Phase diagram of P3HT:PC70BM thin films based on variable-temperature spectroscopic ellipsometry

In this article, we present the research on the influence of the composition of thin films of a blend of poly (3-hexylthiophene ?2,5-diyl) - P3HT with fullerene derivatives 6,6]-phenyl-C71-butyric acid methyl ester – PC70BM and 6,6]-phenyl-C61-butyric acid methyl ester – PC60BM on their thermal transitions. The influence of molar mass (Mw) of P3HT (Mw = 65.2; 54.2 and 34.1 kDa) and PCBM (PC60BM – Mw = 911 g/mol and PC70BM – Mw = 1031 g/mol) is examined in details. The article presents significantly expanded research compared to our previous work on thermal transitions in thin films of blend P3HT (Mw = 65.2 kDa) with PC60BM. For this reason, we also compare current results with previous ones. Here, we present for the first time a phase diagram of thin films of the P3HT(Mw = 65.2 kDa):PC70BM blend using variable-temperature ellipsometry. Our research reveals the presence of characteristic temperatures of pure phases in thin films of P3HT: PCBM blends. It turns out that the cold crystallization temperature of the P3HT phase in P3HT(Mw = 65.2 kDa):PC70BM blend films is lower than corresponding temperature in P3HT(Mw = 65.2 kDa):PC60BM blend films. At the same time, the cold crystallization temperature of the PC70BM phase behaves inversely. We demonstrate also that variable-temperature spectroscopic ellipsometry is a very sensitive technique for studying thermal transitions in these thin films. In addition, we show that the entire phase diagram can be determined based on the raw ellipsometric data analysis, e.g. using a delta angle at wavelength λ = 280 nm.