Improving the Efficiency of Conjugated Polymer Based Devices: Controlling the Conformation of a Conjugated Polyelectrolyte in Solution

Sean Roberts
University of California, Los Angeles

Since the first conjugated polymers were created nearly a decade ago, these materials have been under heavy research due to their potential usage in a number of applications that require visible band-gap semiconductors, such as light emitting diodes (LEDs) and solar cells. Due to their inexpensive production cost and solubility in a wide variety of different organic solvents, devices can be easily and cheaply prepared by a wide variety of different techniques. Although polymer films act as semiconductors in a variety of polymer based devices, the way in which polymer films interact with light cannot be thought of in the same way by which typical inorganic semiconductors (i.e. infinite lattices that can be described by band theory). In addition to single chain excitons, interchain excited states that are delocalized among one or more chains can form and form as low energy sinks that can harvest nearby single chain excitations. In LEDs, the creation of large numbers of interchain states represents a tradeoff between the efficiency of the device and its brightness because interchain states improve charge transport but are weakly emissive. For photovoltaic devices, on the other hand, the weakly emissive character of interchain states improves the efficiency of devices.

Past research by my group suggests that the confirmation that conjugated polymers take on in solution persists through the spin casting process by which films are made and can have profound effects on the performance of polymer based devices. The goal of my research project is to see how to control the conformation of a particular polymer that contains negatively charged side groups (poly(2-methoxy-5-propyloxy sulfonate phenylene vinylene, MPS-PPV) by controlling the ionic strength and pH of solutions of the polymer. By screening charges along the backbone of the polymer, the normally stiff polymer chains should be allowed to fold and form interchain species, thereby causing devices made from solutions with different values of ionic strength or pH to exhibit different properties. Thus, the performance of polymer devices can be optimized through the careful selection of the solvent conditions used to create them.