Abstract

Some of the key questions regarding polymer solar cells are why certain bulkheterojunctions have internal quantum efficiencies higher than 90%, what is the optimal morphology and why fullerenes are by far the best electronic acceptors. We will show that in several highly efficient solar cells there is a three-phase morphology in which there are pure regions of polymer and fullerenes along with a mixed region. We will show that there are energetic offsets that push electrons and holes out of the mixed region and into the pure region. One can control the size and composition of the phases by adjusting factors such as the regioregularity and molecular weight of the polymer, the polymers sidechains, the donor:acceptor ratio, the choice of fullerenes, the use of processing additives and annealing conditions. We will show how one can decide what to adjust to get the ideal morphology and thereby obtain high energy conversion efficiency in solar cells. We will also discuss the mechanisms of degradation and polymer solar cells and how the lifetime can be extended.