Singlet and triplet harvesting enable efficient NIR-II quantum-dot electroluminescence

Recently, near-infrared light-emitting diodes (NIR LEDs) based on PbSe quantum dots (QDs) have attracted considerable attention due to their facilely tunable emission wavelength, as well as high quantum yield. However, the low external quantum efficiency (EQE) of these LEDs has restricted their actual applications because of the non-radiative recombination caused by the aggregation in the solid-state QD films. Therefore, we proposed in this work to employ the liquid-type structure in NIR LEDs base on PbSe QDs, which exhibited the main advantages relying on the fact that the liquid structure could prevent the active layer from self-aggregation and improve the device stability. The emission intensity of these NIR LEDs was optimized by tuning the concentration of PbSe QDs. Besides, the radiation power of PbSe QD-based devices with different emission wavelengths was analyzed under different biases, and the maximum EQE of NIR LEDs was confirmed to be 5.3%. This result represents the highest record among the reported NIR QD-LEDs, indicating this kind of liquid-type NIR LEDs is promising for commercial applications.

We report on the low-frequency current and light noise in 515 nm green GaInN/GaN quantum well LEDs. The current noise was the superposition of the 1/<i>f</i> and the generation-recombination (GR) noise. The characteristic time of the GR process was found to be proportional to the reciprocal current for the entire current range. This dependence is the characteristic for the monomolecular non-radiative recombination. The dominance of the nonradiative recombination is in agreement with the measured low external quantum efficiency (EQE) <10%. Hence, the noise measurements point out that a low EQE is caused by the low internal quantum efficiency and not by an inefficient light extraction. The noise spectra of light intensity fluctuations were close to the 1/<i>f</i> noise and correlated with the LED quantum efficiency and with the recombination current. Higher noise corresponded to a smaller quantum efficiency and to a higher non-radiative recombination current. The relative spectral noise densities of the light intensity fluctuations within the LED spectral line increase with the wavelength decrease. Fluctuations at different wavelengths are found to be correlated.

By alloying GaN with AlN the emission of AlGaN light-emitting diodes can be tuned to cover almost the entire ultraviolet spectral range (210–400 nm), making ultraviolet light-emitting diodes perfectly suited to applications across a wide number of fields, whether biological, environmental, industrial or medical. However, technical developments notwithstanding, deep-ultraviolet light-emitting diodes still exhibit relatively low external quantum efficiencies because of properties intrinsic to aluminium-rich group III nitride materials. Here, we review recent progress in the development of AlGaN-based deep-ultraviolet light-emitting devices. We also describe the key obstacles to enhancing their efficiency and how to improve their performance in terms of defect density, carrier-injection efficiency, light extraction efficiency and heat dissipation. This Review covers recent progress in AlGaN-based deep-ultraviolet light-emitting devices. The key technologies of how to improve their performance, carrier-injection efficiency, light extraction efficiency and heat dissipation are discussed.

Near‐infrared (NIR) phosphor‐converted LEDs (pc‐LEDs) are critical for smart agriculture and night vision, but their efficiency is limited by low external quantum efficiency (EQE) of phosphors. Herein, the introduction of Al 3+ ions via chemical substitution increased the EQE of the GdGa 2.85 (BO 3 ) 4 :0.15Cr 3+ NIR phosphor from 8.3% to 30.7% (a relative increase of 270%). Importantly, an “absorption‐efficiency decoupling” paradigm is identified: while the Al 3+ substitution reduces the absorbance, confirmed by Density Functional Theory, diffuse reflectance spectra, and X‐ray photoelectron spectroscopy, the quantum efficiency is significantly increased. Theoretical analyses and spectroscopic evidence suggest that the superior EQE gain stems from Al 3+ ‐induced crystal field enhancement and subsequent energy level remodeling. This remodeling optimizes the Cr 3+ excited state dynamics by suppressing the non‐radiative transitions pathway and increasing the radiative transitions, thus overcoming the detrimental effects of absorption reduction. The optimized phosphor, combined with a blue LED chip, produced an NIR pc‐LED achieving 41.7 mW output power and 15.7% photoelectric efficiency at 100 mA. The device successfully demonstrated potential for precision agriculture and night vision. This work provides novel insight into “absorption‐efficiency decoupling” and offers a new design strategy for efficient Cr 3+ ‐doped NIR phosphors.

