Currently, most two‐dimensional (2D) metal halide perovskites are of the Ruddlesden–Popper type and contain the thermally unstable methylammonium (MA) molecules, which leads to inferior photovoltaic performance and mild stability. Here we report a new type of MA‐free formamidinium (FA) based low‐dimensional perovskites, featuring a general formula of (PDA)(FA)n−1PbnI3n+1 with propane‐1,3‐diammonium (PDA) as the organic spacer cation. The perovskite films with well‐oriented crystal grains are attained under the assistance of the FACl additive, where the role of Cl is investigated through the grazing‐incidence X‐ray diffraction technique. The photovoltaic device based on the optimized (PDA)(FA)3Pb4I13 film demonstrates a remarkable power conversion efficiency of 13.8 %, the highest record for the FA‐based 2D perovskite solar cells. In addition, compared to (PDA)(MA)3Pb4I13, the MA‐containing analogue and a renowned stable 2D perovskite, both the (PDA)(FA)3Pb4I13 films and their derived devices exhibit exceedingly higher thermal stability.
The
heterogeneous stacking of a thin two-dimensional (2D) perovskite
layer over the three-dimensional (3D) perovskite film creates a sophisticated
architecture for perovskite solar cells (PSCs). It combines the remarkable
thermal and environmental stabilities of 2D perovskites with the superior
optoelectronic properties of 3D materials which resolves the chronic
stability issue with no compromise on efficiency. Herein, we propose
the vapor-assisted growth strategy to fabricate high-quality 2D/3D
heterostructured perovskite films by introducing long-chain organoamine
gases in which the 2D layers have a uniform and tunable thickness.
The 3D to 2D transformation of the widely adopted MAPbI3 (MA = methylammonium) film is initiated by the butylamine vapor
and monitored through the in situ grazing-incidence X-ray diffraction
technique. A variety of 2D species are observed and rationalized by
the different collapsing and reconstruction models of the Pb–I
octahedra. The PSC devices based on the optimized 2D/3D heterostructures
show significant improvements in photovoltaic performances, owing
to better energy level alignments, longer carrier lifetimes, and less
defects as compared to their 3D analogues. In addition, both the butylamine
vapor-treated perovskite films and the derived PSC devices demonstrate
exceptional long-term stabilities.
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