Establishing an sp2-carbon-bonding pattern is one of the efficient accesses to various organic semiconducting materials. However, the less-reversible carbon–carbon bond formation makes it still challenging to spatially construct a well-defined organic framework with π-extended two-dimensional (2D) structure through solution process. Here, a Knoevenagel condensation approach to synthesize two new 2D covalent organic frameworks (COFs) connected by unsubstituted carbon–carbon double bond linkages through activating the methyl carbons of a 2,4,6-trimethyl-1,3,5-triazine monomer is presented. The resulting sp2-carbon-linked triazine-cored 2D sheets are vertically stacked into high-crystalline honeycomb-like structures, endowing this kind of COF with extended π-delocalization, tunable energy levels, as well as high surface areas, regular open channels, and chemical stabilities. On the other hand, their microfibrillar morphologies allow for the facile manipulation of thin films as photoelectrodes without additive. Accordingly, such kinds of COF-based photoelectrodes exhibit photocurrents up to ∼45 μA cm–2 at 0.2 V vs RHE as well as rapid charge transfer rates, in comparison with imine-linked COF-based photoelectrodes. In addition, both COFs are applicable for conducting photocatalytic hydrogen generation from water splitting by visible-light irradiation.

Organic light-emitting diodes (OLEDs) have become a mainstream display technology in consumer electronics. Self-emitting ability, transparency, true dark tone, and capability of being made flexible, are some of the features of OLED displays, leading to a superior performance compared with liquid crystal displays. In addition to displays, OLEDs are also a strong candidate for lighting applications. Despite great advances in improving the internal quantum efficiency of an OLED to nearly 100%, the external quantum efficiency is still lacking behind due to optical losses. This review reports the latest advances in the optical design of OLEDs that address the external coupling efficiency of OLEDs. Discussed at first are the fundamentals of OLED optics and how the refractive indices of different layers in an OLED stack affect the extraction efficiency. Then, this paper reviews how microlens arrays, scattering layers, and corrugated structures can be used to recover the optical losses and improve the external efficiency, and the general optical designs for different optical structures for light extraction are presented.

high photoluminescence quantum yield (PLQY), wide wavelength tunability, and high color purity, [4][5][6] they have been attractive for light-emitting diode (LED) applications. Since the first demonstration of perovskite LEDs in 2014, [7] the device external quantum efficiency (EQE) has risen rapidly from 0.1% [7] to ≈20%, [2,4,8] and the efficiency enhancements are mainly attributed to passivation and compositional engineering, [2,8] improved charge balance by optimization of device structure, [9] and efficient light extraction. [4] More recently, these materials are considered as optical gain medium for lasers. In 2014, the first amplified spontaneous emission (ASE) was observed from CH 3 NH 3 PbI 3 thin films with a threshold of 12 µJ cm −2 and a gain of 250 cm −1 , which is ascribed to the large absorption coefficient, low bulk defect density, and slow Auger recombination rate. [10] These ASE threshold and gain values are comparable to the state of art gain media such as colloidal quantum dots [11] and organic thin films. [12] Since then, optically pumped lasers have been demonstrated based on various microcavity structures such as Fabry-Pérot cavities, [13,14] distributed feedback (DFB) gratings, [3,15] and whispering gallery cavities. [16] The flexibility of fabricating hybrid perovskite lasers using solution-processed methods enables large-scale production and is attractive for the realization of on-chip integration of photonic circuits. [17] Quasi-2D perovskites, which are also known as Ruddlesden-Popper (RP) perovskites, are mixed phases of 2D and 3D nanocrystals. In the mixture, 2D domains exhibit quantumwell-like electronic properties with strong exciton binding energy due to the reduced dimensionality. [18] Typically, the 2D perovskite (A') 2 A n−1 B n X 3n+1 domains consist of multilayers of BX 6 octahedra separated by intercalating ammonium cations A', which is too large to fit into the crystal structure and hinder the growth of 3D ABX 3 crystals (A = methylammonium (MA + ), formamidinium (FA + ), or Cs + , B = Pb 2+ , and X = I − , Br − , Cl − ). As a result, the number of layers determine the bandgap of 2D quantum-well-like domains. [19] Different from 3D perovskites, thin films of qausi-2D perovskites typically contain a mixture of domains with different layers. Within such inhomogenous Quasi-2D Ruddlesden-Popper halide perovskites with a large exciton binding energy, self-assembled quantum wells, and high quantum yield draw attention for optoelectronic device applications. Thin films of these quasi-2D perovskites consist of a mixture of domains having different dimensionality, allowing energy funneling from lower-dimensional nanosheets (high-bandgap domains) to 3D nanocrystals (low-bandgap domains). High-quality quasi-2D perovskite (PEA) 2 (FA) 3 Pb 4 Br 13 films are fabricated by solution engineering. Grazing-incidence wide-angle X-ray scattering measurements are conducted to study the crystal orientation, and transient absorption spectroscopy measurements are conducted to study the charge-carr...

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