## 4 Discussion
The higher ionic conductivity and good gelling properties of PAN [17; 18; 19] probably reduce the disadvantages of a pure PEO GPE. An exact comparison of the DSSCs is difficult due to the different additives in the electrolyte and differences in the general structure of the DSSC. Additionally, the problematic reproducibility makes accurate comparisons between DSSCs developed by different researchers difficult [27]. Furthermore, the approach chosen here was to optimize long-term stability and not efficiency. Another polymer tested for the purpose of GPEs is ABS, shown as pure polymer and blended with PEO. To our knowledge, this is the first time that a combination of these polymers has been tested. As described by [PERSON] et al. [20], ABS is supposed to form a supportive matrix, whereby ABS was mixed with PMMA instead of PEO. There, immiscible ABS coagulates and the butadiene and styrene parts of the polymer form a supportive structure [20; 21]. It is to mention that PMMA offers inert ionic transport characteristics in contrast to the coordinating properties of PEO [7; 9]. The overall long-term stability and efficiency of the ABS/PEO GPE looks promising, but the decrease in U\({}_{\text{OC}}\) and strong increase of I\({}_{\text{SC}}\) (cf. sample 4 in Figure 5) was unexpected. A polymer already used for GPEs in combination with PEO and DMSO as solvent is PVA [23; 42]. While [PERSON] et al. [42] achieved good long-term stability represented by the conductivity over 36 days, we see a steady decrease in efficiency after two weeks. It is to mention that, despite similar PEO and PVA weight ratios, the addition of ethylene carbonate, tetrabutylammonium iodide and lithium iodide were also part of the GPE which [PERSON] et al. [42] characterized and therefore the consistency should vary from the GPE used here. Due to our efforts to develop DSSCs that are easy to produce, low-cost and as non-toxic as possible, adding too many additives is not feasible. [PERSON] et al. [24] described the positive effects of a PVDF/PEO blend and found a mixing ratio of 1:4 (PVDF:PEO) with 3-methoxyppionitrile as solvent and other additives to be optimal, whereas we used a ratio of 1:3 and DMSO as solvent for good coatability. The long-term stability of 500 h (-21 days) shown by them is confirmed and according to our results can also be verified for a period of 120 days (cf. sample 8 in Figure 6). Compared to the mix of PAN and PEO, the value of maximum efficiency is clearly lower, but the course of the curve is straighter overall. In addition to polymers, a suitable solvent is of great importance, since evaporation leads to a drying of the GPE and will result in an increasing viscosity and decreasing ionic conductivity [43]. The here used non-toxic solvent DMSO with its high vapor pressure offers a stable alternative to the other most often used solvents and is compatible with our goal of producing non-toxic DSSCs [15; 41; 44]. Comparing the ionic conductivities shown in Table 2 with the composition of the different GPEs given in Table 1, no direct correlation between the electrolyte salt concentration or the polymer content can be seen. Further, comparing the ionic conductivities with the results of the long-term stability measurement, it can be seen that the ionic conductivities do not seem to correlate with the long-term efficiencies. Sorting the ionic conductivities in descending order, the first six samples would be numbers 5, 6, 8, 9, 1 and 2 and while samples 1, 6 and 8 also have high long term stability and efficiency, unfortunately this is not true for numbers 2, 5 and 9. In addition, the purchased reference electrolyte shows a significantly higher efficiency at the beginning compared to the other GPEs, but has a conductivity that is smaller by a factor of four to a factor of seven. One of the reasons why high ionic conductivity does not guarantee good efficiencies is that the contact between the GPE and the nanoporous catalyst and dye layer is very important.