Squaraine Dyes for Photovoltaic and Biomedical Applications

Herein, a series of novel butterfly-shaped small-molecule organic semiconductors (OSCs) have been designed, synthesized and complexed with single-walled carbon nanotubes (SWCNTs) as p-type thermoelectric materials. The butterfly-shaped molecules exhibit curved molecular structures, which tune their frontier molecular orbitals and increase their interactions with SWCNTs. A systematic study shows that the composites based on butterfly-shaped OSCs exhibit significantly improved thermoelectric performances compared with that of the composite based on the analoguous planar OSC. The enhanced thermoelectric performances are attributable to the higher activation energy, improved doping level and charge transport process between the organic molecules and SWCNTs. The butterfly-shaped OSC and SWCNT composite opens up a new avenue for the design of thermoelectric materials and devices.

Novel butterfly-shaped organic semiconductor and single-walled carbon nanotube composites for high performance thermoelectric generators.

Annealing effect on the morphology, linear and non-linear optical properties of squaraine derivative thin films for optoelectronics applications

Piezochromic fluorescent (PCF) materials that feature high sensitivity and wide-range switching are attractive in intelligent optoelectronic applications but their fabrication remains a significant challenge. Here we present a propeller-like squaraine dye SQ-NMe2 decorated with four peripheral dimethylamines acting as electron donors and spatial obstacles. This precise peripheral design is expected to loosen the molecular packing pattern and facilitate more substantial intramolecular charge transfer (ICT) switching caused by conformational planarization under mechanical stimuli. As such, the pristine SQ-NMe2 microcrystal exhibits significant fluorescence changes from yellow (λem = 554 nm) to orange (λem = 590 nm) upon slight mechanical grinding and further to deep red (λem = 648 nm) upon heavy mechanical grinding. Single-crystal X-ray diffraction structural analysis of two SQ-NMe2 polymorphs provides direct evidence to illustrate the design concept of such a piezochromic molecule. The piezochromic behavior of SQ-NMe2 microcrystals is sensitive, high-contrast, and easily reversible, enabling cryptographic applications.

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Precise peripheral design enables propeller-like squaraine dye with highly sensitive and wide-range piezochromism

Near-infrared organic photodiodes (NIR OPDs) have tremendous potential in industrial, military, and scientific applications, due to their unique features of lightweight, low toxicity, high structural flexibility, cooling-system-free, etc. However, the overall performance of currently available NIR OPDs still lags behind the commercial inorganic photodetectors, ascribed to the critical challenge of realizing organic semiconductors with sufficiently low optical bandgap and excellent optoelectronic properties simultaneously. Among various types of NIR-absorbing organic semiconductors, polymethine dyes not only possess advantages of simple synthesis and structural diversity, but also show fascinating optical and aggregation features in the solid state, making them attractive material candidates for NIR OPDs. In this review, after a brief introduction of NIR OPDs and polymethine dyes, we comprehensively summarize the advances of polymethine dyes for broadband and narrowband NIR OPDs, and further introduce their applications in all-organic optical upconversion devices and photoplethysmography sensors. In particular, the relationship between the chemical structure and the aggregation behaviors of polymethine dyes and the device performance is carefully discussed, providing some important molecular insights for developing high performance NIR OPDs. 

Advances in Polymethine Dyes for
 Near‐Infrared
 Organic Photodiodes

Squaraines have been demonstrated as one class of effective low-bandgap donor materials in bulk-heterojunction organic solar cells (BHJ-OSCs). However, they often suffer from low hole mobility, thereby limiting their photovoltaic performance. Herein, we designed and synthesized three new butterfly-shaped dimeric squaraines (DSQ1–3) with an extended π-conjugated system for BHJ-OSCs. Our results reveal that the modification of connecting bridges between two asymmetric dyes has a minimal effect on their absorption, energy levels and charge transport properties, however, subtle side chain modifications can efficiently improve the charge transport ability in the BHJ films due to enhanced miscibility with fullerene acceptors. By simply replacing the hexyl chains in the fluorene core of DSQ1 by branched 2-ethylhexyl chains, the resulting DSQ3 could possess a hole mobility of 1.10 × 10−4 cm2 V−1 s−1, providing an ∼6 times enhancement of that of DSQ1. Thus, as donor materials in single-junction BHJ-OSCs, DSQ3 afforded a power conversion efficiency (PCE) of 4.02%, while that of DSQ1 was only 2.26%. Moreover, the 2-hexyldecyl substituted carbazole cored DSQ2 showed slightly higher hole mobility than DSQ3, and thus generated the highest PCE of 4.38% among the three dimers. Our work strengthens the importance of side chain engineering for designing high-performance squaraine-based donor materials, even for those already having a large π-conjugated system.

Butterfly-shaped asymmetric squaraine dimers for organic photovoltaics

Photostable squaraine dimers for organic solar cells with a high open circuit voltage exceeding 1.0 V

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Butterfly-shaped asymmetric squaraine dimers for organic photovoltaics

Photostable squaraine dimers for organic solar cells with a high open circuit voltage exceeding 1.0 V