A Computational Re-Evaluation of Se as a Solar Absorber

Trigonal selenium (c-Se) was the first material to exhibit the photovoltaic (PV) effect and has recently experience a renaissance in research interest due to its desirable properties (suitable band gap, high earth abundance, low temperature processing and ‘simple’ chemistry) and potential implementation in silicon tandem cells.¹ Though cell efficiencies have improved much since 1883, c-Se lags behind leading technologies such as silicon and lead-halide perovskites.²<br/><br/>In this work³, we use hybrid density functional theory (DFT) to investigate crystalline selenium’s 5 known synthesisable polymorphs (trigonal, rhombohedral, β-monoclinic, γ-monoclinic, δ-monoclinic) and determine the ideal phase for solar cell operation. Trigonal Se has the lowest direct band gap (1.904 eV with spin-orbit coupling) and highest charge carrier mobilities, allowing it to reach the highest efficiencies. Analysis of the optical response shows that c-Se has a theoretical upper limit to its efficiency as a single-junction solar absorber of 23% at a thickness of 2 μm. We calculate the vacuum alignment of the electronic band edges, providing guidance for further optimisation of the absorber contact layers. In addition, we compute the formation energies and charge transition levels of all intrinsic point defects (vacancies and interstitials) at the hybrid DFT level, to characterise the defect chemistry and doping response in this material.<br/><br/>References:<br/>1 M. Zhu, G. Niu and J. Tang, <i>J. Mater. Chem. C</i>, 2019, <b>7</b>, 2199–2206.<br/>2 T. K. Todorov, S. Singh, D. M. Bishop, O. Gunawan, Y. S. Lee, T. S. Gershon, K. W. Brew, P. D. Antunez and R. Haight, <i>Nat. Commun.</i>, 2017, <b>8</b>, 682.<br/>3 A. E. Samli, S. R. Kavanagh and D. O. Scanlon, Submitted, 2022.