16.1
16.2
time before they are implemented in PV modules.
Multi-junction solar cells
The first limitation discussed in the list above can be tackled using multi-junction solar
cells. Depending on the author, these solar cells are seen as part of the second or third
generation, and indeed we already briefly discussed them in Chapter 13 on thin-film solar
cells when looking at the III-V technology (Section 13.2) and thin-film silicon technology
(Section 13.3). For completeness, we will summarize this technology here.
In multi-junction cells, several cell materials with different bandgaps are combined in
order to maximize the amount of the sunlight that can be converted into electricity, as we
already illustrated in Figure 10.9. To realize this, two or more cells are stacked onto each
other. The top cell has the highest bandgap and will absorb and convert the short
wavelength (blue) light. Light with wavelengths longer than the bandgap wavelength can
traverse the top cell and be absorbed in the cells with lower bandgaps below. The bottom
cell has the lowest bandgap and absorbs the long wavelength (red and near-infrared) light.
In order to optimize the performance of multi-junction solar cells with two electrical
terminals, matching the currents of all the subcells (current matching) is crucial.
Multijunction cells with more terminals do not have this restriction, but their production is
more complicated.
In thin-film silicon tandem cells, an a-Si:H top cell is stacked onto an nc-Si:H bottom
cell. In order to achieve current matching, the top cell is much thinner than the bottom
cell. The cell can be further optimized by using an intermediate reflector between the top
and the bottom cell in order to reflect the blue light back into the top cell while letting the
red light pass to the bottom cell. The reported record efficiency of a-Si:H/nc-Si:H tandem
cells is 12.3% and for a-Si:H/nc-Si:H/nc-Si:H it is 13.4% [77].
Multi-junction cells containing III-V semiconductors are at present the most efficient
solar cells. The current world record efficiency is 46.0% for a four-junction GaInP/GaAs;
GaInAsP/GaInAs cell that is used in a concentrator PV system [48]. Because of the high
concentration factor of 508 suns the overall efficiency increases and hence we also tackle
the second limitation mentioned in the list above. As a result the SQ limit can be exceeded
by more than 10%.
Spectral conversion
16.2
time before they are implemented in PV modules.
Multi-junction solar cells
The first limitation discussed in the list above can be tackled using multi-junction solar
cells. Depending on the author, these solar cells are seen as part of the second or third
generation, and indeed we already briefly discussed them in Chapter 13 on thin-film solar
cells when looking at the III-V technology (Section 13.2) and thin-film silicon technology
(Section 13.3). For completeness, we will summarize this technology here.
In multi-junction cells, several cell materials with different bandgaps are combined in
order to maximize the amount of the sunlight that can be converted into electricity, as we
already illustrated in Figure 10.9. To realize this, two or more cells are stacked onto each
other. The top cell has the highest bandgap and will absorb and convert the short
wavelength (blue) light. Light with wavelengths longer than the bandgap wavelength can
traverse the top cell and be absorbed in the cells with lower bandgaps below. The bottom
cell has the lowest bandgap and absorbs the long wavelength (red and near-infrared) light.
In order to optimize the performance of multi-junction solar cells with two electrical
terminals, matching the currents of all the subcells (current matching) is crucial.
Multijunction cells with more terminals do not have this restriction, but their production is
more complicated.
In thin-film silicon tandem cells, an a-Si:H top cell is stacked onto an nc-Si:H bottom
cell. In order to achieve current matching, the top cell is much thinner than the bottom
cell. The cell can be further optimized by using an intermediate reflector between the top
and the bottom cell in order to reflect the blue light back into the top cell while letting the
red light pass to the bottom cell. The reported record efficiency of a-Si:H/nc-Si:H tandem
cells is 12.3% and for a-Si:H/nc-Si:H/nc-Si:H it is 13.4% [77].
Multi-junction cells containing III-V semiconductors are at present the most efficient
solar cells. The current world record efficiency is 46.0% for a four-junction GaInP/GaAs;
GaInAsP/GaInAs cell that is used in a concentrator PV system [48]. Because of the high
concentration factor of 508 suns the overall efficiency increases and hence we also tackle
the second limitation mentioned in the list above. As a result the SQ limit can be exceeded
by more than 10%.
Spectral conversion