The importance of chain structure in conjugated polymer-based material active layers and its relation to device efficiencies in OPVs, organic field transistors, OLEDs, and other devices has been well established. However, the influence that the absorbance of the light inherent to these devices might have on the conjugated polymer structure is not well understood. Herein, we employ small-angle neutron scattering to investigate structural changes occurring in solutions of poly(3-hexylthiophene-2,5-diyl) with exposure to white light. Results indicate significant decrease in both Kuhn length (b) and radius of gyration (R g) of the polymer upon illumination, coupled with a drop in the second virial coefficient (A 2). We explain this phenomenon through a chain collapse model, proposing that the interaction of light with the polymer backbone alters its thermodynamic interactions with and solubility in the surrounding solvent. The presence of such an effect, which we observe in several conjugated polymers, introduces the possibility of a powerful, nondestructive, and tunable method for controlling polymer conformation in solution. This in turn opens a path to develop a broad range of new light-responsive materials, in that a variety of conjugated polymers could be used as the stimuli-responsive material. Additional implications include the identification of the importance of illumination in the reproducible fabrication of organic electronic active layers from conjugated polymer inks.
The role of counterion sterics on the structure and dynamics of a low glass transition temperature, amorphous poly(isoprene-ran-styrenesulfonate) copolymer was investigated using a series of symmetric, tetraalkylammonium counterions with methyl (TMA), ethyl (TEA), propyl (TPA), and butyl (TBA) pendent groups, in addition to a sodium (Na) control. A detailed analysis of the aggregate structure was achieved by fitting the copolymers’ X-ray scattering profiles with a modified hard sphere model. Increasing the counterion sterics from Na to TEA resulted in slight changes to the aggregates with some ionic groups present in the isoprene matrix. For the more hindered TPA and TBA counterions, considerable disruption of the structure occurred. Using solid-state NMR, dynamic mechanical analysis, and rheology, the effect of the counterion sterics on the copolymer’s dynamics was determined. The T 1ρ relaxation of the copolymers revealed a rigid isoprene fraction, associated with the ion clusters, and a mobile isoprene matrix fraction. Copolymers with larger counterions exhibited an increase in the dynamic moduli at high frequency and a decrease in the dynamic moduli at lower frequencies in addition to possessing faster molecular dynamics. These two observations are attributed to an increased incorporation of ionic groups into the isoprene matrix and screening of the dipole–dipole interactions.
a b s t r a c tThis work addresses the detailed molecular dynamic behavior of miscible blends of Poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and their pure counterparts by quasi-elastic neutron scattering measurements (QENS). The study provides the measure of relaxation processes on pico-to-nanosecond time scales. A single relaxation process was observed in pure P3HT and PCBM while two relaxation processes, one fast and one slow, were observed in the blends. The fast process was attributed to the dynamics of P3HT while the slow process was correlated to the dynamics of PCBM. The results show that the relaxation process is a balance between two opposing effects: increased mobility due to thermal activation of P3HT molecules and decrease mobility due to the presence of PCBM which is correlated to the percent crystallinity of P3HT and local packing density of PCBM in the amorphous phase. The threshold for the domination of the thermally activated relaxation is between 5 and 9 vol.% of PCBM loading. Two distinct spatial dependences of the relaxation processes, in which the crossover length scale depends neither on temperature nor composition, were observed for all the samples. They were attributed to the collective motions of the hexyl side chains and the rotational motions of the CeC single bonds of the side chains. These results provide an understanding of the effects of PCBM loading and temperature on the dynamics of the polymer-fullerene blends which provides a tool to optimize the efficiency of charge carrier and exciton transport within the organic photovoltaic (OPV) active layer to improve the high performance of organic solar cells.
Studying the gelation process of conjugated optoelectronic polymers has often been employed as a means of better understanding the final morphology and assembly in active layers of organic electronic devices due to the correlation between the experimentally observed sol-gel transition and many common solution based fabrication techniques. The nature of the percolated network structures formed through the molecular assembly that occurs during this gelation directly affects device performance in conjugated polymer based active layers. Thus, precise knowledge of the evolution of structures during gelation provides crucial information that is needed to rationally improve device performance by directing the assembly during processing. Additionally, observing the effects of environmental factors such as ambient light exposure upon the gelation process will direct efforts towards improving universally overlooked facets of the typical fabrication procedure. Thus, we have conducted a series of ultra small angle and small angle neutron scattering experiments to probe the temperature-driven gelation process of the conjugated photoactive polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) in both the presence and absence of white light. Analysis of the resultant scattering data shows that the gelation 2 process consists of the creation and steady growth of cylindrical aggregates formed by the agglomeration of free chain P3HT. Furthermore, clear differences in the gel structure and assembly between illuminated and non-illuminated gels are observed across multiple length scales, pointing towards an optically-induced variation in the gelation process. Our results indicate that simple white light exposure sharply retards the growth of conjugated polymer microstructures, which clearly suggests that ignoring illumination conditions throughout organic electronic fabrication processes risks producing inconsistent and non-reproducible active layer architectures and ultimately endangers dependable device performance.
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