Thermal evaporation is attractive for perovskite optoelectronics because it holds considerable promise for scalable manufacturing and pixel patterning of perovskite LEDs. Yet evaporated perovskite LEDs have consistently underperformed solution-processed ones, and the new study pinpoints why: without effective strategies to regulate perovskite crystallization during vacuum deposition, the perovskite phase forms too quickly and uncontrollably, yielding films with defects that drain energy as heat instead of light. The team, whose authors include Yangyu Liu, Yuhan Zhou and Jianing Chang together with Feng Gao and Chunxiong Bao among many colleagues, set out to understand and then repair that crystallization pathway.

The researchers began with FAPbI3, a formamidinium lead iodide perovskite that emits efficiently in the near-infrared. They first optimized a sequential evaporation process, laying down formamidinium iodide and then lead iodide in separate layers rather than co-evaporating them, which produced larger and more uniform grains. Adding 5-aminovaleric acid, an additive known to work well in solution-processed films, boosted the average external quantum efficiency of evaporated devices from 2.3 percent to around 11.3 percent. That was encouraging, but the same additive lifted solution-processed devices from 4.8 percent to 21.2 percent, revealing a persistent shortfall specific to vacuum deposition.

To find out what was going wrong, the team built an in situ spectroscopy system that could watch the film as it formed. Optical fibers carried light into the evaporation chamber and collected absorption and photoluminescence signals in real time, while grazing-incidence wide-angle X-ray scattering at the Shanghai Synchrotron Radiation Facility tracked crystal structure during annealing. The measurements revealed a surprise: during evaporation, the three-dimensional FAPbI3 phase formed regardless of whether 5-aminovaleric acid was present. In solution processing, by contrast, the additive stabilizes a metastable intermediate phase that later guides orderly crystal growth. The key difference turned out to be the solvent itself. Formamidinium iodide and 5-aminovaleric acid react to form a condensation product that populates the beneficial intermediate phase, but pH strip tests confirmed that this reaction requires a solvent such as N,N-dimethylformamide or dimethyl sulfoxide. In the dry vacuum environment, it simply never happens.

The in situ measurements also mapped three distinct stages during post-deposition annealing of the evaporated films. First, the prematurely formed three-dimensional perovskite partially decomposed, with excess formamidinium intercalating into the lead halide octahedral framework to create a low-dimensional, formamidinium-rich phase identified as FA2PbI4. Second, formamidinium iodide gradually deprotonated and decomposed, a process accelerated by the catalytic effect of the zinc oxide layer beneath the film. Third, continued loss of formamidinium drove the intermediate into the final three-dimensional perovskite. Films containing 5-aminovaleric acid followed essentially the same pathway, delaying three-dimensional phase formation only slightly, from 24 seconds to 29 seconds of annealing. The lesson was clear: what matters is preventing the three-dimensional phase from forming during deposition and then letting it emerge slowly during annealing, guided by a stable intermediate framework.

Armed with that insight, the team designed a molecule that could play the role of the missing solvent. They chose 2,6-pyridine dicarboxamide, or PyDA, a dual-amide compound whose two carbonyl groups form hydrogen bonds with the two amine groups of the formamidinium cation. The molecule had to satisfy two demands: it needed multiple hydrogen-bond donors and acceptors to stabilize formamidinium ions and prevent premature perovskite formation, yet it had to remain stable under vacuum and release the cation during annealing. Sequentially evaporated films containing PyDA showed only lead iodide absorption features during deposition, with no detectable three-dimensional perovskite photoluminescence peak or diffraction rings, confirming that phase formation was effectively suppressed in the chamber.

