Jianhai Li

Quantum Dot Light‐Emitting Diodes Based on Inorganic Perovskite Cesium Lead Halides (CsPbX3)

Novel quantum-dot light-emitting diodes based on all-inorganic perovskite CsPbX3 (X = Cl, Br, I) nanocrystals are reported. The well-dispersed, single-crystal quantum dots (QDs) exhibit high quantum yields, and tunable light emission wavelength. The demonstration of these novel perovskite QDs opens a new avenue toward designing optoelectronic devices, such as displays, photodetectors, solar cells, and lasers.

50-Fold EQE Improvement up to 6.27% of Solution-Processed All-Inorganic Perovskite CsPbBr3 QLEDs via Surface Ligand Density Control.

Solution-processed CsPbBr3 quantum-dot light-emitting diodes with a 50-fold external quantum efficiency improvement (up to 6.27%) are achieved through balancing surface passivation and carrier injection via ligand density control (treating with hexane/ethyl acetate mixed solvent), which induces the coexistence of high levels of ink stability, photoluminescence quantum yields, thin-film uniformity, and carrier-injection efficiency.

Monolayer and Few-Layer All-Inorganic Perovskites as a New Family of Two-Dimensional Semiconductors for Printable Optoelectronic Devices.

Printed flexible photodetectors based on 2D inorganic perovskites with atomic thickness show excellent photosensing with fast rise and decay response times. As-synthesized nanosheets can easily be dispersed in various solvents, leading to large-area, crack-free, low-roughness, flexible films after printing. This study demonstrates that all-inorganic perovskite CsPbX3 nanosheets as a new class of 2D semiconductors have huge potential for flexible optoelectronic applications.

Superstable transparent conductive Cu@Cu4Ni nanowire elastomer composites against oxidation, bending, stretching, and twisting for flexible and stretchable optoelectronics.

Low cost and high conductivity make copper (Cu) nanowire (NW) electrodes an attractive material to construct flexible and stretchable electronic skins, displays, organic light-emitting diodes (OLEDs), solar cells, and electrochromic windows. However, the vulnerabilities that Cu NW electrodes have to oxidation, bending, and stretching still present great challenges. This work demonstrates a new Cu@Cu4Ni NW conductive elastomer composite with ultrahigh stability for the first time. Cu@Cu4Ni NWs, facilely synthesized through a one-pot method, have highly crystalline alloyed shells, clear and abrupt interfaces, lengths more than 50 μm, and smooth surfaces. These virtues provide the NW-elastomer composites with a low resistance of 62.4 ohm/sq at 80% transparency, which is even better than the commercial ITO/PET flexible electrodes. In addition, the fluctuation amplitude of resistance is within 2 ohm/sq within 30 days, meaning that at ΔR/R0 = 1, the actual lifetime is estimated to be more than 1200 days. Neither the conductivity nor the performances of OLED with elastomers as conductive circuits show evident degradation during 600 cycles of bending, stretching, and twisting tests. These high-performance and extremely stable NW elastomeric electrodes could endow great chances for transparent, flexible, stretchable, and wearable electronic and optoelectronic devices.

Improving All‐Inorganic Perovskite Photodetectors by Preferred Orientation and Plasmonic Effect

All-inorganic perovskites have high carrier mobility, long carrier diffusion length, excellent visible light absorption, and well overlapping with localized surface plasmon resonance (LSPR) of noble metal nanocrystals (NCs). The high-performance photodetectors can be constructed by means of the intrinsic outstanding photoelectric properties, especially plasma coupling. Here, for the first time, inorganic perovskite photodetectors are demonstrated with synergetic effect of preferred-orientation film and plasmonic with both high performance and solution process virtues, evidenced by 238% plasmonic enhancement factor and 106 on/off ratio. The CsPbBr3 and Au NC inks are assembled into high-quality films by centrifugal-casting and spin-coating, respectively, which lead to the low cost and solution-processed photodetectors. The remarkable near-field enhancement effect induced by the coupling between Au LSPR and CsPbBr3 photogenerated carriers is revealed by finite-difference time-domain simulations. The photodetector exhibits a light on/off ratio of more than 106 under 532 nm laser illumination of 4.65 mW cm-2 . The photocurrent increases from 0.67 to 2.77 μA with centrifugal-casting. Moreover, the photocurrent rises from 245.6 to 831.1 μA with Au NCs plasma enhancement, leading to an enhancement factor of 238%, which is the most optimal report among the LSPR-enhanced photodetectors, to the best of our knowledge. The results of this study suggest that all-inorganic perovskites are promising semiconductors for high-performance solution-processed photodetectors, which can be further enhanced by Au plasmonic effect, and hence have huge potentials in optical communication, safety monitoring, and biological sensing.

A general one-pot strategy for the synthesis of high-performance transparent-conducting-oxide nanocrystal inks for all-solution-processed devices.

