## 2 Materials and Methods
### Building the Simulation Model  
Figure 4 shows the simulation model developed in a Matlab/Simulink package, reflecting the construction of the C3 PV module discussed in this paper. The \"Solar Cell\" [22] component was used to build the model, in which a three-junction cell was implemented according to the scheme shown in Figure 2. The model also considers the deviation from the direction of incident direct radiation perpendicular to the module surface. A switch was used that implements the focusing algorithm through a Fresnel lens. It was experimentally determined that an angular deviation of \(5^{\circ}\) from the perfectly perpendicular direction results in a sharp decrease in the module power. Thus, based on the schematic in Figure 3b, it can be hypothesized that a larger deviation causes focusing of the radiation outside the region of the triple-junction cell with 3 mm dimensions. The simulation model considers the semiconductor material parameters and absorbed radiation ranges, as shown in Figure 2. The simulation results are presented and recorded in current-voltage characteristic plots and power plots. The novelty of the proposed model is that a standard module of the Matlab/Simulink package \"Solar Cell\" was used to build a three-junction cell model (C3 PV). The model is relatively simple and allows other researchers to continue verification studies for similar custom solutions. The \"Solar Cell\" component was developed based on the classical model of a dual diode PV cell. Adapting the proposed model for other multijunction cells requires the determination of the range of the angle of deviation from the direction of perpendicular incident radiation and the degree of focusing. If the module manufacturer provides these data, they can be directly implemented in the model. The basis for constructing the single three-junction cell model shown in Figure 4 is the series connection of three \"Solar Cell\" components, representing three successive layers of material as described in Figure 2. The structure of the C3 PV module was modeled according to the module construction diagram shown in Figure 1. The structure of the modeled C3 PV module was realized using a \"Solar Cell\" component, but in such an  
Figure 3: Models of directional deflection of incident radiation on the cell surface: (**a**) for a classical PV module (plane of array (PAO)), (**b**) for a CPV module with a point-focus Fresnel concentrator. unusual way that three components replace a single cell. The connections between the three materials were modeled with tunnel diodes inserted in series in the equivalent circuit. As a simplifying assumption, the radiation flux distribution was taken as given for each layer of semiconductor material. The part of the model describing the optics of the concentrator was made using standard blocks of the Simulink package. The \"Solar Cell\" component of the Matlab/Simulink engineering computing package is an equivalent model of a dual diode PV cell, the developed form of which is shown in Formulas (1) and (2). The mathematical form of this model is non-linear and entangled and can only be solved by iterative simulation calculations [23]. \[I=I_{ph}-I_{d1}-I_{d2}-I_{sh} \tag{1}\]  
\[I=I_{ph}-I_{s1}\bigg{[}exp\bigg{(}q\frac{U+IR_{s}}{n_{1}k_{B}T_{M}}\bigg{)}-1 \bigg{]}-I_{s2}\bigg{[}exp\bigg{(}q\frac{U+IR_{s}}{n_{2}k_{B}T_{M}}\bigg{)}-1 \bigg{]}-\frac{U+IR_{s}}{R_{sh}} \tag{2}\]  
where \(I\), \(I_{ph}\), \(I_{d1}\), \(I_{d2}\), \(I_{ph}\) mean, respectively, currents of the cell, photocurrent, diodes, and shunt; \(U\)--cell voltage; \(R_{s}\) and \(R_{sh}\)--series and shunt resistances; \(n_{1}\) and \(n_{2}\)--quality factors of the first and second diode; \(I_{s1}\) and \(I_{s2}\)--saturation currents of the first and second diode; \(T_{M}\)--cell temperature; \(q\) and \(k_{B}\)--electron charge and Boltzmann constant.