## 1 Introduction
The construction and working principle of the static n-type dye-sensitized solar cells is presented in Figure 5. DSSCs belong to the group of thin-film solar cells. The working electrode, sensitizer (dye), redox-mediator (electrolyte), and the counter electrode are four key elements of a DSSC [27; 28]. In DSSC, a photon is absorbed by a photosensitizer adsorbed on nano-TiO\({}_{2}\) (semiconductor), generating an exciton (excited electron-hole couple) that self-separates, and the electrons are injected into the conduction band of TiO\({}_{2}\). Then, the electrons are collected on a transparent conducting oxide layer (FTO/ITO). Through the external circuit, electrons reach the counter electrode, where they are conducted to the carbon/nano-Pt layer. The electrons at the surface of the counter electrode reduce \(I_{3}^{-}\) to \(I^{-}\). Ions \(I^{-}\) are transported through the gap between electrodes in the liquid electrolyte. The regeneration of the ground state of the dye takes place due to the acceptance of electrons from \(I^{-}\) ion, \(I^{-}\) gets oxidized to \(I_{3}^{-}\) state, and, again, \(I_{3}^{-}\) is transported back through the electrolyte layer to the counter electrode surface. The generation of current in the DSSC is a multi-stage process whose efficiency depends on the series  
Figure 4: Membrane-less cell configurations: (a) Schematic of the cell design of a single or multiphase co-laminar flow; (b) Schematic of the cell design of a co-laminar flow of electrolytes separated by a flowing stream of a supporting electrolytes. resistance of all of the stages of electric charge transfer. The highest resistance limits the performance of the whole process [29; 30]. The sheet resistance of the FTO layer is between 7 and 15 \(\Omega\)/sq. Thus, this makes the scaling of the device difficult and acts as a limiting factor for an active cell area >1 cm\({}^{2}\)[27]. To increase the efficiency of the cell, silver fingers can be used to collect the current, but this solution may reduce the stability of the cell. Additionally, the diffusive transport of ions in the electrolyte layer may be the bottleneck, especially when the electrode gap is >50 \(\upmu\)m. In this case, the diffusive transport resistances prevail. On the other hand, reducing the electrode gap leads to unstable cell operation and problems with electrolyte leakage. One of the possible solutions is to force electrolyte flow in microchannels placed between electrodes to enhance ion transport by convection. This should allow the gap height between the electrodes to be increased without increasing the mass transport resistance in the electrolyte layer. [PERSON] and [PERSON] [31] have analyzed the influence of electrolyte flow on the DSSC cell performance. While circulating the electrolyte, they observed the photocurrent improved by 38% and 13% compared to the stationary conditions, depending on the system configuration. In their experiments, the microchannel's height does not exceed 100 \(\upmu\)m, and the active surface area of prepared cells varied between 0.25-1.0 cm\({}^{2}\), depending on the experiments. The three configurations of microchannels have been tested for the effectiveness of the controlled delivery of the electrolyte: linear (square chamber), series (serpentine), and parallel, obtaining the highest current densities for parallel configuration. Employing microfluidics in degraded devices allowed for the total replacement of the degraded active layer with a fresh payload of dye, resulting in a greater than 100% photocurrent recovery [31]. [PERSON] et al. [32; 33; 34] investigated a microfluidic all-vanadium photoelectrochemical cell for solar energy storage under various operating conditions. The cell consisted of a TiO\({}_{2}\) photoanode, two microchambers with a depth of 500 \(\upmu\)m, a Nafion 115 membrane separating microchambers, and a Pt coated carbon paper (counter electrode). The active surface area of the cell was 1.0 cm\({}^{2}\).