## 3 Polystyrene in TIPS Pentacene Crystallization
### Medium Molecular Weight PS (20k to 250k)  
Medium Mw PS (Mw between 20k and 250k) balances film formation and crystallinity. It provides moderate improvements in both crystal size and charge carrier mobility. The increased viscosity compared to low Mw PS allows for better film stability and larger crystalline domains, which can enhance the overall performance of the thin films. PS with Mw within this range is often used to achieve a good compromise between processability and electronic properties. [PERSON] et al. integrated PS (Mw 48k) with TIPS pentacene to manipulate the film morphology and crystallization dynamics for improved OTFT performance [111]. The use of PS facilitated the formation of a bilayered structure in the thin films, where the TIPS pentacene and PS components vertically phase-separated. This separation resulted in TIPS pentacene forming nearly single-crystalline structures on the bottom, directly contacting the gate dielectric, while PS created a top layer. The PS presence played a critical role in controlling the crystallization of TIPS pentacene, preventing the formation of large, rough crystalline domains typically observed in pure TIPS pentacene films. This approach led to a more uniform film with significantly reduced surface roughness. The smoother surface contributed to better device reliability and consistency by ensuring uniform electrical characteristics across the active layer. The enhanced structural arrangement also resulted in higher field-effect mobility and better on/off current ratios compared to films produced through traditional spin-coating methods. Additionally, the PS component helped protect \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Author** & **PS M\({}_{\text{w}}\)** & **Result** & **Mobility** \\ \hline [PERSON] et al. [109] & 9.58k & PS enhanced the uniform channel coverage and reduced the orientation variation of backbone & 0.73 cm\({}^{2}\)/Vs \\ \hline [PERSON] et al. [110] & 9.58k & PS was loaded to control morphology and charge transport of inkjet printed TIPS & 0.58 cm\({}^{2}\)/Vs \\  & & pentacene & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._  
Figure 6: The 2D topography captured using SKPM of (**a**) pristine TIPS pentacene and (**b**) blend (67 wt.% TIPS pentacene) transistors. Their corresponding surface potential images are presented in (**c**) and (**d**), respectively. The white dashed lines mark the positions where potential cross-sections were obtained. The solid black lines represent the potential profiles along these marked positions. Reproduced from reference [110], with permission from Elsevier. the TIPS pentacene layer from environmental factors, such as oxygen and moisture, which further contributed to the stability and longevity of the OTFT devices. [PERSON] et al. reported the cross-linking of PS polymeric layer (Mw 140k) by UV radiation to modify the surface wettability, thin film morphology and charge transport of inkjet printed TIPS pentacene crystals [112]. The amorphous PS was employed as the gate dielectric due to its low dielectric contact which prevents the reduction in charge carrier transfer caused by dipolar disorder. PS was spin coated to form a thin layer and then exposed to UV radiation with a wavelength of 254 nm. Cross-linked PS can prevent the dissolution of TIPS pentacene macromolecules and also served as a modifier layer to enhance the crystallization of the organic semiconductor. Contact angle measurement indicated the surface energy of PS layer reduces with increased exposure to UV radiation. In addition, different doses of UV radiation were tested on its effect on cross-linking the PS polymer layer. As shown in Figure 7a, PS layer without UV radiation yielded a large spreading of TIPS pentacene solution, reducing the continuity of TIPS pentacene crystals. With a dose at 0.417 J cm\({}^{-2}\), 0.695 J cm\({}^{-2}\), and 1.854 J cm\({}^{-2}\) as shown in Figure 7b-d, long chain PS molecules were reduced in the solvent, thereby reducing the macromolecule viscosity.