Enhancing the photovoltaic performance of planar heterojunction perovskite solar cells by doping the perovskite layer with alkali metal ions

Perovskite solar cells (PSCs) have attracted much attention due to their impressive photovoltaic performance and low fabrication cost. The perovskite layer plays a critical role in light-to-electricity conversion. Here we study the effects of alkali metal ion dopants on the growth, structure and properties of perovskite films and the photovoltaic performance of PSCs. At a low doping level, the alkali metal ions can significantly affect the formation and properties of the perovskite films. They can enhance the short-circuit current density (J_sc) and power conversion efficiency (PCE) significantly. The J_sc increases significantly from 19.2 mA cm^-2 to 21.0 mA cm^-2 and 20.3 mA cm^-2 for the PSCs with a methylammonium lead triode layer doped with 0.5 mol% K⁺ and 0.25 mol% Na⁺, respectively, and the fill factor (FF) increases from 0.74 to 0.79 for the devices with both doping ions. Correspondingly, the PCE increases from 13.7% to 15.3% and 14.6% for the PSCs, respectively. The efficiency improvement by K⁺ or Na⁺ doping is attributed to their effect in increasing the crystallinity and crystallite size of the perovskite films.