# Emergence of White Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence
## 1 Introduction  
Luminescence is a type of cold body radiation caused by external stimuli, such as electric field, mechanical stress, photoabsorption and chemical reactions [1; 2; 3; 4; 5; 6]. For the electroluminescence, it is referred to the process that holes and electrons are recombined to furnish light. In 1953, [PERSON] et al. first discovered organic materials to produce electroluminescence, by using a cellulose film doped with acridine orange [7]. In 1963, [PERSON] et al. achieved the electroluminescence by utilizing an anthracene single crystal connected to high-field carrier injection electrodes [8]. Since then, there is an increasing interest for the electroluminescence of organic materials. In 1987, [PERSON] et al. reported the first organic light-emitting diode (OLED) [9]. In their device, charges of both polarities were injected into the organic layers and the subsequent charge transport and recombination produced green emission originating from singlet excitons; that is, fluorescence. In general, OLEDs possess many outstanding characteristics, such as high efficiency, low power consumption, fast switching, wide viewing angle, light weight, long lifetime and flexibility [10; 11; 12]. After the invention of OLED, other kinds of LEDs (e.g., polymer LED, quantum-dot LED, nanoplatelet LED and perovskite LED) were also successively reported [13; 14; 15; 16; 17; 18; 19]. By dint of the strategies used in OLEDs, the performance of other kinds of LEDs can be greatly enhanced. Besides, with the increasinglyunderstanding the insight of OLEDs, the concepts utilized in OLEDs can also be applied to other optoelectrical devices, which is beneficial to the development of related fields [20; 21; 22; 23]. Nowadays, the OLED field has evolved to the extent that commercial applications for cell-phones, televisions and lamps are available. Furthermore, to satisfy the requirements of energy-saving lighting and high-quality displays, attentions have been gradually paid on the white OLED (WOLED) technology [24; 25; 26; 27; 28; 29; 30]. In 1994, [PERSON] et al. made the pioneer works of WOLEDs [31; 32]. Over the past two decades, the power efficiency (PE) of WOLEDs has been improved from 0.83 lm W\({}^{-1}\) to \(>\)100 lm W\({}^{-1}\)[33; 34; 35; 36], demonstrating the great potential of WOLEDs for the lighting and displays. In the case of lighting applications, WOLEDs should meet the demand of bright luminance, high efficiency and long lifetime simultaneously. More specifically, WOLEDs require standard fluorescent tube efficiency (40-70 lm W\({}^{-1}\)) and \(\geq\)10,000 h of lifetime at the luminance of \(\geq\)1000 cd m\({}^{-2}\)[24; 25; 26; 27; 28; 29; 30]. Besides, the color rendering index (CRI) above 80 is required for the indoor lighting and the Commission International de L'Eclairage (CIE) chromaticity coordinates of WOLEDs should be located near white light equal-energy point (0.33, 0.33). Moreover, for the high-quality lighting, other characterization parameters (e.g., correlated color temperature (CCT), color stability and driving voltage) of WOLEDs are also needed to be taken into account [37; 38; 39; 40; 41]. Based on the adopted emissive organic materials, WOLEDs can be fabricated by using fluorescence, phosphorescence and thermally activated delayed fluorescence (TADF) materials [42]. Particularly, the TADF material has been considered as the third-generation OLED emitter after [PERSON] et al. made a breakthrough on it in 2012 [43; 44; 45]. For the conventional fluorescent emitters, only the singlet excitons (25%) can emit light since the radiative decay of triplet excitons (75%) is spin forbidden. In the case of the TADF emitter, it could harness both singlet and triplet excitons since triplets can be harvested as delayed fluorescence through their up-conversion from a lowest triplet state to a lowest singlet state by inducing efficient reverse intersystem crossing (RISC) [46; 47; 48; 49; 50].