Memory-Enabled Quantum-Dot Light-Emitting Diodes.

Quantum-dot light-emitting diodes (QLEDs) with memory capability can provide multifunctional integration properties in on-chip and intelligent electronic applications. Herein, memory properties are achieved by inserting a tungsten oxide (WOx) film between the ZnO electron-transporting layer and cathode. Pentavalent tungsten ions (W5+) in this nonstoichiometric WOx film can be oxidized to W6+ by storing holes, inducing significant electrons in the adjacent ZnO layer. Hole storage in the WOx layer suppresses electron injection into the quantum dot emissive layer, hence reducing electroluminescence intensity on the onset stage of the QLEDs. This operation-history correlation for the electroluminescence intensity means a memory behavior for the QLEDs. Furthermore, the power efficiency of the devices is greatly improved after inserting the WOx layer due to electrical field-dependent self-adaptive electron injection into the quantum dots (QDs). We anticipate this type of QLEDs have potential applications in on-chip integration applications, such as the optical computing field and storage.

On the electroluminescence overshoot of quantum-dot light-emitting diodes.

The charge-carrier dynamics is a fundamental question in quantum-dot light-emitting diodes (QLEDs), determining the electroluminescence (EL) properties of the devices. By means of a hole-confined QLED design, the distribution and storage/residing of the charge carriers in the devices are deciphered by the transient electroluminescence (TrEL) spectroscopic technology. It is demonstrated that the holes stored in the quantum dots (QDs) are responsible for the EL overshoot during the rising edge of the TrEL response. Moreover, the earlier electroluminescence turn-on behavior is observed due to the holes residing in the hole-confined structure. The hole storage effect should be attributed to the ultralow hole mobility of QD films and large barrier for hole escape from the cores of the QDs. Our findings provide a deep understanding of the charge transport and storage at the most critical interface between QDs and hole-transport layer, where the excitons are formed.

Efficient Quantum-Dot Light-Emitting Diodes Enabled via a Charge Manipulating Structure.

Charge carriers are the basic physical element in an electrically driven quantum-dot light-emitting diode (QLED), which acts as a converter transforming electric energy to light energy. Therefore, it is widely sought after to manage the charge carriers for achieving efficient energy conversion; however, to date, there has been a lack of understanding and efficient strategies. Here, an efficient QLED is achieved by manipulating the charge distribution and dynamics with an n-type 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) layer embedded into the hole-transport layer. Compared with the control QLED, the maximum current efficiency of the TPBi-containing device is enhanced over 30%, reaching 25.0 cd/A, corresponding to a 100% internal quantum efficiency considering the ∼90% photoluminescence quantum yield of the QD film. Our results suggest that there is still a great deal of room to further improve the efficiency in a standard QLED by subtly manipulating the charge carriers.

Improving the voltage tolerance of perovskite light-emitting diodes via a charge-generation layer.

A high electrical field is necessary to achieve a high brightness for halide perovskite light-emitting diodes (PeLEDs). Charge accumulation in the perovskite film becomes more serious under a high electrical field owing to the imbalanced charge injection in PeLEDs. Concomitantly, the perovskite film will suffer from a higher electrical field increased by the accumulated-charge-induced local electrical field, dramatically accelerating the ion migration and degradation of PeLEDs. Here we construct a voltage-dependent hole injection structure consisting of a ZnO/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) bilayer, which can properly adjust the hole injection according to the driving electrical field, matching with the injected electrons. As a result, the ZnO/PEDOT:PSS-containing PeLED can be operated under higher driving voltage with a higher peak brightness of 18920 cd/m2, which is 84% higher than the reference device based on a PEDOT:PSS single layer. Moreover, the ZnO/PEDOT:PSS-containing PeLED delivers a much higher power efficiency than the reference device under high driving voltages.

Efficient flexible quantum-dot light-emitting diodes with unipolar charge injection.

