High-Resolution, Highly Transparent, and Efficient Quantum Dot Light-Emitting Diodes.

Aiming at next-generation displays, high-resolution quantum dot light-emitting diodes (QLEDs) with high efficiency and transparency are highly desired. However, there is limited study involving the improvements of QLED pixel resolution, efficiency, and transparency simultaneously, which undoubtedly restricts the practical applications of QLED for next-generation displays. Here, the strategy of electrostatic force-induced deposition (EF-ID) is proposed by introducing alternating polyethyleneimine (PEI) and fluorosilane patterns to synergistically improve the pixel accuracy and transmittance of QD patterns. More importantly, the leakage current induced by the void spaces between pixels that is usually reported for high-resolution QLEDs is greatly suppressed by substrate-assisted insulating fluorosilane patterns. Finally, high-performance QLEDs with high resolution ranging from 1104 to 3031 pixels per inch (PPI) and a high efficiency of 15.6% are achieved, among the best performances of high resolution QLEDs. Notably, the high resolution QD pixels greatly enhance the transmittance of the QD patterns, thus prompting an impressive transmittance of 90.7% for the transparent QLEDs (2116 PPI), which represents the highest transmittance of transparent QLED devices. Consequently, this work contributes an effective and general approach for high-resolution QLEDs with high efficiency and transparency.

Highly stable lanthanide-doped CsPbI3 perovskite nanocrystals with near-unity quantum yield for efficient red light-emitting diodes.

CsPbI3 perovskite nanocrystals (NCs) are gaining popularity as promising photoactive materials for optoelectronic devices. However, their poor phase stability has caused substantial limitations in their practical application. Herein, the small-sized rare earth La cation is strategically introduced to fundamentally improve the NC phase stability against the environment, heat, and UV radiation by the partial substitution of Pb ions to suppress structural distortion and increase the formation energy. The strong interaction between La and I of the octahedra has been demonstrated to enable the effective suppression of the trap states, which promotes strengthened radiative recombination for a near-unity photoluminescence quantum yield (PLQY) of 99.3%. High energy bands have also been found for the La-doped NCs to narrow down the energy barrier for efficient hole injection. The superior optoelectronic properties of La-doped NCs promote great improvements in the perovskite light-emitting diode (PeLED) performances with a 5-fold improvement in external quantum efficiency (EQE) from 1.19 to 6.01% and 2-fold longer lifetime from 1451 to 2956 s. This work provides an effective method for small-sized metal ion-doped CsPbI3 NCs to realize high emission efficiency and phase stabilization for efficient PeLEDs.

Suppressing Nickel Oxide/Perovskite Interface Redox Reaction and Defects for Highly Performed and Stable Inverted Perovskite Solar Cells.

The inorganic hole transport layer of nickel oxide (NiOx ) has shown highly efficient, low-cost, and scalable in perovskite photovoltaics. However, redox reactions at the interface between NiOx and perovskites limit their commercialization. In this study, ABABr (4-(2-Aminoethyl) benzoic acid bromide) between the NiOx and different perovskite layers to address the issues has been introduced. How the ABABr interacts with NiOx and perovskites is experimentally and theoretically investigated. These results show that the ABABr molecule chemically reacts with the NiOx via electrostatic attraction on one side, whereas on the other side, it forms a strong hydrogen bond via the NH3 + group with perovskites layers, thus directly diminishing the redox reaction between the NiOx and perovskites layers and passivating the layer surfaces. Additionally, the ABABr interface modification leads to significant improvements in perovskite film morphology, crystallization, and band alignment. The perovskites solar cells (PSCs) based on an ABABr interface modification show power conversion efficiency (PCE) improvement by over 13% and maintain over 90% of its PCE after continuous operation at maximum power point for over 500 h. The work not only contributes to the development of novel interlayers for stable PSCs but also to the understanding of how to prevent interface redox reactions.

Highly Emissive Quasi-2D Perovskites Enabled by a Multifunctional Molecule for Bright Light-Emitting Diodes.

