with multiple n-values. Crucially, the phosphonic acid group does double duty: nuclear magnetic resonance and X-ray diffraction showed that the PO(OH)2 group interacts with lead iodide, forming P-O-Pb bonds that passivate uncoordinated lead ions, a well-known source of trap states. In this design, one molecule simultaneously builds the protective layer and heals the defects beneath it.

To construct the working heterostructure, the researchers deposited HPA onto the surface of a 3D perovskite of composition FA0.85MA0.1Cs0.05PbI3. Grazing-incidence wide-angle X-ray scattering showed new 2D perovskite peaks emerging alongside a weakening of the residual lead iodide signal, indicating that HPA consumed unreacted precursors to form the 2D phase. Cross-sectional scanning transmission electron microscopy directly imaged the 2D capping layer, and in-situ photoluminescence during annealing captured the 2D phase appearing within minutes as characteristic signals at 501 and 592 nanometers grew with time. X-ray photoelectron spectroscopy showed the Pb 4f peaks shifting to lower binding energy as P-O-Pb bonds formed, while metallic lead peaks, a signature of deep defects, vanished entirely after treatment.

The electronic consequences were striking. Femtosecond transient absorption spectroscopy revealed that the treated films carried an additional bleaching signal from the 2D phase, and the 3D bleaching peak shifted from 772 to 786 nanometers, consistent with excited-state energy transfer from the 2D layer into the 3D absorber. Photoluminescence quenching at the C60 contact became faster, with the fast decay lifetime shortening from 388 to 233 picoseconds, evidence of more rapid interfacial carrier transfer. Ultraviolet photoelectron spectroscopy showed the work function dropping from 4.95 to 4.14 electron volts and the gap between the Fermi level and conduction band narrowing from 0.58 to 0.32 electron volts, giving the surface stronger n-type character that aligns favorably with the C60 electron transport layer. Kelvin probe force microscopy confirmed the contrast: a PEAI-treated surface swings p-type, which is poorly suited to this interface, while the HPA-treated surface swung decisively the other way.

The devices reaped the rewards. With 1.0 milligram per milliliter HPA, the champion cell improved from 24.29 percent efficiency to 26.32 percent, with open-circuit voltage of 1.16 volts, current density of 26.51 milliamperes per square centimeter and fill factor of 85.6 percent. Across twenty independently fabricated devices the average efficiency rose to 25.79 percent from 23.91 percent. External quantum efficiency exceeded 93 percent with an integrated current matching the measured photocurrent. Space-charge-limited current measurements showed trap density falling from 7.77 x 10^14 to 2.08 x 10^14 per cubic centimeter, impedance spectroscopy showed recombination resistance nearly doubling, and the ideality factor dropped from 2.25 to 1.34, all pointing to suppressed trap-assisted recombination. The strategy also scaled: cells of 1.012 square centimeters reached 24.91 percent, and a 5 by 5 square centimeter rigid module delivered 19.24 percent.

Stability is where the in-situ approach truly separates itself from conventional spacer cations. Density functional theory calculations found that the formation energy of A-site vacancy defects in the HPA-based 2D perovskite exceeds that of both Ruddlesden-Popper and Dion-Jacobson analogues, meaning the n-value stays fixed because formamidinium ions cannot easily migrate away. Time-of-flight secondary ion mass spectrometry after 8 hours at 180 degrees Celsius showed far less formamidinium and halide accumulation in the C60 layer of HPA-treated devices than in untreated ones. While PEAI-treated films showed spectral signatures of deprotonation after an hour at 180 degrees, the HPA-treated films held their chemistry. In aging tests, unencapsulated devices retained 95.5 percent of their initial efficiency after 3000 hours in nitrogen, 92.5 percent after 3000 hours in ambient air at 45 plus or minus 5 percent relative humidity, and about 90 percent after 800 hours at 85 degrees Celsius under the ISOS-D-2I protocol, while untreated devices lost half their efficiency under the same thermal stress.

The broader significance is hard to overstate. Perovskite photovoltaics have long been caught between two demands: the 2D layers that protect the absorber tend to undermine themselves through ion exchange and photochemical decomposition, while the untreated absorber degrades through defect-driven ion migration. By letting the protective cations assemble themselves from the perovskite’s own ingredients, held together by hydrogen bonds rather than fragile ammonium chemistry, this work dissolves that trade-off with a molecule as simple as a hexyl phosphonic acid. Champion efficiencies above 26 percent, working modules, and stability results that clear the thousand-hour mark in air suggest the strategy could translate directly into manufacturing. If in-situ cation construction proves robust across other compositions and module formats, it may become a standard step in the recipe that finally carries perovskite solar cells from the laboratory bench to the rooftop.

Subject of Research: In-situ construction of hydrogen-bonded spacer cations for stable and efficient 2D/3D perovskite heterostructure solar cells

Article Title: In‐Situ Constructed Cations for 2D/3D Perovskite Heterostructure for Stable and Efficient Photovoltaics

Article References: Liu, M., You, Q., Liu, L., Shi, X., Wang, Z., Wu, B., Yang, Z., Zhou, G., Liu, J.-M., Gao, J., & Jiang, Y. (2026). In‐Situ Constructed Cations for 2D/3D Perovskite Heterostructure for Stable and Efficient Photovoltaics. Advanced Science, 13(55), Article e76508. https://doi.org/10.1002/advs.76508

Image Credits: AI Generated

DOI: 10.1002/advs.76508

Keywords: perovskite solar cells, 2D/3D heterostructure, n-hexyl phosphonic acid, hydrogen bonds, ion migration, defect passivation, power conversion efficiency, self-assembled monolayer, stability, photovoltaics, spacer cations, ISOS protocols