Elucidating the aspect of "phase separation" in organic blends by means of thermal analysis

Abstract

Control of morphology is a key issue in order to improve the performance of organic bulk heterojunction solar cells. Solar cells consisting of a blend of regioregular P3HT (poly(3-hexylthiophene)) and PCBM ([6-6]-phenyl C-61 butyric acid methyl ester) have demonstrated the highest efficiencies until now (up to 5 %). This performance was achieved by applying a post-production annealing, which is considered to induce a dual crystallization behavior. In order to control and tune the morphology, the phase behavior needs to be described in terms of the underlying fundamental thermodynamics. Hence, it is essential to obtain a phase diagram of the blend. In this study, the state diagram of P3HT:PCBM blends is measured by means of standard and modulated temperature differential scanning calorimetry (DSC). For the first time, the glass transition (T-g) of PCBM could be determined. All blends evidenced a single T-g, indicating an homogeneous blend is formed. Phase separation is thus only induced from crystallization and no "intrinsic" phase separation is occurring in the blend.