Impact of film morphology upon the performance of polymer / fullerene solar cells

Abstract

Polymer:fullerene blend microstructure has been recognised as a key, but poorly understood, factor in optimising performance of polymer:fullerene based solar cells. This thesis focuses on investigating the impact of material chemical structure upon the blend microstructure, and thereby on device efficiency and stability for polymer:fullerene solar cells. It will be shown in the first results chapter that polymer fluorination can promote phase segregation in polymer:fullerene blend, limiting its charge generation as demonstrated using transient absorption spectroscopy. The promotion in phase segregation is shown to vary from polymer to polymer. With a careful modification of the polymer backbone, the negative impact of polymer fluorination can be avoided, resulting in an improvement in device performance. The following chapter focuses on the impact of polymer side chain on device performance. Linear side chain for PTTV polymers is found to offer a better polymer:fullerene mixing in comparison to branched side chains, potentially due to its better ability to accommodate fullerene molecules. This subsequently addresses limitations of poor of fullerene exciton dissociation efficiency. The next chapter provides new insights into the effects of polymer molecular weight on device performance. A high molecular weight of two DPP-containing polymers is shown to be beneficial to device performance, by offering a higher degree of material mixing in the polymer:fullerene blend. The last results chapter compares two polymers with different crystallinity in terms of their blend microstructure and device performance upon thermal annealing. It is shown that devices employing a crystalline DPP based polymer exhibits a sharp collapse in efficiency for annealing at temperatures beyond 140 °C, which is assigned to the polymers poorer miscibility with fullerene, making it incapable to suppress fullerene clusters formation. In contrast, an amorphous IDT based polymer shows a smaller decrease in efficiency under the same condition, consistent with its greater miscibility with fullenere.