The exfoliation between the electrode film and the adjacent functional layer is still a big challenge for the flexible light emitting diodes, especially for the devices dependent on the direct charge injection from the electrodes. To address this issue, we design a flexible quantum-dot light-emitting diodes (QLEDs) with a charge-generation layer (CGL) on the bottom electrode as the electron supplier. The CGL consisting of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/ZnO can provide sufficient electron injection into the QDs, enabling a balanced charge injection. As a result, the CGL-based QLED exhibits a peak external quantum efficiency 18.6%, over 25% enhancement in comparison with the device with ZnO as the electron transport layer. Moreover, the residual electrons in the ZnO can be pulled back to the PEDOT:PSS/ZnO interface by the storage holes in the CGL, which are released and accelerates the electron injection during the next driving voltage pulse, hence improving the electroluminescence response speed of the QLEDs.

Perovskite-WS2 Nanosheet Composite Optical Absorbers on Graphene as High-Performance Phototransistors.

High-responsivity phototransistors with a structure of perovskite-WS2 nanosheet composite optical absorber and a reduced graphene oxide (rGO) channel layer is demonstrated via a facile and low-cost solution-processing method. The WS2 nanosheets are dispersed within the perovskite matrix, forming the perovskite-WS2 bulk heterojunction (BHJ). The hybrid phototransistor exhibits excellent figures of merit including high photoresponsivity of 678.8 A/W, high specific detectivity of 4.99 × 1011 Jones, high EQE value of 2.04 × 105% and rapid response to photoswitching. The high photoresponsivity could be attributed to the WS2 nanosheets induced photo-generated electron-hole separation promotion effects due to the selective electron trapping effects in the WS2 nanosheets, together with the high carrier mobility of the rGO channel. This work provides a promising platform for constructing high-responsivity photodetectors.

Intaglio-type random silver networks as the cathodes for efficient full-solution processed flexible quantum-dot light-emitting diodes.

Flexible quantum-dot light-emitting diodes (FQLEDs) hold great promise as a leading display and lighting technology due to their light weight, low-cost, and saturated emission color. However, there remain many challenges in the development of high quality electrodes on flexible substrates for device fabrication and operation. In this work, we present a robust flexible transparent conductive film with embedded random Ag networks in the PET substrate (named PRAN). The PRAN composite film exhibits an average transmittance of 85%, and the sheet resistance reaches near 5.3 Ω sq-1 without any obvious change after bending 3000 times, indicating excellent flexibility of this type of conductive film. A highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer was employed to smooth the surface of the PRAN electrode. Consequently, FQLEDs based on these flexible electrodes are successfully fabricated and the peak power efficiencies of 42.3, 101.9, and 6.4 lm W-1 are achieved for the red, green and blue devices, respectively. To the best of our knowledge, these are the best efficiencies for the FQLEDs reported to date. These results lay the foundation of the realization of ITO-free, high-efficiency FQLEDs for use in flexible lighting and display applications.

Crystal structures of transition metal pernitrides predicted from first principles.

We have extensively explored the stable crystal structures of early-transition metal pernitrides (TMN2, TM = Ti, V, Cr, Mn, Zr, Nb, Mo, Hf, and Ta) at ambient and high pressures using effective CALYPSO global structure search algorithm in combination with first-principles calculations. We identified for the first time the ground-state structures of MnN2, TaN2, NbN2, VN2, ZrN2, and HfN2 pernitrides, and proposed their synthesis pressures. All predicted crystal structures contain encapsulated N2 dumbbells in which the two N atoms are singly bonded to a [N2]4- pernitride unit utilizing the electrons transferred from the transition metals. The strong nature of the single dinitrogen bond and transition metal-nitrogen charge transfer induce extraordinary mechanic properties in the predicted transition metal pernitrides including large bulk modulus and high Vickers hardness. Among the predictions the hardness of MnN2 is 36.6 GPa, suggesting that it is potentially a hard material. The results obtained in the present study are important to the understanding of structure-property relationships in transition metal pernitrides and will hopefully encourage future synthesis of these technologically important materials.