## 3 Polystyrene in TIPS Pentacene Crystallization
### Medium Molecular Weight PS (20k to 250k)
This effect further caused the contact line to recede fast and resulted in small crystallites. At the lowest surface energy corresponding to the highest dose of 4.403 J cm\({}^{-2}\), the contact angle is maintained while receding, and the absence of nucleation sites eventually led to the formation of coffee rings. A highest hole mobility of 0.48 cm\({}^{2}\)/Vs was obtained at a UV dose of 0.417 J cm\({}^{-2}\), which was attributed to the uniform formation of large crystal ribbons. [PERSON] et al. reported the electrostatic spray deposition of TIPS pentacene/PS (Mw 180k) for fabrication of OTFTs on a flexible substrate [113]. The mix solution of TIPS pentacene/PS blend was deposited onto the substrate at a feed rate of 1.5 mL/min while the substrate was heated up to 40 \({}^{\circ}\)C. The vertical stratification between TIPS pentacene and PS was formed after the film deposition, resulting in TIPS pentacene layer in the top and a PS layer in the bottom. The selective etching of the top layer TIPS pentacene by using an orthogonal solvent dodecane exposed the bottom PS layer. AFM results indicated a flat  
Figure 7: AFM images of inkjet printed TIPS pentacene crystals on PS polymeric films with different doses of UV light irradiation, including (**a**) 0 J cm\({}^{-2}\), (**b**) 0.417 J cm\({}^{-2}\), (**c**) 0.695 J cm\({}^{-2}\), (**d**) 1.854 J cm\({}^{-2}\). Reproduced from reference [112], with permission from Elsevier. surface with a root-mean square roughness of 0.78 nm, despite the observation of a few pit holes with height larger than 5 nm. The formation of the pit holes was attributed to the partial solvent evaporation as the mixture solution dropped onto the substrate. A hole mobility of up to 0.95 cm\({}^{2}\)/Vs and a current on/off ratio of \(\sim\)10\({}^{2}\) was reported from the TIPS pentacene/PS blend based OTTFs. The relatively small on/off ratio was attributed to the large thickness of TIPS pentacene (\(\sim\)400 nm) as well as the small capacitance of PS. [PERSON] et al. investigated the impact of integrating PS (Mw 230k) with TIPS pentacene on the morphology and gas sensing properties of OTTFs [114]. Blending TIPS pentacene with PS, the researchers aimed to enhance the performance of OTTFs by overcoming the limitations of single-component semiconductors. The study focused on how varying spin-coating times during the fabrication process influenced the phase separation and crystallization modes within the blend films. When the spin-coating time was short, there was excess residual solvent, which induced convective flow and resulted in the formation of one-dimensional (1D) TIPS pentacene crystals. These 1D crystals, characterized by thick and needle-like structures, were formed due to slow solvent evaporation and convective flow during spin-coating. However, when the spin-coating time was extended, it allowed for an optimal amount of residual solvent, promoting the growth of two-dimensional (2D) spherulitic crystals with larger grain sizes and a continuous film structure. The 2D crystals exhibited superior field-effect mobility compared to the 1D structures due to their higher surface coverage and fewer inter-crystal gaps, which facilitated more efficient charge transport. Additionally, the porous nature of the 2D crystalline film enhanced gas permeability, thereby improving the sensitivity and response time of the OTTT based gas sensors. A mobility of 0.653 cm\({}^{2}\)/Vs was reported based on 2D crystal spherulites. The research concluded that controlling the crystallization mode through precise adjustment of spin-coating times is crucial for optimizing the structural and electrical properties of TIPS pentacene/PS blend films, ultimately leading to better performance in OTTT-based gas sensors. Table 2 summarizes an overview of the studies of TIPS pentacene blended with medium molecular weight PS discussed in this section.