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Room-temperature spin injection across a chiral perovskite/III–V interface

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To achieve highly efficient and bright thin-film LEDs, a combination of factors is necessary for the light-emitting materials. These include high PLQE, high charge mobilities and low quantum efficiency droop. Traditional thin-film LEDs, such as organic LEDs, face challenges in achieving high efficiency at high brightness owing to their low charge mobility and susceptibility to Auger or excitonic quenching⁷. In recent studies, it has been demonstrated that low-dimensional perovskites with multiple-quantum-well or quantum dot structures, making use of quantum confinement effects, can effectively suppress nonradiative recombination^8,9,10. This phenomenon allows them to achieve near 100% PLQE⁹. However, these low-dimensional perovskite materials often exhibit low charge mobility and suffer from severe Auger recombination, which limits their potential for efficient LEDs at high brightness^8,11. On the other hand, 3D perovskites have emerged as promising materials for the development of efficient and bright thin-film LEDs. They have high charge mobility and show low quantum efficiency droop. Furthermore, 3D perovskites can naturally form discrete, sub-micrometre-scale structures, leading to an enhanced light outcoupling efficiency greater than 30% (ref. ¹). Recent advancements have showcased large-sized, flexible, efficient and bright LEDs based on discrete 3D perovskites with excellent stability¹². However, one challenge faced by 3D perovskites is their slow radiative recombination rate, making their PLQE highly susceptible to defects^4,13. To address this issue, various passivation strategies have been used to reduce the defect density in 3D perovskite films, approaching levels comparable with single crystals³. Despite substantial efforts, the maximum achievable PLQE only reaches approximately 80%, with resulting LEDs exhibiting peak EQEs of less than 25% (refs. ^3,4).

Here we demonstrate efficient, near-infrared 3D perovskites by accelerating the radiative recombination through formation of tetragonal FAPbI₃ perovskite using a dual-additive method. The fabrication process is shown in Fig. 1a. A precursor solution was prepared using 1-aminopyridinium iodide (PyNI), 5-aminovaleric acid (5AVA), formamidinium iodide (FAI) and PbI₂ with a molar ratio of 0.15/0.25/2.40/1.00 dissolved in N,N-dimethylformamide (DMF; 8.5 wt%) (see Methods for details), in which PyNI and 5AVA are both additives. For comparison, a control sample using only a single additive (5AVA) was also prepared¹. We note that the role of 5AVA has been investigated previously, which can markedly enhance the LED efficiency by facilitating the formation of low-defect-density FAPbI₃ perovskite with sub-micron discrete structures¹. The control and dual-additive perovskites exhibit similar dispersed sub-micron grain morphologies (Fig. 1a and Extended Data Fig. 1), which can contribute to a light outcoupling efficiency greater than 30% in LED device architectures¹ (Supplementary Note 1). Notably, the dual-additive sample shows a much higher PLQE of 96% compared with the control sample (about 70%) (Fig. 1b).

**Fig. 1: Fabrication process and characterization of perovskite films.**

Then we fabricated and characterized LEDs based on the two samples mentioned above. The device structure was indium tin oxide (ITO)/polyethylenimine ethoxylated (PEIE)-modified zinc oxide (ZnO; 30 nm)/perovskite (approximately 60 nm)/poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)diphenylamine) (TFB; 20 nm)/molybdenum oxide (MoO_x; 5 nm)/gold (Au; 80 nm)^1,3 (Fig. 2a and Extended Data Fig. 2a). The dual-additive perovskite LEDs exhibit a peak EQE as high as 32.0% at a current density of 20 mA cm⁻², accompanied by a maximum brightness of 390 W sr⁻¹ m⁻² at a low voltage of 3.6 V (Fig. 2b,c). The EQE remains high, with a value of 30% at a high current density of 100 mA cm⁻². The peak energy conversion efficiency (ECE) reaches 25.5% at a current density of roughly 1 mA cm⁻² and is maintained at 16.0% at a high current density of 100 mA cm⁻² (Supplementary Note 2). The electroluminescence (EL) peak is located at 805 nm (Fig. 2d), which is slightly redshifted compared with the control device (801 nm) (Extended Data Fig. 3c). The devices demonstrate good reproducibility, with an EQE histogram for 70 devices showing an average peak EQE of 28.5% (Fig. 2e). By contrast, the control device exhibits a peak EQE of approximately 20% (Extended Data Fig. 3a). Considering they have similar light outcoupling efficiency (Supplementary Note 1), we believe that the improved performance of the dual-additive device can be mainly attributed to its higher PLQE (Fig. 1b). Also, we conducted measurement on the half-life (T₅₀) stability of our devices under a constant current density of 100 mA cm⁻². The T₅₀ lifetimes of the devices were found to be comparable, with the control sample lasting for 19 h (Extended Data Fig. 3d), which aligns closely with the reported result¹, and the dual-additive device lasting for 17 h (Fig. 2f). We believe that the slightly shorter lifetime of the dual-additive device could be because of the slightly more excess iodide ions with the dual-additive perovskite film, as the ion migration is the main cause of the device degradation¹⁴.

