The Preparation and Properties of Polycrystalline Bi2O2Se—Pitfalls in Reproducibility and Charge-Transport Limiting Factors

Thermoelectric materials enable the direct conversion of heat into electricity, but progress is often limited by challenges in reproducibility and stability. Bi2O2Se has recently attracted attention as a promising candidate; however, reported transport properties of undoped polycrystalline samples vary by several orders of magnitude, complicating its use as a baseline for doping studies. In this work, we investigate the sources of variability and identify key factors including precursor contamination, reactions with quartz ampoules and graphite dies, grain size effects, and surface oxidation. To mitigate these issues, we employed calcination of Bi2O3 precursors, synthesis with controlled temperature gradients, coarse-fraction powders, and hot pressing in Si3N4 dies. The resulting polycrystalline Bi2O2Se exhibits improved reproducibility, reduced sensitivity to thermal cycling, and characteristic transport values around σRT ≈ 500 S·m−1 and S ≈ −300 μV·K−1 at room temperature. This is a good starting point for further doping studies and a prerequisite of thermoelectric efficiency studies in the future, which can reveal the true thermoelectric potential of this material.