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Register a new userEfficient light-emitting diodes based on oriented perovskite nanoplatelets
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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.
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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.
Lead-halide perovskites have been attracting attention for potential use in solid-state lighting. Following the footsteps of solar cells, the field of perovskite light-emitting diodes (PeLEDs) has been growing rapidly. Their application prospects in lighting, however, remain still uncertain due to a variety of shortcomings in device performance including their limited levels of luminous efficiency achievable thus far. Here we show high-efficiency PeLEDs based on colloidal perovskite nanocrystals (PeNCs) synthesized at room temperature possessing dominant first-order excitonic radiation (enabling a photoluminescence quantum yield of 71% in solid film), unlike in the case of bulk perovskites with slow electron-hole bimolecular radiative recombination (a second-order process). In these PeLEDs, by reaching charge balance in the recombination zone, we find that the Auger nonradiative recombination, with its significant role in emission quenching, is effectively suppressed in low driving current density range. In consequence, these devices reach a record high maximum external quantum efficiency of 12.9% reported to date and an unprecedentedly high power efficiency of 30.3 lm W-1 at luminance levels above 1000 cd m-2 as required for various applications. These findings suggest that, with feasible levels of device performance, the PeNCs hold great promise for their use in LED lighting and displays.
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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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