New research has helped realize a significant advance in the understanding of charge-transfer coupling in crystalline assemblies of organic semiconductors. A study by a team from the University of Massachusetts Amherst has detected a surprising property within an organic semiconductor molecule that could introduce increasingly cost-effective and more efficient materials for the manufacture of solar and opto-electronic devices, including lasers, light-emitting diodes and fiber optic communications, as well as cell phone and laptop displays.

The research, which was reported in the journal Nature Communications [Labastide et al. Nat. Commun. (2015) DOI: 10.1038/ncomms10629], explained the discovery of directional intrinsic charge separation in crystalline nanowires of an organic semiconductor known as 7,8,15,16-tetraazaterrylene (TAT). They found not only efficient separation of charges in TAT, but also an extremely specific directionality, which adds control, rather than depending on inefficient random movement, describing an aspect of the nanoscopic physics operating within individual crystals.

"a well-defined directionality in the charge-separation means that we can selectively turn ‘on’ or ‘off’ this process with the orientation of optical polarization used for excitation".Michael Barnes

With a charge-transfer interaction in the molecule’s charge-conducing nanowires being able to be programmed, such a structure could make it more straightforward to use the molecule in new applications, especially those using polarized light input for optical switching. This intrinsic charge separation is believed not to occur in polymers, but only in this family of small organic molecule crystalline assemblies or nanowires, although a few other materials could share this property, such as pentacene crystals, perhaps leading to the findings about TAT being advantageous in other research.

On harvesting solar energy using organic or carbon-based organic materials, it is usually believed that the organic active layers operating in the devices are absorbing light, leading to the excited state known as an exciton. For a while, the standard view of organic solar energy harvesting devices was that excitons had to migrate/diffuse relatively large distances to find an interface where charge-separation could take place. However, this study demonstrates that exciton transport need not be a design constraint.

The scientists are currently exploring the effect of TAT crystal size and shape on charge-separation efficiency, photocurrent imaging and correlating spectral properties with surface electronic properties, and are interested in achieving a better understanding of how the optical/physical properties of the nanowire crystals can be altered by interactions with conducting or semiconducting interfaces. As researcher Michael Barnes points out, “In terms of application we are now exploring ways to arrange the crystals in a uniform pattern and from there we can turn things on or off depending on optical polarization, for example”.