Self-Assembly of Semiconducting Polymers and Fullerenes for Photovoltaic Applications
In this thesis we present methodologies for control and characterization of nanoscale film morphology and self-assembly in systems containing semiconducting polymers and fullerenes for use in photovoltaic devices. These materials are of interest to the photovoltaic community due to their facile processing and relative low cost. Organic photovoltaics consist of a photo- absorbing electron-donating polymer and an electron-accepting fullerene, upon exposure to light an electron-hole pair (excition) is formed. This exciton can travel 10-20 nm before if finds a polymer-fullerene interface or it will recombine. Due to this small exciton diffusion length, the study of the nanoscale morphology is pivotal in understanding and improving device properties.
Here we first explore how the crystallinity of different molecular components of a blended film affects device performance. Using grazing incidence wide-angle X-ray scattering (GIWAXS), we find that different device fabrication techniques are optimized by polymers with different crystallinities. Additionally we studied all-polymer solar films through GIWAXS, which shows that these blends are approximately an addition of the two polymers; however, shifts of the polymer peaks elucidated how the polymers are mixing. To further these X-ray studies we used time-resolved microwave conductivity to study the local mobilities of fullerenes.
In the second half of this thesis, we examine a hydrogel network formed from a charged amphiphilic polymer, poly(fluorene-alt-thiophene) (PFT). This polymer self-assembles into rod- like structures in water and also shows improved conductivity in dried films due to its assembled structure. Here we use small angle X-ray scattering (SAXS) and TEM to confirm the nanoscale rod-like assembly, and employ rheology to study how the three dimensional network is held together. Finally, we examine photophysical changes upon the addition of a water-soluble fullerene, C60-N,N-dimethylpyrrolidinium iodide, to PFT, as a step towards water-processable organic solar cells. Photoexcitation of aqueous assemblies of cationic polymers and fullerenes result in the formation of free charge carriers (polarons). These separated charge carriers are stable for days to weeks, which is unprecedented in polymer/fullerene assemblies. We have shown that through these fundamental studies of device architectures and intelligent molecular design, self-assembly has the power to provide a pathway towards improved photovoltaic device properties.