Chromic effects on the interfacial morphology of P3HT-CdS nanowire nanohybrids

Interfaces play an important role in bulk heterojunction organic/inorganic hybrid photovoltaic devices, but directly probing the interface to improve device characteristics is exceedingly difficult. The purpose of this work is to develop a method to create a discrete polymer-nanowire system that is free from bulk effects using fabrication methods that will most easily translate to traditional thin film photovoltaic device fabrication. Using these individual nanohybrids, we seek to explore the nature of the polymer-nanowire interface and develop methods to manipulate the morphology of this interface. To this end, we report on a method to form core-shell inorganic nanowire-polymer hybrids of a conducting polymer, poly (3 -hexlthiophene 2,5 diyl) (P3HT) and a semiconducting nanowire, cadmium sulfide (CdS) using solution processing to create the polymer shell around the nanowire in order to study the polymer-nanowire interface directly without interference from bulk effects. The solution processing technique yielded solution color changes from an orange colored solution to a purple colored solution. This color change is an intrinsic property of conjugated polymers such as P3HT that represents a change in polymer chain conformation from a 'coil' dominated conformation where the thiophene are predominately non-aligned to a 'rod' dominated conformation of predominately aligned thiophene monomers called 'chromism' or the rod-coil transition. Nanohybrids produced in an environment where the polymer conformation is predominately in a coil conformation produced polymer-nanowire interfaces that lacked crystallinity. In order to promote a more crystalline interface, we have used the rod coil transition with temperature (thermochromism) in P3HT to create a polymer chain conformation in solution in order to effect a change in the morphology of the polymer-nanowire interface. As the temperature of the solution drops further below the rod-coil transition temperature (10 °C), the P3HT conformation changes to a predominately rod conformation. This rod dominated chain conformation has been shown through TEM characterization to enhance the crystallinity at the polymer nanowire interface. Also, the rod coil transition with respect to solvent quality (solvatochromism) has been shown to have a very strong effect on the crystallinity at the polymer-nanowire interface. Solvents with a large solubility parameter with respect to P3HT, such as methanol and isopropanol, create very large crystalline domains at the polymer nanowire interface compared to solvents that have solubility parameters closer to P3HT, such as pyridine and hexanol. We have shown that creating more order within the P3HT main chain prior to deposition on the nanowire, by controlling the temperature and the solvent quality, can increase the extent of polymer crystallinity present at the polymer-nanowire interface. These results lead the way to more advanced manipulations of the polymer-nanowire interface in order to increase efficiency in bulk heterojunction photovoltaic devices.