4.3. Effect of thickness for CIGS and CIGS-Si
The variations of the imaginary part of the complex impedance according to the real part as well as the variation of the real part of the complex impedance according to the frequency of the samples CIGS and CIGS-SI are displayed respectively in Fig .6. (a), (b), (c) and (d). The important parameters that can be extracted from a Cole-Cole plot after fitting with equivalent circuit model are the capacitances (C), resistances (R) and time constants ( τ ) corresponding to various contributions. The plot of the complex impedance in the Fig.6. (a), (b) Nyquist plane corresponds to a semicircle of diameter Rp, distant from the axis of imaginary Rs which was very low and whose cutoff frequency F0 is located at the abscissa Rs + Rp/2. The minimum frequency (F = 0) is at the impedance point Rs + Rp and the maximum frequency at the impedance point Rs. The cell capacitance is calculated from these resistors. The other representations to were used to show the variations of the imaginary part and the real part according to the frequency and to determine the frequencies of relaxations.
The effect of the thickness of the silicon layer on the polarization resistance of the solar cell in Fig. 6. The thickness increases the polarization resistance of the solar cell (CIGS-Si) related to (CIGS). This last one resistance increases by a step depends on the thickness of Si. We can clearly see that the thickness begins to influence the performance of the cell from the thickness x=4µm. On the other hand, the series resistance of the solar cell decreased with the increase in the thickness of the silicon layer.
Fig.6.Variation of the imaginary part according the real part of impedance (a), (b) and as a function of frequency (c-with doping), (d –reference) of the all thickness.
The figure shows the separation of the processes one at low frequency and the other at high frequency in order to say which there are two relaxations or hence there are two regions, which contribute to the conduction and the conversation of solar energy into electrical energy. SCAPS software is one-dimensional electrical characterization software the use of impedance spectroscopy shows in this process separation is by jump conduction between the two junctions formed by this solar cell. The two loops are not identical, which justifies the fact that the contribution is predominant in the recombination interface region and the small radius loop for the diffusion interface region.
Fig.7. Nyquist separation diagram of the thickness 2µm for CIGS at temperature 300K
Fig.8. (a-b) variation of the imaginary part of impedance complex as a function of frequency for CIGS without Si and With Si respectively
In Fig. 9 (a), it is clearly shown that the effect of thickness on the resistance of the cell starts from the frequency 0.1Hz to 1 KHz. It is independent of thickness over the rest of the study frequency range. In Fig. 8 (b), it is shown that the effect of the thickness of the doping layer starts to influence the resistance of the cell from the thickness of 4µm and at the frequency 1MHz.
The thickness, which allowed us to obtain a theoretical yield of very good value, is 2µm. This confirmation on the thickness will be to decide, that it is the optimal thickness of a cell by the relaxation frequency determined from the representation of the imaginary part of the complex impedance as a function of the frequency in fig.7 (a, b).
Fig.9. (a), (b) Variation of the real part of impedance complex according of frequency for sample CIGS and CIG-Si
An equivalent circuit of a simple heterojunction (HJ) generally consists of three elements: a series resistance (Rs), a parallel circuit with a parallel resistance (Rp), and a constant capacitance (C) or “capacitance-like element,” i.e., the CPE. The CPE is impedance that displays non-ideal, frequency-dependent properties and a constant phase over the entire frequency range. The non-ideal behavior generally originates from the current density distribution due to the material in homogeneity. The impedance of the CPE (ZCPE) is defined by the CPE index p and constant T as follows.