First-Principles Calculations of Black Phosphorene-Like MX (M = Ge, Sn; X = S, Se) Bilayers: Implications for Photovoltaic Solar Cells

Black phosphorene-like MX materials (M = Ge, Sn; X = S, Se) have enormous application potentials in solar cells due to their novel electronic structures and the good overlap of their band gaps with the solar spectrum. In this paper, we comprehensively searched the different stacking structures of bilayer two-dimensional (2D) MX materials (MXs) employing first-principles calculations based on density functional theory (DFT). Except for the well-known most stable AB and tAB stackings for all the MXs, we also found metastable 2D stacking structures: AD stackings for all the MXs, tAD stacking for tSnSe, AA stacking for GeSe, tAδ stackings for tGeSe and tSnS, and tAδ1 and tAδ2 stackings for tGeS. Moreover, all the stable and metastable bilayer 2D MXs have suitable band gaps ranging from 0.95 to 2.17 eV for absorbing sunlight, thus favoring their applications in the photoelectric conversion. As a result, we fabricated a range of junctions using different or identical bilayer 2D MXs and calculated their power conversion efficiency (PCE). Among them, the GeSe-AA/GeSe-AD stacking has the highest PCE of 22.86%, which is superior to most of the junction-type solar cells reported so far, thus showing great potential in solar cells.