Ultrafast scanning calorimetry on thin film active layers for organic photovoltaics, 3rd Korea-Belgium Bilateral Symposium on Functional Materials, Sept 15-16 (2014), Seoul, Korea

Bulk-heterojunction (BHJ) organic photovoltaics are based on an active layer consisting of a donor polymer and an acceptor material blend. For an optimal BHJ, this active layer has to form a phase separated co-continuous nanoscale morphology. In practice, thermal treatments (annealing) are often used to obtain the desired morphology. In this work, 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 benchmark BHJ system, were studied by Rapid Heating Cooling Calorimetry (RHC) [1,2] and Fast Scanning Differential Chip Calorimetry (FSDCC) [3]. Rapid-scanning thermal analysis techniques like RHC made it possible to restrict the processes that occur during heating or cooling at slower rates (e.g., nucleation), enhancing the study of the effects of an annealing treatment. However, the active layers used in organic photovoltaics, have thicknesses in the order of 100 nm. In this range of layer thicknesses, the transitions and transformations in these layers may be influenced by the presence of the layer-substrate and layer-air interfaces. Fast Scanning Differential Chip Calorimetry permitted for the first time the thermal characterization of the solar cell blends in layers as thin as used in actual organic photovoltaic devices. Due to the very high heating and cooling rates reached in FSDCC (up to 106 K.s-1), it is now also possible to perform a similar study for pure P3HT, which is impossible using more conventional techniques due to its very fast crystallization kinetics.