## 5 Results and Discussion
### Influence of Electrolyte Flow Hydrodynamics on \(\mu\)DSSC Cell Performance
same time, the resistances at the phase boundaries can reach up to 2.9-3.1 \(\Omega\) and 5.8-21.4 \(\Omega\), respectively, for R\({}_{3}\) and R\({}_{1}\)[57]. These resistances are combined with R\({}_{8}\) resistance and subsequently affect the deterioration of the efficiency of the cell. Figure 21: The resistance distributions and silver paths arrangement on TCO glass: (**a,b**) The electrode without silver path; (**c,d**) The electrode with silver paths for the three paralel circular channels arrangement; (**e,f**) The electrode with silver paths for the three paralel hexagonal channels arrangement; (**g,h**) The electrode with silver fingers for the serpentine channel arrangement. The properly chosen hydrodynamic conditions in the electrolyte can reduce not only the diffusive resistance R\({}_{2}\) but also the interfacial resistances R\({}_{1}\) and R\({}_{3}\), reducing the interfacial electron transfer resistances contained in them. To investigate the effect of the electrolyte hydrodynamics on the microfluidic cell performance, we have analyzed the influence of electrolyte flow rate on the cell efficiency (Equation (10)), fill factor (Equation (11)), and the maximal power of the system (\(P_{mp}\) in Figure 22). \[\eta_{cell}=\frac{P}{P_{m}}\cdot 100\%=\frac{J_{sc}\cdot V_{oc}\cdot FF}{P_{ in}} \tag{10}\]  
where  
\[FF=\frac{J_{mp}\cdot V_{mp}}{J_{sc}\cdot V_{oc}}\cdot 100\% \tag{11}\]  
\(J_{sc}\) [mA/cm\({}^{2}\)] is the short-circuit current density, \(V_{oc}\) [V] is the open-circuit voltage, \(J_{mp}\) [mA/cm\({}^{2}\)] and \(V_{mp}\) [V] are current density and voltage at maximum power \(P_{mp}\) [mW/cm\({}^{2}\)], and \(FF\) [\(\%\)] is the fill factor, which is the measure of the ideality of the solar cell. \(FF\) is the ratio of maximal power \(P_{mp}\) to the theoretical maximal power \(P_{max}\) that would be output at \(V_{oc}\) and \(J_{sc}\) (Figure 22). The experimental investigations were performed for serpentine, hexagonal, and circular channels. The results of the experiments are shown in Figures 22 and 23. The flow rate was adjusted in the range of 0-40 mL/h. Each measurement was repeated several (a minimum of three) times. The mean standard deviation (SD) of measured power densities varied between 1-4%, depending on the flow conditions and channel configuration. For the serpentine channel and flow  
Figure 23: Current-Voltage characteristics of \(\upmu\)DSSCs with: (**a**) The circular channel configuration; (**b**) The serpentine channel configuration; (**c**) The hexagonal channel configuration. Figure 22: J–U–P characteristics of \(\upmu\)DSSCs at a 20 mL/h electrolyte flow rate: (**a**) The circular channel configuration; (**b**) The serpentine channel configuration; (**c**) The hexagonal channel configuration. rates between 5 and 20 mL/h, the SD varied between 1-2%, while, for no-flow conditions and the flow rate of 40 mL/h, the SD varied between 3-4%. The highest flow rate was limited by the maximal possible pressure induced by the syringe pump. The value of the cell's maximal power has changed little with the flow rate but was always noticed at a voltage of 0.4 V (Figure 22) that is mainly influenced by the properties of the dye used and the quality of the manufacture of each cell. We have not noticed the large influence of the channel's configuration on the \(\mu\)DSSC maximal power density. Only for the hexagonal channel configuration was the measured power density a little bit lower compared to the other two configurations. The electrolyte flow hydrodynamics have also little impact on the I-V curve shapes (Figure 23). The influence of the electrolyte flow rate on the short-circuit current density, fill factor, efficiency, and the open-circuit voltage is analyzed in Figure 24. For serpentine configuration (Figure 24b) the increase of \(V_{OC}\) from initial 0.715 V to 0.745 V is clearly noticed, while for circular and hexagonal channels (Figure 24a,c), the change of \(V_{OC}\) with the flow rate is not clear.