3 Solar Cells: Basics
65
Fig. 3.22 a V oc as a function of light intensity: theoretical curves for three materials (amorphous
silicon, CdTe, and crystalline silicon); b normalized cell efficiencies / 1−sun : experimental curves
for three types of silicon solar cells (amorphous silicon, polycrystalline silicon and monocrystalline
silicon)—measurements done at IMT, PV-Lab, Neuchâtel
because kT /q is approximately 26 mV at room temperature.
14 The overall result
is shown in Fig. 3.22a, for amorphous silicon (a-Si), crystalline silicon (c-Si) and
cadmium telluride solar cells.
2. A practical effect, depending on the imperfections of the solar cell
According to Fig. 3.15b, c, FF (and therefore also η) will decrease with increasing
light intensity, under the influence of the series resistance R series , which is responsible for a voltage drop under all operation conditions, except for open-circuit
conditions.
On the other hand, V oc and FF will decrease, with decreasing light intensity,
under the influence of the shunt resistance R shunt , which siphons off, at lower
light intensities, a correspondingly larger fraction of the photo-generated current
J ph (see Fig. 3.15b, c) leaving less current to build-up V oc .
The phenomena described here (under point 2), depend very much on solar cell
type, solar cell design, and solar cell fabrication process. A practical example is
given in Fig. 3.22b for three cells, which were measured at IMT PV-Lab Neuchâtel. By comparing Fig. 3.22a, b, one can deduce that at low light intensities, the
imperfections of the solar cell (in this case, the shunts within the solar cell) are
the predominant factor responsible for reducing the efficiency.
3. Concluding remarks
The exact behaviour of solar cell efficiency η in function of light intensity cannot
be predicted in a general manner, but depends (as stated above) on solar cell type,
solar cell design, and solar cell fabrication process. Amorphous silicon solar cells
have, in most cases, a better efficiency at very low light intensities than waferbased crystalline silicon solar cells: for this reason, they are often used for indoor
applications, where the light intensity can be very low.
14 This is only valid for relatively high light intensities, say for light intensities in the range between
0.1 and 1 sun. Remember: 1 sun is equivalent to an irradiation of 1000 W/m 2 ; STC conditions are
based on 1 sun and on the AM 1.5 spectrum (see Chap. 2).
65
Fig. 3.22 a V oc as a function of light intensity: theoretical curves for three materials (amorphous
silicon, CdTe, and crystalline silicon); b normalized cell efficiencies / 1−sun : experimental curves
for three types of silicon solar cells (amorphous silicon, polycrystalline silicon and monocrystalline
silicon)—measurements done at IMT, PV-Lab, Neuchâtel
because kT /q is approximately 26 mV at room temperature.
14 The overall result
is shown in Fig. 3.22a, for amorphous silicon (a-Si), crystalline silicon (c-Si) and
cadmium telluride solar cells.
2. A practical effect, depending on the imperfections of the solar cell
According to Fig. 3.15b, c, FF (and therefore also η) will decrease with increasing
light intensity, under the influence of the series resistance R series , which is responsible for a voltage drop under all operation conditions, except for open-circuit
conditions.
On the other hand, V oc and FF will decrease, with decreasing light intensity,
under the influence of the shunt resistance R shunt , which siphons off, at lower
light intensities, a correspondingly larger fraction of the photo-generated current
J ph (see Fig. 3.15b, c) leaving less current to build-up V oc .
The phenomena described here (under point 2), depend very much on solar cell
type, solar cell design, and solar cell fabrication process. A practical example is
given in Fig. 3.22b for three cells, which were measured at IMT PV-Lab Neuchâtel. By comparing Fig. 3.22a, b, one can deduce that at low light intensities, the
imperfections of the solar cell (in this case, the shunts within the solar cell) are
the predominant factor responsible for reducing the efficiency.
3. Concluding remarks
The exact behaviour of solar cell efficiency η in function of light intensity cannot
be predicted in a general manner, but depends (as stated above) on solar cell type,
solar cell design, and solar cell fabrication process. Amorphous silicon solar cells
have, in most cases, a better efficiency at very low light intensities than waferbased crystalline silicon solar cells: for this reason, they are often used for indoor
applications, where the light intensity can be very low.
14 This is only valid for relatively high light intensities, say for light intensities in the range between
0.1 and 1 sun. Remember: 1 sun is equivalent to an irradiation of 1000 W/m 2 ; STC conditions are
based on 1 sun and on the AM 1.5 spectrum (see Chap. 2).