Investigating annealing in organic photovoltaics using advanced calorimetry

Bulk-heterojunction organic solar cells rely on an active layer consisting of a co-continuous morphology of donor and acceptor phases in order to reach their optimum efficiency. A conjugated, light-excitable polymer is most often used as an electron donor, and fullerene derivatives are the most widespread type of electron acceptor. Post-production isothermal annealing plays an important role for these systems, both for fine-tuning the morphology and crystallinity and thus increasing the efficiency, but also for retaining the desired morphology during long-term operation [1-2]. Optimal thermal annealing can only be carried out when the thermal transitions and annealing kinetics for the used systems are known. 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 as used in organic photovoltaics was studied by Rapid Heating Cooling Calorimetry (RHC) [3] and Fast Scanning Differential Chip Calorimetry (FSDCC) [4].
In order to simulate thermal annealing in a complete way, both annealing directly from the molten state and by first cooling to the glassy state were studied. From RHC results a double bell shaped peak is found for the crystallization rates, but a clear rate difference between the melt and glass pathways is visible [5]. The consistently higher rates for the glass pathway can be attributed to the effect of nucleation, which was not sufficiently avoided by using RHC. This rate difference is clearly reduces when using the much increased rates allowed by FSDCC, resulting in an effective simulation of isothermal annealing.

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