## 2 Overview of Polymer Additives in Crystallization
### Amorphous Polymers  
When combined with organic semiconductors, amorphous polymers significantly influence the thin film structure by minimizing misalignment and enhancing the parallel orientation of organic crystals [71; 72; 73]. Such morphological enhancements have been reported  
Figure 3: Optical images of TIPS pentacene incorporating the semicrystalline polymer additive PEO at different weight ratios. The Mw of PEO are 8k for images (**a**–**e**), 100k for (**f**–**j**), 300k for (**k**–**o**), and 900k for (**p**–**t**). Each image in (**a–**t**) uses the same 500 \(\upmu\)m scale bar as in (**a**). Reproduced from reference [70], with permission from [PERSON]. in amorphous polymers including PS [74; 75], poly(\(\alpha\)-methylstyrene) (P\(\alpha\)MS) [76; 77; 78], poly(2,5-bis(3-hexadecylthiophene-2-yl)thieno[3,2-b]thiophene (PTAA) [80; 81; 82], and poly(methyl methacrylate) (PMMA) [83; 84; 85; 86; 87]. [PERSON] et al. reported using two different insulating polymers, PMMA and P\(\alpha\)MS to modulate the crystallization and morphology of spin-coated TIPS pentacene [88]. During the spinning process, phase separation occurred, leading to a vertical arrangement with TIPS pentacene at the top. PMMA accelerated solidification due to its hydrophilic nature, resulting in faster formation of TIPS pentacene crystals. P\(\alpha\)MS, having a slower solidification rate, acted as a reservoir for residual solvent, which facilitated a controlled crystallization environment. As shown in the polarized optical images of Figure 4a,b, the presence of PMMA led to more rapid solidification and, consequently, smaller grain sizes and less organized crystallization compared to P\(\alpha\)MS (Figure 4c,d), which enabled larger, more well-ordered crystals. This difference in polymer interaction impacted the film morphology, with PMMA yielding quicker but less perfect crystallization, while P\(\alpha\)MS allowed for slower, more orderly crystal formation, enhancing the material charge transport properties. Moreover, amorphous polymers can alleviate thermal stress and prevent thermal cracks. For instance, mixing P\(\alpha\)MS with TIPS pentacene could resolve the charge transport blockage caused by thermal cracks [45]. While external heat (i.e., temperature gradient technique) introduced thermal cracks in the film (Figure 1c,d) [89], integrating P\(\alpha\)MS into the organic semiconductor solution effectively averted these cracks while maintaining crystal alignment [45]. Beyond these advantages, amorphous polymers can also cause organic semiconductors to segregate laterally or vertically [90]. Consequently, multiple benefits arise from such lateral or vertical phase segregation. For example, a vertically phase-segregated TIPS pentacene sublayer with a higher semiconductor concentration forms when blended with P\(\alpha\)MS, enhancing charge carrier mobility [91; 92]. Similarly, a  
Figure 4: Polarized optical images of TIPS pentacene incorporating **(a,b)** PMMA and **(c,d)** P\(\alpha\)MS polymer additives, based on different spin coating times as noted in each image. All images share the same scale bar of 250 \(\upmu\)m. Reproduced from reference [88], with permission from [PERSON]. layer of the polymer was observed to be in the top portion of the active layer, shielding the device from external elements and thereby boosting overall device stability [93,94]. In this section, we have discussed the significant role polymer additives play in influencing the crystallization of organic semiconductors by altering thin film morphology and enhancing charge transport properties. We have covered how conjugated polymers improve molecular alignment and crystallinity, which leads to distinct crystalline structures and enhanced charge mobility. Semicrystalline polymers offer in-situ crystallization control, improving film morphology and crystallinity. We have also explored amorphous polymers, such as PS, which help minimize misalignment and promote parallel orientation of crystals, critical for effective charge transport.