Thermal Annealing and Crystallization Kinetics of P3HT:PCBM Blends for Organic Solar Cells Studied by Rapid Heat-Cool Calorimetry (RHC)

In bulk heterojunction solar cells an essential step of raising power conversion efficiency is the post production annealing [1]. Since defining appropriate annealing temperatures and times is vitally important, the isothermal crystallization of poly(3-hexyl thiophene) (P3HT) and [6,6]-phenyl C61 - butyric acid methyl ester (PCBM) blend is investigated in this study in order to better understand the annealing process. The study is done by a "Rapid Heat-Cool" Calorimetry (RHC) that contains a micro-sized furnace for operation at high scan rates of up to 2000 K.min-1 [2]. Using high cooling rates, crystallization during cooling can be avoided and a glassy blend can be achieved. Hence, it becomes possible to crystallize isothermally at temperatures closer to the glass transition temperature (Tg) than possible by conventional DSC techniques.
As the annealing and the deterioration of the long-term stability both involve crystallization, the crystallization rate of the P3HT:PCBM 1:1 blend was investigated at temperatures in between the glass transition and melting. The evolution of the crystallization rate with temperature was compared for annealing from the glassy state and from the melt state.
Plotting the melting enthalpy of isothermally obtained crystals as a function of the annealing time, results in sigmoidal curves. These curves were fitted using Avrami kinetics model [3] in order to obtain the crystallization time constants. The reciprocal of the crystallization time constant is a measure of the crystallization rate. Plotting the isothermal crystallization rate versus crystallization temperature (Tc) (reached both from the melt and glassy states) gives bimodal bell-shaped curves with two maxima, one at lower temperatures (70°C and 80°C) and the second at higher temperature (125°C) (Figure 1).
In addition to RHC, P3HT/PCBM blends are now being studied by fast scanning chip calorimetry [4] in order to strengthen the understanding of the correlation between morphology and annealing.

Figure1. Isothermal crystallization rate vs crystallization temperature of P3HT:PCBM 1:1 blend.
References
1. Padinger F, Rittberger RS, and Sariciftci NS. Advanced Functional Materials 2003;13(1):85-88.
2. Danley RL, Caulfield PA, and Aubuchon SR. American Laboratory 2008;40(1):9-11.
3. Avrami M. J.Chem.Pys. 1940; 8:212-224.
4. Minakov AA and Schick C. Review of Scientific Instruments 2007;78(7).