Hybrid Materials As Potential NIR Absorbers For Organic Electronics

Modern society is largely a plastic-based culture in which organic (carbon-based) plastics have become more ubiquitous than other common materials such as metals, glass, or ceramics. As a result, some have postulated that there is sufficient justification to refer to the period beginning with the 20th century as the Age of Plastics. Although common organic plastics comprised of polymers such as polyethylene or polystyrene are electrically insulating materials, it was discovered in the 1960s that certain types of organic polymers could be made to exhibit semiconducting properties.

By the late 1970s, this had been expanded to even include plastics with metallic conductivity. This subclass of organic polymers, known as conjugated or conducting polymers, has thus led to materials that combine the properties of common plastics with the electronic properties of classic inorganic semiconductors. The development of these materials has then resulted in the current field of organic electronics, which can, in turn, provide the realistic promise of commercially-available flexible electronics in the near future.  Current examples of these types of electronic devices include plastic solar cells, plastic transistors, and organic light-emitting diodes (OLEDs), the last of which are finding growing use in modern smartphone displays and HDTVs.

One current limitation of these materials, however, is their inability to effectively absorb light very deep into the near-infrared (NIR) spectrum (700-2500 nm). Materials capable of absorbing NIR light could find application as the active sensing elements of NIR photodetectors, which are an important component of light-based telecommunications which utilize wavelengths of 1150, 1350, and 1550 nm to transmit data through fiber-optic cables. Although many conjugated polymers can absorb light down to 1000 nm, only a handful of organic polymers can absorb light at longer wavelengths. Even those few rare examples are still not capable of absorbing light at the two longest wavelengths, which precludes their ability to serve as NIR photodetectors for these applications.

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Developing New Hybrid Materials

To address this problem, the Rasmussen group at North Dakota State University has been developing hybrid materials that combine segments of the conjugated polymer polythiophene with metal complexes known as metal dithiolenes. Metal dithiolenes were first pioneered in the early 1960s and have generated significant interest due to their conductive and magnetic properties in the solid state. More importantly for the focus here, however, is that many metal dithiolenes also strongly absorb in the NIR.

A few real-world examples coming from the Rasmussen lab: A freestanding conjugated polymer film (A); Electrochromic films (B); Solar cell architecture and device (C); and OLED architecture and emitting device (D). Credit: Seth C. Rasmussen

By appending short segments of thiophene chains (oligothiophenes) to a nickel dithiolene core, the Rasmussen group has successfully generated materials that are structurally and electronically similar to polythiophenes, while also exhibiting strong NIR absorbance down to 1400 nm.  In addition, the energy of this NIR absorbance has been shown to be dependent on the length of the oligothiophene units, thus even lower energy absorbance should be possible with the application of longer oligothiophene segments.

Lastly, as with typical conjugated polymers, the properties of these hybrid materials can be tuned by changing various aspects of their molecular composition. As an example, it has most recently been found that by replacing two carbons of the molecular structure with nitrogen, the hybrid materials can be made to be more electronically stable and thus more promising for the targeted device applications.

This study, Thiophene-Extended Nickel Thiazoledithiolene: π-Extended Fused-Ring Metal Dithiolenes with Stabilized Frontier Orbitals, was recently published in the European Journal of Inorganic Chemistry.

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