The addition of small quantities of polystyrene (PS), a typical insulator, was recently shown to increase the power conversion efficiencies of solution deposited molecular organic solar cells. The performance is further improved by the presence of diiodooctane (DIO), a more conventional solvent additive. How the two additives function in tandem to provide optimal enhancement was not initially understood mechanistically. Thus, we probed how the addition of PS and DIO affects film formation kinetics of the p-DTS(FBTTh2)2 and (6,6)-phenyl C71-butyric acid methyl ester (PC71BM) system, a bulk heterojunction blend film with high-performance organic photovoltaic (OPV) applications. This thin film characterization was accomplished through in-situ monitoring of absorbance, thickness and crystallinity during spin-casting – using ellipsometry, UV-Vis spectrometry and grazing-incidence wide-angle X-ray diffraction, respectively. These techniques probe the complex “wet” film environment and allow for high temporal resolution during the solution-solid transition.
In-situ measurement shows that the PS and DIO additives promote donor crystallite formation on different time scales and through different mechanisms. PS-rich films retain chlorobenzene solvent, slightly extending evaporation time and promoting phase separation earlier in the casting process. However, this extended time and chlorobenzene alone is not sufficient to attain the right morphology for optimal PCE results before dryness. Here is where the solvent additive, DIO, comes into play: its very low vapor pressure extends the time scale of film evolution for a considerably longer time and allows for crystalline rearrangement of the donor phase for more than a minute after casting. This behavior is not observed for chlorobenzene, retained by PS-rich films.