an n-type CIGS layer between the p-type CIGS and the n-type CdS layers, as already
mentioned above. The p − n junction within the CIGS is called a buried junction; there the
electron-hole pairs are separated. In p-CIGS, which is Cu-deficient, the dominant
recombination mechanism is Shockley–Read–Hall recombination in the bulk. In contrast,
in Cu-rich CIGS films the SRH recombination at the CIGS/CdS interface becomes
dominant.
A very important issue in the development of CIGS solar cells is the role of sodium
(Na). A low contamination with sodium appears to reduce the recombination in p-type
CIGS materials because of better recombination of the grain boundaries. The reduced
recombination rate results in a higher bandgap utilization and thus a higher open circuit
voltage. Typically, the optimal concentration of sodium in the CIGS layers is about 0.1%.
Often, as Na source soda-lime glass is used, which is also the substrate for the solar cell. If
no soda-lime glass is used, the Na has to be intentionally added during the deposition
process. The scientific question of why Na significantly improves several properties of the
CIGS films is still not completely solved. Currently, the influence of potassium (K) is also
being heavily investigated.
Fabrication of CIGS solar cells
CIGS films can be deposited with various different technologies. Because many of these
activities are developed within companies, not much detailed information is available on
these processing techniques. One approach is co-evaporation under vacuum conditions.
Using various crucibles of copper, gallium, indium and selenium, the precursors are
coevaporated onto a heated substrate. The second approach is sputtering onto a substrate
at room temperature. After that, the substrate is thermally annealed under the presence of
selenium vapour, such that the CIGS structure can be formed. Alternatively, a
seleniumrich layer can be deposited on top of the initially deposited alloy, followed by an
annealing step. Because of the variety and complexity of the reactions taking place during
the ‘selenization’ process, the properties of CIGS are difficult to control. Companies that
use or have used co-evaporation processes are Würth Solar, Global Solar and Ascent Solar
Technologies. Among CIGS companies using sputter approaches are Solar Frontier,
Avancis, MiaSolé and Honda Soltec.
An alternative approach to produce CIGS layers is based on a wafer bonding
technique. Two different films are deposited onto a substrate and a superstrate,
respectively. Then, the films are pressed together under high pressure. During annealing,
the film is released from the superstrate and a CIGS film remains on the substrate.
Non-vacuum techniques can be based on depositing nanoparticles of the precursor
materials on a substrate after which the film is sintered. During the sintering process films
mentioned above. The p − n junction within the CIGS is called a buried junction; there the
electron-hole pairs are separated. In p-CIGS, which is Cu-deficient, the dominant
recombination mechanism is Shockley–Read–Hall recombination in the bulk. In contrast,
in Cu-rich CIGS films the SRH recombination at the CIGS/CdS interface becomes
dominant.
A very important issue in the development of CIGS solar cells is the role of sodium
(Na). A low contamination with sodium appears to reduce the recombination in p-type
CIGS materials because of better recombination of the grain boundaries. The reduced
recombination rate results in a higher bandgap utilization and thus a higher open circuit
voltage. Typically, the optimal concentration of sodium in the CIGS layers is about 0.1%.
Often, as Na source soda-lime glass is used, which is also the substrate for the solar cell. If
no soda-lime glass is used, the Na has to be intentionally added during the deposition
process. The scientific question of why Na significantly improves several properties of the
CIGS films is still not completely solved. Currently, the influence of potassium (K) is also
being heavily investigated.
Fabrication of CIGS solar cells
CIGS films can be deposited with various different technologies. Because many of these
activities are developed within companies, not much detailed information is available on
these processing techniques. One approach is co-evaporation under vacuum conditions.
Using various crucibles of copper, gallium, indium and selenium, the precursors are
coevaporated onto a heated substrate. The second approach is sputtering onto a substrate
at room temperature. After that, the substrate is thermally annealed under the presence of
selenium vapour, such that the CIGS structure can be formed. Alternatively, a
seleniumrich layer can be deposited on top of the initially deposited alloy, followed by an
annealing step. Because of the variety and complexity of the reactions taking place during
the ‘selenization’ process, the properties of CIGS are difficult to control. Companies that
use or have used co-evaporation processes are Würth Solar, Global Solar and Ascent Solar
Technologies. Among CIGS companies using sputter approaches are Solar Frontier,
Avancis, MiaSolé and Honda Soltec.
An alternative approach to produce CIGS layers is based on a wafer bonding
technique. Two different films are deposited onto a substrate and a superstrate,
respectively. Then, the films are pressed together under high pressure. During annealing,
the film is released from the superstrate and a CIGS film remains on the substrate.
Non-vacuum techniques can be based on depositing nanoparticles of the precursor
materials on a substrate after which the film is sintered. During the sintering process films