A fast scanning calorimetry study of post-production annealing in organic photovoltaics (P087)

Organic photovoltaics of the bulk-heterojunction type rely on an active layer consisting of a co-continuous morphology of donor and acceptor phases in order to reach their optimum efficiency. In these active layers, the donor is a conjugated polymer, and the acceptor is most often a fullerene derivative. In order to fine-tune the morphology and crystallinity and thus increasing the efficiency a post-production isothermal annealing is performed on these systems before operation [1-2]. An optimal annealing treatment can only be carried out when the thermal transition and annealing kinetics are known for the used active layer. Using advanced fast-scanning thermal analysis techniques, the thermal effects that occur during heating or cooling (e.g. nucleation) can be avoided, making it possible to study only the effects of isothermal annealing. In this study, the thermal transitions and isothermal crystallization kinetics of the P3HT:PCBM (poly(3-hexyl thiophene: [6,6] - phenyl C61 - butyric acid methyl ester) system, a model system for organic photovoltaics, was studied by Rapid Heating Cooling Calorimetry (RHC) [3] and Fast Scanning Differential Chip Calorimetry (FSDCC) [4].
For a thorough simulation of thermal annealing, both isothermal crystallization directly from the molten state and by first cooling to the glassy state were studied. A double bell shaped peak is found for the crystallization rates, obtained by applying the Avrami model on RHC results. However, a clear rate difference between the melt and glass pathways is visible [5]. The higher rates for the glass pathway can be attributed to the effect of nucleation, which was not sufficiently avoided by the scanning rates of RHC. This rate difference is clearly reduced when using the much increased scanning rates allowed by FSDCC, resulting in an effective simulation of isothermal annealing.