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In the past, considerable effort are focused on advancing blue organic light‐emitting diodes (OLEDs) based on 1,3‐bis(N‐carbazolyl)benzene (mCP). To enhance and stabilize the device performance in vacuum‐processed OLEDs, it is essential to optimize the thermally stable organic materials constituting the emitting layer. The proposed novel carbazole derivative, Ad‐mCP, is designed and synthesized as a host material for blue thermally activated delayed fluorescence OLEDs, which are ideal for vacuum processing. Ad‐mCP exhibits a high glass transition temperature and triplet energy (T1), making it highly compatible with the blue dopant 2,11,14‐tri‐tert‐butyl‐N,N,5‐tris(4‐(tert‐butyl)phenyl)−5H‐5,8b‐diaza‐15b‐borabenzo[a]naphtho[1,2,3‐hi]aceanthrylen‐7‐amine emitter. The incorporation of a rigid adamantane unit into the carbazole group modifies the molecular structure of mCP, preserving crucial photophysical properties, such as T1,while enhancing the morphological and thermal stability of the host material in its film state. Moreover, the spatially hindered molecular structure of Ad‐mCP improves the photoluminescent quantum yield of the emissive layer and enhances charge balance, crucially contributing to the overall efficiency of the OLEDs. As a result, the exceptional performance of Ad‐mCP as a host material for OLEDs resulted in a remarkably high maximum external quantum efficiency of 29.9%, demonstrating superior stability in efficiency even after thermal annealing compared to mCP devices. [ABSTRACT FROM AUTHOR]

To achieve high efficiency and reliability in thermally activated delayed fluorescence (TADF) and phosphorescent organic light-emitting diodes (OLEDs), developing a novel host system has emerged as a critical research theme. In this study, we synthesized CzCzPh-mAd (9-(3-((1s,3s)-adamantan-1-yl)phenyl)-9H-3,9′-bicarbazole) as a p-type host by incorporating a bulky adamantane moiety fused with three cyclohexanes into CzCzPh (9-phenyl-9H-3,9′-bicarbazole). By incorporating a large and rigid adamantane group into the CzCzPh host (CzCzPh-mAd), we aimed to maintain the electrochemical and photophysical properties of CzCzPh while increasing its molecular weight, film-forming characteristics, and thermal stability. Ultimately, the solution-processed narrowband blue TADF-OLED employing the CzCzPh-mAd host exhibited a high external quantum efficiency greater than 13% and demonstrated superior thermal stability and reproducibility in device efficiency compared to devices based on CzCzPh. [ABSTRACT FROM AUTHOR]

All‐polymer photovoltaics (all‐PPVs) that operate under both indoor and outdoor lighting conditions require active layers with appropriately adjusted optical‐absorption ranges. However, the optical absorption of a conventional donor–acceptor binary blend is restricted to the combined absorption bands of its components. Herein, a new conjugated block copolymer (CBC) acceptor, b‐PYT, is designed by integrating polymer acceptor blocks of wide and narrow bandgaps in a single structure. Such combination results in the wide absorption range (550–850 nm) of b‐PYT that matches the emission of both artificial and solar light. The b‐PYT CBC acceptor is more crystalline than the corresponding random terpolymer, r‐PYT, owing to improved interactions between its macromolecular acceptor units. Despite exhibiting slightly inferior outdoor performance compared to that of devices using the homopolymer BTTP‐T, the PM6:b‐PYT‐based devices deliver superior power conversion efficiency (PCE) under indoor light‐emitting diode (LED) light owing to better matched absorption and emission spectra of b‐PYT and a cold white LED, respectively. Additionally, it is worth highlighting that PM6:b‐PYT‐based all‐PPVs can maintain approximately 87% of the initial PCE even after 600 min of thermal aging at 150 °C, which demonstrates the superior thermal stability compared with those of all‐PPVs that use traditional binary active layers. [ABSTRACT FROM AUTHOR]

Multiple‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters exhibit enormous potential for use in organic light‐emitting diodes (OLEDs), owing to their exceptional external quantum efficiencies (EQEs) and narrowband emission spectra. However, planar MR‐TADF emitters often suffer from aggregation‐caused quenching (ACQ) and spectral broadening at high doping concentrations because of strong interchromophore π–π interactions. A method of sterically encapsulating the planar MR skeleton with four bulky 9‐phenyl‐fluorene (Fl) units is devised, resulting in the development of a bright bluish‐green emitter (4FlCzBN). This steric shielding effectively reduces intermolecular interactions, suppresses ACQ, and improves solubility. Consequently, by utilizing 4FlCzBN as a doping‐insensitive MR emitter, solution‐processed OLEDs are fabricated with doping concentrations of 2–16 wt.%, and they show EQEs of 10.1–10.9% with a bandwidth of 28–30 nm. Furthermore, a TADF‐sensitizer‐based device using 4FlCzBN demonstrates a significantly reduced efficiency roll‐off while achieving an EQE of 12.2%. This is a remarkable improvement that overcomes the disadvantages that are difficult to achieve in previously reported MR‐TADF OLEDs. The current work provides valuable insights into the design of efficient MR‐TADF emitters with minimized aggregation and reduced efficiency roll‐off for solution processing. [ABSTRACT FROM AUTHOR]