Photovoltaic cells, which are the basic component of photovoltaic panels, are made from monocrystalline, polycrystalline and amorphous silicon wafers. Technologies used in their production are constantly being modernised in order to improve their technical parameters and reduce costs, so that they become more and more attractive to buyers. Laboratories are currently working on thin-film cells made of both inorganic and organic compounds. Photovoltaic panels in the form of a thin layer, which could be placed on the roof, adjusted to its shape and roof colour, would certainly attract the attention of homeowners, architects and designers.
The thickness of the active layer in a thin film cell made of inorganic compounds usually does not exceed 20 \( \mu m \). The consumption of semiconductor raw material is consequently limited [1], the potential flexibility of the cell is also ensured. It is also important that such an active layer has a high absorption coefficient value, as this affects the efficiency of light energy conversion to electricity. When organic materials are used for photovoltaic cells, the thickness of the active layer does not exceed 1 \( \mu m \), because the absorption coefficient of these materials is much higher than that of semiconductor materials.
Thin film cells work efficiently even when the cell is shaded; the temperature has less effect on their power output, and they can be made in almost any shape and design. A drawback of these cells may be the lower efficiency when a material other than silicon is used and the fact that the material used may be toxic.
The active layers in thin-film photovoltaic cells are deposited on transparent or opaque substrates and different deposition methods are used. In the case of a transparent substrate, the transparent electrode is applied first, then the photovoltaic active layers and finally the second electrode. On the other hand, in the case where the substrate is opaque, an electrode which may be opaque is applied first, over it the photovoltaically active layers and finally the transparent electrode. The thickness of the individual layers ranges from 100 to 500 nm.
Materials with different properties are used to build photovoltaic cells. It is worth emphasising that substrates should ensure appropriate mechanical durability of the cell, and in the case of flexible cells, the material should be resistant to bending. Electrodes should be made of materials so selected that the resistance between the photovoltaic material and the electrode is as low as possible.

At present, the most favourable materials for thin film photovoltaic cells are polycrystalline structures containing gallium indium copper diselenide \( CuIn_{x}Ga_{1-x}Se_{2} \), or cadmium telluride CdTe [2]. Photovoltaic cells with a CdTe layer under laboratory conditions achieve efficiencies in the range of \( 16\% \) (in production unfortunately about \( 10\% \)). However, work is underway to realise the full potential of these cells, and critical issues in improving their efficiency are:

• identification and reduction of defect density at grain boundaries,
• increase in the concentration of hole carriers in the CdTe layer,
• elimination or control of parallelism,
• development of the manufacturing process,
• identifying and overcoming back contact formation problems,
• it is also necessary to develop a better hermetic enclosure as these cells interact strongly with the atmosphere ( \( O_{2}, H_{2}O \)), which reduces their lifetime.

