Defect Engineering

Defects in materials are known to have a negative effect on the device performance. Removing or decreasing the number of such defects is a common challenge in both traditional inorganic and in organic semiconductors. In fact, it has been observed that supramolecular order is beneficial for conduction in organic semiconductors that are used for example in field-effect transistors, while local molecular orientation and lack of well-defined interfaces affect the efficiency of photovoltaic devices.  

Thus, to advance the field of organic semiconductors, the defects have to be engineered, or the structure has to be engineered around the defect to mitigate its detrimental effect on the device performance. Polymers can also exhibit microscopic defects under the influence of environmental stresses. These defects can develop into macroscopic voids (cracks) that severely deteriorate mechanical properties. Healing of these defects becomes possible with a mobile phase that fills the cracks and then solidifies, a repair that occurs either autonomously or subject to external stimuli such as light. 

However, in some cases, it appears useful to intentionally introduce defects. One possible strategy is to turn vice into virtue, and when synthetic materials perfection is impossible, employ ‘defect engineering’ to achieve otherwise inaccessible properties. For instance, by introducing defects during the synthesis, it is possible to exploit a high charge-carrier mobility of graphene nanoribbons for use in transistors, as well as affect the graphene mechanical and catalytic properties. Another example of "good" defects are the nitrogen-vacancy centers (point defects in the lattice structure) in nano-diamonds which can be exploited for applications in biomedicine. Nano-diamond are also very promising for single molecule detection with atomic resolution under ambient conditions. 

The future challenge for defect engineering is to explore improved synthetic protocols and building blocks to make defined materials with high accuracy and reproducibility and at the same time develop more sensitive methods for detecting, characterizing and theoreti­cally understanding structural deviations. 

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