4.1. Characteristics of a CIGS and CIGS –Si by SCAPS
We varied the thickness of the absorber from 1µm to 4µm and we plotted the variation of the photovoltaic parameters (the short-circuit current density Jsc, open-circuit voltage Voc, form factor FF and conversion efficiency η) as a function of thickness. The graphs obtained are represented in fig.3. (a-b) and fig.4. (a-b) respectively show the influence of the thickness and the temperature on the J-V characteristics of the ZnO/CdS/CIGS-Si solar cell. We can observe that when the thickness of the CIGS layer increases, all the photovoltaic parameters increase, and then stabilize from a thickness of 3.5µm. It is therefore not necessary to develop absorbers beyond 3.5 µm thick. To understand these results, we plotted the recombination rate at the level of the active layer (Fig.3) as well as the external quantum yield of the cell (Fig.4) for the two extreme thicknesses of the absorber (d = 0.6µm for Si and d = 1.6µm for CIGS). According to the curves obtained, we notice that the recombination rate is lower at the CIGS/CdS interface when the thickness of the active layer increases. These results can be explained by the fact that the increase in the volume of the active layer will allow to absorb more photons and thus generate more charge carriers.

Fig. 3a-b. Variation of JSC; VOC, FF (%) and η (%) at different thickness of Si
Fig.4 a-b: Current versus voltage at various temperature for 1μm (a) of CIGS and 1μm (b) CIGS-Si
Fig. 5(a) (b) depicts the capacitance and theC-2 versus applied voltage plots obtained from capacitance-voltage measurements of CIGS and CIGS-Si solar cells respectively. The curves are linear at reverse voltages, which indicate the presence of traps acting as charge carrier in the solar cell [22, 23]. The slope of curve can allows estimation of charge carrier density [24] and the built-in voltage can obtain from the extrapolated intercept on the voltage axis. The depletion capacitance can be examined using the following relation [24]:
Fig. 5 (a), (b). Variation of the capacitance (a) and (b)C -2 versus applied voltage plots of solar cell irradiated with voltage