The past few years have seen a significant improvement in the efficiency of organometal halide-perovskite-based light-emitting diodes (PeLEDs). However, poor operation stability of the devices still hinders the commercialization of this technology for practical applications. Despite extensive studies on the degradation mechanisms of perovskite thin films, it remains unclear where and how degradation occurs in a PeLED. Electroabsorption (EA) spectroscopy is applied to study the degradation process of PeLEDs during operation and directly evaluates the stability of each functional layer (i.e., charge transporting layers and light-emitting layer) by monitoring their unique optical signatures. The EA measurements unambiguously reveal that the degradation of the PeLEDs occurs predominantly in the perovskite layer. With finite-element method-based device modeling, it is further revealed that the degradation may initiate from the interface between the perovskite and hole transporting layers and that vacancy, antisite, or interstitial defects can further accelerate this degradation. Inspired by these observations, a surface-treatment step is introduced to passivate the perovskite surface with phenethylammonium iodide. The passivation leads to a drastic enhancement of the PeLED stability, with the operation lifetime increased from 1.5 to 11.3 h under a current density of 100 mA cm −2 .light emission properties of PeLEDs, and the external quantum efficiency (EQE) and radiance of the best -performing nearinfrared PeLED has already reached 21.6% and 308 W (sr × m 2 ) −1 , respectively. [4] However, most reported PeLEDs degrade rapidly during operation, losing luminescence within several hours. [5][6][7] Stability has been a general issue for almost all perovskite-based optoelectronic devices, but it is particularly problematic in PeLEDs where the current density is high and the energy conversion efficiency is still low. Some research efforts have been made to study the degradation mechanisms of perovskite solar cells and LEDs through characterization of perovskite films, which include in situ X-ray diffraction (XRD) monitoring of phase evolution, [8][9][10][11][12][13] time-of-flight secondary ion mass spectrometry investigation of water penetration in perovskite films, [14] and time-resolved photoluminescence measurements. [12] Prakasam et al. peeled off the top electrode layer of a PeLED after bias stressing and studied the underlying perovskite layer using scanning electron microscopy, structural characterization techniques, and energy-dispersive X-ray profiling. [15] The results suggest decomposition of the perovskite layer during operation, leading to local delamination of the top electrode layer. Studies have also been done to directly improve the stability of the perovskite layer through selection of stable cations, [16] composition tuning and inclusion of additives in the precursor, [17] as well as introduction of a doped electron transport layer, [18] and development of a core-shell perovskite grain structure. [7] De...

Many advanced materials have been developed for organic field-effect transistors (OFETs) or thin-film transistors (TFTs) based on organic and organic hybrid materials. However, although many new OFETs exhibit superior characteristic parameters (such as high mobility), most of them show nonideal performances that have strongly limited progress in the design of molecules, the understanding of transport mechanisms, and the circuit applications of OFETs. In this review, the device physics of ideal and nonideal OFETs is discussed first to understand the factors that limit effective mobility in semiconducting channels, distort the potential distribution, or reduce the drift electric field. Then, recent advances in optimizing the material combinations, device structures, and fabrications of OFETs toward ideal transistors are discussed. Based on the good control of materials and interfaces, some new and novel concepts to utilize the nonideal properties of OFETs to build low-power circuits and integrated sensors are also discussed.

Colloidal quantum wells (CQWs) have emerged as a promising family of two-dimensional (2D) optoelectronic materials with outstanding properties, including ultranarrow luminescence emission, nearly unity quantum yield, and large extinction coefficient. However, the performance of CQWs-based light-emitting diodes (CQW-LEDs) is far from satisfactory, particularly for deep red emissions (≥660 nm). Herein, high efficiency, ultra-low-efficiency roll-off, high luminance, and extremely saturated deep red CQW-LEDs are reported. A key feature for the high performance is the understanding of charge dynamics achieved by introducing an efficient electron transport layer, ZnMgO, which enables balanced charge injection, reduced nonradiative channels, and smooth films. The CQW-LEDs based on (CdSe/CdS)@(CdS/CdZnS) ((core/crown)@(colloidal atomic layer deposition shell/hot injection shell)) show an external quantum efficiency of 9.89%, which is a record value for 2D nanocrystal LEDs with deep red emissions. The device also exhibits an ultra-low-efficiency roll-off and a high luminance of 3853 cd m −2 . Additionally, an exceptional color purity with the CIE coordinates of (0.719, 0.278) is obtained, indicating that the color gamut covers 102% of the International Telecommunication Union Recommendation BT 2020 (Rec. 2020) standard in the CIE 1931 color space, which is the best for CQW-LEDs. Furthermore, an active-matrix CQW-LED pixel circuit is demonstrated. The findings imply that the understanding of charge dynamics not only enables high-performance CQW-LEDs and can be further applied to other kinds of nanocrystal LEDs but also is beneficial to the development of CQW-LEDs-based display technology and related integrated optoelectronics.

