The production of solar cells is a highly precise and technologically demanding process that consists of a large number of chemical, physical and thermal process steps. Solar cells, the basis for photovoltaic systems, convert solar energy directly into electrical energy and play a key role in sustainable energy generation. The industrial production of solar cells requires extensive water treatment and wastewater treatment systems, as there are high purity requirements for the process water and wastewater with specific pollutant loads is produced.

Overview of the production of solar cells

Solar cells consist mainly of semiconductor materials, in particular silicon, and are manufactured in several successive steps. A distinction is made between the production of monocrystalline, polycrystalline and thin-film solar cells.

1. production of silicon wafers

The raw material for solar cells is high-purity silicon, which is obtained using energy-intensive processes:

• Melting process:
  • Pure silicon is melted and crystallized at high temperatures (Czochralski process for monocrystalline silicon, ingot casting process for polycrystalline silicon).
• Sawing and cutting:
  • The silicon blocks are sawn into wafer-thin wafers (typically 150-200 µm).

2. surface preparation

The wafer surfaces are chemically treated to optimize the texture and eliminate defects:

• Etching:
  • Removal of saw marks and contamination by chemical or alkaline wet etching.
  • Typical chemicals: hydrofluoric acid (HF), potassium hydroxide (KOH).
• Cleaning:
  • Use of ultrapure water (UPW), which is free of particles, ions and organic compounds.

3. doping and layer formation

The electrical conductivity of the solar cells is specifically influenced by doping with foreign atoms such as phosphorus or boron:

• Diffusion:
  • Generation of a pn transition layer by high-temperature treatment with doping gases (e.g. phosphorus trichloride, boron trioxide).
• Anti-reflective coating:
  • Application of silicon nitride (Si₃N₄) or other materials to minimize light reflection.

4. metallization

• Contact structure:
  • Printing and sintering of metallic conductor tracks (e.g. silver pastes) on the front and back of the wafers.

5. assembly and quality assurance

• Lamination:
  • Solar cells are laminated with protective layers and glass panes.
• Electrical tests:
  • Testing the cells for efficiency, short-circuit current and open-circuit voltage.

Water demand and requirements in solar cell production

The production of solar cells requires large quantities of process water, in particular ultra-pure water (UPW), which must be almost completely free of impurities. The quality of the water is crucial, as even the smallest particles or chemical residues can impair the efficiency and quality of the solar cells.

Water applications in production

1. Cleaning and rinsing:
   • Removal of chemicals and particles after etching, diffusion or coating processes.
2. Chemical process baths:
   • Use of ultrapure water as a solvent in wet etching and cleaning baths.
3. Cooling:
   • Use of water in cooling systems for thermal processes (e.g. diffusion or sintering).

Important quality parameters

1. Conductivity:

   • Reinstwasser weist eine extrem niedrige Leitfähigkeit auf (< 0,1 µS/cm). Idealerweise liegt die Leitfähigkeit bei 0,055 µS/cm (theoretischer Wert für vollständig deionisiertes Wasser bei 25 °C).
   • A low conductivity is an indicator of the minimum content of dissolved salts and ions.

2. Total organic carbon (TOC):

   • TOC values must be below 5 ppb to minimize the presence of organic compounds.
   • Organic substances can be deposited on the silicon wafers and reduce the efficiency of the solar cells.

3. Particles:

   • Partikelfreiheit ist entscheidend, da schon mikroskopische Partikel (< 0,1 µm) die Struktur der Wafer beschädigen können.
   • The particle concentration must be close to zero.

4. Bacteria and pyrogens:

   • Bakterien müssen vollständig entfernt werden (< 1 KBE/mL).
   • Pyrogens (bacterial residues) must not be present.

5. Ion-free:

   • Cations (e.g. sodium, potassium) and anions (e.g. chloride, sulphate) must be almost completely removed in order to avoid corrosion and chemical reactions.

6. Silica (SiO₂):

   • Siliziumdioxid ist ein natürlicher Bestandteil vieler Wässer und muss vollständig entfernt werden (< 1 ppb), um Ablagerungen auf den Wafern zu vermeiden.