**Fig. 2: Device structure and performance of dual-additive perovskite LEDs.**

To verify why the dual-additive perovskite exhibits enhanced PLQE, we conducted time-resolved photoluminescence (TRPL) measurements under various excitation intensities. At an extremely low excitation intensity (with an initial carrier density of about 10¹³ cm⁻³), the TRPL of both the dual-additive perovskite and the control sample exhibit similar decay behaviour, indicating comparable trap densities between them¹⁵ (Extended Data Fig. 4). As the excitation intensities increase, the dual-additive perovskite exhibits accelerated photoluminescence (PL) decay compared with the control sample (Fig. 3a,b), which can be because of enhanced radiative recombination. To quantify this observation, we determined the first-order (trap-assisted and excitonic, k₁) and second-order (bimolecular, k₂) recombination rate constants for both samples by fitting the transient PL data, while keeping the third-order Auger recombination rate constant (k₃)⁴ (Supplementary Note 3). The fitting results indicate that, regardless of the injected carrier density, the k₂ value of the dual-additive perovskite surpasses that of the control sample by several folds, and k₁ is approximately one order of magnitude higher for the dual-additive perovskite compared with the control sample (Extended Data Table 1). We found negligible effects on the fitting results by increasing/decreasing k₃ for one order of magnitude (Supplementary Note 3). Because the trap densities between them are similar, the enhanced k₁ in dual-additive perovskite is mainly because of the enhancement of excitonic (radiative) recombination instead of trap-assisted (nonradiative) recombination, which is consistent with the high PLQE observed in Fig. 1b under low excitation intensities. On the basis of these findings, we can conclude that the enhanced PLQE of the dual-additive perovskite film is associated with increased radiative recombination rates (both excitonic and bimolecular), rather than a decreased nonradiative recombination rate typically observed through defects passivation in previous studies^1,2,3.

**Fig. 3: Optical properties of perovskite films.**

We then investigate the underlying mechanism behind the enhanced radiative recombination rates. We observe that the absorption edge of the dual-additive perovskite film become more obvious (Fig. 3c), indicating a stronger excitonic feature compared with the control sample. By fitting the absorption spectra near the band edge based on Elliott’s theory¹⁶ (Supplementary Note 4), we obtain an exciton binding energy (E_b) of 13.9 meV for the dual-additive perovskite film, which is much higher than that of the control sample (3.9 meV) (Extended Data Fig. 5). The increased E_b can lead to higher population of excitons in the excited states and enhance radiative recombination^17,18, which is consistent with the fitting results (Extended Data Table 1). The more excitonic recombination can be further confirmed by power-dependent TRPL measurements at the zero time (I_PL[t = 0]) (Fig. 3d). In these experiments, the PL intensities were measured immediately after the ultra-fast (fs) excitation, which can precisely determine the carrier-density-dependent PL intensity¹³. We then fit the PL intensities using a power-law function of the form PL ≈ P^k, in which P represents excitation fluence and k is a real-number exponent providing information on the order of recombination. For dual-additive perovskite, a k value of 1.7 indicates the coexistence of free carriers (bimolecular recombination) and excitons (monomolecular recombination) (Supplementary Note 5). Conversely, I_PL[t = 0] of the control perovskite increases quadratically (meaning a k value of 2) with the excitation fluence, suggesting predominantly bimolecular recombination. Therefore, we believe that the accelerated radiative recombination of the dual-additive perovskite is because of the increased E_b, which is consistent with previous studies on the 3D FAPbI₃ perovskite at low temperatures¹⁹.

