Dye-sensitized solar cells - also called dye-sensitized solar cells (DSSCs, DSCs or DYSCs) - are, along with organic cells, categorized as the third generation of photovoltaic cells. They were developed by Brian O'Regan and Michael Grätzel in 1991 [1]. Dye cells are also referred to as Grätzel cells due to the inventor's name.
Both electrodes in a dye cell can be transparent or semitransparent, allowing the solar cell to be illuminated from either side. A schematic of the cell design is shown in Fig. 1.
The dye is an important component in the photovoltaic process. The transparent anode is covered with a semiconducting layer (usually oxide) on which the dye is deposited. The most commonly used oxide material is titanium dioxide ( \( TiO_{2} \)), zinc oxide ( \( ZnO \)), niobium ( \( Nb_{2}O_{5} \)). Among others, metal complex compounds, porphyrin derivatives, and phthalocyanines were used as dyes.

In addition to the electrode containing the dye, the cell contains a redox couple (e.g. iodide/triiodide \( I^{3-} \)) dissolved in a suitable medium (electrolyte) and a cathode (must be the material catalyzing the redox reaction). A redox reaction is a chemical reaction in which both reduction (the process by which an atom or group of atoms move from a higher oxidation state to a lower one) and oxidation (the atom moves from a lower oxidation state to a higher one, which is equivalent to donating electrons) occurs [2].

The operation of a dye cell begins with the absorption of light by the dye particles, which moves an electron from the ground state to the excited state. Now an electron is injected into the conduction band of the semiconductor (e.g. \( TiO_{2} \)). From there it is transported, first in the porous semiconductor layer to the conducting electrode by an external circuit to the cathode. Removal of an electron leads to a reduction of the excited state of the dye molecules (regeneration of the dye by oxidation \( I^{-} \) do \( I^{3-} \)). Fig. 2 shows a block diagram of how a dye cell works.

The mechanism of action of dye-sensitized solar cells is used by plants to produce organic compounds, occurring in cells containing chlorophyll or bacteriochlorophyll with the help of light (photosynthesis). The principle of operation is presented in the video "C8 Dye Sensitized Solar Cells," while the method of manufacture is shown in the video "Dye-Sensitised Solar Cells: Animation".

Richard Thornley, C8 Dye Sensitized Solar Cells, DSSC – HL IB Chemistry (Principle of operation of a dye cell), 26.03.2017 (accessed 10.09.2020). Available on YouTube: https://youtu.be/g1TfQ9rypHI.

TheSolarSpark, Dye-Sensitised Solar Cells: Animation (captions) (Method of fabricating a simple dye cell), 20.05.2011 (accessed). Available on Vimeo: https://vimeo.com/24015119.

The maximum efficiency that has been achieved in dye cells is \( 12.6\% \). Dye cells have many advantages. The first is the manufacturing process - requiring no complicated equipment or prerequisites. Dye cells are relatively inexpensive, despite not having an easy synthesis process. By using different dye colors, we have control over the appearance of the cell, which increases the usability. Device design can be, in addition to color change, controlled by the oxide layer used, and this increases the aesthetic value. Due to their structure, dye cells are resistant to mechanical damage and are less negatively affected by temperature than silicon cells [3], [4]. The graph in Fig. 3 shows the increasing levels of efficiency of organic, dye, quantum dot-based and tandem cells (the name of the center is on marked grey) [5].

Figure 3: Performance of dye and organic cells. Source: National Renewable Energy Laboratory (NREL), CC0 license, source: Wikimedia Commons.