## 3 Results and Discussion
### The Effects of Using PET  
Table 1 summarizes the main properties of both commercial ITO substrates employed in this work. To preserve their mechanical properties, the ITO films in flexible substrates are thinner in comparison with those on ITO-glass. This causes significant differences in sheet resistance and average roughness values.  
Table 2 shows the results of devices prepared on both types of substrates under identical experimental conditions (see Materials and Methods) and after using thermal annealing at 110 \({}^{\circ}\)C as post-fabrication treatment. As expected, SM-BHJ solar cells fabricated on ITO-PET showed lower performance than those fabricated on ITO-glass. Several reasons account for these differences. First, it has been reported that the PCE decreases as ITO surface roughness increases, influencing cell performance and even causing short circuits [20]. This observation agrees with our results since the roughness of the ITO-PET substrate is five times higher than that of the glass-based film (see Table 1). Second, we measured the optical transmittance of the substrates in the wavelength range of 300-800 nm. As can be seen in Figure 2a the comparison of the area under the curve indicates that ITO-PET substrate shows 11% less transmittance than ITO-glass, which is assigned to the higher roughness of the flexible ITO surface that scatters more light, and to the lower transmittance of the PET substrate at wavelengths under 420 nm and above 600 nm. This difference is also seen in the UV-Visible spectra for the partially fabricated solar cell devices (see Figure 2b), where the molecules deposited onto the flexible substrate absorb less light than those on glass. The differences in light absorption are reflected in the lower photogenerated J\({}_{\rm{sc}}\) of the flexible devices (see Table 2).  
\begin{table}
\begin{tabular}{c c c c} \hline \multirow{2}{*}{**Substrates**} & **Average Thickness** & **Sheet Resistance** & **RMS Average (nm)** \\  & **of ITO Film (nm)** & **(f/sq)** & **RMS Average (nm)** \\ \hline ITO-glass & 205 & 10 & 1.07 \\ ITO-PET & 100 & 50 & 4.77 \\ \hline \end{tabular}
\end{table}
Table 1: Summary of different parameters of the ITO covered glass or PET used in this work. RMS stands for Roughness Mean Square.  
On the other hand, [PERSON] et al. [21] attributed the low fill factor and/or a non-optimized charge transport due to unbalanced charge carrier mobility. Additionally, [PERSON] et al. [22] recognized sheet resistance (Rs) as one of the key parameters affecting the performance of organic photovoltaic devices through reduction of the FF. In fact, this suits well with the results observed in our devices since the PET-based devices with higher Rs showed lower FF. It is worth noting that the Rs is related to the energy loss in carrier transport and is strongly affected by the interfacial barriers crossed by charges in their way to the electrodes. This includes the ohmic contributions of the electrodes, the contact between the organic semiconductor and the metal, and the resistance of the active materials. Comparably, the parallel resistance (Rp) illustrates the potential leakage current through the device and around the edges of the active area. Transparent contact layers, such as indium tin oxide ITO, and carrier transporting interlayers of different kinds could increase Rs significantly [23]. In general, these layers may add more resistance because of less-than-optimum energy level alignment and differences in polarity, which affects interfacial charge transfer. Therefore, we believe that the higher Rs and Rp values in the flexible substrates are the origin of the lower V\({}_{\text{oc}}\) and FF, and consequently lower device performances. However, these observations are, by no means, affecting the purpose of this study on the impact of post-deposition treatment on PV characteristics.