Planar deep-ultraviolet (DUV) light emitting diodes (LEDs) suffer from extremely low external quantum efficiencies (EQEs) due to poor light extraction efficiencies (LEE) which are often less than 1%, hindering their widespread use. In AlGaN DUV LEDs with high Al-content, the positioning of the valence subbands leads to dominant transverse magnetic (TM)-polarized emission which is difficult to extract from planar devices. To improve the LEE of DUV LEDs, techniques such as surface roughening and nanowire formation have been used. Nanowires are especially promising for DUV LEDs because they allow for very efficient extraction of TM-polarized light through their sidewalls. In this work, we demonstrate a novel "inverse taper" profile in AlGaN nanowires, in which the base of the nanowire can be narrowed to have a smaller diameter than the top through a KOH-based wet etch process. Hydroxyl-based chemistries are known to have a lower etch rate against the c-plane of wurtzite AlGaN alloys. Here, we report on observations of 0.8% KOH at 80℃ exhibiting a unique selectivity to a different wurtzite crystal plane, believed to be the (202̅ 1) plane, allowing for formation of an inverse taper structure. Finite difference time domain (FDTD) simulations at 280 nm reveal that AlGaN nanowire LEDs with high sidewall inverse taper angles can have greater than 75% and 90% LEE for TE and TM-polarized light respectively, ~2.5x higher than the LEE of vertical sidewall nanowires. This novel phenomenon may allow for significant improvements in the LEE of DUV nanowire LEDs.

Perovskite light-emitting diodes (PeLEDs) have gained significant attention due to their promising optoelectronic properties and potential applications in the fields of lighting and display devices. Despite their potential, PeLEDs face challenges related to stability, high turn-on voltage, and low external quantum efficiency (EQE) which has restricted their broad acceptance. Most research efforts have predominantly focused on refining the properties of the perovskite films. However, it is becoming more apparent that interfacial layers and device architecture are crucial for achieving stability and high efficiency, making them indispensable components in PeLED development. This perspective highlights remarkable advancements in PeLED devices, with a primary focus on modifying adjacent layers interfacing with the perovskite film.

Organometal halide perovskite is a promising material to fabricate light-emitting diodes (LEDs) via solution processing due to its exceptional optoelectronic properties. However, incomplete precursor conversion and various defect states in the perovskite light-emitting layer lead to low luminance and external quantum efficiency of perovskite LEDs. We show here the addition of an optimum amount of sodium bromide in the methylammonium lead bromide (MAPbBr3) precursor during a one-step perovskite solution casting process can effectively improve the film coverage, enhance the crystallinity, and passivate ionic defects on the surface of MAPbBr3 crystal grains, resulting in LEDs with a reduced turn-on voltage from 2.8 to 2.3 V and an enhanced maximum luminance from 1059 to 6942 Cd/m2 when comparing with the pristine perovskite-based device.

Although high quantum efficiency has been achieved in large-sized InGaN/GaN LEDs operating at relatively high current densities (above 35 A/cm2), the operating current density of mini-LEDs (around 1 A/cm2) is far less than that of traditional large-sized LEDs. The low external quantum efficiency (EQE) of mini-LEDs at small current densities seriously hinders their practical applications, highlighting the importance of investigating the radiative recombination mechanisms of mini-LEDs at small current densities. By using microscopic hyperspectral imaging, the cryogenic electroluminescence (EL) of GaN-based green mini-LEDs mainly originating from localized excitons was demonstrated experimentally. Based on the dependence of electron–phonon coupling on current and temperature, Coulomb screening of the polarization field weakens the electron–phonon coupling, whereas the band-filling effect enhances the coupling. Coulomb screening of the polarization field can also reduce the deviation of the localization. The EL from edge regions of the mesa adjacent to the sidewall possesses relatively higher peak energy due to the strain relaxation. Results of this work also suggest that optimization of the chromatic characteristics and efficiency can be achieved by strain engineering of GaN-based green mini-LEDs.

The performance of organic photodetectors (OPDs) sensitive to the short-wavelength infrared (SWIR) light lags behind commercial indium gallium arsenide (InGaAs) photodetectors primarily due to the scarcity of organic semiconductors with efficient photoelectric responses exceeding 1.3µm. Limited by the Energy-gap law, ultralow-bandgap organic semiconductors usually suffer from severe non-radiative transitions, resulting in low external quantum efficiency (EQE). Herein, a difluoro-substituted quinoid terminal group (QC-2F) with exceptionally strong electron-negativity is developed for constructing a new non-fullerene acceptor (NFA), Y-QC4F with an ultralow bandgap of 0.83eV. This subtle structural modification significantly enhances intermolecular packing order and density, enabling an absorption onset up to 1.5µm while suppressing non-radiation recombination in Y-QC4F films. SWIR OPDs based on Y-QC4F achieve an impressive detectivity (D*) over 1011 Jones from 0.4 to 1.5µm under 0V bias, with a maximum of 1.68 × 1012 Jones at 1.16µm. Furthermore, the resulting OPDs demonstrate competitive performance with commercial photodetectors for high-quality SWIR imaging even under 1.4µm irradiation.

