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.