During annealing, the hydrogen-bonded network did its job gradually. Iodide ions diffused from the formamidinium iodide layer into the lead iodide layer and converted its edge-sharing lattice into corner-sharing octahedra, with the growth of this phase saturating after about 69 seconds as free iodide was depleted. Formamidinium was then released slowly from the PyDA network over roughly 420 seconds to build the three-dimensional perovskite, a formation time almost seven times longer than with 5-aminovaleric acid. Fourier transform infrared spectroscopy and proton solid-state nuclear magnetic resonance verified the hydrogen bonds between the two molecules, and in situ infrared measurements showed the coupled carbonyl and formamidinium vibrational peaks splitting apart as annealing broke the network down. Fitting the temperature-dependent formation rates to the Arrhenius equation yielded apparent activation energies of 1.11, 1.17 and 1.98 electronvolts for the control, 5-aminovaleric acid and PyDA films respectively, with PyDA exhibiting the highest barrier, approaching that of spin-coated films containing 5-aminovaleric acid. Density functional theory calculations independently supported the picture, showing stronger binding for the PyDA-formamidinium complex and a higher barrier to cation release.

The controlled crystallization paid off dramatically in film quality and device performance. Photoluminescence quantum efficiency climbed from 12.6 percent in control films and 38.8 percent with 5-aminovaleric acid to 82.4 percent with PyDA, while the photoluminescence lifetime lengthened from 214 nanoseconds to 809 nanoseconds. Modeling of the carrier dynamics showed that the defect-assisted non-radiative recombination rate fell by roughly two orders of magnitude, from 1.80 times ten to the sixth per second in control films to 1.86 times ten to the fourth per second, while the radiative bimolecular rate stayed comparable, meaning the efficiency gain came primarily from suppressing defect-assisted non-radiative recombination. The resulting LEDs turned on at just 1.3 volts, reached a maximum radiance of about 185 watts per square meter per steradian at 3.1 volts, and delivered a champion external quantum efficiency of 25.0 percent with an average of 22.1 percent, competitive with state-of-the-art solution-processed near-infrared perovskite LEDs. Emission was uniform across a 10-by-10-centimeter film, with device efficiencies consistently exceeding 20 percent across the area, and the team also demonstrated a large-area LED with an active area of 5 by 5 centimeters. Related dual-amide molecules, including 2-butenediamide, succinimide and 1,3-benzene dicarboxamide, also suppressed premature crystallization more effectively than 5-aminovaleric acid and yielded device efficiencies of 18.7, 16.8 and 18.0 percent respectively, suggesting the design principle is transferable, though PyDA remained the strongest performer.

To showcase the practical advantage of vacuum deposition, the researchers replaced lead iodide with lead bromide to tune emission across the near-infrared and deep-red, achieving peak external quantum efficiencies of 12.9 to 17.3 percent at wavelengths from 748 to 771 nanometers, with nearly identical current density-voltage curves across the series. They then integrated LEDs emitting at 805 and 759 nanometers onto the same substrate, creating monolithic dual-color pixels that point toward perovskite pixelated arrays. As a demonstration, they built a wearable pulse oximeter in which the dual-color perovskite emitter and a silicon photodiode exploit the different absorption of oxygenated and deoxygenated hemoglobin at the two wavelengths. The device measured blood oxygen saturation between 96 and 100 percent, with a mean absolute deviation of only 0.95 percent relative to a commercial oximeter used for calibration. Because both light sources share one thin monolithic film rather than discrete LEDs, the architecture offers strong potential for integration into wearable systems.

The study establishes a molecular strategy for intermediate-phase regulation in thermally evaporated formamidinium-based perovskites, a challenge that has limited evaporated devices across the field. By substituting a rationally designed hydrogen-bonding mediator for the solvent coordination that solution chemistry provides, the researchers showed that the intermediate-phase strategy long exploited in spin-coated films can be recreated under high vacuum. The authors note that the hydrogen-bond-mediated interaction is proposed as an important factor contributing to the regulated evolution of the intermediate phase, which subsequently undergoes a gradual conversion into the three-dimensional perovskite during annealing. Their in situ spectroscopic toolkit offers a way to test such mechanisms directly as films grow, and the demonstration of dual-color pixels on a single substrate and a functioning oximeter highlights the potential of evaporated perovskites for multicolor pixelation and wearable sensing applications.

