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The radiative recombination of injected charge carriers gives rise to electroluminescence (EL), a central process for light-emitting diode (LED) operation. It is often presumed in some emerging fields of optoelectronics, including perovskite and organic LEDs, that the minimum voltage required for light emission is the semiconductor bandgap divided by the elementary charge. Here we show for many classes of LEDs, including those based on metal halide perovskite, organic, chalcogenide quantum-dot and commercial III-V semiconductors, photon emission can be generally observed at record-low driving voltages of 36%-60% of their bandgaps, corresponding to a large apparent energy gain of 0.6-1.4 eV per emitted photon. Importantly, for various classes of LEDs with very different modes of charge injection and recombination (dark saturation current densities ranging from ~10^-35 to ~10^-21 mA/cm2), their EL intensity-voltage curves under low voltages exhibit similar behaviors, revealing a universal origin of ultralow-voltage device operation. Finally, we demonstrate as a proof-of-concept that perovskite LEDs can transmit data efficiently to a silicon detector at 1V, a voltage below the silicon bandgap. Our work provides a fresh insight into the operational limits of electroluminescent diodes, highlighting the significant potential of integrating low-voltage LEDs with silicon electronics for next-generation communications and computational applications.
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Solution-processed planar perovskite light-emitting diodes (LEDs) promise high-performance and cost-effective electroluminescent (EL) devices ideal for large-area display and lighting applications. Exploiting emission layers with high ratios of horizontal transition dipole moments (TDMs) is expected to boost photon outcoupling of planar LEDs. However, LEDs based on anisotropic perovskite nanoemitters remains to be inefficient (external quantum efficiency, EQE <5%), due to the difficulties of simultaneously controlling the orientations of TDMs, achieving high photoluminescence quantum yields (PLQYs) and realizing charge balance in the films of the assembled nanostructures. Here we demonstrate efficient EL from an in-situ grown continuous perovskite film comprising of a monolayer of face-on oriented nanoplatelets. The ratio of horizontal TDMs of the perovskite nanoplatelet films is ~84%, substantially higher than that of isotropic emitters (67%). The nanoplatelet film shows a high PLQY of ~75%. These merits enable LEDs with a peak EQE of 23.6%, representing the most efficient perovskite LEDs.
We present the combined analysis of the electroluminescence (EL) as well as the current-voltage (I-V) behavior of single, freestanding (In,Ga)N/GaN nanowire (NW) light-emitting diodes (LEDs) in an unprocessed, self-assembled ensemble grown by molecular beam epitaxy. The data were acquired in a scanning electron microscope equipped with a micromanipulator and a luminescence detection system. Single NW spectra consist of emission lines originating from different quantum wells, and the width of the spectra increases with decreasing peak emission energy. The corresponding I-V characteristics are described well by the modified Shockley equation. The key advantage of this measurement approach is the possibility to correlate the EL intensity of a single NW LED with the actual current density in this NW. This way, the external quantum efficiency (EQE) can be investigated as a function of the current in a single NW LED. The comparison of the EQE characteristic of single NWs and the ensemble device allows a quite accurate determination of the actual number of emitting NWs in the working ensemble LED and the respective current densities in its individual NWs. This information is decisive for a meaningful and comprehensive characterization of a NW ensemble device, rendering the measurement approach employed here a very powerful analysis tool.
Short wavelength ultraviolet (UV-C) light deactivates DNA of any germs, including multiresistive bacteria and viruses like COVID-19. Two-dimensional (2D) material-based UV-C light emitting diodes can potentially be integrated into arbitrary surfaces to allow for shadow-free surface disinfection. In this work, we perform a series of first-principles calculations to identify the core components of ultrathin LEDs based on hexagonal boron nitride (hBN). The electrons and holes are predicted to be confined in multiple quantum wells (MQWs) by combining hBN layers with different stacking orders. Various p- and n-doping candidates for hBN are assessed, and the relative p- and n-type metal contacts with low Schottky barrier heights are identified. The findings are summarized in a concrete UV-C LED structure proposal.
Light-emitting diodes (LEDs) based on III-V/II-VI materials have delivered a compelling performance in the mid-infrared (mid-IR) region, which enabled wide-ranging applications, including environmental monitoring, defense and medical diagnostics. Continued efforts are underway to realize on-chip sensors via heterogeneous integration of mid-IR emitters on a silicon photonic chip. But the uptake of such approach is limited by the high costs and interfacial strains, associated with the process of heterogeneous integrations. Here, the black phosphorus (BP)-based van der Waals (vdW) heterostructures are exploited as room temperature LEDs. The demonstrated devices can emit linearly polarized light, and their spectra cover the technologically important mid-IR atmospheric window (3-4 um). Additionally, the BP LEDs exhibit fast modulation speed as well as exceptional stability, and its peak extrinsic quantum efficiency (QE~0.9%) is comparable to the III-V/II-VI mid-IR LEDs. By leveraging the integrability of vdW heterostructures, we further demonstrate a silicon photonic waveguide-integrated BP LED. The reported hybrid platform holds great promise for mid-IR silicon photonics.
Perovskite-based optoelectronic devices have gained significant attention due to their remarkable performance and low processing cost, particularly for solar cells. However, for perovskite light-emitting diodes (LEDs), non-radiative charge carrier recombination has limited electroluminescence (EL) efficiency. Here we demonstrate perovskite-polymer bulk heterostructure LEDs exhibiting record-high external quantum efficiencies (EQEs) exceeding 20%, and an EL half-life of 46 hours under continuous operation. This performance is achieved with an emissive layer comprising quasi-2D and 3D perovskites and an insulating polymer. Transient optical spectroscopy reveals that photogenerated excitations at the quasi-2D perovskite component migrate to lower-energy sites within 1 ps. The dominant component of the photoluminescence (PL) is primarily bimolecular and is characteristic of the 3D regions. From PL quantum efficiency and transient kinetics of the emissive layer with/without charge-transport contacts, we find non-radiative recombination pathways to be effectively eliminated. Light outcoupling from planar LEDs, as used in OLED displays, generally limits EQE to 20-30%, and we model our reported EL efficiency of over 20% in the forward direction to indicate the internal quantum efficiency (IQE) to be close to 100%. Together with the low drive voltages needed to achieve useful photon fluxes (2-3 V for 0.1-1 mA/cm2), these results establish that perovskite-based LEDs have significant potential for light-emission applications.
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