## 1 Introduction  
Standard silicon PV modules, which directly convert solar energy into electricity, continue to dominate the market, and this dominance is estimated to last for another 15 years or so [1; 2; 3; 4]. Recently, solar energy conversion technology has included intensively developed concentrated photovoltaic (CPV) technology, which converts solar energy into electricity radiation. Concentrated solar technologies use multilayer photovoltaic cells, which make the efficiency of converting solar energy into electricity about 2.5 times higher than that of conventional single-junction photovoltaic cells [5; 6; 7; 8; 9]. The subject of this paper is another less common photovoltaic module constructed from point-focused Fresnel radiation concentrators (CPVs) and high-efficiency three-junction (3J) cells. This type of photovoltaic module design is also often denoted by the symbol C3 PV. The internal structure of this module is different from the standard one, and in this particular case for the model designated with the symbol CX-75/200, it consists of 200 three-junction cells, which are connected in four parallel connected series made of 50 cells each (50s4p). Each cell has a point-focusing Fresnel concentrator. The size ratio of the concentrator lens to the cell is 40/3 mm, and the distance between them is 80 mm. According to the CPV classification of concentric photovoltaic cells due to the degree of focusing on solar radiation, point-focusing Fresnel concentrators are classified as high concentrator photovoltaic (HCPV). Table 1 gives the generally accepted classification of photovoltaic systems using concentrators (CPV), which is a division based on the degree of focusing of the radiation measured in multiples, the so-called \"suns\", or in W/cm\({}^{2}\) of the cell [10,11]. In-service testing of this type of PV system has shown a very significant dependence of performance on the direction of direct solar radiation and the level of lens contamination [13]. Fresnel concentrators take the form of individual lens blocks that focus sunlight from each side of their upper surface toward a single point at which a photovoltaic cell is located. In a single point-focusing Fresnel concentrator in the shape of a square (40 mm), rings are formed on the surface, which makes use of the whole surface of the lens and results in a high concentration factor, which translates into higher efficiencies achieved by the cell. The advantage of this type of module is the high total efficiency exceeding 30%. The disadvantage is that they require the use of biaxial precision tracking mechanisms because even a small deviation of the direction of direct solar radiation from the direction perpendicular to the surface of the module causes a large and abrupt drop in efficiency. The structure of the internal design of three-junction cells means that the focused incident radiation is captured by three cascaded layers of different materials, and overall, radiation from a wide range of wavelengths (300-1800 nm (for silicon, this range is 350-1100 nm)) is used. The different materials of the three-junction cell are stacked in such a way that each material absorbs a specific range of sunlight and, in a simplified way, this type of C3 PV cell can be considered to absorb approx. 44% of photons are from sunlight, while an ordinary PV cell can do the same with an efficiency of about 20%. The order is also important here--the outer layer must absorb the radiation with the smallest wavelength, that is, with the largest energy gap value of the semiconductor material. The materials used in the construction of the cells of the module under consideration are GaInP-GaInAs-aGe in order from the top outer layer. Two conditions determine the higher efficiency of C3 PV modules: the concentration of solar radiation and the fact that the photovoltaic phenomenon is realized in parallel by three junctions with different spectral ranges [14,15].