Effect of alkaline earth metal chloride additives BCl2 (B = Mg, Ca, Sr and Ba) on the photovoltaic performance of FAPbI3 based perovskite solar cells

Abstract

Additive engineering is known to be an effective method for inducing a simultaneous effect of enlarging the grain size and surface passivation. As compared to the monovalent halides frequently used as additives, divalent halides are relatively less investigated in the role of additives. In this work, we report effects of alkaline earth metal halides BCl₂ (B = Mg, Ca, Sr, Ba) as additives on the opto-electronic properties and photovoltaic performance of FAPbI₃ based perovskite solar cells (PSCs). A significant improvement in power conversion efficiency (PCE) from 17.27% to 21.11% is observed by MgCl₂ addition in the FAPbI₃ precursor solution, while a marginal increment for CaCl₂ or BaCl₂ and a negative effect for SrCl₂ is observed. The lattice constant of cubic FAPbI₃ is hardly changed by additives, while the crystallinity is improved by MgCl₂. The carrier lifetime increases from 40 ns to 287 ns and the trap density is reduced from 1.08 × 10¹⁶ cm⁻³ to 3.19 × 10¹⁵ cm⁻³ by addition of 5 mol% MgCl₂, which is responsible for the enhancement in photovoltaic parameters. The steady-state PCE of the PSC with the MgCl₂-additive-treated FAPbI₃ measured under continuous illumination at the maximum power point remains unchanged for 1500 s.