2D organic semiconductor crystals are emerging as a fascinating platform with regard to their applications in organic field-effect transistors (OFETs), attributed to their enhanced charge transport efficiency and their new optoelectronic functions, based on their unique morphological features. Advances in material processing techniques have not only enabled easy fabrication of few-monolayered 2D nanostructures but also facilitated exploration of the interesting properties induced by characteristic 2D morphologies. However, to date, only a limited number of representative organic semiconductors have been utilized in organic 2D optoelectronics. Therefore, in order to further spur this research, an intuitive crystal engineering principle for realizing organic 2D crystals is required. In this regard, here, not only the important implications of applying 2D structures to OFET devices are discussed but also a crystal engineering protocol is provided that first predicts molecular arrangements depending on the molecular factors, which is followed by realizing 2D supramolecular synthon networks for different molecular packing motifs. It is expected that 2D organic semiconductor crystals developed by this approach will pave a promising way toward next-generation organic 2D optoelectronics.

A highly sensitive short‐wave infrared (SWIR, λ > 1000 nm) organic photodiode (OPD) is described based on a well‐organized nanocrystalline bulk‐heterojunction (BHJ) active layer composed of a dicyanovinyl‐functionalized squaraine dye (SQ‐H) donor material in combination with PC61BM. Through thermal annealing, dipolar SQ‐H chromophores self‐assemble in a nanoscale structure with intermolecular charge transfer mediated coupling, resulting in a redshifted and narrow absorption band at 1040 nm as well as enhanced charge carrier mobility. The optimized OPD exhibits an external quantum efficiency (EQE) of 12.3% and a full‐width at half‐maximum of only 85 nm (815 cm−1) at 1050 nm under 0 V, which is the first efficient SWIR OPD based on J‐type aggregates. Photoplethysmography application for heart‐rate monitoring is successfully demonstrated on flexible substrates without applying reverse bias, indicating the potential of OPDs based on short‐range coupled dye aggregates for low‐power operating wearable applications.

Dye–dye interactions affect the optical and electronic properties in organic semiconductor films of light harvesting and detecting optoelectronic applications. This review elaborates how to tailor these properties of organic semiconductors for organic solar cells (OSCs) and organic photodiodes (OPDs). While these devices rely on similar materials, the demands for their optical properties are rather different, the former requiring a broad absorption spectrum spanning from the UV over visible up to the near‐infrared region and the latter an ultra‐narrow absorption spectrum at a specific, targeted wavelength. In order to design organic semiconductors satisfying these demands, fundamental insights on the relationship of optical properties are provided depending on molecular packing arrangement and the resultant electronic coupling thereof. Based on recent advancements in the theoretical understanding of intermolecular interactions between slip‐stacked dyes, distinguishing classical J‐aggregates with predominant long‐range Coulomb coupling from charge transfer (CT)‐mediated or ‐coupled J‐aggregates, whose red‐shifts are primarily governed by short‐range orbital interactions, is suggested. Within this framework, the relationship between aggregate structure and functional properties of representative classes of dye aggregates is analyzed for the most advanced OSCs and wavelength‐selective OPDs, providing important insights into the rational design of thin‐film optoelectronic materials.

A series of indolo[3,2-b]indole (IDID) derivatives comprising the core unit of N , N -dihexyl-IDID with different aromatic and aliphatic substituents at 2-and 7-position are designed and synthesized to construct highperformance organic semiconductors by different processing routes. Structure-property relationship of the derivatives is comprehensively studied in terms of their photophysical, electrochemical, structural, and electrical characteristics. IDID derivatives are either evaporated in vacuum or dissolved in common organic solvents to ensure applicalbility in different processing routes toward outstanding p-type semiconductor fi lms. Among others, the excellently soluble compound 4H4TIDID (with 2-and 7-substituents of 5-hexyl-2,2′-bithiophene moiety, solubility >20 wt% in chloroform), shows the highest fi eld-effect hole mobility of 0.97 cm 2 V −1 s −1 in a device constructed by vacuum-deposition and 0.18 cm 2 V −1 s −1 in device cosntructed by spincoating, respectively. The 2D grazing incidence X-ray diffraction of 4H4TIDID fi lms in both devices identically show the 2D molecular orientation favorable for the high transistor mobility.

Fluorophores emitting in the near-infrared (NIR) are highly desired for various applications, but increasing nonradiative rates cause severe fluorescence quenching for wavelengths beyond 800 nm. Here, a bis(squaraine) dye is reported that bears two NIR dyes in a head-to-tail chromophore arrangement. This arrangement leads to intramolecular J-type exciton coupling, resulting in an absorption maximum at 961 nm and a fluorescence peak at 971 nm with a quantum yield of 0.33% in chloroform. In less polar toluene, the bis(squaraine) self-assembles into nanofibers, affording another bathochromic shift with an absorption maximum at 1095 nm and a fluorescence peak at 1116 nm originating from intermolecular J-type coupling.