Narrowband photodetectors (PDs) are extensively utilized in fields such as image sensing, machine vision, and optical communications. Traditional approaches to achieving wavelength‐selective light detection typically involve methods such as voltage‐modulated charge collection or injection the incorporation of optical filters, or the use of multi‐stacked active layer materials. Herein, an innovative dual‐mode perovskite PD characterized by unbalanced charge carrier transport (UCCT) properties are introduced. By altering the direction of light incidence, the PD exhibits two distinct narrowband spectral responses, thereby significantly simplifying conventional strategies that complicate PD fabrication. This novel approach also mitigates issues related to slow response times, limited band selectivity, and low external quantum efficiency (EQE). In the Red (R)‐Mode, a detection peak at 700 nm is identified, with a full‐width at half‐maximum (FWHM) of 30 nm, a specific detectivity (D*) of 2.6 × 1010 Jones, and a responsivity (R) of 179 mA W−1. Conversely, in Green (G‐Mode, a narrowband response is observed with a peak at 500 nm, a FWHM of 75 nm, a D* of 1.1 × 1010 Jones, and an R of 75 mA W−1. Furthermore, a dual‐mode narrowband single‐PD imaging prototype, integrated with an advanced Fourier imaging algorithm, has been developed, enabling the reconstruction of high‐quality images with 128 × 128 pixels within a time frame of 100 seconds. This research represents a significant advancement in the realm of filter‐free dual‐mode narrowband PDs and lays the groundwork for the evolution of next‐generation optoelectronic applications.

Effects of quantum well number on spectral response of InGaN/GaN multiple quantum well (MQW) solar cells are investigated. It is found that the strain relaxation by forming defects occurs when the number of quantum wells (QWs) increases to 30. It results in the low luminescence intensity and external quantum efficiency (EQE) due to the poor crystal quality of MQW region and the high non-radiative recombination rate in MQW region. However, the EQE of 25-period InGaN/GaN MQW solar cell with relatively high crystal quality of MQW region does not increase at all wavelengths compared with 15-period one. This may be attributed to the lower carrier collection efficiency resulting from the lower electric field of i-MQW region.

Highly efficient light-emitting diodes based on an organic-soluble poly ( p-phenylenevinylene) derivative carrying both the hole-transporting carbazole moiety and the 2-ethylhexyloxy group

Modeling of flexible light extraction structure: Improved flexibility and optical efficiency for organic light-emitting diodes

Near-infrared light-emitting diodes (NIR LEDs) based on perovskite quantum dots (QDs) have produced external quantum efficiency (EQE) of ~15 %. However, these high-performance NIR-QLEDs suffer from immediate carrier quenching because of the accumulation of migratable ions at the surface of the QDs. These uncoordinated ions and carriers-if not bound to the nanocrystal surface-serve as centers for exciton quenching and device degradation. In this work, we overcome this issue and fabricate high-performance NIR QLEDs by devising a ligand anchoring strategy, which entails dissolving the strong-binding ligand (Guanidine Hydroiodide, GAI) in the mediate-polar solvent. By employing the dye-sensitized device structure (phosphorescent indicator), we demonstrate the elimination of the interface defects. The treated QDs films exhibit an exciton binding energy of 117 meV: this represents a 1.5-fold increase compared to that of the control (74 meV). We report, as a result, the NIR QLEDs with an EQE of 21 % which is a record among NIR perovskite QLEDs. These QLEDs also exhibit a 7-fold higher operational stability than that of the best previously reported NIR QLEDs. Furthermore, we demonstrate that the QDs are compatible with large-area QLEDs: we showcase 900 mm2 QLEDs with EQE approaching 20 %.

Light-emitting diodes (LEDs) based on perovskite quantum dots (QDs) have achieved external quantum efficiencies (EQEs) exceeding 20% at a practical luminance (>1000 cd m-2) for real-world applications in the green and red regions. However, the pure-blue perovskite QD LEDs still suffer from low efficiency at high luminance: the EQE of the reported best device at 600 cd m-2 is below 7.0%. One of the limiting factors is weak ligand-surface binding because of the dynamic ligand-binding nature. In this work, we employ first-principles calculations to evaluate the hard-soft-acid-base interaction strength and search for the appropriate alkali metal ions to resolve this limiting factor. We found that rubidium ions have the most compatible softness with bromine dangling bonds in the strongly confined QDs, leading to stronger bonding. This strategy enabled the photoluminescence quantum yield of the QD film to be 81%. The low-defect QD films ensured efficient carrier recombination, enabling pure-blue perovskite QD LEDs to generate an EQE peak of 15.7% with a luminance of 3916 cd m-2, and the EQE remains at 14.9% under a higher luminance of 5371 cd m-2. This represents an achieved record EQE × luminance: a 35-fold improvement compared to the best previous pure-blue perovskite LED.