Microscopic Analysis of Interdiffusion and Void Formation in CdTe_(1-x)Se_x and CdTe Layers

Area of Science:

• Materials Science
• Photovoltaics
• Semiconductor Physics

Background:

• Cadmium telluride (CdTe) photovoltaics are a promising solar energy technology.
• Incorporating selenium (Se) to form cadmium telluride-selenide (CST) phases can enhance CdTe solar cell performance.
• The diffusion of Se and its impact on CdTe grain structure require further investigation.

Purpose of the Study:

• To investigate the impact of different window layer structures (CdS, CdSe, CdS/CdSe) on Se diffusion and CdTe grain structure.
• To understand the intermixing behavior between CdS, CdSe, and CdTe layers.
• To evaluate the effectiveness of CdSe incorporation for Se grading in CST phases.

Main Methods:

• Transmission electron microscopy (TEM) for microstructural analysis.
• Energy-dispersive X-ray spectroscopy (EDS) for elemental composition analysis.
• Electron backscatter diffraction (EBSD) for crystallographic orientation and grain structure analysis.

Main Results:

• Extensive intermixing between CdS, CdSe, and CdTe layers was observed, leading to void formation at the front interface and potential device performance degradation.
• The use of CdS/CdSe bilayer structures resulted in the formation of a parasitic cadmium sulfide-selenide (CdSSe) phase.
• Removing the CdS layer facilitated effective CdTe and CdSe intermixing, but sputtered CdSe showed limited success in achieving Se grading within the CST phase.

Conclusions:

• Intermixing at the front interface, particularly with CdS, is a significant challenge for CdTe photovoltaics incorporating Se.
• Bilayer CdS/CdSe structures can lead to undesirable parasitic phase formation.
• Optimizing window layer strategies, potentially by excluding CdS, is crucial for successful Se grading and improved CdTe solar cell performance.