Revealing the intrinsic electrical properties is the basis of understanding new functional materials and developing their applications. However, in nonideal field-effect transistors (FETs), conventional current-voltage characterizations do not accurately probe charge transport, particularly for newly developed semiconductors. Here, a generalized gated four-probe (G-GFP) technique is developed, which detects dynamic changes in carrier accumulation and transport. The technique is suitable for exploring the intrinsic properties of semiconductors in FETs with arbitrary contacts and in any operational regimes above the threshold. Application to simulated transistors confirms its accuracy in probing the evolution of channel potential, drift field, and gate-dependent carrier mobility for devices with a contact-limited operation and disordered semiconductors. Comparative experiments are performed based on FETs with various materials, device structures, and operational temperatures. The G-GFP technique proves to exclude the various injection properties, to detect in situ how carriers are accumulated, and to clarify carrier mobility of the semiconductors. In particular, the well-known "double-slope" features in the current-voltage relations are controllably generated and their origins are identified. The approach could be used to explore electronic properties of newly developed materials such as organic, oxide, or 2D semiconductors.

Artificial synapses, combining sensing and computing functions, have played an important role in emerging human-like sensory systems. In particular, organic electrochemical transistors (OECTs) are highly sought as promising candidates because...

White organic light-emitting diodes (WOLEDs) with ultrahigh color rendering index (CRI ≥ 90) are ideal for lighting. However, ultrahigh CRIs are usually obtained via complicated architectures and blue molecular emitters are required. Herein, with the combination of blue exciplex/electroplex and green/red phosphors, blue molecular emitter-free and doping-free WOLEDs have been developed. An ultrahigh CRI of 95 is achieved, which is the highest for blue molecular emitter-free and doping-free WOLEDs. Additionally, a maximum total external quantum efficiency of 7.5% and power efficiency of 21.3 lm W −1 are obtained, which are the highest for blue molecular emitterfree WOLEDs with CRI ≥ 95. The origin of ultrahigh CRIs is unveiled by studying effects of the phosphor thickness, blue molecular emitter-free system, phosphor location, and pick of phosphors. Such results not only give an insightful understanding of simplified but high-performance WOLEDs, but also provide an alternative strategy to achieve ultrahigh CRIs. Index Terms-Doping-free, electroplex, exciplex, white organic light-emitting diode. I. INTRODUCTION W HITE organic light-emitting diodes (WOLEDs) have great potential to displays and lighting due to their superior properties, including high efficiency, low voltage, and flexibility [1]. For high-quality white emissions, the color rendering index (CRI) should be high enough since the

In article number 1903889, Chuan Liu and co‐workers review the origins and critical factors that lead to organic field‐effect transistors (OFETs) with deviations from the ideal device models in terms of device physics. The recent progress in the optimization strategies and new perspectives for utilizing nonideal OFETs are also presented.

Electronic doping has endowed colloidal quantum wells (CQWs) with unique optical and electronic properties, holding great potential for future optoelectronic device concepts. Unfortunately, how photogenerated hot carriers interact with phonons in these doped CQWs still remains an open question. Here, through investigating the emission properties, we have observed an efficient phonon cascade process (i.e., up to 27 longitudinal optical phonon replicas are revealed in the broad Cu emission band at room temperature) and identified a giant Huang–Rhys factor (S ≈ 12.4, more than 1 order of magnitude larger than reported values of other inorganic semiconductor nanomaterials) in Cu-doped CQWs. We argue that such an ultrastrong electron–phonon coupling in Cu-doped CQWs is due to the dopant-induced lattice distortion and the dopant-enhanced density of states. These findings break the widely accepted consensus that electron–phonon coupling is typically weak in quantum-confined systems, which are crucial for optoelectronic applications of doped electronic nanomaterials.