The enhanced exciton binding energy in perovskites can result from increased space confinement of crystal grains or phase transitions^19,20,21. In both the EL and PL spectra of the dual-additive perovskite, we observe an approximately 4 nm redshift compared with those of control sample (Figs. 2d and 3c and Extended Data Fig. 3c). This indicates that the dual-additive perovskite does not have enhanced space confinement. Otherwise, one would anticipate a blueshift in the emission. So, we can infer that a phase transition is responsible for the observed increase in binding energy. Also, because the dual-additive perovskite shows a similar trap density to the control sample, we believe that the trap-induced redshift does not occur in this case²². To investigate the crystal structure of the perovskite films, we conducted grazing-incidence wide-angle X-ray scattering (GIWAXS) analysis, varying the incidence angles (0.05° to 0.50°) to examine both the surface and bulk regions of the films. As shown in Fig. 4a, both the dual-additive perovskite and the control sample exhibit characteristic scattering at q ≈ 1.00 Å⁻¹, which is attributed to the cubic or tetragonal perovskite^23,24. Notably, the control sample shows the overlapping of discrete Bragg scattering spots and scattering rings (or arcs), whereas the dual-additive perovskite shows a scattering ring isotropic in polar angle. This result suggests a preferential stacking of vertical-oriented octahedrons along the surface-normal direction in the control sample, along with a small amount of random crystalline orientation (Extended Data Fig. 6), whereas the dual-additive perovskite has a random crystalline orientation (Extended Data Fig. 7). This observation is consistent with our morphology characterization that shows several types of grain packed in the dual-additive film (Extended Data Fig. 2b). We then analysed the q_z direction of 1D GIWAXS at various probing angles (or depths) (Fig. 4b). It shows that the control sample exhibits a uniform distribution of both cubic and tetragonal phases throughout the film. By contrast, the upper part of the dual-additive film is predominantly composed of a single tetragonal phase (0.05° to 0.25°), whereas the lower part exhibits mainly tetragonal phase with a small amount of cubic phases (Fig. 4c). It is consistent with the observed redshifted EL and PL spectra of the dual-additive perovskite film, as the tetragonal phase of FAPbI₃ (refs. ^19,25). On the basis of these investigations, we can conclude that the increased exciton binding energy in the dual-additive film is primarily attributed to the higher proportion of the tetragonal phase, which is consistent with the observation of the perovskite under low-temperature conditions^19,25. Also, our transient PL measurement further shows that there is no substantial energy transfer occurring between the two phases (Supplementary Fig. 4), probably because of their very close bandgaps²⁵ (Supplementary Note 6). Moreover, because the ZnO electron transport layer has a much higher charge mobility than the TFB hole transport layer^26,27, we can expect that the electron–hole recombination mainly occurs near the interface of perovskite and TFB, which is predominantly of single tetragonal phase.

