To ensure the highest possible efficiency in energy conversion, it is necessary to create a design in such a way that as much of the sunlight's energy as possible reaches the absorber. For this purpose, conductive metal oxides are used in solar cells as transparent electrodes.

Most materials that transmit visible light well are insulators. The materials to be used as electrodes become semiconductors after doping and should have a resistivity higher or equal to \( 10^{-3} \Omega \cdot cm \). A charge carrier concentration of the order of \( 10^{20} cm^{-3} \). Oxides meeting the above conditions are called transparent conductive oxides (TCO) [1].

The most common material used as a transparent electrode is indium tin oxide (ITO). When used as a transparent electrode, ITO is a mixture of \( 74\% \) In, \( 18\% \) \( O_{2} \) i \( 8\% \) Sn, by weight. Its energy gap is 3.5- 3.7 eV [2]. The gap width determines the optical properties of ITO. Due to the high concentration of carriers ( \( 10^{20} - 10^{21} cm^{-3} \)) ITO conductivity is approaching the level of metallic conductivity. The average thickness of an ITO electrode layer in a photovoltaic system is about 100 nm.

A second common semiconductor material is fluorine-doped tin oxide (FTO) [3]. In the visible range, it achieves a transmission of up to \( 80\% \) and has a lower surface resistivity (resistance of the material calculated per unit area) than ITO (7~13 \( \Omega \cdot m \)) [4]. It is a promising material due to its time stability of performance under atmospheric conditions, chemical inertness, strength and abrasion resistance. The FTO energy gap is about 3.80 eV [5].
The transmission of both ITO and FTO is over \( 80\% \), and the differences in properties (e.g., absorption spectrum, conductivity) are strongly dependent on the degree of doping, method of application, thickness and other parameters [6], [7].

Zinc oxide ZnO:Al (AZO) doped (usually aluminium) is also used as a transparent electrode [8].
Depending on the amount and type of doping, it shows a change in electrical conductivity in a wide range: from metal conductivity to insulators. For electrode applications, a wide energy gap (3.3 - 3.6 eV) and high transparency are also important [9], [10], [11].

Other transparent metal oxides include \( ZnSnO_{4} \), \( Cd_{2}SnO_{2} \), and also \( CdO \) (because od their toxicity) Cd oxides have not come into use.
Attempts are being made to use electrode layers made of polymeric materials, e.g., PEDOT, i.e. poly(3,4-ethyl-1,4-dioxytiophene) or its mixture with the polymer PSS (polystyrene sulphonate). PEDOT:PSS is a mixture of polymers with an exit work of - 5.2 eV. PEDOT alone is insoluble, only its combination with the water-soluble PSS polymer allows wet application on flexible surfaces [12].
Graphene electrodes are also proposed as part of research into new materials [13], [14], however, despite promising results from both transparency and resistance studies, they are not yet widely used.

Despite the development of new materials, ITO and FTO remain the most common transparent electrodes. The price of indium, however, is regularly increasing, which translates into rising costs of electrodes made of this material (FTO is cheaper than ITO). Due to the lower price, the popularity of FTO is increasing [15].