Enhancing CdTe Solar Cell Efficiency through CSS Heat Treatments

• Filip Růžek, Mechanical Engineering, University of Delaware

• William Shafarman, Materials Science and Engineering, University of Delaware

Enhancing CdTe Solar Cell Efficiency through CSS Heat Treatments

Closed space sublimation (CSS) is a key vapor transport technique used as a deposition process in the thin film cadmium telluride (CdTe) solar cell production. This study presents essential steps implemented within the CSS system to achieve high-performance solar cell devices. 

The CSS system has been used for both high-temperature annealing (HTA) steps and the critical CdCl₂ treatment in the CdTe fabrication process. HTA-1 is the first heat treatment in the CSS process, applied after the deposition of the thermally evaporated front stack CdSeTe/CdTe layer. During this treatment, CdTe source is being evaporated onto the sample at 575°C. HTA-1 treatment helps improve the crystal quality by increasing grain size and reducing defects in the front layer, which makes it easier for charges to move through the material. Following CdTe layer deposition using the vapor transport deposition (VTD) method, the sample undergoes HTA-2 in the CSS system as the second heat treatment in the process. It helps to intermix the front and VTD layers into one uniform layer with improved crystallinity and uniform grain size. Following this, a post-deposition CdCl₂ treatment is carried out at 425 °C for 1 hour. To ensure optimal deposition in the CSS system, all treatments should include both a controlled preheating phase and an argon gas purge. Preheating the substrate and source materials stabilizes thermal gradients and supports consistent deposition rates. Purging with argon displaces oxygen and moisture from the chamber, minimizing oxidation risk. Nevertheless, some studies have suggested that the presence of a small amount of oxygen during the final and most critical CdCl₂ treatment, may be beneficial. The CdCl₂ treatment is a crucial step that helps passivating grain boundaries and promoting recrystallization and grain growth. Together, these steps greatly enhance the quality of the CdTe layer and boost solar cell performance.