Cation optimization for bifacial surface passivation in efficient and stable perovskite solar cells

Abstract

Passivating the exposed surface of perovskite films is a well-established method for improving the performance of perovskite solar cells (PSCs), but the effect of similar processes on the buried interfaces remains poorly understood. Using methylammonium thiocyanate (MASCN) with a 2D perovskite passivator is known to passivate both exposed and buried surfaces through perovskite recrystallization, allowing the 2D passivator to penetrate the buried interface. However, systematic studies on the role of the thiocyanate anion (SCN⁻) and different cations in this recrystallization process are lacking. Herein, we investigate thiocyanate salts with various cations – ammonium (NH₄⁺), methylammonium (MA⁺), ethylammonium (EA⁺), and n-butylammonium (n-BA⁺). Our results reveal that the SCN⁻ anion significantly improves the solubility of organic perovskite precursors, facilitating recrystallization regardless of the cation. The choice of cation influences the recrystallization process: MA⁺, EA⁺, and n-BA⁺ lead to the formation of a horizontally oriented 2D perovskite layer that hinders charge collection, while NH₄⁺ allows the 2D passivator (in our case, 4-methoxy-phenylethylammonium iodide, MEO-PEAI) to remain on the surface, enhancing device performance. Devices treated with MEO-PEAI and NH₄SCN achieved the highest open-circuit voltage and power conversion efficiency of 24.3% (V_OC = 1.17 V, J_SC = 25.1 mA cm⁻², FF = 82.9%). These devices also demonstrated improved stability under thermal stress at 65 °C and continuous light illumination.