(4db) Structural Characterization of Conjugated Polymer/Fullerene Dispersed and Thin-Film Composites for Organic Photovoltaic Applications

Understanding the structural morphology of conjugated polymer/fullerene composites is an important step toward improving the performance of photovoltaic devices. The efficiency of exciton dissociation and carrier transport to the working electrodes are both important factors to optimize with respect to thin-film morphology.

In thin-film composites, we have shown that generally the films can be viewed as composed of three main phases:  a polymer aggregate phase, a fullerene aggregate phase, and a disordered polymer/fullerene phase. The concentration, distribution, and composition of these phases are directly tied to how devices are processed and the specific film treatment history. However, regardless of the specific thermal treatment we have shown that the performance of the thin-film composite can be understood by examining its instantaneous morphology. Coupled with spectroscopic and microscopy techniques, Grazing Incidence Small Angle X-Ray (GISAXS) scattering techniques provide detailed structural information about the structural morphology of these thin-films and provides a means to understand performance of thin-film composites.

Similar to their thin-film counterparts, the performance of photovoltaic devices derived from conjugated polymer/fullerene composite nanoparticles (CNPs) is a function of their internal structural morphology. However, whereas the structural morphology of the thin-film composite is a function of the detailed history of the coating process, the structure of CNPs is fixed as a result of their fabrication. Because of this fact, the structure can be identified in the dispersed phase and tied, using single particle characterization, to its performance before it is deposited into a device. Therefore, the optimization of devices derived from CNPs can be broken into two discrete problems: improving the intrinsic properties of a nanoparticle as a function of how it is produced and optimizing the extrinsic properties of devices through the deposition of optimized CNPs. Our work has addressed both of these challenges. First, using model conjugated polymer and fullerenes, poly(3-hexlythiophene) (P3HT)/[6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), we show that internal phase segregation of P3HT/PCBM CNPs is a complex function of their production and composition. Second, we demonstrate the deposition of monolayers of these CNPs onto device relevant electrodes. The combination of these two separate approaches opens up the potential for novel roll-to-roll processes to produce high performance photovoltaic modules of arbitrary complexity with flexible form factors.