Quasi-two-dimensional (quasi-2D) perovskite has exhibited great potential to be an ideal luminescent material for perovskite light-emitting diodes (PeLEDs). However, the low-order phases (especially n = 1 phase) and the inevitable defects result in massive nonradiative recombination and poor emission efficiency. Herein, a multifunctional molecule of tetrabutylammonium dihydrogen phosphate (TDP) is introduced to simultaneously suppress the low-n phase, passivate the defects, and increase the exciton binding energy of the quasi-2D perovskite for massive radiative recombination and thus high emission efficiency. The multifunctional roles of TDP are realized by the synergistic effects of tetrabutylammonium cation and dihydrogen phosphate anion, both of which show strong interaction with the lead bromide octahedron of the perovskite. As a result, the TDP-incorporated perovskite films show a great enhancement of the emission efficiency with a remarkable increase in photoluminescence quantum yield (PLQY) from 34.6 to 96.9% at the wavelength of 522 nm. The strengthened radiative recombination promotes efficient emission efficiency with over 2.5-fold improvement in external quantum efficiency (EQE) and current efficiency (CE) from 3.27% and 10.83 cd A-1 to 9.25% and 28.35 cd A-1, respectively, as well as high brightness with over 37% enhancement from 12713 to 17536 cd m-2. Consequently, this work contributes to an efficient approach to employ a multifunctional molecule for highly emissive quasi-2D perovskites and enhanced quasi-2D PeLED performances.

In Situ Growth Mechanism for High-Quality Hybrid Perovskite Single-Crystal Thin Films with High Area to Thickness Ratio: Looking for the Sweet Spot.

The development of in situ growth methods for the fabrication of high-quality perovskite single-crystal thin films (SCTFs) directly on hole-transport layers (HTLs) to boost the performance of optoelectronic devices is critically important. However, the fabrication of large-area high-quality SCTFs with thin thickness still remains a significant challenge due to the elusive growth mechanism of this process. In this work, the influence of three key factors on in situ growth of high-quality large-size MAPbBr3 SCTFs on HTLs is investigated. An optimal "sweet spot" is determined: low interface energy between the precursor solution and substrate, a slow heating rate, and a moderate precursor solution concentration. As a result, the as-obtained perovskite SCTFs with a thickness of 540 nm achieve a record area to thickness ratio of 1.94 × 104  mm, a record X-ray diffraction peak full width at half maximum of 0.017°, and an ultralong carrier lifetime of 1552 ns. These characteristics enable the as-obtained perovskite SCTFs to exhibit a record carrier mobility of 141 cm2 V-1 s-1 and good long-term structural stability over 360 days.

High-Performance Blue Quasi-2D Perovskite Light-Emitting Diodes via Balanced Carrier Confinement and Transfer.

Extensive investigation of the passivating agents has been performed to suppress the perovskite defects. However, very few attentions have been paid to rationally design the passivating agents for the balance of the carrier confinement and transfer in quasi-2D perovskites, which is essential to achieve high-performance perovskite LEDs (PeLEDs). In this work, tributylphosphine oxide (TBPO) with moderate carbon chain length is demonstrated as a decent passivator for the quasi-2D perovskites by strengthening the carrier confinement for massive radiative recombination within the perovskites, and more importantly providing efficient carrier transfer in the quasi-2D perovskites. Benefiting from these interesting optoelectronic properties of TBPO-incorporated perovskites, we achieve high-efficient blue PeLEDs with an external quantum efficiency up to 11.5% and operational stability as long as 41.1 min without any shift of the electroluminescence spectra. Consequently, this work contributes an effective approach to promote the carrier confinement and transfer for high-performance and stable blue PeLEDs.

High-Performance Blue Perovskite Light-Emitting Diodes Enabled by Efficient Energy Transfer between Coupled Quasi-2D Perovskite Layers.

