Unravelling the Thermomechanical Properties of Bulk Heterojunction Blends in Polymer Solar Cells

Glass transition temperature is a critical parameter for achieving favorable and thermally stable bulk heterojunction morphology as it determines the kinetics of molecular organization of polymeric semiconducting materials. This study presents a sensitive method of precisely determining the glass transition temperature (T_g) of conjugated polymers and polymer-PCBM blends using dynamic mechanical thermal analysis (DMTA). The method presented here is very versatile in which polymer or polymer-molecule films are reinforced using a woven glass fiber and utilizes only 5-10 mg of the material. This makes the method superior to differential scanning calorimetry (DSC) for determining the thermal properties of conjugated polymers. The effects of PCBM loading, solvents, and additive on the T_g of polymer-PCBM blends and on the miscibility of different phases are investigated using the novel DMTA method. For the P3HT:PC₆₁BM system, two different thermal transitions were found corresponding to P3HT-rich and PCBM-rich phases when cast using CHCl₃, while chlorobenzene was found to result in a single T_g for the blend which was between those of the pure components, indicating greater miscibility when cast from chlorobenzene. On the other hand, miscibility of PCBM in TQ1 was found to be relatively low, and two thermal transitions were found for all TQ1:PCBM blends. The total PCBM content or the solvent used was found to have little influence on the resultant PCBM miscibility in TQ1. T_g of a range of other polymers as measured using DMTA is also reported to prove the versatility of this technique.