In Fig. 1 the structure of a thin-film photovoltaic cell is presented. A metal layer is placed on a glass substrate to form ohmic contact with a p-type semiconductor, i.e., the absorber layer (CIS, CIGS or CGS). The cadmium sulphide (CdS) layer forming the p-n junction with it - a semiconductor with n-type conductivity - is preceded by an Ordered Vacancy Compound, OVC). The CdS buffer layer is designed to match the edges of the conductivity bands of the CIGS layer and the which is ZnO zinc oxide.
A soda-lime glass with a thermal expansion coefficient of about \( 9\cdot 10^{-6} K^{-1} \) is the most commonly used substrate for CIGS thin-film photovoltaic panels. A typical composition of this glass contains oxides such as \( Na_{2}O \) and CaO, which are sources of impurities in the other layers of the panel.
The absorber layer, responsible for absorbing photons and generating electric current carriers, is the most important material in a thin film cell. It usually consists of two ternary alloys: \( CuInSe_{2} \) and \( CuGaSe_{2} \), with the ratio y = Ga/(Ga+In) ranging from 0 to 1. The most effective layer used in photovoltaics is obtained for y = 0.11 - 0.26. The alloy \( CuInSe_{2} \) is a semiconductor with a simple energy gap of 1.05 eV (up to ca. 1.65 eV in the case of the \( CuGaSe_{2} \) ) and with a very large absorption coefficient \( \alpha \) = 105 \( cm^{-1} \) for photons with energies > 1.4 eV.
Group II-(III)-VI materials are called chalcopyrite because they crystallize in the same arrangement as chalcopyrite \( CuFeS_2 \) – a common mineral in the sulfide cluster. The crystal structure is based on a regular arrangement, the so-called zinc blende structure. The semiconducting properties of chalcopyrite are related to its electrical and structural similarity to group IV semiconductors, such as silicon or germanium. One of the main features of \( CuIn_{x}Ga_{1-x}Se_{2} \) is the insensitivity of the structure's opto-electronic parameters to significant variations in material composition. The optical and electrical properties of \( CuInSe_{2} \) depend strongly on the Cu/In ratio and the crystal structure of the material. The concentration of holes depends on the excess of selenium and on the Cu/In ratio. As the Cu/In ratio decreases, the concentration of holes decreases. The resistivity of the p-type layer increases by more than five orders of magnitude as the Cu/In ratio decreases from 1.1 to 0.9.
The buffer layer in CIGS thin-film photovoltaic cells is n-type conductive cadmium sulfide CdS, which together with the absorber layer forms an n-CdS/p-CIGS heterojunction structure. The cadmium sulfide has a large energy gap, \( E_{g} \) = 2.4 eV. The advantage of such a junction is that the material with the larger energy gap is transparent to the radiation absorbed in the material with the smaller energy gap. This causes the material with the larger energy gap to provide a window for radiation to be absorbed in the layer with the smaller energy gap width. Disadvantages include the phenomenon of radiative recombination in a semiconductor with a smaller energy gap [3], [4].
All currently manufactured CdTe cells are mainly made as heterostructures with the design shown in Fig. 1. Light falls on the heterojunction from the substrate side through a TCO (Transparent Conducting Oxide) electrode. The CdS layer acts as an optical window and helps to reduce the influence of the recombination process in the n-CdS/p-CdTe contact area.
All currently produced cells with CIGS layer are made as heterostructures, in which light falls on the heterojunction through a transparent conductive oxide layer TCO. It is usually formed by two layers of zinc oxide ZnO - one with high resistivity and the other heavily doped with n+ type conductivity. The energy gap of ZnO, \( E_{g} \) = 3.3 eV, allows photons of wavelength 350 nm and above to pass through the material deep into the structure.
Production costs of cells made on the basis of cadmium telluride CdTe are relatively low. However, their wider dissemination is hampered by the fact that they contain significant amounts of cadmium, which is a toxic element. Thin film photovoltaic cells with a layer of copper indium gallium diselenide CIGS are described as the most promising cells due to their manufacturing technology and low production costs. Under laboratory conditions, the efficiency of these panels is close to \( 20\% \). The size of the energy gap width close to the optimum value and the possibly wide choice of structures working with the CIGS layer, make them highly attractive from the point of view of application in the photovoltaic industry [5].

Thin film cells ( Fig. 2 ) are made using:

• cadmium telluride (CdTe cells) – the most widely known technology; cadmium telluride contains significant amounts of cadmium, which is toxic,
• amorphous silicon (a-Si cells) – the technology most closely resembling standard silicon panels,
• combinations of copper, indium, gallium, and selenide (CIGS cells), gallium,
• arsenide (GaAs cells) – very expensive technology, used primarily in spacecraft.

Photovoltaic thin film cells, thanks to small light absorbing layers, with efficient semiconductors, thinner and much lighter than their traditional counterparts, with very aesthetic appearance and almost any shape, are a very attractive offer to use. In summary, their undisputed advantages include:

• less effect of high temperatures on power output,
• reduced number of materials used in their production,
• efficient operation with little light,
• shading of the panel affects its power output to a lesser extent than in the case of typical panels,
• very aesthetic appearance,
• can take many different shapes, even fancy ones.

Manufacturers such as Sharp and First Solar, in addition to mono and polycrystalline silicon panels, offer thin-film panels. The name describes their construction well - the light-absorbing layers are about 350 times thinner than in standard silicon cells. They are also flexible and adapt to the shape of the roof, and are less than 20 \( \mu m \). Placing them on the roof, instead of putting thick and rigid silicon panels, is a very interesting proposition.