While there has been extensive investigation into modulating quasi-2D perovskite compositions in light-emitting diodes (LEDs) for promoting their electroluminescence, very few reports have studied approaches involving enhancement of the energy transfer between quasi-2D perovskite layers of the film, which plays very important role for achieving high-performance perovskite LEDs (PeLEDs). In this work, a bifunctional ligand of 4-(2-aminoethyl)benzoic acid (ABA) cation is strategically introduced into the perovskite to diminish the weak van der Waals gap between individual perovskite layers for promoting coupled quasi-2D perovskite layers. In particular, the strengthened interaction between coupled quasi-2D perovskite layers favors an efficient energy transfer in the perovskite films. The introduced ABA can also simultaneously passivate the perovskite defects by reducing metallic Pb for less nonradiative recombination loss. Benefiting from the advanced properties of ABA incorporated perovskites, highly efficient blue PeLEDs with external quantum efficiency of 10.11% and a very long operational stability of 81.3 min, among the best performing blue quasi-2D PeLEDs, are achieved. Consequently, this work contributes an effective approach for high-performance and stable blue PeLEDs toward practical applications.

Establishing Multifunctional Interface Layer of Perovskite Ligand Modified Lead Sulfide Quantum Dots for Improving the Performance and Stability of Perovskite Solar Cells.

While organic-inorganic halide perovskite solar cells (PSCs) show great potential for realizing low-cost and easily fabricated photovoltaics, the unexpected defects and long-term stability against moisture are the main issues hindering their practical applications. Herein, a strategy is demonstrated to address the main issues by introducing lead sulfide quantum dots (QDs) on the perovskite surface as the multifunctional interface layer on perovskite film through establishing perovskite as the ligand on PbS QDs. Meanwhile, the multifunctions are featured in three aspects including the strong interactions of PbS QDs with perovskites particularly at the grain boundaries favoring good QDs coverage on perovskites for ultimate smooth morphology; an inhibition of iodide ions mobilization by the strong interaction between iodide and the incorporated QDs; and the reduction of the dangling bonds of Pb2+ by the sulfur atoms of PbS QDs. Finally, the device performances are highly improved due to the reduced defects and non-radiative recombination. The results show that both open-circuit voltage and fill factor are significantly improved to the high values of 1.13 V and 80%, respectively in CH3 NH3 PbI3 -based PSCs, offering a high efficiency of 20.64%. The QDs incorporation also enhances PSCs' stability benefitting from the induced hydrophobic surface and suppressed iodide mobilization.

Enhanced light emission of quantum dot films by scattering of poly(zinc methacrylate) coating CdZnSeS/ZnS quantum dots and high refractive index BaTiO3 nanoparticles.

Quantum dots (QDs) have received considerable attention in information displays owing to their high quantum yield, high colour purity and low-cost fabrication. However, light emission for ultra-thin QD films with low mass percentage of QDs still need to be improved because the blue light can directly transmit the films, leading to insufficient energy to excite the QDs. In this study, we report QD films based on a poly(zinc methacrylate) coating with alloyed green-emitting CdZnSeS/ZnS quantum dots (QDs@PZnMA) together with high refractive-index BaTiO3 nanoparticles to enhance the scattering coefficient of the QD films. Results demonstrate a 7.5-fold increase in the absorption coefficient, 11.3-fold increase in the scattering coefficient, 8.5-fold increase in the optical density (OD) and 8.6-fold increase in the green-light emission of QD films, compared with films that have the same mass percentage of pristine QDs. This approach provides a promising strategy for developing QD optical films with high scattering and enhanced light emission for flexible displays.

Bilayer PbS Quantum Dots for High-Performance Photodetectors.

