# Transition Metal Coordination Compounds as Novel Materials for Dye-Sensitized Solar Cells
## 2 Characterization and Principle of Operation of Dye-Sensitized Solar Cells  
There are two types of dye-sensitized solar cells based on the physical state of the electrolyte used in the solar cell: liquid state and solid state. In the case of DSSCs with liquid electrolytes, a redox couple (e.g., \(\mathrm{I}^{-}/\mathrm{I}^{3-}\), \(\mathrm{Fc}/\mathrm{Fc}^{+}\), or \(\mathrm{Cu}^{+}/\mathrm{Cu}^{2+}\), \(\mathrm{Co}^{2+}/\mathrm{Co}^{3+}\)) is typically dissolved in the electrolyte [1]. The redox couple is one of the most important components of DSSCs due to it acting as a redox mediator, facilitating charge transports from the surface of the dye to the counter electrode of the DSSC. The redox couple also affects the electrochemical potential of the TiO\({}_{2}\) electrode through the kinetics of recombination among the electrons in the TiO\({}_{2}\) and the oxidised redox species [13]. This couple, as a result of metal reduction, is capable of regenerating the dye, and the solution is transferred to the cathode by diffusion. To seal a system consisting of an anode, cathode, spacer and electrolyte, thermoplastic films are often employed. For solid-state systems, the electrolyte is replaced by a solid transport material (HTM), and transport, in exchange for diffusion, occurs by inter-molecular charge jumps from the oxidized species to the nearest reduced species [14]. The principle of operation of dye-sensitized solar cells is based on the separation of n-type (electron selective layer) and p-type (hole selective layer) charges at the semiconductor junction under the influence of energy from electromagnetic radiation. The dye forms the outer layer of the system, and the supplied energy triggers the excitation of electrons from the valence band to the conduction band due to photon absorption. As a result of charge separation, some of the electrons are transported deep into the material (into the n layer) towards the anode, while the holes are transported towards the cathode (into the p layer). As a consequence of this process, a voltage appears in the conductive layer, and a closed circuit causes an electric current to flow [14]. An important parameter characterizing cells is the photopotential (V\({}_{\mathrm{OC}}\)), which is the maximum voltage value that can be reached with an open circuit when the current is 0 A. This quantity is also defined as the difference of the Fermi levels of electrons in TiO\({}_{2}\) (E\({}_{\mathrm{Fn}}\)) and the electrochemical Nernst potential of the redox mediator (E\({}_{\mathrm{ redox}}\)). The efficiency of a working system is associated primarily with V\({}_{\mathrm{OC}}\). As the V\({}_{\mathrm{OC}}\) decreases, the efficiency decreases, which is related, for instance, to the low regeneration of the dye used (Figure 1). Moreover, the V\({}_{\mathrm{OC}}\) value depends on the material and surface area of the counter electrode [15]. As a result, the highest possible solar cell efficiencies are currently being pursued, and the use of complex compounds as dyes promotes this process. The efficiency of photoelectric conversion depends primarily on the absorption capacity of the photo-sensitizing dyes of solar energy, i.e., as the intensity of solar energy absorption increases, the efficiency increases [16]. An issue relevant to the proper design and prediction of compound properties is to study the effect of the ligand on the redox potential, e.g., in the case of copper compounds. The density functional theory (DFT) method is used to calculate the energetic properties. The paper [17] presents how redox potentials will affect the catalytic properties of bis(\(\beta\)-diketonato)copper(II) compounds, especially compounds containing **1** and **2** (Figure 2). The work indicates that, depending on the reduction potential of the copper-containing compound, the properties of the dyes can be modified.