## 3 Results and Discussion
### Post-Treatment of the Active Layer  
In this work, we used three different post-treatments: thermal annealing (TA), solvent vapor annealing (SVA), and joule-heating annealing (JHA). On the one hand, TA is easy to apply, but on the other hand, it is limited by the difficulties inherent in achieving high heating or cooling rates, since heat must transfer throughout the sample by thermal conduction [24]. Moreover, extended thermal annealing has been shown to create large crystals, from a few to up to 100 \(\upmu\)m, that could be detrimental to the performance of the device [25]. Additionally, many cracks can be generated in the ITO/PET interface during the TA process due to the lower thermal stability of PET compared to glass [26]. On the other hand, SVA is a versatile method where thin films are exposed to an atmosphere saturated with solvent vapors in which the solid film constituents are soluble. The solvent molecules diffuse into the deposited layer, the extent of which is dependent on, and thus controllable, the solubility and exposure time. However, the application of SVA to large areas is more difficult in comparison to TA, especially when dealing with high throughput opened processing. For the TA treatment, a range of temperatures between 65 and 110 \({}^{\circ}\)C was applied to the samples by using a hotplate. The treatment temperature was first set to 110 \({}^{\circ}\)C and later reduced to 65 \({}^{\circ}\)C after the results obtained by JHA (vide infra). The SVA processing was done with dichloromethane (SVA-DCM) and tetrahydrofuran (SVA-THF) to compare chlorinated and non-chlorinated solvents. For both techniques, the  
Figure 2: (**a**) Transmission spectra of commercial ITO-glass and ITO-PET substrates employed in this work and (**b**) UV-Visible spectra of a DPP(TBFu)\({}_{2}\):PC\({}_{61}\)BM photoactive layer fabricated onto these two substrates. most convenient way to stop the treatment was to follow the color change from blue to purple (Figure S2), since it marked the formation of more ordered domains between the donor and acceptor. Figure 3 shows the AFM images of the devices annealed using TA and SVA treatments on glass and on PET substrates. For the two different substrates, the devices annealed using the TA treatment (Figure 3a,b) display larger rod-domains than the devices annealed by SVA (Figure 3b,c,e,f), which showed similar size and more homogeneous round-shape domains. The bright dots observed in the phase images are due to PC\({}_{61}\)BM, which has a higher elastic (Young's) modulus [27]. It is noteworthy to observe that, while the roughness of the ITO on PET is higher than the roughness of the ITO on glass, after spin-coating and post-deposition annealing treatment of the photoactive layer, the final roughness of this layer is no longer dependent on the roughness of the ITO substrate. The results of the AFM image analysis show that the dimensions of domains are affected by the post-treatment technique and the annealing time. Thus, the domains for the samples treated by TA are about 200 nm, whereas those treated by SVA (for both DCM and THF solvents) are found to be below 100 nm. Treatment time is also a crucial parameter, and when a sample was overexposed to SVA, the domain size increased significantly up to 300 nm (Figure S3). Regarding the SVA-treated films, similar results were observed by absorption spectroscopy. The films show an increase in the absorption intensity after exposure to either DCM or THF, with this effect slower on THF-treated samples (Figure S4). In addition, there is a shift in the absorbance maximum from 605 nm to 595 nm, pointing to an increase in the domain size [19]. The characteristics of the devices exposed during different times are shown in Table 3. DCM has been selected as an example since the effect of non-chlorinated THF is similar. The cell performance trend is similar in both ITO-glass and ITO-PET, pointing to an optimum time for SVA post-treatment of 2 min. At longer times, all the parameters decrease progressively.