Intrinsic and Extrinsic Point Defect Chemistry of Selenium

Selenium was the first material used in a photovoltaic (PV) solar cell and has recently experienced a renaissance in research interest as a candidate solar absorber, due to several desirable properties (suitable band gap, earth abundance, low-temperature processing), its potential implementation in silicon tandem cells and ‘simple’ elemental chemistry.^1,2 While cell efficiencies have improved significantly since its initial application in 1883,³ <i>t</i>-Se still lags behind leading PV technologies primarily due to a low open-circuit voltage relative to the band gap. The atomistic origins of this voltage deficit and fundamental limitations of <i>t</i>-Se-based solar cells remain unclear, however, with controversy regarding both the doping concentrations of as-grown selenium films and calculated defect levels muddying our understanding of defect activity in this material.<br/><br/>In this work, we investigate the intrinsic and extrinsic defect chemistry of <i>t</i>-Se using both theoretical and experimental approaches. We find a strong ability for the helical selenium chains to reconstruct and valence alternate in order to compensate charged defects, yielding amphoteric behaviour for most low-energy extrinsic species. We demonstrate that intrinsic point defects are not the cause of apparent high hole concentrations in as-grown samples, but could be active for non-radiative electron-hole recombination.<br/><br/><b>References:</b><br/>1 M. Zhu, G. Niu and J. Tang, <i>J. Mater. Chem. C</i>, 2019, <b>7</b>, 2199–2206.<br/>2 R. Nielsen et al., <i>J. Mater. Chem. A</i>, 2022, <b>10</b>, 24199–24207.<br/>3 C. E. Fritts, <i>American Journal of Science</i>, 1883, <b>3</b>, 465–472.