Construction of a 3D/2D heterojunction based on a fluorinated cyclohexylamine 2D Ruddlesden-Popper perovskite for highly efficient and stable perovskite solar cells

Nonradiative recombination at perovskite/charge transport layer interfaces is caused by surface defects and instability, and it is known to limit the long-term development of perovskite solar cells (PSCs). To overcome this issue, the three-dimensional/two-dimensional (3D/2D) perovskite heterojunction has emerged as a possible solution to improve the stabilities and efficiency of PSCs. Herein, we employ a simple one-step method to prepare n-i-p-structured PSCs using a 3D/2D perovskite heterojunction as the absorption layer. For this purpose, the large and non-centrosymmetric 4,4-difluorocyclohexylammonium (DFCHA⁺) cation, which has been confirmed to be a valid organic spacer in 2D Ruddlesden-Popper (RP) phase perovskites, is employed as an organic ligand for post-treatment of the surfaces of MAPbI₃ films. The presence of an ultrathin 2D RP phase perovskite was confirmed on the surface, and the 3D/2D perovskite heterojunction was successfully constructed. Benefitting from surface post-treatment, the density of the surface trap states was reduced with effective passivation. In addition, nonradiative recombination was suppressed, and the interface bands were aligned. As a result, the optimal device achieved a power conversion efficiency of 21.93% with a remarkable open-circuit voltage (V_OC) of 1.14 V, a current density (J_SC) of 23.71 mA cm⁻², and a fill factor of 0.82. Furthermore, owing to the hydrophobicity of the DFCHA⁺ cation, the unencapsulated device was able to maintain an initial efficiency of 82.3% after storage for 500 h at a relative humidity of ∼45%. We believe that this post-treatment strategy has wide application potential in the field of photovoltaic devices.