Calculations for blends of TIPS-pentacene and poly indicated an optimum ratio of 1:4 that was experimentally confirmed for phase-separated blend films with high charge carrier mobilities. (30) The calculation of Δ G m is also valuable to estimate the optimal blend ratio for efficient phase separation. For this reason, blends of high- M w poly(α-methylstyrene) (PαMS) and 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) tend to phase-separate in thin films, while both components are homogeneously intermixed in the case of low- M w PαMS. The entropy of mixing is decreased and Δ G m is increased for high- M w insulators. (29) One important factor among others is the molecular weight ( M w) of the insulating polymer. (28) Both factors are controlled by a proper choice of components, blending ratio, and processing conditions to adjust favorable phase separation with the desired blend morphology.
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(24−27) The phase separation of the two components is determined by the change of Gibbs free energy of mixing (Δ G m) and the interaction parameter χ from the Flory–Huggins theory. But so far, such binary blends have been mainly applied to improve the semiconductor morphology. This method is today commonly referred to as solvent vapor annealing (SVA). (23) Solvent vapor penetrates the polymer film and induces diffusion, nucleation, and subsequent crystallization of the small molecules in the blend film. Another modification was the “two-step reticulate doping”, in which the as-cast blend film is exposed to solvent vapor using a good solvent for the polymer, but rather poor ones for the dispersed small molecules. The idea of crystallization of small molecules in an insulating polymer matrix was introduced already in the 80s as the so-called “reticulate doping” (21,22) that allowed creating conducting composites with percolation thresholds below 1 wt %. (19,20)Īn alternative strategy to circumvent the solution deposition on plastic substrates is based on free-standing films obtained from organic semiconductor/insulating polymer blends. Finally, the choice of processing solvents for the semiconductor is limited to avoid damage of the thermoplastic substrate during the solution deposition of the active film. (17,18) In addition, a dielectric layer is needed to separate the gate electrode from the semiconductor. Their high surface roughness typically reduces the molecular order in the semiconducting film and therefore the device performance. (13−16) Flexible OFETs also require plastic substrates, which, however, show disadvantages. (8−12) As a great advantage, the relatively low elastic modulus of organic semiconductors compared to their inorganic counterparts permits certain stretching and bending of the devices without serious degradation of the electrical performance within the mechanical properties of the active material. (6−8) A high molecular order in large domains ensures an unhindered transport of charge carriers, especially in organic field-effect transistors (OFETs). (1−5) The processing conditions determine to a great extent the molecular organization that is important for the charge carrier transport. Solution-processable organic semiconductors offer great advantages for thin-film processing of mechanically flexible electronic devices.
The performance of the flexible transistors remains stable up to a strain of 1.8%, while above this deformation, a close relation between current and strain is observed that is required for applications in strain sensors. Due to the distinct bilayer morphology, the resulting flexible field-effect transistors reveal similar charge carrier mobilities as rigid devices and additionally pronounced environmental and bias stress stabilities. The crystallinity and blend morphology strongly depend on the molecular weight of polystyrene, and under optimized conditions, distinct phase separation with a well-defined and trap-free interface between both fractions is achieved. The phase separation is induced by solvent vapor annealing of initially amorphous blend films, leading to crystallization of TIPS-pentacene as the top layer. Free-standing and flexible field-effect transistors based on 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene)/polystyrene bilayers are obtained by well-controlled phase separation of both components.