Charge transport behavior in SnO2 nanoparticle (NP) films is rather crucial to the optoelectronic devices. Temperature-dependent electrical results show that the electron transport in SnO2 NP films is dominated by the Mott variablerange hopping processes, i.e., the electrons are transported between different NPs through surface states rather than the conduction band of the nanocrystals, which is identical to the commonly used ZnO NP solids. Compared with ZnO, SnO2 films exhibit similar electron mobility but lower density of states. Therefore, we deduce that the low density of states in the SnO2 NP films should be the key factor limiting the device performance in compared with the ZnO as reported in most of the quantum-dot light-emitting diodes. Our work sheds light on optimizing SnO2 nanoparticle films for quantum-dot light-emitting diodes. Moreover, we believe that the SnO2 remains a desirable candidate as the electron transport material for the QLEDs due to its excellent physicochemical stability.

of high-quality QDs, QLEDs have gained huge progress and the performance is comparable with that of state-of-the-art organic light-emitting diodes. Especially, the red and green QLEDs have delivered the external quantum efficiency (EQE) over 20% and operating lifetime up to a few millions of hours. [13,14] Currently, blue QLEDs become the short slab for the QLED-based display technology.The introduction of wide bandgap shells (such as ZnS), which help to confine the charges/excitons to the cores and to passivate various defects, pave the way to high photoluminescence (PL) quantum yields (QYs), and stability for blue QDs. [3,[15][16][17] However, the other side of the coin is that the wide bandgap shells increase the holeinjection barrier inevitably, which is particularly pronounced in blue QLEDs due to the inherently lower valance band (VB) of blue QDs. Therefore, how to improve hole injection or charge balance becomes a core research area for further development of QLEDs. Efficient blue QLEDs with EQE over 16% were achieved by introducing poly(methyl methacrylate) (PMMA) or tert-butyldimethylsilyl chloride-modified poly(p-phenylene benzobisoxazole) (TBS-PBO) to impede the electron injection and improve the charge-injection/transfer balance. [18,19] CNPr-TFB hole transporting polymer has been developed by Wu and co-workers, which exhibits a superior hole conductivity and much more stabilized highest-occupied molecular orbital (HOMO) in comparison with TFB. Therefore, much more holes are delivered into the QD emissive layers when it was used as hole-transport layer (HTL). [20] Currently, Se used throughout ZnCdSe/ZnSe QDs and/or cadmium-doped zinc sulfide (CdZnS) as the outermost shell was proposed, which reduced the hole-injection barrier and enhanced hole injection. Then, the resulted blue QLED achieved outstanding luminance up to 62 600 cd m −2 . However, the EQE of this blue QLED was only 8.5% and/or 8.4%. [14,21] Even though these are optimizing strategies, the performance of blue QLEDs still lags behind that of red and green ones. The fundamental and essential issues in the devices need to be uncovered and resolved. To achieve efficient blue QLEDs, HTLs with low HOMO energy levels are commonly employed to achieve balanced charge injection. To date, poly-N-vinylcarbazole (PVK) is the most popular HTL for the blue QLEDs. [15,19,22] Currently, blue quantum-dot light-emitting diodes (QLEDs) remain the bottleneck limiting the development of QLED-based applications. To achieve high-performance blue QLEDs, poly-N-vinylcarbazole (PVK) is usually employed as the hole-transport layer (HTL) to reduce the hole injection barrier. However, fabrication of efficient blue QLEDs with PVK HTL remains challenging and empirical/accidental. Here, it is demonstrated that PVK layer can trap electrons and hence resulting in low device efficiency. This is why the performance of blue QLEDs is highly dependent on the PVK batch received from the manufacturers. As an interlayer, ZnSe/ZnS quantum dots (QDs) are inserted between PVK and bl...

To date, measuring the carrier mobility in semiconductor films, especially for the amorphous organic small-molecule films, is still a big challenge. Here, we demonstrate that transient electroluminescence (TrEL) spectroscopy with quantum-dot light-emitting diodes as the platform is a feasible and reliable method to evaluate the carrier mobility of such amorphous films. The position of the exciton formation zone is precisely determined and controlled by employing a quantum dot monolayer as the emissive layer. The electrical field intensity across the organic layer is evaluated through the charge density at the electrode calculated by the transient current. Then, the charge carrier mobility is obtained by combining the electroluminescence (EL) delay time and the thickness of the organic layer. Additionally, we demonstrate that the large roughness of the organic layer leads to serious charge accumulation and, hence, a high localized electrical field, which provides preferred charge injection paths, reducing the EL delay time and underestimating the EL delay time. Therefore, a thick organic film is the prerequisite for a reliable measurement of charge carrier mobility, which can circumvent the negative effect of film roughness.

Even in the most efficient hybridstructure QLEDs, the injected electrons are also considered to exceed the holes owing to the high electron mobility of ZnO films and low injection barrier between ZnO and QDs. [1c,2b,6] Therefore, the apparent charge injection is believed to be imbalanced evaluated by the singlecarrier devices. [6b,c] Although the device performance has been highly improved by optimizing this apparent charge balance properties, [2b,7] there is still a lack of in-depth understanding on the chargecarrier distribution and dynamics in the blue devices. [8] Moreover, we believe there are some more fundamental physical mechanisms responsible for the enhancement of device performance rather than the apparent charge balance. [9] The chargecarrier dynamics in the QLEDs is the most essential factor deciding the device features, including electroluminescence (EL) turn-on, luminous efficiency, operation lifetime, as well as exciton formation. Among these, the EL turn-on process is closely related to the charge-carrier injection and distribution as well as energy transfer or/and direct charge injection. Therefore, it is desired to disclose this EL turn-on process to exploit optimization strategy of blue QLEDs.Due to the multilayer device structure, the injected charge carriers (electrons and holes) are accumulated at various interfaces due to the injection barriers between different layers. These accumulated charges will change the electrical field within the functional layers, the exciton recombination, and the EL turn-on process. It is well known that the EL turn-on is just resulted from the accumulated charges at the interface adjacent QDs. As reported previously, [10] the excitons in QLEDs are formed at the QDs/hole-transport layer (HTL) interface in the hybrid QLEDs. Therefore, the mechanism of EL turn-on can be accessed by exploring the injection and distribution of charges near the QDs/HTLs interface. Currently, much attention is paid to the influence of this interface on the holes, while the electron distribution is rarely reported. [11] However, the influence of hole-injection barrier and hole mobility of HTLs on charge distribution and dynamics remains so far unclear. [8] In this work, the distribution and dynamics of both holes and electrons at the interface between QDs and HTLs in blue QLEDs are investigated in detail. It is demonstrated that both device efficiency and EL turn-on mechanism are highly A clear understanding on the fundamental working mechanisms of quantumdot light emitting diodes (QLEDs), especially the blue devices, is always sought after. Here the electroluminescence (EL) turn-on mechanism of blue QLEDs is unraveled by structure engineering of the hole-transport layers (HTLs). It is demonstrated that the EL turn-on in blue QLEDs is highly dependent on the charge-transport polarity of the HTLs. Energy transfer between HTLs and the blue quantum dots (QDs) occurs for the QLEDs based on bipolar HTLs, and, in contrast, charge injection is responsible for EL turn-on of the blue de...

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