The Effects of Polymer Conformation on Charge Transfer in Organic Solar Cells

Organic solar cells are cheaper alternatives to traditional Si-based solar cells. We discuss the advantages and disadvantages of organic solar cells. Then we discuss what limits the efficiency of organic solar cells and current approaches to optimizing the efficiency. Then, we review two time-dependent density functional theory approaches to modeling the active materials in organic solar cells. Finally, one of the key objectives to optimize the efficiency of these devices is determining the effects of molecular geometry on charge transfer within the device. We know that charge transfer in bulk heterojunction organic solar cells is driven by an energy offset between the donor and acceptor molecules. We manipulate this energy offset by changing the donor energy. We achieve this by introducing torsions that disrupt the π-conjugation along the polymer chains thereby creating different sized chromophores with larger π-π ∗ energy gaps. We then characterize the lowest-lying excited states of the donor:acceptor heterodimer as an excitonic state or a charge transfer state. We found that the lowest-lying state of the heterodimer with the largest energy offset between the donor and acceptor is a charge transfer state, and from this, we predict the minimum energy offset necessary for a charge transfer state to occur.