Illumination Alters the Structure of Gels Formed from the Model Optoelectronic Material P3HT

One method by which conjugated optoelectronic polymers function is by transferring charge carriers through active layers. It is therefore vital that the polymer active layers are fabricated in such a way that optimum transfer of charge carrier is achieved. In order to achieve effective active layers, control of the gelation process of conjugated optoelectronic polymers is needed to create assemblies with strong junctions, which augment optimal transfer of charge carriers. For example, the aggregation of the conjugated polymer, poly (3-hexylthiophene-2, 5-diyl) during gelation undergoes assembly of its free chain to form nanofibril aggregates, which further aggregate into a larger gel. However, more knowledge is needed to understand the detailed assembly that occurs during the gelation process, and parameters that impact this assembly process.

The impact of the presence of excitons during the gelation process of optoelectronic polymers is not well understood, and yet can play a significant role on these structural assembly processes.  Therefore it is important to understand the effects of illumination on the assembly and structural changes of conjugated polymer chains during the gelation process.

New research conducted by Professor Mark Dadmun and his graduate student Brian Morgan from the University of Tennessee provides insight into these effects, where they investigated the thermally driven gelation process of an optically active conjugated polymer in the presence and absence of white light using ultra-small and small angle neutron scattering experiments. The research work is now published in the journal, Polymer.

In their small angle neutron scattering experiments, they monitored the assembly of conjugated polymer gels with decreasing temperature in both illuminated and dark conditions.  All samples exhibited an increased scattering intensity as the temperature decreases, signifying a continuous growth of the gel during the in gelation process. The authors observed stronger scattering for the samples formed in the dark relative to the sample created in the light.  At the same time, scattering at high q also indicates that the free polymer chains exhibit variation in their structure with illumination.

The authors utilized the elliptical cylinder and polymer excluded volume models to analyze the scattering data and provide quantification to the structural changes of the nanofibril aggregates and free polymer chains, respectively. For example, the elliptical cylinder model provides essential information about the structural size and surface area of the polymer gels, indicating larger nanofibril aggregates in the gels formed in the dark relative to those formed when illuminated. At the same time, the analysis of the free chains with the polymer excluded volume model revealed a larger radius of gyration for the free chain forms in the illuminated samples relative to those assembled in the dark.

This study therefore provides profound knowledge of the effects of illumination on the structure and assembly of optically active conjugated polymers, providing insight into designing fabrication processes that improve the structure and performance of organic electronic devices.