Subject of Research: Crystallization regulation of thermally evaporated perovskite films for efficient near-infrared light-emitting diodes

Article Title: Regulating the crystallization of thermally evaporated perovskites for efficient near-infrared light-emitting diodes

Article References: Liu, Y., Zhou, Y., Chang, J., Yang, J., Yuan, D., Xu, W., Wang, W., Li, Z., Feng, X., Fu, L., Wang, J., Li, Z., He, B., Yu, T., Zou, Z., Gao, F., & Bao, C. (2026). Regulating the crystallization of thermally evaporated perovskites for efficient near-infrared light-emitting diodes. Nature Photonics. https://doi.org/10.1038/s41566-026-02036-5

Image Credits: AI Generated

DOI: 10.1038/s41566-026-02036-5

Keywords: perovskite LEDs, thermal evaporation, crystallization, hydrogen bonding, near-infrared, external quantum efficiency, FAPbI3, pulse oximetry, vacuum deposition, intermediate phase, PyDA, wearable sensors

The research team, composed of Vilya K and Shaik Kaleemulla from the Department of Physics at Vellore Institute of Technology, set out to answer a deceptively simple question: when vanadium pentoxide and molybdenum trioxide are combined, is it better to mix them into a single composite film or to stack them into a layered heterostructure? Both approaches use the same two transition metal oxides, both films are deposited by the same thermal evaporation technique, and both are subjected to the same annealing treatment. The only difference is geometry, and that difference turns out to be decisive. The heterostructure, arranged as a V2O5/MoO3/V2O5 sandwich, outperformed the mixed composite in optical modulation by roughly eleven percent, achieving a coloration efficiency of 40.42 square centimeters per coulomb along with improved cycling stability.

To understand why this comparison matters, it helps to consider what happens inside an electrochromic film at the atomic scale. When a negative voltage is applied, positively charged ions from an electrolyte insert themselves into the oxide lattice, a process called intercalation. Electrons follow to maintain charge neutrality, and the resulting change in the electronic structure of the material alters which wavelengths of light it absorbs. In cathodic coloring materials such as the oxides studied here, this insertion process shifts the film from a relatively transparent state to a colored one. The efficiency of this transformation depends on how easily ions can travel through the film, how many insertion sites are available, and how well the material withstands repeated swelling and contraction as ions shuttle in and out over thousands of cycles.

Thermal evaporation, the deposition method chosen by the researchers, is one of the oldest and most scalable techniques in thin film fabrication. Material is heated in a vacuum chamber until it vaporizes, and the vapor condenses onto a substrate to form a film. The technique is attractive for electrochromic applications because it produces dense, adherent coatings without the solvents or binders that can contaminate electrochemical performance. In this study, the same evaporation process was used to create two distinct architectures. For the composite, vanadium and molybdenum oxides were co-deposited into a single mixed layer. For the heterostructure, the deposition was staged sequentially, first vanadium pentoxide, then molybdenum trioxide, then vanadium pentoxide again, creating three distinct layers with sharp interfaces. An annealing step followed in both cases, and this thermal treatment modified the morphology of the films in ways that proved central to their electrochemical behavior.

Before any electrochromic testing began, the team subjected both film types to a rigorous structural interrogation. X-ray diffraction revealed the crystalline phases present in the deposited layers, confirming that the films had formed the expected oxide structures. Raman spectroscopy provided complementary vibrational fingerprints that further verified the lattice structure, while X-ray photoelectron spectroscopy probed the oxidation states of the metal atoms at the film surface. This last measurement is particularly important for electrochromic materials because the ratio of different oxidation states, such as the presence of reduced molybdenum species alongside fully oxidized ones, directly influences how the material responds to ion insertion. Field emission scanning electron microscopy paired with energy dispersive spectroscopy then mapped the physical landscape of the films, revealing a rod-shaped morphology with uniform elemental distribution across both architectures.

With the structural groundwork established, the electrochemical characterization delivered the study’s most consequential findings. Cyclic voltammetry, a technique that sweeps the voltage back and forth while measuring current, confirmed that both films exhibit cathodic coloring behavior, darkening when ions insert and bleaching when they are extracted. The shape and magnitude of the voltammetric curves provided insight into how reversibly each film could accommodate the intercalation process. Electrochemical impedance spectroscopy added a second layer of understanding by probing how readily ions diffuse through the films and how the films behave as capacitive elements. Together, these measurements painted a picture of two materials that are chemically similar but kinetically distinct, with the layered heterostructure offering more favorable pathways for ion transport.

