Understanding Local and Macroscopic Electron Mobilities in the Fullerene Network of Conjugated Polymerbased Solar Cells: TimeResolved Microwave Conductivity and Theory

There are a number of factors that can limit the power conversion effi ciency (PCE) of organic photovoltaics, including the effi ciency of exciton generation and separation, [ 1–4 ] the mobilities of the subsequently generated electrons and holes, [ 5–8 ] and the nanometer-scale morphology of the bulk heterojunction (BHJ) network that determines how easily carriers can reach the electrodes or recombine. [ 9–14 ] The importance of the network morphology can be seen in the fact that a number of organic electron donors and acceptors with seemingly optimally matched energy levels produce poorly performing solar cells because they have an improper degree of phase segregation when blended together. [ 15,16 ] Because of this, there has been a great deal of effort to use processing conditions to control BHJ morphology, including the use of solvent additives, [ 17–19 ] post-fabrication thermal annealing, [ 20–22 ] and sequential deposition of the donor and acceptor layers. [ 23–30 ] All of these techniques greatly increase the parameter space for optimizing power conversion effi ciency, which is detrimental when most of the progress in increasing PCE for a given set of materials is made via exploring this parameter space through trial-and-error. In previous work, we developed a method to control the nanometer-scale morphology of the BHJ network in polymer/ fullerene photovoltaics using pentaaryl-substituted fullerenes that self-assemble into one-dimensional stacks. [ 31–33 ] The idea is that pentaaryl substitution creates fullerene molecules with a self-complementary shape that promotes stacking; because of their shape, we (and others [ 34–37 ] ) have referred to this class of fullerene derivatives as ’shuttlecocks’ (SCs). When exploring the behavior of SCs in photovoltaic blends with poly(3-hexylthiophene) (P3HT), we found that the SCs that formed 1-dimensional stacks had signifi cantly improved photovoltaic performance compared with nearly identical fullerenes that had meta Understanding Local and Macroscopic Electron Mobilities in the Fullerene Network of Conjugated Polymer-based Solar Cells: Time-Resolved Microwave Conductivity and Theory