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Chapter 4
current, which has an impact on the electrical output of the solar cells. Within the
two batches, small modifications were also introduced, including variations in the
contact material, the alkali content, and the addition of a second cell with monolithic
interconnection, in order to study the impact of these modifications.
In this study, we show the temperature dependencies of the efficiency, open circuit
voltage, fill factor, ideality factor, series and shunt resistances and photo-, saturation
and short circuit current density. Additionally, we propose background theory behind
these dependencies. This will also help the estimation of the temperature dependency
of CIGS solar cells and modules not included in this study.
4.2 Experimental
4.2.1 CIGS solar cells
In this study, both CIGS solar cells deposited on soda lime glass and polyimide were
used. The general structure of these solar cells is shown in Figure 4.1, while the general
structure of a CIGS solar cell is extensively described in chapter 1.3.
The samples used in this study were made following these procedures:
• The SLG samples are depicted in Figure 4.1a, b, c and g. The back and front contacts of
the test samples were covered with soldered indium or evapourated gold, allowing
contact with the measurement system. More information about these samples
can be found in see chapter 3.2. Alongside with the single cells, also monolithically
interconnected cells or 'minimodules' were used in order to simulate the behaviour
of a module with a scribe zone ( Figure 4.1g).
Additionally, small differences between the samples was obtained by the control
of the sodium and potassium contents. This was obtained by the modification of
the molybdenum back contact or by the introduction of a SiN barrier, leading to
x
differences in sodium and potassium concentrations between the samples. Examples
of this variation are shown in chapter 7 and reference [18]. It should be noted that the
cells that are very low in sodium and potassium have a low initial efficiency (10-11%
at room temperature). In the rest of the chapter, we refer to the sodium content, but
it should be noted that the potassium content also varied between the samples.
• The CIGS cells on polyimide substrates were also tested ( Figure 4.1d, e and f). These
samples were deposited via a semi-industrial process and also have the substrate/
Mo/CIGS/CdS/i-ZnO/ZnO:Al stack sequence. The CIGS layer is deposited via ion-
beam assisted coevapouration. These cells were covered by a screen-printed
108
current, which has an impact on the electrical output of the solar cells. Within the
two batches, small modifications were also introduced, including variations in the
contact material, the alkali content, and the addition of a second cell with monolithic
interconnection, in order to study the impact of these modifications.
In this study, we show the temperature dependencies of the efficiency, open circuit
voltage, fill factor, ideality factor, series and shunt resistances and photo-, saturation
and short circuit current density. Additionally, we propose background theory behind
these dependencies. This will also help the estimation of the temperature dependency
of CIGS solar cells and modules not included in this study.
4.2 Experimental
4.2.1 CIGS solar cells
In this study, both CIGS solar cells deposited on soda lime glass and polyimide were
used. The general structure of these solar cells is shown in Figure 4.1, while the general
structure of a CIGS solar cell is extensively described in chapter 1.3.
The samples used in this study were made following these procedures:
• The SLG samples are depicted in Figure 4.1a, b, c and g. The back and front contacts of
the test samples were covered with soldered indium or evapourated gold, allowing
contact with the measurement system. More information about these samples
can be found in see chapter 3.2. Alongside with the single cells, also monolithically
interconnected cells or 'minimodules' were used in order to simulate the behaviour
of a module with a scribe zone ( Figure 4.1g).
Additionally, small differences between the samples was obtained by the control
of the sodium and potassium contents. This was obtained by the modification of
the molybdenum back contact or by the introduction of a SiN barrier, leading to
x
differences in sodium and potassium concentrations between the samples. Examples
of this variation are shown in chapter 7 and reference [18]. It should be noted that the
cells that are very low in sodium and potassium have a low initial efficiency (10-11%
at room temperature). In the rest of the chapter, we refer to the sodium content, but
it should be noted that the potassium content also varied between the samples.
• The CIGS cells on polyimide substrates were also tested ( Figure 4.1d, e and f). These
samples were deposited via a semi-industrial process and also have the substrate/
Mo/CIGS/CdS/i-ZnO/ZnO:Al stack sequence. The CIGS layer is deposited via ion-
beam assisted coevapouration. These cells were covered by a screen-printed
108
