(Invited) Understanding the Role of Organic Alloys in Polymer-Fullerene Solar Cells

While the efficiency of organic solar cells increases beyond 10%, it is still clear that there is much room for improvement in efficiency, lifetime, and cost-effective synthetic and fabrication approaches. We are investigating ternary blend solar cells based on two donor components and one acceptor component (or one donor and two acceptors), which have been recognized as a potential route to increase the absorption breadth of a solar cell and consequently the short-circuit current density. Recently, using a three-component system, we demonstrated for the first time that the open-circuit voltage of ternary blend solar cells is composition dependent and can be tuned across the full range defined by the corresponding limiting binary blends without negatively impacting the fill factor or the short circuit current. As a result, with judicious choice of components, the attainable product of short circuit current and open circuit voltage (and by extension the efficiency) in a single-layer ternary blend solar cell could be higher than is achievable with a standard binary blend solar cell. We have successfully demonstrated higher efficiencies in ternary blends based on two donor polymers and one fullerene acceptor than could be achieved in either of the limiting binary blends. It has become apparent in the systems we have investigated that the formation of so-called organic alloys is critical to the synergistic function of components in these ternary blends. Here, the electronic and physical structure of these organic alloys will be presented and the broader ramifications of organic alloys will be discussed. Efforts toward developing a deeper understanding of the mechanism of operation in these ternary blends will also be discussed.