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For organic photodetectors (OPDs), cathode interlayer (CIL) with low‐lying highest occupied molecular orbital (EHOMO) energy level is required to reduce dark current density (Jd). In this study, toward application as CIL in OPDs, an alcohol‐processable n‐type conjugated polymer (n‐PT8) based on imide/cyano‐substituted polythiophene backbone and branched oligo(ethylene glycol) side chain is developed. Owing to the electron‐accepting imide/cyano groups on the backbone, n‐PT8 exhibits deep‐lying EHOMO of −5.80 eV and a high conductivity of 5.12 × 10−6 S cm−1. OPD with n‐PT8 as CIL exhibits dark current density one order of magnitude lower than that with the state‐of‐the‐art CIL material because of the large energy barrier for reverse hole injection. Moreover, the device performance is insensitive to the n‐PT8 layer thickness, which is very desirable for the mass production of OPDs. The near‐infrared (NIR) OPDs based on n‐PT8 exhibit an ultra‐low dark current density of 1.76 × 10−9 A cm−2 and a specific detectivity of 2.88 × 1012 cm Hz1/2 W−1 at −1 V. To the best knowledge, this is the first report on n‐type polythiophene derivatives as CIL in organic opto‐electronic devices. [ABSTRACT FROM AUTHOR]

Organic photodetectors (OPDs) have received great interest for their tremendous potential in the field of optical communication, medical monitoring and intelligent buildings. Interface layers are commonly introduced between the electrodes and active layer of OPDs, to facilitate charge extraction and increase reverse charge injection, thus achieving low dark current density (Jd) and high specific detectivity (D*). However, high preparation cost and hygroscopicity of the interface layer limit their further development in practical applications. Herein, we report a direct modification strategy of the indium tin oxide (ITO) anode to construct high-performance OPDs. The ITO anode modified by dilute hydrochloric acid (ITO-Cl) increases the work function and surface hydrophobicity of the substrate due to the surface coverage of Cl atoms. Using ITO-Cl as the anode, the shifted work function increases the electron injection barrier from the anode to the active layer and the enhanced surface hydrophobicity induces a more favorable vertical stratification morphology of the active layer deposited on the anode surface. As a direct consequence, the OPD device based on the ITO-Cl anode exhibits an ultra-low Jd of 0.90 nA cm−2 at −1 V bias and high peak D* of 2.31 × 1012 cm Hz1/2 W−1 (or 1.79 × 1013 cm Hz1/2 W−1 calculated from the shot noise), which is superior to the control device based on traditional anode ITO/poly(3,4-ethylenedioxythiophene):poly-styrene sulfonate (PEDOT:PSS) due to the effective reduction of the dark current. These results demonstrated that the ITO chlorination strategy is a competitive candidate for developing high-performance OPDs. [ABSTRACT FROM AUTHOR]

Near‐infrared organic photodetector (NIR OPD) is promising for emerging wearable biosensing applications. However, their practical application is hindered by the high dark currents of devices that limit the detection of faint light. In this work, highly sensitive flexible NIR OPDs are presented with ultralow dark currents, fabricated using a layer‐by‐layer blade coating (LBL‐BC) technique. In the active layer, a fully fused‐ring molecule FM2, featuring a fixed molecular skeleton, is employed as an electron acceptor to reduce the trap density. The LBL‐BC method enhances the film order and phase purity of the active layer, significantly reduces the trap density of states, and optimizes the vertical phase separation structure of the thin film, thereby preventing reverse charge injection. As a result, a highly sensitive flexible NIR OPD is developed exhibiting an ultralow dark current density of 4.83 × 10−9 A cm−2 at −0.3 V bias and an extremely low noise current density of 7.65 × 10−15 A Hz−1/2 at 10 Hz, comparable to commercial silicon photodiodes. Furthermore, this flexible device is successfully applied for real‐time monitoring of human heartbeat rate, oxygen saturation, and motion recognition. These findings advance the development of highly sensitive NIR OPDs and their application in wearable biosensing technologies. [ABSTRACT FROM AUTHOR]

