10
C. Ballif
W p at the module level are now reported for mono-crystalline Si modules
4 . Assuming
a module lifetime of 25 years, the corresponding module emissions depends on the
location, but would be in the typical range of 10–13 g of CO 2 per kWh, to be compared
to 400 g for a gas power plant and 900 g for a modern coal power plant. Efficiency
improvements, which lead to a direct reduction per Watt-peak of all other material
contributions, such as glass and encapsulates, and the ongoing reduction in wafer
thickness (Fig. 1.6), will continue to improve the ecological impact of photovoltaics,
even more if the sources of electricity used for the production of PV modules are
also decarbonized. Finally, thin-film modules can have even less embodied energy
thanks to the reduced usage of semiconductors.
1.6 Beyond Silicon Single-Junction Solar Cells
Current record efficiencies for solar cells of size larger than 1 cm
2 : 22% for CdTe,
23.3% for CIGS, 26.7% for c-Si [8]. What could come next, in terms of efficiency?
A possibility is given by single-junction GaAs solar cells, where record cells reach
up to 29.1% [8]. The 2.4% difference between the record values for GaAs and c-Si
can be attributed in a large part to intrinsic limitation of silicon, namely its indirect
bandgap and Auger recombination. These material properties limit the efficiency of
Silicon solar cells to 29.4–29.6% [9, 10].
The only proven concept to increase efficiency significantly is the combination of
solar cells in a multi-junction configuration, i.e. where solar cells are stacked on top
of each other. This allows for a better utilization of the light spectrum, thanks to the
fact that each partial cell within a multi-junction configuration can be optimized for
a part of the solar spectrum. The top cell absorbs the short-wavelength light (blue,
green) and delivers a high voltage. The bottom cell absorbs the long-wavelength light
(red, infrared) and delivers a lower voltage. Figure 1.7a illustrates the two classical
configurations of 4 and 2 terminal devices. In the 4-terminal configuration the two
partial solar cells are made separately and work independently, and each partial cell
needs to be contacted separately. In the monolithic 2-terminal configuration, the top
solar cell is directly grown on the bottom solar cell. It is easier to manufacture, but
requires a similar current generation in the top and bottom cells, as the two cells are
connected in series.
The highest stable efficiencies were usually reached by multi-junction devices
made from materials within the GaAs system (alloys of Ga, Al, In etc.,), in combination with a Ge bottom cell. Recently solar cells having efficiencies up to 38.8%
with 5 junctions using deposition on GaAs and InP wafers were reported [12]. As
costly substrates are used and as the deposition process is expensive, such cells are,
4 These carbon footprint values are certified and required for PV tenders in France.
See for instance https://www.pv-magazine.com/press-releases/q-cells-modules-earn-further-lowcarbon-certification-for-french-tenders/.
C. Ballif
W p at the module level are now reported for mono-crystalline Si modules
4 . Assuming
a module lifetime of 25 years, the corresponding module emissions depends on the
location, but would be in the typical range of 10–13 g of CO 2 per kWh, to be compared
to 400 g for a gas power plant and 900 g for a modern coal power plant. Efficiency
improvements, which lead to a direct reduction per Watt-peak of all other material
contributions, such as glass and encapsulates, and the ongoing reduction in wafer
thickness (Fig. 1.6), will continue to improve the ecological impact of photovoltaics,
even more if the sources of electricity used for the production of PV modules are
also decarbonized. Finally, thin-film modules can have even less embodied energy
thanks to the reduced usage of semiconductors.
1.6 Beyond Silicon Single-Junction Solar Cells
Current record efficiencies for solar cells of size larger than 1 cm
2 : 22% for CdTe,
23.3% for CIGS, 26.7% for c-Si [8]. What could come next, in terms of efficiency?
A possibility is given by single-junction GaAs solar cells, where record cells reach
up to 29.1% [8]. The 2.4% difference between the record values for GaAs and c-Si
can be attributed in a large part to intrinsic limitation of silicon, namely its indirect
bandgap and Auger recombination. These material properties limit the efficiency of
Silicon solar cells to 29.4–29.6% [9, 10].
The only proven concept to increase efficiency significantly is the combination of
solar cells in a multi-junction configuration, i.e. where solar cells are stacked on top
of each other. This allows for a better utilization of the light spectrum, thanks to the
fact that each partial cell within a multi-junction configuration can be optimized for
a part of the solar spectrum. The top cell absorbs the short-wavelength light (blue,
green) and delivers a high voltage. The bottom cell absorbs the long-wavelength light
(red, infrared) and delivers a lower voltage. Figure 1.7a illustrates the two classical
configurations of 4 and 2 terminal devices. In the 4-terminal configuration the two
partial solar cells are made separately and work independently, and each partial cell
needs to be contacted separately. In the monolithic 2-terminal configuration, the top
solar cell is directly grown on the bottom solar cell. It is easier to manufacture, but
requires a similar current generation in the top and bottom cells, as the two cells are
connected in series.
The highest stable efficiencies were usually reached by multi-junction devices
made from materials within the GaAs system (alloys of Ga, Al, In etc.,), in combination with a Ge bottom cell. Recently solar cells having efficiencies up to 38.8%
with 5 junctions using deposition on GaAs and InP wafers were reported [12]. As
costly substrates are used and as the deposition process is expensive, such cells are,
4 These carbon footprint values are certified and required for PV tenders in France.
See for instance https://www.pv-magazine.com/press-releases/q-cells-modules-earn-further-lowcarbon-certification-for-french-tenders/.