**Fig. 4: Structural characterization of perovskite films.**

To investigate how the dual additives can induce an increased proportion of the tetragonal phase, we conducted in situ absorption-spectra measurements during the film-annealing process (Extended Data Fig. 8). We observe that PyNI-additive-alone perovskite film exhibits dominant absorption peaks at 445 nm (PbI₄²⁻) and 490 nm (PbI₆⁴⁻), similar to the samples without additive or with 5AVA alone²⁸. Notably, the PyNI-additive-alone and dual-additive films show a rapid decrease in absorption at 445 nm, along with an increase at 490 nm, indicating that PyNI can facilitate the conversion of PbI₄²⁻ to PbI₆⁴⁻ during annealing. In the cases of the 5AVA-alone and dual-additive films, we observe a further absorption peak at 545 nm, which can be attributed to the formation of a 5AVA-related low-dimensional intermediate phase³. These results indicate the coexistence of two crystallization pathways in the dual-additive system induced by the two additives separately. Previous studies have shown that the competition between these two crystallization pathways can lead to the confinement of the crystals, resulting in a higher proportion of the tetragonal phase^29,30. Notably, from scanning transmission electron microscopy (STEM) measurements, we can observe the variation in Kikuchi line patterns among various regions within each particle for the dual-additive perovskite (Extended Data Fig. 2b). This suggests that the particles in dual-additive perovskite consist of several grains that are tightly connected or interlocked. We believe that our result of the formation of more tetragonal phase in dual-additive perovskite is consistent with previous studies.

To achieve high-efficiency LEDs with superior brightness, it is crucial to use light-emitting materials that exhibit high PLQEs, minimal Auger or excitonic quenching, high charge mobilities and preferably have structures conducive to efficient light outcoupling. Existing thin-film light-emitting materials, including organic semiconductors, quantum dots and low-dimensional perovskites, have fallen short of meeting all these criteria simultaneously. In our study, we present a straightforward approach to address this challenge by using 3D perovskites with increased exciton binding energy, which facilitate an accelerated rate of radiative recombination. By promoting the formation of tetragonal FAPbI₃ perovskite, we have successfully achieved a near-unity PLQE in the 3D perovskite film. This remarkable achievement has enabled us to realize LEDs with an unprecedented EQE record of 32.0%. Our work holds pivotal importance in paving the way for continuous advancements in breaking the efficiency limits of perovskite LEDs and unlocks their full capabilities in next-generation display and lighting technologies.

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Welcome to the Laser Wars

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The age of the laser weapon is finally upon us.

The United States Army has officially sent a pair of high-energy laser weapons overseas to defend American troops and US allies against enemy drones, the service recently revealed, marking the first publicly known deployment of a directed-energy system for air defense in military history. And, according to a top official, those weapons are actively blasting threats out of the sky.

The weapon, known as the Palletized High Energy Laser (P-HEL) and developed by the American defense contractor BlueHalo based on the company’s 20-kilowatt Locust Laser Weapon System, first arrived in an unspecified location overseas and “commenced operational employment” in November 2022, according to an April press release from the company. A second system arrived overseas “earlier this year.”

While the Army initially declined to indicate where the P-HEL systems were sent and whether they had achieved a “kill” against an adversary drone, citing operational security concerns, the service’s top acquisition official recently confirmed that the new laser weapons had in fact succeeded in neutralizing incoming threats in the Middle East.

“They’ve worked in some cases,” Doug Bush, the Army’s assistant secretary for acquisition, logistics, and technology, told Forbes this month. “In the right conditions, they’re highly effective against certain threats.”

News of the P-HEL’s deployment comes as the US military seeks to aggressively bolster its air defense capabilities amid a dramatic increase in drone and missile attacks against US troops by Iran-backed militias in the Middle East, as well as against US Navy warships operating in the Red Sea by Houthi rebels in Yemen following the October 7 attack in Israel by Hamas.

Since the beginning of the Israel-Hamas conflict, the US Defense Department has been slowly but surely hinting at the use of laser weapons downrange. But the arrival of the P-HEL in the Middle East for operational use is a technological victory for the US military, which has actively pursued research related to directed-energy weapons since the 1970s. Even more significantly, it may also represent a tipping point for the development and use of laser weapons more broadly by militaries around the world.

Light at the End of the Tunnel

Following its creation in 1960 by American engineer and physicist Theodore Maiman, the laser—technically an acronym for “light amplification by stimulated emission of radiation”—almost immediately became a futuristic weapon of choice among both science fiction writers and military planners. This wasn’t surprising: While Maiman touted the potential scientific applications of his discovery when he first unveiled it to the country later that year, the laser immediately conjured up visions in the public consciousness of the Martian “heat ray” from H. G. Wells’ War of the Worlds, so much so that many of the contemporary headlines from its debut were variations of the Los Angeles Herald’s “L.A. Man Discovers Science Fiction Death Ray,” according to Jeff Hecht’s book Beam: The Race to Make the Laser. “In reality, the laser was more of a Life Ray than a Death Ray,” Maiman would later recall thinking of his invention’s medical applications, according to his memoir.

