UCLA

Semiconducting polymers are a versatile class of materials used for a variety of electronic applications. They are a cheap source of material relative to their inorganic counterparts, are flexible, and with solution processing, are easily scalable However, they are intrinsically poor conductors, which can result in low device efficiencies and shorter carrier lifetimes. The conductivity can be improved using a variety of methods, including controlling the morphology to improve carrier transport or introducing charge carriers using chemical dopants. In this dissertation, we describe experiments that use a combination of X-ray and neutron scattering techniques to understand how we can use morphology in semiconducting polymers to improve their charge transport properties.

The first part of this dissertation focuses on the design of an amphiphilic conjugated polyelectrolyte model system to control the aggregation of polymer chains in solution, with the goal of straightening chains to reduce carrier trap sites caused by kinks that disrupt the π-conjugation. After showing good control of the nanoscale morphology in a well-defined, ideal system in solution, we then look at the controllably improving the electron transfer process in an actual organic photovoltaic (OPV) device. We then look to control and improve the electron transfer process in full organic photovoltaic devices. We show that using sequential processing, where the polymer and fullerene are deposited in two separate steps, we can control the device level mixing of polymer and fullerene.

We end with a discussion of studies focused on the morphological effects of molecular doping of semiconducting polymers. Using small molecule dopants, we show that with SqP, the polymer morphology is maintained as compared to conventional blend casting (BC) method. This allows more dopant to incorporate into thin films, further increasing the conductivity. Thereafter, we focus on how SqP provides the opportunity to tune the polymer morphology prior to doping and investigate how controlling the polymer crystallinity affects the optical and electronic properties in its doped state. Using statistical copolymers, we investigate the effect of polymer crystallinity and energy level offset between polymer and dopant. Finally, we conclude with a study on how the interplay between polymer chain ordering and the location of the dopant counterion in the lattice controls polymer conductivity. Overall, these results emphasize the importance of understanding and controlling the morphology of semiconducting polymers on multiple length-scales.