Sensitive detection of near-infrared (NIR) light enables many important applications in both research and industry. Current organic photodetectors suffer from low NIR sensitivity typically due to early absorption cutoff, low responsivity and/or large dark/noise current under bias. Herein, organic photodetectors based on a novel ultranarrow bandgap non-fullerene acceptor, CO1-4Cl, are presented, showcasing a remarkable responsivity over 0.5 A W-1 in the NIR spectral region (920-960 nm), which is the highest amongst organic photodiodes. By effectively delaying the onset of the space charge limited current and suppressing the shunt leakage current, the optimized devices show a large specific detectivity around 10 12 Jones for NIR spectral region up to 1010 nm, close to that of a commercial Si photodiode. The presented photodetectors can also be integrated in photoplethysmography for real-time heart rate monitoring, suggesting its potential for practical applications. Near-infrared (NIR) light usually corresponds to the region of electromagnetic radiation with wavelength spanning from about 750 nm to 1400 nm. [1] Despite being invisible to human visual perception, NIR sensing finds applications in a variety of technologies, including medical monitoring, [2] quality inspection, [3] machine vision, [4] and bio-imaging. [5] NIR sensing has been conventionally realized with detectors based on single-crystal inorganic semiconductor materials (e.g. Si, Ge, GaInAs), which typically have drawbacks including costly processing, mechanical inflexibility, and sensitivity to temperature. [6-8] Owing to the low cost, solution processing, material tunability, unique structure-property relationships and good mechanical flexibility, organic semiconductors emerged as an exciting candidate for integrated electronics, lighting, solar cells and photodetection. Particularly, photodetectors based on organic semiconductors have witnessed increasing research endeavor, Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff))

Fused‐ring core nonfullerene acceptors (NFAs), designated “Y‐series,” have enabled high‐performance organic solar cells (OSCs) achieving over 18% power conversion efficiency (PCE). Since the introduction of these NFAs, much effort has been expended to understand the reasons for their exceptional performance. While several studies have identified key optoelectronic properties that govern high PCEs, little is known about the molecular level origins of large variations in performance, spanning from 5% to 18% PCE, for example, in the case of PM6:Y6 OSCs. Here, a combined solid‐state NMR, crystallography, and molecular modeling approach to elucidate the atomic‐scale interactions in Y6 crystals, thin films, and PM6:Y6 bulk heterojunction (BHJ) blends is introduced. It is shown that the Y6 morphologies in BHJ blends are not governed by the morphology in neat films or single crystals. Notably, PM6:Y6 blends processed from different solvents self‐assemble into different structures and morphologies, whereby the relative orientations of the sidechains and end groups of the Y6 molecules to their fused‐ring cores play a crucial role in determining the resulting morphology and overall performance of the solar cells. The molecular‐level understanding of BHJs enabled by this approach will guide the engineering of next‐generation NFAs for stable and efficient OSCs.

The environmental impact of solution processed organic solar cells (OSCs) can be mitigated by introducing so‐called green solvents during the fabrication processes. However, the effects of such green solvents on the molecular‐level structures and optoelectronic properties lack in‐depth characterization. Here, insights into the structure–processing–property correlation of a PPDT2FBT:PC61BM bulk‐heterojunction (BHJ) system processed from a green solvent, ortho‐xylene (o‐XY), is investigated in comparison with the same blend processed from a traditional halogenated solvent, chlorobenzene (CB). The BHJ blends are characterized with various techniques probing at difference length scales, and an increased donor:acceptor (D:A) interfacial area as well as well‐mixed features in the bulk morphologies of the active layer are observed for the o‐XY processed BHJ blend. Furthermore, molecular‐level differences in the D–A intermolecular interactions at the BHJ interfaces in o‐XY and CB cast films are elucidated by 2‐dimensional solid‐state nuclear magnetic resonance (ssNMR) measurements and analysis. These results are consistent with the device properties, suggesting that the green‐solvent‐processed devices have longer charge carrier lifetimes and faster charge carrier extraction. The optimized PPDT2FBT:PC61BM devices processed from o‐XY can achieve a noteworthy higher power conversion efficiency (PCE) owing to a higher short‐circuit current density and fill factor.

A series of A−π−D−π−A-type nonfullerene acceptors (NFAs) was designed and synthesized with the goal of optimizing light absorption and energy losses in nearinfrared (NIR) organic solar cells (OSCs) principally through the use of side-chain engineering. Specific molecules include p-IO1, o-IO1, p-IO2, and o-IO2 with optical bandgaps of 1.34, 1.28, 1.24, and 1.20 eV, respectively. Manipulating the optoelectronic properties and intermolecular organization by substituting bulky phenylhexyl (p-) for linear octyl chains (o-) and replacing bisalkoxy (-O2) with alkyl-alkoxy combination (-O1) allows one to target energy bandgaps and achieve a favorable bulk heterojunction morphology when in the presence of the donor polymer PTB7-Th. Solar cells based on o-IO1 and PTB7-Th exhibit an optimal power conversion efficiency of 13.1%. The excellent photovoltaic performance obtained with the o-IO1 acceptor can be attributed to a shortcircuit current of 26.3 mA cm −2 and energy losses on the order of 0.54 eV. These results further highlight how side-chain engineering is a straightforward strategy to tune the molecular design of n-type molecular semiconductors, particularly in the context of NIR high-efficiency organic photovoltaics.

Organic photodetectors (OPDs) capable of detecting visible to near-infrared light provide a ubiquitous platform for emerging flexible and wearable electronics. In the process of implementing OPDs into a Si-based manufacturing process, organic semiconductors undergo ≥ 200 °C thermal stress, leading to the deterioration of photosensing capability. Here, we combine multiscale characterization and device physics to unravel the impact of thermal stress on the optoelectronics characteristics of PTB7-Th:non-fullerene acceptor blends (NFAs: SiOTIC-4F, COTIC-4F, CO1-4F, and CO1-4Cl). For as-cast devices, favorable intermixing and phase separation between PTB7-Th and the NFA facilitate charge generation and extraction. Reductions in the OPD performance after thermal annealing (200 °C for 5–120 min) are observed due to the morphological degradation, regardless of the NFA choice, but the reduction is more severe for the PTB7-Th:SiOTIC-4F blend. Thermally induced morphological changes are examined using atomic force microscopy, wide-angle X-ray scattering, and solid-state NMR spectroscopy. This study provides essential insights into morphology-driven deteriorations, which will help in developing structure–stability–performance relationships in high detectivity OPDs.