The headline result came from chronoamperometry combined with optical transmission measurements. When the researchers quantified the optical modulation, the change in transmittance between the bleached and colored states, the heterostructure films delivered approximately eleven percent more modulation than the composite films. In practical terms, this means the layered architecture can swing more dramatically between transparent and colored states for the same electrical input, which translates directly into deeper, more saturated blue hues in a display application. The coloration efficiency, defined as the optical density change achieved per unit of charge inserted, reached 40.42 square centimeters per coulomb for the heterostructure, a figure that indicates the film converts electrical energy into optical change with notable economy. Equally important, the heterostructure demonstrated enhanced cycling stability compared to the composite, meaning it can endure many more coloration and bleaching cycles before its performance degrades.

The physical explanation for the heterostructure’s advantage likely lies in the interfaces between its layers. In a sandwich structure, each vanadium pentoxide layer is bounded by molybdenum trioxide, and these heterointerfaces can act as highways for ion transport while also buffering the mechanical stress that accompanies repeated ion insertion. The composite, by contrast, distributes both oxides randomly throughout the film volume, which can create regions of poor connectivity and uneven ion distribution. The rod-shaped morphology observed in both films, with its high surface area and open channels, benefits both architectures, but the layered design appears to organize those channels more effectively. This interpretation is consistent with a growing body of literature on molybdenum oxide and vanadium oxide hybrid systems, where bilayer and multilayer configurations have repeatedly shown superior electrochromic kinetics compared to homogeneous mixtures.

The broader significance of this work extends well beyond the laboratory bench. Displays that can reproduce the full RGB color space using electrochromic materials would enable a new generation of low-power, reflective screens that remain legible in bright sunlight without backlights. Smart windows based on the same chemistry could dynamically regulate solar heat gain in buildings, reducing air conditioning loads in an era of rising energy demand. Electrochromic materials are also finding unexpected applications in biosensing, where color changes signal the presence of target molecules, and in adaptive camouflage and automotive glazing. For all of these applications, the blue channel has been the bottleneck, because few materials combine the deep blue coloration, fast switching, and long-term stability that commercial products require. By demonstrating that a simple change in film architecture can meaningfully improve blue hue modulation using two abundant and well-characterized oxides, the Vellore team has added a practical design rule to the electrochromic engineer’s toolkit.

What makes the study especially compelling is its methodological discipline. Rather than introducing new dopants or exotic synthesis routes, the researchers held every variable constant except the arrangement of the two oxides, isolating architecture as the sole determinant of performance. The annealing step, applied identically to both film types, modified the morphology in parallel, ensuring that the comparison remained fair. The comprehensive characterization suite, spanning diffraction, spectroscopy, microscopy, and multiple electrochemical techniques, provides the kind of complete evidence base that allows other researchers to reproduce and extend the findings. As electrochromic technology moves from laboratory curiosity toward commercial reality in displays, windows, and sensors, studies of this kind, which carefully disentangle the factors that govern performance, will be essential guides. The message from Vellore is clear: in the quest for better blue, how you stack the layers matters as much as what you put in them.

Subject of Research: Electrochromic performance of V2O5-MoO3 composite and heterostructure thin films for blue hue modulation

Article Title: Comparative study of V2O5-MoO3 based composite and heterostructure films prepared by thermal evaporation for enhanced electrochromic blue hue modulation

Article References: K, V., & Kaleemulla, S. (2026). Comparative study of V2O5-MoO3 based composite and heterostructure films prepared by thermal evaporation for enhanced electrochromic blue hue modulation. Ionics. https://doi.org/10.1007/s11581-026-07494-z

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07494-z

Keywords: electrochromic, V2O5, MoO3, heterostructure, composite films, thermal evaporation, thin films, coloration efficiency, metal oxides, ion intercalation, smart windows, display technology