Ultrafast thermal analysis of thin polymer:fullerene active layers for organic photovoltaics

Organic photovoltaics using the bulk-heterojunction (BHJ) concept are based on an active layer consisting of a donor polymer and an acceptor material blend. For an optimal BHJ, this active layer should phase separate into a co-continuous nanoscale morphology. By using advanced fast-scanning thermal analysis techniques, it has become possible to avoid the thermal effects that occur during heating or cooling (e.g. nucleation) and only study the effect of an annealing treatment. However, while conventional thermal analysis techniques work with bulk materials, the active layers used in organic photovoltaics have thicknesses in the order of 100 nm. 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 benchmark BHJ system, was studied by Rapid Heating Cooling Calorimetry (RHC) [1,2] and Fast Scanning Differential Chip Calorimetry (FSDCC) [3]. This allows for thermal characterization of the blend in bulk and, for the first time, in thin layers as used in actual organic photovoltaic devices. Due to the high scanning rates reached in FSDCC, it is now also possible to perform an isothermal crystallization study of pure P3HT.

[1] R.L. Danley, P.A. Caulfield, S.R. Aubuchon, American Laboratory 40 (2008) 9.
[2] F. Demir, N. Van den Brande, B. Van Mele, S. Bertho, D. Vanderzande, J. Manca, G. Van Assche, Journal of Thermal Analysis and Calorimetry 105 (2011) 845.
[3] A.A. Minakov, A.W. van Herwaarden, W. Wien, A. Wurm, C. Schick, Thermochimica Acta 461 (2007) 96.