The development process, molecular design principles, material systems, structure–property relationships and OLED applications of hot exciton materials are comprehensively summarized.

Blue emissions in organic light-emitting devices (OLEDs) are of great significance for their application in full color flat-panel displays and white lightings. [1] However, high-performance blue emitters are still relatively rare. In OLEDs, the injected electrons and holes recombine to form singlet and triplet excitons in the ratio of 1:3, according to the spin statistics, whereas only singlet exciton can decay radiatively in fluorescent materials. [2] Approximately 75% of the triplet excitons are wasted in nonradiative processes, leading to an upper limit of the internal quantum efficiency (IQE) of only 25% in conventional fluorescent devices. One of the methods to enhance the efficiency of OLEDs is to make use of the nonemissive triplet excitons. [3] Phosphorescent OLEDs (PhOLEDs) based on Ir, Pt, and Os organic-metal complexes can approach 100% IQE, which is attributed to the heavy-atom effect. [4] Yet, pure-blue and deep-blue phosphors with Commission Internationale de l'Eclairage (CIE) y values smaller than 0.15 are particularly scarce due to the inherently great challenge in their molecular design; similarly, proper host materials with a large band gap that allows for the refinement of the triplet excitons in devices are also rare. Therefore, it is important to find a way to develop efficient, stable, pure-and deep-blue fluorescent materials. In principle, new-generation, purely organic fluorescent materials can also utilize the nonemissive triplet excitons and achieve high efficiency by converting triplet excitons into singlet excitons. The main mechanisms involve triplet-triplet annihilation (TTA), thermally activated delayed fluorescence (TADF) and the "hot exciton" channel. [5] Essentially, both the TTA and TADF processes can promote the external quantum efficiency (EQE) of the devices by converting excitons from the lowest triplet excited state (T 1 ) to the lowest singlet excited state (S 1 ). Experimental results have confirmed that devices based on TTA and TADF materials can realize a high EQE with a breakthrough of the spin statistical limitation. [6] Although a high EQE has been obtained in TTA and TADF materials, pure-and deep-blue emitters with high efficiency and stability are still exiguous. Unlike TTA and TADF materials, the "hot exciton" materials reported by our group highlight the reverse intersystem crossing from Purely organic electroluminescent materials, such as thermally activated delayed fluorescent (TADF) and triplet-triplet annihilation (TTA) materials, basically harness triplet excitons from the lowest triplet excited state (T 1 ) to realize high efficiency. Here, a fluorescent material that can convert triplet excitons into singlet excitons from the high-lying excited state (T 2 ), referred to here as a "hot exciton" path, is reported. The energy levels of this compound are determined from the sensitization and nanosecond transient absorption spectroscopy measurements, i.e., small splitting energy between S 1 and T 2 and rather large T 2 -T 1 energy gap, which are expected to...

Because of their readiness and high power bandwidth, batteries are the preeminent source of power supply for portable electronics, but are subject to periodic recharging and replacement. Hence, a key challenge is to design suitable and sustainable power sources for portable electronic devices. Harvesting energy from the human body is suitable for providing consistent and uninterrupted energy for wearable electronic devices. The daily activities of a 68-kg adult can generate over 100 W of power, through breathing, heating, blood transport, and walking. [3] Thus, converting 1% of the power generated by the human body may be enough to support the work of most portable electronics. Various energy-harvesting technologies, such as, triboelectric nanogenerators (TENGs), [4][5][6] piezoelectric nanogenerators, [7] and thermoelectric generators (TEGs), [8] have been developed to convert human energy (from human motion and body heat) into electricity. In line with recent developments in wearable electronics and e-skins, the use of a self-powered direct-current (DC) electric power supplier is inevitable for activating human body-adjustable electronic systems. [9] Among the various energy-autonomous devices, [10] only TEGs can permanently produce DC electric power without complex power managing components, and they are maintenance free. Moreover, solar energy and vibration-based energy harvesters areThe emergence of artificial intelligence and the Internet of Things has led to a growing demand for wearable and maintenance-free power sources. The continual push toward lower operating voltages and power consumption in modern integrated circuits has made the development of devices powered by body heat finally feasible. In this context, thermoelectric (TE) materials have emerged as promising candidates for the effective conversion of body heat into electricity to power wearable devices without being limited by environmental conditions. Driven by rapid advances in processing technology and the performance of TE materials over the past two decades, wearable thermoelectric generators (WTEGs) have gradually become more flexible and stretchable so that they can be used on complex and dynamic surfaces. In this review, the functional materials, processing techniques, and strategies for the device design of different types of WTEGs are comprehensively covered. Wearable self-powered systems based on WTEGs are summarized, including multi-function TE modules, hybrid energy harvesting, and all-in-one energy devices. Challenges in organic TE materials, interfacial engineering, and assessments of device performance are discussed, and suggestions for future developments in the area are provided. This review will promote the rapid implementation of wearable TE materials and devices in self-powered electronic systems.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.202102990.

Organic fluorescent emitters with narrowband emissions are highly desirable for high‐resolution organic light‐emitting diode (OLED) display technology. In principle, this can be achieved by specifically controlling the intrinsic structural relaxation and vibronic coupling in the excited state. Here, a design strategy to realize narrowband emission of organic fluorescent emitters is proposed by significantly enhancing the low‐frequency vibronic coupling strength (Λ) while simultaneously reducing the high‐frequency Λ of the commonly involved stretching modes. The quinolino‐[3,2,1‐de]acridine‐5,9‐dione (QAO) species is found to be directly associated with this design principle. By introducing single bond‐linked peripheral moieties into the QAO core, the constructed QAO derivatives are shown to exhibit better performance, by achieving a full width at half‐maximum of 23 nm/0.13 eV in toluene for the narrowest band as well as 27 nm/0.15 eV in doped devices, with negligible dependence on the doping concentrations. The maximum external quantum efficiency of the fabricated blue OLED is 17.5%.