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Register a new userNear-Infrared Lead Chalcogenide Quantum Dots: Synthesis and Applications in Light Emitting Diodes
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This paper reviews recent progress in the synthesis of near-infrared (NIR) lead chalcogenide (PbX; PbX=PbS, PbSe, PbTe) quantum dots (QDs) and their applications in NIR QDs based light emitting diodes (NIR-QLEDs). It summarizes the strategies of how to synthesize high efficiency PbX QDs and how to realize high performance PbX based NIR-QLEDs.
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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.
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.
The magnetoelectroluminescence of conjugated organic polymer films is widely accepted to arise from a polaron pair mechanism, but their magnetoconductance is less well understood. Here we derive a new relationship between the experimentally measurable magnetoelectroluminescence and magnetoconductance and the theoretically calculable singlet yield of the polaron pair recombination reaction. This relationship is expected to be valid regardless of the mechanism of the magnetoconductance, provided the mobilities of the free polarons are independent of the applied magnetic field (i.e., provided one discounts the possibility of spin-dependent transport). We also discuss the semiclassical calculation of the singlet yield of the polaron pair recombination reaction for materials such as poly(2,5-dioctyloxy-paraphenylene vinylene) (DOO-PPV), the hyperfine fields in the polarons of which can be extracted from light-induced electron spin resonance measurements. The resulting theory is shown to give good agreement with experimental data for both normal (H-) and deuterated (D-) DOO-PPV over a wide range of magnetic field strengths once singlet-triplet dephasing is taken into account. Without this effect, which has not been included in any previous simulation of magnetoelectroluminescence, it is not possible to reproduce the experimental data for both isotopologues in a consistent fashion. Our results also indicate that the magnetoconductance of DOO-PPV cannot be solely due to the effect of the magnetic field on the dissociation of polaron pairs.
Scalability and foundry compatibility (as for example in conventional silicon based integrated computer processors) in developing quantum technologies are exceptional challenges facing current research. Here we introduce a quantum photonic technology potentially enabling large scale fabrication of semiconductor-based, site-controlled, scalable arrays of electrically driven sources of polarization-entangled photons, with the potential to encode quantum information. The design of the sources is based on quantum dots grown in micron-sized pyramidal recesses along the crystallographic direction (111)B theoretically ensuring high symmetry of the quantum dots - the condition for actual bright entangled photon emission. A selective electric injection scheme in these non-planar structures allows obtaining a high density of light-emitting diodes, with some producing entangled photon pairs also violating Bells inequality. Compatibility with semiconductor fabrication technology, good reproducibility and control of the position make these devices attractive candidates for integrated photonic circuits for quantum information processing.
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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