The Pentagon began exploring the military applications of lasers almost immediately, from relatively practical uses like designators for laser-guided bombs to more far-fetched concepts like the Strategic Defense Initiative of the 1980s, also known as “Star Wars.” But only in the past few decades has the underlying technology advanced to the point where laser weapons are effective against their intended targets.

In the mid-2000s, the Air Force successfully used its Boeing 747-based YAL-1 airborne laser to defeat ballistic missiles in flight during tests, while the Army’s Humvee-mounted Zeus-HMMWV Laser Ordnance Neutralization System system deployed to Afghanistan and Iraq to zap landmines, improvised explosive devices, and unexploded ordnance. By 2014, the Navy’s AN/SEQ-3 Laser Weapon System (LaWS) was successfully disabling drones and small boats during testing from the bow of the Austin-class amphibious transport dock USS Ponce in what the service billed at the time as the world’s first “active laser weapon.” (When the Ponce was decommissioned in 2017, the LaWS’s successor system, the Technology Maturation Laser Weapon System Demonstrator, was installed on the San Antonio-class amphibious transport dock USS Portland, which successfully tested it in 2020 and 2021).

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May 13, 2024

‘Sound laser’ is the most powerful ever made

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A “phonon laser” emits particles of sound instead of light

Dmytro Razinkov/Alamy

A tiny, levitated bead is at the core of an unprecedentedly bright laser that shoots particles of sound instead of light.

Just as a ray of light is made up of many particles called photons, sound consists of particle-like chunks called phonons. For several decades, researchers have been creating “phonon lasers” that output these particles in a narrow beam, similar to the way optical lasers emit photons.

Now, Hui Jing at Hunan Normal University in China and his colleagues have created the brightest phonon laser yet.

The heart of their device was a roughly micrometre-long silica bead, about the size of a typical bacterium. They used two beams of light to levitate the bead and surrounded it with a reflective cavity. Any small vibration of this bead created phonons, which were then trapped and amplified in the cavity. This continued until there were enough phonons to make up a laser-like beam.

Several research groups had tested similar designs before. But Jing and his colleagues added an electrode right below the bead, which produced carefully selected electromagnetic signals. This modification enhanced the laser’s “brightness” – the amount of power it delivered at each phonon frequency – tenfold, as well as making its beam tighter and helping it last longer. Jing says that past devices, from his team and others, worked for dozens of minutes only, but the latest phonon laser could operate for over an hour.

Phonons are less affected by moving through liquids, so they could be more effective than conventional lasers for imaging watery tissues in biomedicine or in some deep-sea monitoring devices, Jing says.

But Richard Norte at the the Delft University of Technology in the Netherlands says current experimental set-ups, which require precise tuning of every component, are too intricate. Phonon lasers may require years of research and engineering before they match the usefulness of their optical counterparts.

“There is excitement about phonon lasers given the impact that optical lasers have had on modern life, but time will tell if there will be an equivalent impact,” he says.

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March 15, 2024

Lasers smaller than a human hair emit doughnut-shaped light

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Schematic image of GaN hollow nanowires on a sapphire substrate. — An artist’s impression of a hollow nanowire emitting doughnut-shaped laser light

Masato Takiguchi et al./ACS Photonics/American Chemical Society 2024

Tiny, hollow wires can produce doughnut-shaped laser light that could be used to levitate small objects or transmit information.

Conventional lasers typically make beams that appear as a single, small point of light when they hit a surface. But for some novel communication technologies that use light to transfer information, it can be better to use lasers that produce hollow beams like a drinking straw, which appear as a ring of light when they hit a surface.

Such hollow laser…

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February 15, 2024

Tag: lasers

Light at the End of the Tunnel