Opto‐electronic properties and device performance of organic semiconductors are mainly determined by energy levels of their frontier molecular orbitals, e.g. lowest unoccupied molecular orbital (ELUMO) and highest occupied molecular orbital (EHOMO) in the ground state, first singlet state (ES1) and first triplet state (ET1) in the excited state. These energy levels are always intricately intertwined. Herein, we report a series of monodisperse oligomers based on double B←N bridged bipyridine (BNBP) units. With the increasing number of repeating units, the oligomers exhibit gradually downshifted ELUMO and nearly unchanged EHOMO due to the different distribution of the frontier molecular orbitals of the oligomers. Moreover, the oligomers exhibit gradually decreasing ES1 and nearly unchanged ET1 because of the different contributions of the charge transfer component in the excited state. This work provides new insight into energy level tuning of organic semiconductors, which is important for high‐performance organic opto‐electronic devices. [ABSTRACT FROM AUTHOR]

Organic photodetectors (OPDs) hold immense promise for stretchable and wearable applications, but their photoactive films demonstrate brittleness, stiffness, and mechanical instability, posing challenges for future applications. Herein, the nanoconfinement effect is reported to substantially improve the stretchability and mechanical stability while reducing the stiffness of photoactive films, simultaneously maintaining the high light detection performance. The microstructure, mechanical properties, and photoelectric performance of the quaternary blend films with varying elastomer contents are thoroughly characterized. Additionally, the elastic modulus of quaternary blends can be accurately predicted by the Coran–Patel model. Cyclic tensile tests demonstrate enhanced mechanical stability, resulting in a specific detectivity of the quaternary blend with the nanoconfinement effect being 19 times higher than that of the ternary blend after 200 cycles. Overall, this study provides novel insights into constructing stretchable blend films for wearable electronics. [ABSTRACT FROM AUTHOR]

Abstract: Most of efficient polymer electron acceptors for polymer solar cells (PSCs) are based on naphthalene diimide or perylene diimide as the electron deficient building block. In this paper, for the first time, we report polymer electron acceptors based on fluorinated isoindigo (F‐IID) as the electron deficient building block. We synthesized two polymer electron acceptors consisting of alternating F‐IID unit and thiophene/selenophen unit. They show low‐lying LUMO/HOMO energy levels of –3.69/–5.69 eV, high electron mobilities of 1.31×10–5 cm2·V–1·s–1 and broad absorption spectra with the optical bandgap of 1.61 eV. PSC devices using the two F‐IID‐based polymers as polymer electron acceptors show encouraging power conversion efficiencies (PCEs) of up to 1.50% with an open‐circuit voltage (VOC) of 0.97 V, a short‐circuit current density (JSC) of 2.91 mA·cm–2, and a fill factor (FF) of 53.2%. This work suggests a new kind of polymer electron acceptors based on F‐IID unit. [ABSTRACT FROM AUTHOR]

For polymer solar cells (PSCs), efficient polymer electron acceptors are always based on strong electron-withdrawing imide unit or boron-nitrogen coordinative bond (B←N). In this paper, a new polymer electron acceptor based on thiophene-S,S-dioxide (TDO) unit is reported. The polymer electron acceptor, PBDT-TDO, consists of alternating TDO unit and 4,8-bis(alkylthienyl)-2-yl]benzo[1,2-b:4,5-b']dithiophene (BDT) unit. For comparison, a control polymer with alternating BDT unit and thiophene unit has also been synthesized. Replacing thiophene unit with TDO unit in the polymer backbone leads to large downshift of lowest unoccupied molecular orbital/highest occupied molecular orbital energy levels by 0.9 eV/0.4 eV, which is attributed to the dearomatization and electron deficiency of TDO unit. The replacement also leads to redshift of absorption spectra by ≈110 nm. PSC device with PBDT-TDO as the electron acceptor shows photovoltaic response with the preliminary power conversion efficiency of 0.64%. This work suggests a new approach to design polymer electron acceptors using the TDO unit., (© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)