Mixed dimensionality of 2D/3D heterojunctions for improving charge transport and long-term stability in high-efficiency 1.63 eV bandgap perovskite solar cells

Developing 2D/3D heterojunctions on 3D perovskite surfaces is an attractive strategy to improve the photovoltaic efficiency and stability of perovskite solar cells (PSCs). Particularly, the mixed dimensionality (MD) of 2D phases is found to be beneficial to both interfacial charge transport and device stability. Nevertheless, the control of dimensionality in aromatic spacers for 2D perovskites is very difficult. In addition to that, the use of 2D/3D heterojunctions on wider (∼1.63 eV) bandgap perovskite solar cells is also uncommon. In this work, halogen para -substituted aromatic ligands based on R-α-methylbenzylammonium (R-α-MBA) were employed to form 2D capping layers on top of the wide-bandgap perovskite surface. The MD of the aromatic 2D capping layers ( n = 1 and n = 2) was achieved through para -substitution with bromine (R-α-BrMBA), which increases the formation energy of the n = 1 phase. Thus, the interfacial charge transport was improved with the MD heterojunctions that had Type I band alignment, resulting in reduced energy loss. Furthermore, the introduction of aromatic ligands also flattens the surface morphologies and reduces the trap density. As a result, a champion power conversion efficiency (PCE) of 21.48% was delivered with an ultrahigh FF of 82.44% for 1.63 eV bandgap PSCs. Besides, long-term storage and thermal stabilities of the unencapsulated PSCs modified with R-α-BrMBA are substantially enhanced owing to the increased surface hydrophobicity and thermally stable MD passivation layers. By using R-α-MBA with bromine terminal to develop 2D perovskite with mixed dimensionality on the 3D perovskite surface, an improved PCE of 21.48% with a FF of 82.44 was achieved for 1.63 eV bandgap perovskite solar cells.