For high-purity Cadmium Telluride (CdTe), the purity level of 99.999% (5N) is the most critical factor, meaning total metallic impurities must be less than 10ppm. Buyers should demand a Certificate of Analysis (COA) that specifies levels of trace elements like Copper (Cu), Iron (Fe), and Lead (Pb), as these significantly impact semiconductor performance. Additionally, specify the physical form required, such as powder, granules, or sputtering targets, as the particle size distribution affects the melting rate and film uniformity in thin-film solar cell production.