Atom-Scale Chemistry in Chalcopyrite-Based Photovoltaic Materials Visualized by Atom Probe Tomography

Area of Science:

• Materials Science
• Solid-State Physics
• Photovoltaics

Background:

• Polycrystalline chalcopyrite materials like copper indium gallium selenide (CIGS) are vital for photovoltaic devices.
• Structural imperfections, particularly at grain boundaries (GBs), can influence device performance.
• Understanding GB chemistry is crucial for optimizing CIGS solar cell efficiency.

Purpose of the Study:

• To characterize grain boundaries (GBs) in co-evaporated CIGS films using atom probe tomography.
• To investigate the impact of sodium (Na) and potassium (K) doping on CIGS GBs.
• To elucidate correlations between matrix elements and alkali dopants at CIGS GBs.

Main Methods:

• Atom probe tomography (APT) was employed to analyze CIGS films on flexible substrates.
• Films were studied with and without Na doping, and with post-deposition K treatment.
• Density functional theory (DFT) calculations were used to interpret experimental observations.

Main Results:

• Compositional fluctuations were observed at GBs, regardless of alkali doping.
• [Cu-poor and Se/In,Ga-rich] GBs were more prevalent than [Cu-rich and Se/In,Ga-poor] ones.
• Anti-correlations between Cu and other elements were linked to alkali segregation at GBs, with Na and K showing intra-GB correlation.

Conclusions:

• CIGS grain boundaries exhibit specific compositional characteristics and alkali dopant associations.
• The study reveals that CIGS GBs can be benign or even beneficial for device performance.
• Fundamental insights into CIGS GB chemistry are provided, aiding in material optimization for photovoltaics.