• Van Den Brande, Niko
• Sabine, Bertho
• Vanderzande, Dirk
• Mele, Bruno Van
• Demir, Fatma
• Manca, Jean
• Van Assche, Guy

Abstract

In this study, the isothermal crystallization kinetics of poly(3-hexyl thiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend of a 50 wt% PCBM (1:1) were investigated by means of Rapid Heating Cooling Calorimetry (RHC). P3HT:PCBM blends are the state-of-the-art material for organic solar cells based on the bulk heterojunction concept. Their phase diagram was recently determined.1 Attaining the best performance requires a finely dispersed phase morphology, with crystalline P3HT and PCBM structures of 10-nm dimensions. During the production of the solar cells, the solvent-cast P3HT:PCBM blend is annealed at temperatures of typically 100°C to 140°C to create the desired phase morphology and increase the solar cell efficiency. However, if the annealing is too long, the efficiency again deteriorates again as a result of the formation of µm-sized PCBM needles. Recent research showed that the rate of efficiency loss shows a maximum around 125°C. As the annealing and the deterioration of the long-term stability both involve crystallization, the isothermal crystallization process of a P3HT:PCBM 1:1 blend was investigated by RHC. <br/>RHC, recently developed by TA Instruments, is a fast-scanning DSC designed for operation at scanning rates of up to 2000°C/min.2 It presents great benefits for the investigation of fast time-dependent transitions such as fast isothermal crystallization. Using the high cooling rates, crystallization during cooling can be avoided in certain cases, resulting in a glassy blend. Hence, it becomes possible to crystallize isothermally at temperatures closer to the glass transition Tg.<br/>The crystallization rate was obtained by fitting an Avrami kinetics model onto the evolution of the relative crystallinity versus time measured with RHC. Plotting the rate versus crystallization temperature (Tc) reached from the melt state gives a bimodal bell-shaped curve with maxima at 70°C and 110°C. Crystallization from the glassy state also exhibits a bimodal bell-shaped curve with two maxima, at 80°C and 125°C, and overall a higher crystallization rate (Figure 1). Similar observations were made for homopolymers. 3 <br/> <br/> <br/>Figure 1.Isothermal crystallization of a<br/>P3HT:PCBM 1:1 blend. <br/> <br/> <br/> <br/>References <br/>1) Zhao J., Swinnen A., Van Assche G., Manca J., Vanderzande D. &amp; B. Van Mele (2009) Journal of Physical Chemistry B, 113(6) 1587-1591. <br/>2) Danley, R. L., P. A. Caulfield &amp; S. R. Aubuchon (2008) American Laboratory, 40, 9-11. <br/>3) Supaphol, P. &amp; J.E. Spruiell (2001) Polymer, 42, 699-712

Topics

• impedance spectroscopy
• melt
• glass
• glass
• thermogravimetry
• differential scanning calorimetry
• annealing
• homopolymer
• phase diagram
• ester
• crystallization
• crystallinity
• crystallization temperature