Over 21% Efficiency Stable 2D Perovskite Solar Cells

Owing to their insufficient light absorption and charge transport, 2D Ruddlesden–Popper (RP) perovskites show relatively low efficiency. In this work, methylammonium (MA), formamidinum (FA), and FA/MA mixed 2D perovskite solar cells (PSCs) are fabricated. Incorporating FA cations extends the absorption range and enhances the light absorption. Optical spectroscopy shows that FA cations substantially increase the portion of 3D‐like phase to 2D phases, and X‐ray diffraction (XRD) studies reveal that FA‐based 2D perovskite possesses an oblique crystal orientation. Nevertheless, the ultrafast interphase charge transfer results in an extremely long carrier‐diffusion length (≈1.98 µm). Also, chloride additives effectively suppress the yellow δ‐phase formation of pure FA‐based 2D PSCs. As a result, both FA/MA mixed and pure FA‐based 2D PSCs exhibit a greatly enhanced power conversion efficiency (PCE) over 20%. Specifically, the pure FA‐based 2D PSCs achieve a record PCE of 21.07% (certified at 20%), which is the highest efficiency for low‐dimensional PSCs (n ≤ 10) reported to date. Importantly, the FA‐based 2D PSCs retain 97% of their initial efficiency at 85 °C persistent heating after 1500 h. The results unambiguously demonstrate that pure‐FA‐based 2D PSCs are promising for achieving comparable efficiency to 3D perovskites, along with a better device stability. Pure formamidinum (FA)‐based 2D perovskite solar cells (PSCs) achieve a record power conversion efficiency (PCE) of 21.07% (certified over 20%), the highest efficiency for low‐dimensional PSCs (n ≤ 10) reported to date, together with the improved device stability. The high‐efficiency device exhibits a narrowed bandgap and unique 2D–3D intermixing phase distribution for improved light absorption and superior charge transport.