Divalent cation replacement strategy stabilizes wide-bandgap perovskite for Cu(In,Ga)Se2 tandem solar cells

Abstract

Despite improvements in the power conversion efficiency (PCE) of perovskite solar cells (PSCs), stability issues due to ion migration and phase separation remain critical concerns. Given the ionic crystal nature of perovskites, the use of multivalent cations is supposed to effectively suppress ionic migration. However, multivalent metal cations produce deep-level trap states, thus impairing device efficiency. Therefore, a multivalent cation replacement strategy that minimizes interstitial defects is desirable. Here we develop a divalent cation replacement strategy that mitigates ionic migration while limiting phase segregation. We demonstrate that the replacement of the A-site cations in the perovskite lattice with methylenediammonium cations (MDA²⁺) substantially suppresses the above issues in wide-bandgap perovskites. This is mainly due to the bivalent state of MDA²⁺ generating a strong interaction with the inorganic framework and reducing the mobility of halide ions and the formation of defects. As a result, the stability and efficiency of the fabricated PSCs are substantially improved. We demonstrate a champion PCE of 23.20% (certified 22.71%) for a single-junction PSC with a bandgap between 1.67 eV and 1.68 eV. Furthermore, a PCE of 30.13% is obtained for mechanically stacked perovskite/Cu(In,Ga)Se₂ tandem devices, and a PCE of 21.88% for translucent perovskite devices. Finally, we obtain a steady-state PCE of 23.28% (certified 22.79%) for flexible monolithic perovskite/Cu(In,Ga)Se₂ tandem cells.