Stanford Synchrotron Radiation Lightsource An Office of Science User Facility

Figure 1. Chemical formula of the semiconducting material used, P(NDI2OD-T2).

Discovering high performing organic semiconductors is a hot area of research, as we look for efficient, low-cost materials that can be used in inexpensive electronic devices, such as flexible solar cells and radio frequency ID tags. To design effective materials, the relationship between a material’s structure and its semiconductive properties must be found. Research on p-type (hole conducting) organic semiconductors has shown π-bond stacking to be important in determining the semiconducting properties. The newer,  n-type (electron conducting) class of organic semiconductors has not been as extensively studied.

A group of scientists at Stanford University, including graduate student Jonathan Rivnay and Prof. Alberto Salleo, working with SSRL staff scientist Mike Toney and Antonio Facchetti from Polyera Corp., investigated the structure and properties of an n-type semiconducting polymer thin film using SSRL beam lines 11-3, 7-2, and 2-1. They found that the polymer molecules, previously thought to be disordered, packed in a face-on manner with the π-bonds mostly perpendicular to the film, unlike the p-type semiconducting materials.

This structure is unexpected, since a molecular arrangement with the π-stacking along the film surface (usually found in p-type polymers) was thought to be critical for conducting charges. The researchers hypothesize that extensive interconnectivity in the polymer structure allows multiple pathways for electrons to travel through the material. These results highlight the importance of understanding how structure influences function in semiconducting organics. This work was published on the Advanced Materials website on July 9, 2010.