Wet Pretreatment-Induced Modification of Cu(In,Ga)Se₂/Cd-Free ZnTiO Buffer Interface

Area of Science:

• Materials Science
• Renewable Energy
• Semiconductor Physics

Background:

• Copper indium gallium selenide (CIGS) solar cells are a promising photovoltaic technology.
• Cadmium sulfide (CdS) is a conventional buffer layer, but its toxicity necessitates alternatives.
• Developing efficient Cd-free buffer layers is crucial for sustainable solar energy.

Purpose of the Study:

• To investigate a novel cadmium-free zinc titanate (ZnTiO) buffer layer deposited by atomic layer deposition for CIGS solar cells.
• To explore the effects of various wet pretreatments on the CIGS absorber surface and the CIGS/ZnTiO interface quality.
• To analyze the chemical states and band alignment at the interface and their impact on device performance.

Main Methods:

• Atomic layer deposition (ALD) for ZnTiO buffer layer fabrication.
• Wet pretreatments using NH4OH, H2O, and Cd2+ ion solutions on CIGS absorbers.
• X-ray photoelectron spectroscopy (XPS) for surface chemical state analysis and depth profiling.
• Solar cell device fabrication and performance testing.

Main Results:

• Aqueous solutions effectively removed detrimental sodium compounds from the CIGS surface.
• NH4OH treatment alone reduced photocurrent due to thinning of the ordered vacancy compound (OVC) layer.
• Addition of Cd2+ ions to NH4OH suppressed OVC etching, mimicking conventional CdS buffer layer benefits.
• A small cliff-like conduction band offset (-0.11 eV) was observed at the CIGS/ZnTiO interface.
• Optimized treatments enabled ZnTiO buffer application, achieving 80% efficiency relative to CdS-treated cells.

Conclusions:

• ZnTiO is a viable Cd-free buffer layer for CIGS solar cells.
• Surface pretreatment strategies are critical for optimizing the CIGS/ZnTiO interface.
• Further improvements in band alignment could enhance device efficiency.
• This work contributes to the development of environmentally friendly and high-performance solar cells.