Book Description

Organic photovoltaics (OPVs) attract interest because they can be printed rapidly and at low cost. Over the past decade, research directed towards increasing the initial power conversion efficiency has pushed the record device efficiency above 10%. However, in order to be a competitive technology, the performance must remain relatively stable over time. In this talk, I discuss mechanisms of degradation in materials and solar cells as well as suggest materials properties that will improve stability. First I discuss the bleaching of the absorber materials in air. I will show that dense, ordered film morphologies can stabilize materials -- in one case, by a factor of 2000. Next, I show that temperature induced degradation can occur in well-encapsulated solar cells when the materials have glass transition temperatures at or below operating conditions. Similarly, I discuss the effect of mobile molecules present in the films and show that purification aimed at reducing their content can improve thermal stability. In a solar cell made from a polymer with a high glass transition temperature, lifetimes can exceed 20 years. Finally, I discuss the light-induced degradation that occurs in the first several hundred hours of solar cell operation, even in the absence of oxygen. This degradation can reduce the short circuit current via a fullerene dimerization reaction, or it can reduce the open circuit voltage via the introduction of energetic disorder on the semiconducting polymer. I show that both degradation modes are reduced in films with a more ordered morphology. These findings provide guidelines for reducing thermal and photo-induced degradation and improving the long-term stability of OPVs.