Due to their wide tunable bandgaps, high absorption coefficients, easy solution processabilities, and high stabilities in air, lead sulfide (PbS) quantum dots (QDs) are increasingly regarded as promising material candidates for next-generation light, low-cost, and flexible photodetectors. Current single-layer PbS-QD photodetectors suffer from shortcomings of large dark currents, low on-off ratios, and slow light responses. Integration with metal nanoparticles, organics, and high-conducting graphene/nanotube to form hybrid PbS-QD devices are proved capable of enhancing photoresponsivity; but these approaches always bring in other problems that can severely hamper the improvement of the overall device performance. To overcome the hurdles current single-layer and hybrid PbS-QD photodetectors face, here a bilayer QD-only device is designed, which can be integrated on flexible polyimide substrate and significantly outperforms the conventional single-layer devices in response speed, detectivity, linear dynamic range, and signal-to-noise ratio, along with comparable responsivity. The results which are obtained here should be of great values in studying and designing advanced QD-based photodetectors for applications in future flexible optoelectronics.

Inorganic Ligand Thiosulfate-Capped Quantum Dots for Efficient Quantum Dot Sensitized Solar Cells.

The insulating nature of organic ligands containing long hydrocarbon tails brings forward serious limitations for presynthesized quantum dots (QDs) in photovoltaic applications. Replacing the initial organic hydrocarbon chain ligands with simple, cheap, and small inorganic ligands is regarded as an efficient strategy for improving the performance of the resulting photovoltaic devices. Herein, thiosulfate (S2O32-), and sulfide (S2-) were employed as ligand-exchange reagents to get access to the inorganic ligand S2O32-- and S2--capped CdSe QDs. The obtained inorganic ligand-capped QDs, together with the initial oleylamine-capped QDs, were used as light-absorbing materials in the construction of quantum dot sensitized solar cells (QDSCs). Photovoltaic results indicate that thiosulfate-capped QDs give excellent power conversion efficiency (PCE) of 6.11% under the illumination of full one sun, which is remarkably higher than those of sulfide- (3.36%) and OAm-capped QDs (0.84%) and is comparable to the state-of-the-art value based on mercaptocarboxylic acid capped QDs. Photoluminescence (PL) decay characterization demonstrates that thiosulfate-based QDSCs have a much-faster electron injection rate from QD to TiO2 substrate in comparison with those of sulfide- and OAm-based QDSCs. Electrochemical impedance spectroscopy (EIS) results indicate that higher charge-recombination resistance between potoanode and eletrolyte interfaces were observed in the thiosulfate-based cells. To the best of our knowledge, this is the first application of thiosulfate-capped QDs in the fabrication of efficient QDSCs. This will lend a new perspective to boosting the performance of QDSCs furthermore.

Improving Loading Amount and Performance of Quantum Dot-Sensitized Solar Cells through Metal Salt Solutions Treatment on Photoanode.

Increasing QD loading amount on photoanode and suppressing charge recombination are prerequisite for high-efficiency quantum dot-sensitized solar cells (QDSCs). Herein, a facile technique for enhancing the loading amount of QDs on photoanode and therefore improving the photovoltaic performance of the resultant cell devices is developed by pipetting metal salt aqueous solutions on TiO2 film electrode and then evaporating at elevated temperature. The effect of different metal salt solutions was investigated, and experimental results indicated that the isoelectric point (IEP) of metal ions influenced the loading amount of QDs and consequently the photovoltaic performance of the resultant cell devices. The influence of anions was also investigated, and the results indicated that anions of strong acid made no difference, while acetate anion hampered the performance of solar cells. Infrared spectroscopy confirmed the formation of oxyhydroxides, whose behavior was responsible for QD loading amount and thus solar cell performance. Suppressed charge recombination based on Mg2+ treatment under optimal conditions was confirmed by impedance spectroscopy as well as transient photovoltage decay measurement. Combined with high-QD loading amount and retarded charge recombination, the champion cell based on Mg2+ treatment exhibited an efficiency of 9.73% (Jsc = 27.28 mA/cm2, Voc = 0.609 V, FF = 0.585) under AM 1.5 G full 1 sun irradiation. The obtained efficiency was one of the best performances for liquid-junction QDSCs, which exhibited a 10% improvement over the untreated cells with the highest efficiency of 8.85%.