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Perylene diimide (PDI) derivatives have great potential to replace the commonly used fullerene derivatives in bulk heterojunction (BHJ) polymer solar cells (PSCs). However, PDI-based molecules have a strong tendency to aggregate in thin films, which prevents the formation of appropriate phase separation. To alleviate this problem, much effort has been devoted to developing twisted PDI dimers with functional bridges at bay positions. Unfortunately, the bay-functionalized PDI dimers exhibit relatively low electron mobilities due to the excessive twist geometry. To solve this dilemma, an efficient method is to functionalize the PDI at less sterically-encumbered ortho-positions. In this article, we synthesized two novel ortho-functionalized PDI dimers, namely PDI-2Th-PDI and PDI-ThFTh-PDI, and investigated the influence of the conjugated bridge length on the physical and photovoltaic properties of PDIs. After introducing a fluorene bridge between the thiophene units, PDI-ThFTh-PDI with the extended conjugate length possessed a relatively low-crystalline nature and therefore exhibited one order of magnitude lower electron mobility compared to PDI-2Th-PDI. As a result, the power conversion efficiency (PCE) of solar cells fabricated with PDI-2Th-PDI as the electron acceptor and PTB7-Th as the electron donor is almost twice that of the solar cell with PDI-ThFTh-PDI as the acceptor. Our findings provide important guidelines for the design of more efficient ortho-functionalized PDI acceptors. [ABSTRACT FROM AUTHOR]

Solution-processable organic photodiodes compensate well for the shortages of the traditional inorganic photodetectors in terms of their unique features, such as rich in varieties, low-cost manufacturing, light weight, high flexibility, and large-area scalability. Owing to the tunable optoelectronic properties of organic materials, both panchromatic and narrowband organic photodiodes have been achieved. In this review, we provide a comprehensive overview of the recent progress of organic near-infrared (NIR) photodiodes, mainly focusing on diverse device architectures toward superior performance. The key to achieving a high specific detectivity is a high responsivity while keeping dark current low. Interfacial engineering plays a critical role in suppressing the dark current, and has been identified as an essential approach for maximizing detectivity of organic photodiodes. Besides, optimization of photoactive layer thickness and morphology is also highly desirable. As to narrowband organic NIR photodiodes, we highlight three primary strategies: (i) the use of truly narrowband absorbers; (ii) the manipulation of internal quantum efficiency via charge collection narrowing; and (iii) the incorporation of a resonant optical microcavity structure to exploit charge-transfer absorption. The latter two creative approaches allow for response tuning by simply adjusting the thickness of photoactive layer and cavity, respectively. [ABSTRACT FROM AUTHOR]

Light is not the only stimulus that can induce linear-to-cyclic isomerization of donor-acceptor Stenhouse adducts (DASAs). Here we demonstrate the water-induced linear-to-cyclic isomerization of DASAs. The mechanism of the water-induced linear-to-cyclic isomerization of DASAs is investigated by density functional theory (DFT) calculations. Water molecules coordinate with DASAs and stabilize the intermediates and cyclic isomers, which favors cyclization thermodynamically. Moreover, the linear-to-cyclic isomerization is reversible. Heating removes the coordinated H2O molecules, which further triggers cyclic-to-linear isomerization. DASAs have been applied in information hiding/displaying and color switching under water vapor and heating control. Donor-acceptor Stenhouse adducts are a relatively new class of photoswitches with promising applications in medical and materials sciences. Here the role of water in inducing isomerisation is explored experimentally and theoretically, showing that water can induce a thermal isomerisation pathway and stabilise the cyclic product. [ABSTRACT FROM AUTHOR]