Efficient Narrow Band Gap Cu(In,Ga)Se₂ Solar Cells with Flat Surface

Area of Science:

• Materials Science
• Photovoltaics
• Semiconductor Physics

Background:

• Narrow band gap CuInSe2 (CIS) and Cu(In,Ga)Se2 (CIS:Ga) absorbers are crucial for tandem solar cell applications.
• Surface properties and electrical characteristics significantly impact solar cell efficiency.

Purpose of the Study:

• To investigate the effects of bromine (Br) etching on the properties of narrow band gap CIS and CIS:Ga solar cells.
• To understand how Br etching influences surface morphology, optical, and electrical properties.
• To determine the role of gallium grading in conjunction with Br etching for improved solar cell performance.

Main Methods:

• Fabrication of narrow band gap absorbers (1.0-1.02 eV) using a modified three-stage process.
• Application of bromine-based etching (Br etching) to the fabricated absorbers.
• Characterization of surface flatness (root-mean-square deviation, Rq), optical properties, electrical properties (leakage current, shunt resistance, Voc deficit), minority carrier lifetime, and diode parameters.

Main Results:

• Br etching significantly reduced surface roughness (Rq) for CIS:Ga absorbers, though voids limited reduction in some CIS absorbers.
• Br etching decreased leakage current and increased shunt resistance (Rsh) to over 10 kΩ·cm², but sometimes increased Voc deficit.
• Back-surface recombination increased with decreasing absorber thickness in CIS and low-Ga CIS:Ga cells, but was reduced by higher Ga grading.

Conclusions:

• Br etching is effective in improving surface flatness and shunt resistance of narrow band gap CIGS solar cells.
• Optimized gallium grading is essential for mitigating back-surface recombination, particularly in thinner absorber layers.
• Combining Br etching with appropriate Ga grading leads to narrow band gap CIGS solar cells with enhanced surface flatness and high open-circuit voltage (Voc).