Surface-directed spinodal decomposition of solvent-quenched organic transistor blends

This paper describes the first example of the application of a combination of the Flory-Huggins and Cahn-Hilliard theories to model and simulate microstructure evolution in solution-processed functional blend layers of organic semiconductors, as used in organic electronics devices. Specifically, the work considers phase separation of the active blend components of organic transistors based on triisopropylsilylpentacene (TIPS-pentacene) and poly(α-methylstyrene) (PαMS). By calculation and estimation of relevant physical parameters, it is shown that the vertically phase-separated structure observed in as-cast blend layers containing PαMS of a sufficiently high molecular weight (of the order of 10(2) kDa) evolves via surface-directed spinodal decomposition. The surface-directed effect can already be triggered by small differences in substrate- and/or air-interface interaction energies of the separating phases. During phase separation, which commences at the interfaces, bulk features of the TIPS-enriched phase formed by thermal noise collapse to give the experimentally observed trilayer structure of TIPS-PαMS-TIPS. The reported near absence of solution-state phase separation of as-cast blend layers containing a low molecular weight PαMS (of the order of 1 kDa) is also reproduced.