For all-solution-processed (ASP) devices, transparent conducting oxide (TCO) nanocrystal (NC) inks are anticipated as the next-generation electrodes to replace both those synthesized by sputtering techniques and those consisting of rare metals, but a universal and one-pot method to prepare these inks is still lacking. A universal one-pot strategy is now described; through simply heating a mixture of metal-organic precursors a wide range of TCO NC inks, which can be assembled into high-performance electrodes for use in ASP optoelectronics, were synthesized. This method can be used for various oxide NC inks with yields as high as 10 g. The formed NCs are of high crystallinity, uniform morphology, monodispersity, and high ink stability and feature effective doping. Therefore, the inks can be readily assembled into films with a surface roughness of 1.6 nm. Typically, a sheet resistance of 110 Ω sq(-1) can be achieved with a transmittance of 88%, which is the best performance for TCO NC ink-based electrodes described to date. These electrodes can thus drive a polymer light-emitting diode (PLED) with a luminance of 2200 cd m(-2) at 100 mA cm(-2).

Stabilizing Cesium Lead Halide Perovskite Lattice through Mn(II) Substitution for Air-Stable Light-Emitting Diodes

All-inorganic cesium lead halide perovskite (CsPbX3, X = Cl, Br, and I) quantum dots (QDs), possessing high photoluminescence quantum yields and tunable color output, have recently been endowed great promise for high-performance solar cells and light-emitting diodes (LEDs). Although moisture stability has been greatly improved through separating QDs with a SiO2 shell, the practical applications of CsPbX3 QDs are severely restricted by their poor thermal stability, which is associated with the intrinsically low formation energies of perovskite lattices. In this regard, enhancing the formation energies of perovskite lattices of CsPbX3 QDs holds great promise in getting to the root of their poor thermal stability, which hitherto remains untouched. Herein, we demonstrate an effective strategy through Mn2+ substitution to fundamentally stabilize perovskite lattices of CsPbX3 QDs even at high temperatures up to 200 °C under ambient air conditions. We employ first-principle calculations to confirm that the significantly improved thermal stability and optical performance of CsPbX3:Mn2+ QDs arise primarily from the enhanced formation energy due to the successful doping of Mn2+ in CsPbX3 QDs. Benefiting from such an effective substitution strategy, these Mn2+-doped CsPbX3 QDs can function well as efficient light emitters toward the fabrication of high-performance perovskite LEDs.

All-inorganic quantum-dot light-emitting diodes based on perovskite emitters with low turn-on voltage and high humidity stability

Recently, both light-to-electricity and electricity-to-light conversion efficiencies of perovskite achieved a breakthrough, e.g. 22.1% for solar cells and 11.7% for light-emitting diodes (LEDs), so the next fatal problem towards practical application, the device stability, became the key issue in this field. Here, we report all-inorganic LEDs including inorganic perovskite emitters (CsPbBr3) and inorganic charge transport layers (CTLs), with an emphasis on the significantly improved device stability. The quantum dot LEDs (QLEDs) were fabricated according to ITO/NiO/CsPbBr3 QDs/ZnO/Al device configuration. On the one hand, the all-inorganic LED lifetime under 65% humidity corresponding to a 70% electroluminescence (EL) conservation rate can be improved up to 3.5 times when compared with LEDs adopting conventional organic CTLs due to the intrinsic chemical stability of these inorganic CTLs and their less hydrophilic surfaces. Furthermore, as a surprise, the bare all-inorganic LED without encapsulation can work in water for about 20 seconds, which is over 10 times more sustainable than the organic–inorganic LED, which proves the excellent water-isolation ability. On the other hand, the all-inorganic QLEDs show the lowest turn-on voltage of 2.4 V among all the reported CsPbBr3 QLEDs because the inorganic CTLs possess well-matched energy band alignments with CsPbBr3, and hence result in efficient carrier injection. This work paves the way to constructing all-inorganic devices for stable perovskite photovoltaic and light-emitting devices.

High Performance Metal Halide Perovskite Light‐Emitting Diode: From Material Design to Device Optimization

Metal halide perovskites have drawn significant interest in the past decade. Superior optoelectronic properties, such as a narrow bandwidth, precise and facile tunable luminance over the entire visible spectrum, and high photoluminescence quantum yield of up to ≈100%, render metal halide perovskites suitable for next-generation high-definition displays and healthy lighting systems. The external quantum efficiency of perovskite light-emitting diodes (LEDs) increases from 0.1 to 11.7% in three years; however, the energy conversion efficiency and the long-term stability of perovskite LEDs are inadequate for practical application. Strategies to optimize the emitting layer and the device structure, with respect to material design, synthesis, surface passivation, and device optimization, are reviewed and highlighted. The long-term stability of perovskite LEDs is evaluated as well. Meanwhile, several challenges and prospects for future development of perovskite materials and LEDs are identified.

Constructing Mie-Scattering Porous Interface-Fused Perovskite Films to Synergistically Boost Light Harvesting and Carrier Transport

Light harvesting (LH) and carrier transport abilities of a photoactive layer, which are both crucial for optoelectronic devices such as solar cells and photodetectors (PDs), are typically hard to be synergistically improved. Taking perovskite as an example, a freeze-drying recrystallization method is used to construct porous films with improvements of both LH and carrier transport ability. During the freeze-drying casting process, the rapid solvent evaporation produces massive pores, the sizes of which can be adjusted to exploit the Mie scattering for enhancement of the LH ability. Meanwhile, owing to the strong iconicity, the interface between perovskite nanocrystals fused during recrystallization, which favors carrier transport. Subsequently, PDs based on these Mie porous and interface-fused films show a high on/off ratio of more than 104 and an external quantum efficiency value of 658 % under 9 V bias and 520 nm light irradiation.