Thermal relaxations in polymer semiconductors and the impact on organic solar cell stability

Abstract

As organic solar cell efficiencies continue to climb to heights that were hard to envision a decade ago they are becoming increasingly attractive for commercial adoption. However, there remains a need to address organic solar cell stability, both intrinsic operational stability as well as physical stability in likely applications, such as flexible power sources. In this talk, we discuss how the thermal relaxation behavior of the polymer semiconductor donor and small molecule acceptor can inform the expected stability of the solar cells. The thermal relaxation is probed in detail using dynamic mechanical analysis. The mechanical behavior is then correlated with the morphological stability of the active layers. We show that organic semiconductors that have a high elastic modulus and glass transition temperature have the greatest morphological stability. Interestingly these systems also show poor miscibility highlighting the relationship between processing, morphology, and stability. In addition, we show how the thermomechanical behavior of polymer: small molecule and polymer: polymer blends dictate film ductility and fracture toughness, which have direct implications on the stability of flexible devices.