The Effect of Blend Composition on Bulk Heterojunction Organic Solar Cells

Abstract

A shift to renewable sources of electricity generation is required to address the threat of climate change. Organic photovoltaics (OPV) are a promising renewable energy technology. The solution-based processing employed by OPV allows for fast and cheap manufacturing, with extremely low embodied energy. OPV devices require an electron donor and electron acceptor material in the semiconductor active layer. The composition of the organic blend solution, in particular, the ratio and concentration of the donor and acceptor, influences a variety of properties which determine the operation of an OPV device. As such, studies investigating the mechanisms underlying the influence of blend composition are of great interest. In this thesis, the influence of the blend composition on the operation of OPV devices was studied in the context of two material systems. The first system was composed of a binary blend of PCPDTBT:PC71BM. In this system, PC71BM is mixed on a molecular scale with amorphous PCPDTBT chains. The optimum polymer:fullerene blend composition (2:7) occurs at high PC71BM concentrations. This is required to facilitate efficient electron transport. The second system was composed of a ternary blend of P3HT:Si-PCPDTBT:PC71BM. A threshold polymer concentration of 40 wt% is required for P3HT to transition from an amorphous to crystalline phase. This governs the charge transport throughout the active layer. The optimum polymer concentration was determined to be 50 wt%, leading to a device efficiency of 3.6%. A comparison between the results of the two studies provides insight into the blend compositional dependencies in bulk heterojunction polymer:fullerene systems. The physical mixing of the components is fundamentally different in the two systems studied. Consequently, the blend compositional dependence on the performance of both systems is different, leading to vastly different optimal conditions. This highlights the importance of the polymer crystallinity and molecular mixing of the components in determining the influence of the blend composition in polymer:fullerene organic systems.